Using a Car Battery to Recharge a Laptop

I had occasionally wondered whether I could use a car battery to recharge a laptop computer battery. This question usually arose in connection with a desire to go camping in a relatively remote location for a period of several days, where I might not benefit much from advice that would be helpful in more conventional settings (e.g., get a long-life battery, recharge at restaurants). This time around, I decided to write down some notes that accumulated as I investigated this question. (Another post talks about using a car battery to power a desktop computer.)


Maxx Battery.


Considering Possibilities
Setting Up a Deep Cycle Battery
Charging a Deep Cycle Battery
Charging the Laptop from the Deep Cycle Battery



A person certainly could use the cigarette lighter to charge a laptop. You would plug an inverter into the car’s lighter socket, plug the laptop’s AC adapter into the inverter, and connect the adapter to the laptop. This seemingly straightforward possibility raised several questions, however:

  • I wondered whether voltage surges or other aspects of automotive electronics could damage the laptop. A couple of searches led to a thread suggesting that the laptop’s adapter would typically be adequate to take care of that, and also to one user’s claim to have lost two laptops due to the car’s impure power. Another commenter in that same thread said that an inverter capable of handling that concern would likely cost over $300. Others, disputing that, said it would be the inverter, not the laptop, that would be damaged. There were also indications that, in any case, the cigarette lighter was powered by the battery, not by the car’s electrical system or by some other kind of generator, and therefore the supplied current would not be plagued by fluctuations. Two commenters reported using a laptop with an inverter with no problem; the recommendation seemed to be that the focus should be, not on the inverter – that any brand would do there (or perhaps not) – but rather on the AC adapter, which should be supplied by the laptop manufacturer. In other words, it might be a bad idea to run an electronic device requiring 12V input directly from the 12V car battery.  Further investigation turned up a New York Times article from 2007 praising the Sears Die Hard inverter for powering a laptop and also keeping an eye on the car’s power level.  Sears offered several DieHard inverters, supplying 120 to 750 watts (i.e., connecting through the cigarette lighter or directly to the car battery), in the $40-70 price range.  In addition, a Top Ten Reviews article (influenced, I think, by advertising dollars) listed ten inverters that, it said, were suitable for use with laptops.  Eight of the ten were in the $30-45 range.  Moreover, an eHow article treated inverters as safe for laptops and noted that some models had circuitry to prevent excessive car battery drain.  It seemed that a brand-name inverter would probably be fine with the laptop.
  • The preceding concern assumed that the car was running at the time. But in a wilderness camping scenario, I did not plan to be driving down the road anywhere for two or more hours a day while my laptop or spare battery was recharging. (A Dell webpage, and others, had indicated that a typical laptop might take two hours to recharge.) I would apparently have to let the car sit there and run for those hours. Not a very green solution. If the car averaged 30 MPG at 60 MPH, that would be two gallons per hour while driving down the freeway. I figured I might get twice that, or more, at an idle. So a tankful of gas would probably last for some days. But it would still be wasteful and relatively expensive to burn up a gallon or two every time I needed to recharge the laptop.
  • If the car was turned off, clearly it would be a case of charging one battery from another, with dirty power but without voltage fluctuations. This seemed unlikely to damage the laptop, but now there would be another problem. The laptop’s AC adapter would always be running, even when the laptop was fully charged. So would the inverter. Neither the inverter nor the adapter would know that the laptop was full. They would keep converting electricity into desired forms until the car battery was worn down. Some said this could happen overnight or possibly even within a few hours. Car batteries were not designed to be drained, and would be damaged by such usage. After a few times, they would have to be replaced.  But I was not sure how quickly a battery would run down.  It seemed to me that a childhood friend had run a car radio for days, using a car battery sitting on a workbench.  (Apparently the warning against leaving car batteries on concrete floors was a false alarm.)  Power calculations seemed to suggest that a car battery might indeed have that kind of staying power.  For instance, one website estimated that a laptop would draw 65-90 watts, and another put it closer to half that; meanwhile, a Consumer Reports webpage, describing how some people have used their cars and power inverters to power their homes (one appliance at a time, if necessary) during power outages, said, “We used a fully charged, spare car battery to run a refrigerator for roughly six hours before the battery ran down.”  They didn’t say how big a fridge, or what the temperature was.  But various sources suggest that a refrigerator might draw between 300 and 800 watts, with the average apparently above 700.  This suggested that a laptop might run for many hours before a car battery would need recharging.

One possibility, I thought, might be to install a second battery in the car. This would, in fact, be a useful thing, not only as a source of backup power, but also as an emergency way to start the car if the first battery was dead. An informative video offered several suggestions for that option. Basically, I would bolt the second battery into a somewhat shock-absorbing and plastic-lined (Tupperware?) space in the car’s trunk. As usual in primitive car electronics, I would arrange to power and recharge it by running a single cable from the positive side of the battery up front, and then cabling the negative side to “ground” (i.e., to the car’s metal frame or body). Either of these two batteries would be able to start a fire and burn up the car, so I would have to install fuses near each of the two batteries. I would want to charge the rear battery while the car was running, but otherwise I would not want that rear battery and its various uses to mess with the car’s starting battery, not unless I wished to walk home from the primitive camping site. So I would want a switch, located somewhere along the power cable running from front to back. Then, instead of using the car’s cigarette lighter to power the laptop (except possibly when the car was running), I would use the rear battery. Given the risk of occasionally running down that rear battery due to unattended recharging of electronic devices, I would buy a deep-cycle battery for the rear. Then I would just have to remember to switch it on, perhaps recharging the laptop and the rear battery simultaneously, when the car was running.  (Note the risks and precautions regarding charging of lead acid car batteries, and the option of using more expensive AGM cells.)

One of the main problems of this whole scenario was that I would have to use both an inverter, to convert 12-volt car electricity to 120-volt AC, and the laptop’s adapter, to convert 120V AC into 20-volt juice for the laptop. I knew some electronic devices would run on straight 12V DC, which would mean that I could power them directly from the cigarette lighter or second car battery without any transformers or cooling fans running in those adapters and converters. In my understanding, this meant that the laptop would draw the current that it needed to recharge, but then the drain would stop. One response, I guessed, would have been to link various batteries in series and parallel, so as to produce a 20V DC power supply. Even then, I would have to rig up a way to get that power into the laptop through its battery connections; it did not have a DC power input jack.

Along these lines, Wikipedia said that some cars had USB ports instead of, or in addition to, cigarette lighter adapters. A search suggested that, at this writing, plans were underway to develop laptops that could be charged via USB 3.0 connections. I didn’t know whether car manufacturers would be using this technology. I guessed that it would still require transformation to produce the desired voltage, as well as laptops that were ready for it. It sounded intriguing, but I could not presently see how it would help me to charge my laptop in my car.

There was, in addition, the workaround of using the laptop’s adapter to charge just the battery (or a second battery), separate from the laptop, if the adapter had that capability.  My laptop, a Lenovo ThinkPad Edge E430, would require both a battery ($80 from Newegg) and a poorly rated “Combo Adapter” ($100) or an External Battery Charger ($120) if I wanted to charge a second battery at the same time as I was charging the one in the laptop.  Searches failed to turn up less expensive alternatives for those charging devices.  To save that $100-120, I would have to charge one battery inside the laptop, and then replace it and charge the other.  An alternative, if I wanted the extra baggage, was perhaps to buy a broken E430 on eBay and use it as a separate charging station.

Considering Possibilities

The foregoing information caused me to pause and collect what I had learned.  Additional thoughts emerged as I did so. My speculations about a second car battery panned out as follows: I would have to buy a deep cycle car battery. These were heavy, so I probably would buy it locally rather than having it shipped. This battery would apparently cost between $70 and $100 at Wal-Mart (with tax) – or $100-200, if I wanted to go for a presumably better battery at Sears or through an Interstate dealer (although some sources said most batteries were made by the same manufacturers). If I wanted to charge the battery while driving, I would have to bolt it into the trunk, cable it to the front (with fuses and switch), and attach a cigarette lighter adapter to it, either in the trunk or via cables into the passenger compartment; the latter would let me keep the laptop up front while traveling. I also might be spooked into buying a more expensive inverter.  In short, I did not see how I would get out of this project for much less than $150, and it could possibly run over $300, plus the time investment to set it up, plus whatever risk there would be of doing it wrong and converting my car into a bomb.

It might occasionally be possible to go into town to recharge the deep cycle battery, assuming I could find a place that would let me plug it in.  It presently appeared that, with a battery charger, deep cycle batteries were best left to recharge for at least four hours and possibly overnight or even longer, especially when attempting to follow the advice not to discharge more than 50% and not to recharge too fast.  Given a weight or 40 to 50 pounds or more, this was not shaping up as a job for the public library, nor for recharging from a running automobile.  The frequent advice to use a smart charger that would taper the charge, and would not overcharge, suggested that I would be looking at maybe $50 for a charger.

The deep cycle battery approach would work only for camping near my car. If I wanted to fly somewhere, I would have to think of something entirely different. In that case, I could perhaps use the developing-nation approach of charging a battery with a bicycle generator. I would need access to a bike (buy one at Walmart and then sell via Craigslist?) and would have to have a way of putting the bike into some kind of stationary stand (suspend it from a tree branch?).  My information at present was that it could take ten hours of pedaling to recharge a large auto battery.  TotallyTandem reported that there were also problems where fluctuations in pedaling pace would trigger laptop surge protection (i.e., end of recharge); it seemed the dynamo would instead have to power an intermediate battery.  There were lots of cool bike devices out there, but they weren’t cheap, and it didn’t sound like everybody had now found the obvious solution in these regards. Alternately, I could take a solar charger, though my preliminary glance suggested that one of these powerful enough to recharge a laptop would still cost hundreds of dollars, and would not necessarily be very portable.  I guessed that solar recharging might achieve commodity pricing levels within the next few years.  There would still be limitations while working at night or on cloudy days, and issues of moving the solar device as the sun moved across the sky.

Later, I ran across external battery packs like the Energizer XP18000A.  This device was billed as a one-pound external battery that could “double your notebook battery life.”  Apparently it came with some but not all of the tips (i.e., adapter plugs) that would be needed to power various kinds of devices; fortunately, it sounded like others could be had cheaply.  The Energizer webpage included a Tip Finder that looked like it listed tips for hundreds of devices.  It had a tip for the Lenovo ThinkPad Edge E530, but not for my E430.  I contacted Energizer’s Customer Service to ask if they had one for the E430.  They offered a Free Tips for Life service through which a registered product would qualify for two “free” tips per year ($4 shipping).  Eventually they did provide information on a tip for the E430.

Meanwhile, I pursued Energizer’s specifications.  It looked like there was an XP18000, an XP18000A, and an XP18000AB, with the later versions apparently adding improved Apple compatibility.  Several user reviews complained that the device wore out not long after the expiration of its six-month warranty.  A search took me to a page that said the XP18000A would extend my laptop battery by only six hours — perhaps equal to one or two backup laptop batteries.  A Lifehacker page suggested that the Anker Astro Pro2 was more popular and had more power than the XP18000A, but its webpage indicated that it did not come with a connector for my laptop, and anyway it was limited to 60W 20V, whereas my laptop’s power brick said that it output 65W.  I also noticed some other Energizer-type devices, including units by IncrediCharge,Turcom, and Tekkeon, and others that looked good but were not compatible with my Lenovo ThinkPad, notably the Intocircuit.

Finally, it occurred to me that there might be a very different approach.  Maybe I could divide up the workload, so that I would work near the car while the laptop battery held out, and then grab some books, smaller electronic devices (e.g., an eReader, or a DVR), and/or paper and pencil and head off to a more remote location for another day or more.  Options of that nature, yet to be explored, could make a significant difference in this post’s concern with laptop battery power.

Setting Up a Deep Cycle Battery

After some months of exploring options and thinking about it, I bought a Maxx Marine 29DC deep cycle marine battery.  This was basically the largest auto-type battery available at my local Walmart.  With tax and the battery core deposit, it cost $120.  It came with a red plastic cap tightly sealed onto the positive post.  The sales person informed me that I could return the battery as long as I had not yet connected it electrically.  I guessed that tearing off that red plastic cap would be their hint.  Basically, this was a $120 step into the unknown.

The Maxx had a two-year replacement warranty.  I could have bought another model with similar capacity with a one-year warranty for about $20 less, after tax, and possibly I should have. The Maxx battery promised 114 amp-hours at one amp.  This seemed to mean that, if I was drawing current at a rate of 1 amp, the battery would go for 114 hours.  As a deep-cycle battery, it was supposedly designed to be drained and recharged many times — hundreds of times, according to some reports — though the advice seemed to be that, for maximum battery life, I should recharge it when it dropped to 50% of capacity.  Another tip was to recharge it as soon as possible after use.  I was not sure that would be feasible in every case.  It was also advised not to undercharge.  Run it down into the 50% to 85% range, and then run it back up to 100%.

The Maxx weighed 62 pounds.  It came with a plastic handle that I could use to carry it.  Carrying it would be least awkward if I had something heavy to carry in the other hand, for balance.  For the most part, I expected to leave it sitting in my car, in the floor space behind the driver’s seat, where it would be braced against tipping over.  Walmart had a plastic container designed for these batteries.  I had gathered that it might be a good idea to use something like that, not so much for spills as for acid fumes that might otherwise go wafting out and chew a hole through the floorboard.  I wasn’t sure how much of a worry that was, but in any case the plastic box came with a vented lid, so as to protect against the risk that some metal object would accidentally contact both battery posts simultaneously — which, as I had learned in childhood, would cause the metal object to become immediately welded to the posts, possibly melting and starting a fire in the process, not to mention draining and probably damaging the battery’s storage capacity. The picture at the beginning of this post shows the Maxx in its plastic container.

I was under the impression that you were supposed to pop the caps on the cells when using a charger, but an eHow page said, “Opening the ports to the cells can void the battery’s warranty.”  There was no such warning among the words printed on the caps on the Maxx.  The eHow page also seemed to say that I should have gotten warranty paperwork with the battery.  Walmart’s FAQs echoed both points; but someone else said the warranty information was entered automatically when I bought it. The easily removable lid on that plastic container would allow me to connect a battery charger without removing the Maxx from the car. The consensus appeared to be that I could also charge the Maxx from the car’s alternator (i.e., with the car running), though preferably with this sequence:  start the car, let it run awhile to recharge its installed battery, connect the deep cycle battery to the car’s battery via jumper cables, and let it charge.

The consensus appeared to be that a car would normally recharge its own battery within maybe 10 minutes after starting. Then it should be ready to do some jump starting. Multiple sources said that recharging a standard-sized dead battery would take a half-hour of driving after a jump start. It would take longer if the car was only idling. In the extreme case, Home Depot observed that a battery charger set to its low-speed mode could take as long as several days to fully charge the battery, but presumably an hour would suffice in most cases of charging from a car. More charge time might be required when charging a large marine battery.

Today’s automotive electronics were hopefully not so delicate that they could not handle something akin to giving another driver a jump start. But a large battery could apparently risk straining the car’s alternator, demanding more than it was designed to put out. To shorten the charge time and perhaps also the duration of strain on the alternator, I wondered whether it would help to use Cruise Control, or brick (better, perhaps, an adjustable tripod or selfie stick) on the gas pedal, to experiment with slightly increasing the speed at which the car’s engine was running during recharge. Instead, one could install the wiring needed to charge the battery inside the vehicle during extended driving periods.

For the most part, I planned to use a charger.  I was advised to keep the Maxx hooked up to a trickle charger when it was in storage, or at least recharge it monthly; apparently it would lose capacity if I let it sit in a discharged state.  Since the battery was not in storage, the mission was to use a more robust charger (i.e., not a trickle charger) to recharge the Maxx in relatively rapid terms.  (In fact, trickle-charging a marine battery that needs an actual recharge would apparently reduce its capacity, potentially ruining the battery.)  A modern charger would also monitor recharge rates, so as to avoid overcharging.  In addition, since I had little familiarity with any of this, I would probably benefit from a charger (or, optionally, a separate monitor) with a digital readout that would tell me whether the battery was at 50% discharge or had returned to 100% charge.

So I bought a Schumacher XCS15 charger for $50.  It offered 15A, 10A, and 2A (trickle) modes.  Its manual provided safety tips for working with lead acid batteries, and also recommended positioning the charger at least 18″ above the floor (or in my case, ground) level. There were criticisms of Schumacher chargers, specifically from people who claimed the charger had ruined deep cycle batteries.  It seemed, however, that others had used these chargers with marine batteries without problems.  Advice was split on the charging rate.  Some recommended 10A (ten amps); others said 15A.  The Schumacher charger’s manual (p. 5) seemed to say that auto and marine batteries needed a 12A to 15A rate, but it did not actually indicate “Not Recommended” for the 10A setting.  Schumacher’s FAQs page was somewhat muddy on this.  One source said I should shoot for 10% of the amp-hour rating, which would indicate an 11.4A charger.

How long would it take to recharge the battery?  Walmart’s page was uninformative, but another source said that the Maxx 29DC had a reserve capacity (RC) of 210.  The Schumacher manual didn’t have a line in its recharging chart for that; the highest it went was 180.  It said that, at the 10A setting, the charger would take 6.5 hours to recharge a 50%-discharged battery with an RC of 180, and that the 15A setting would take 4.5 hours.  Extrapolating from other values shown in the chart, I estimated that the times for a 210 RC might range between about 5 hours (15A) and 7.5 hours (10A).  Basically, when the battery’s charge declined to 50% of capacity (which I could test by hooking up the charger and viewing its readout), I would need to find a place to run an extension cord to the car for a good part of the day, or overnight.  This assumed that the AC power outlet I was using would supply 15 amps without blowing a fuse — not a safe assumption, especially if others were using the same circuit (especially if they were running high-demand items like hair dryers or hot plates).

Finally, I needed a voltage inverter, to convert the Maxx’s 12VDC into 120VAC for the laptop’s adapter.  I wanted one with minimal overhead:  I did not want the Maxx to be wearing itself out, trying to power an unnecessary 400-watt inverter, if all I needed was ~65W for the laptop.  Along with all the other stuff I was buying that day, Walmart offered a Schumacher XI14 inverter that boasted a no-load draw of less than a quarter of an amp.  In other words, its little fan would not be draining the Maxx anytime soon.  The inverter was designed to plug into a cigarette lighter, which was not what I wanted, so I stopped at an Auto Zone store and bought a Bell 39011-8 Battery Clip Power Adapter for $7.  This was essentially a cable with clips on one end (to connect to the Maxx) and a cigarette lighter socket on the other end (to accommodate the inverter).  I would later have to reconnect the wires to the clips, as the factory had done a poor job and the wires were about to fall off.  Amazon offered an alternative:  an inverter with two different cables, one for plugging into a cigarette lighter and the other for connecting to a battery.  But its price was more than my inverter-plus-Bell combination, and Amazon did not say how much it would draw from the battery.

Charging a Deep Cycle Battery

I connected the charger to the Maxx battery and plugged the charger into a power outlet.  It read 12.4V and 76%.  It alternated between voltage and percentage every few seconds.  Once I chose a charging speed, however, it no longer showed the voltage decimal place:  that is, it just showed 12V (not 12.4V). The manual said the battery would need charging if its voltage was less than 12.8V.  Apparently the battery had lost some of its juice while sitting in the Walmart, or maybe they didn’t charge it fully at the factory.  A blue sticker on its top said “5/13” — indicating, I guessed, that it had been manufactured two months earlier than this writing of July 2013.  I hoped that sitting around for two months without being recharged had not damaged its capacity.

The charger was not particularly quiet.  It didn’t exactly have a whine to it, but neither did it have a reassuring whoosh.  Sort of like a mouse-sized vacuum cleaner. I put the charger on the 15A setting at 5:30 PM.  In the first half-hour, it was just flying along:  it looked like we would be back up to 100% in no time.  But then it seemed to get hung at 88%.  It stayed there for at least an hour.  Then it went up to 89%.  At 8:30 PM, three hours after connecting it, that’s where it remained.  Given the estimate (above) of five hours for a 50% discharged deep cycle battery with an RC of 210 at a 15A rate, I would have expected it to be completely charged in 2.5 hours (given my actual starting point of 76%).  We were well past that.  So was it overcharging?  When it said 88%, was it actually at 99%, and had it now arrived at (or above) 100%?  How could I tell? I didn’t have a voltage meter to see for myself.

I put the charger into a closet, without disconnecting it, so that I could hear if the battery was making any noise.  It was bubbling.  Was it supposed to?  A search suggested that bubbling was normal — but not “an expanding case or hissing sound or a hot case or visible smoke.”  I took the Maxx out of the plastic case.  No, the sides were not bulging.  Nothing was leaking out.  I just had a quiet little bubbling.  This seemed to be just the natural chemical reaction, especially toward the end of the charging process.  Some said that, in fact, a lack of bubbling would be a sign of a dead cellOthers said they commonly hooked up the charger overnight at 10A.  So probably I was just watching the clock too closely.  Still, this was about the time when I started to get a little nervous about the warnings about explosions of hydrogen gas produced by charging batteries.  I double-checked my cross-ventilation and turned on the ceiling fan.  I also reflected that this was a downside of the huge battery:  if proper charging meant full charging, I was at risk of being tethered to a power outlet for, what, ten hours? until the thing finished stuffing itself.

I did not check again until about 11 PM, which was 4.5 hours after starting the recharge.  The battery was not bubbling.  I opened the closet.  The charger said 100%.  We were done.  But — what’s this?  The stupid charger was on the “Gel” setting.  This was not a gel battery!  (Referring, here, to the contents of the cells:  mine had liquid, water and acid, not gel.  Apparently I had bumped the button while moving the charger?  Had I ruined the battery?  As I looked at it, I saw that it was also on the trickle setting.  I put the buttons where they belonged:  Standard charge, Fast setting.  It said 95%.  Then 96%, then 97%.  In a few minutes, we were back at 100%.  A quick search yielded no immediate indications that I had destroyed the battery forever.  It seemed we were good to go.  Whew!  Have to pay more attention to those easily bumped charger buttons!

Charging the Laptop from the Deep Cycle Battery

I unplugged and disconnected the charger (in that order) and set it aside.  Now the big question was, how does this thing do at charging my laptop?  It was time to find out.  And I had an opportunity to do so.  I had just been investigating laptop battery endurance, and now my laptop’s battery was at only 6.3%.  In other words, it was almost 94% discharged.  It needed to be charged.

Would the Maxx do the Big Job? I connected the laptop’s AC power supply to the laptop.  I plugged the AC power supply into the Schumacher inverter.  I connected the Battery Clip Power Adapter to the Maxx.  I took a deep breath and plugged the inverter into the adapter.  Everything was connected:  adapter to inverter to AC power to laptop.  I looked at the laptop’s screen.  BatteryBar said we were at 6.4%.  Then 6.5%.  The Maxx was charging the laptop!  It was working. The inverter was putting out a slight whistling sound.  It definitely was using some of the juice to do its conversion and power its little cooling fan.

I wondered whether some inverters would be significantly more efficient than others.  A search led to a West Marine page that said that a 12V DC-to-AC inverter would typically operate in a range of 85% to 95% efficiency, meaning that as much as one-sixth of the Maxx’s power might be chewed up by the inverter.  I guessed that my cheap little Schumacher adapter was probably on the inefficient side.  Then I thought to check its manual.  It said 85% was the unit’s optimum efficiency.  Optimum!  So it might be running at 80% or even 75% efficiency?  Turns out that my focus on its minimum draw had not necessarily led me to an efficient unit. One comment in a discussion thread suggested that “all inverters have a sweet spot where they can reach 95%.”  Taking the sweet spot as meaning the optimum, it seemed that this was not true in the case of this Schumacher.  They said it would reach the optimum at a certain load, e.g., 80% of capacity.  So the search for a maximally efficient inverter would ideally take into account the specific load that the laptop’s AC adapter was placing on the inverter.

I assumed that the 65W figure written on its power brick was its maximum draw, not necessarily its average.  Someone else in that same thread said that inverters (at least in the 1000-2000W range) were around 80-85% efficient, but less than that when running below 10% of capacity.  That seemed to be a matter of simple arithmetic:  I was going to lose some current to the inverter’s fan and other internal workings in any case; that fixed loss would be a larger share of the total when I was drawing less current.  For example:  draw 16W, spend 8W on the fan; net = 50% efficiency.  But draw 160W, spend 8W on the fan; net = 95% efficiency.  A more efficient unit for a larger load might be less efficient for my purposes, if I had to spend more energy to power a bigger unit (with e.g., a larger fan). So the efficiency figure could be misleading:  the real question was, how much total current am I using when I charge the laptop?  One site said that running a laptop could take 25W.  But I was charging and running simultaneously.  Mine was a Lenovo ThinkPad Edge E430:  not a very demanding laptop.  Taking it halfway between 25W and the AC adapter’s 65W capacity, I guessed maybe 40-50W.  What would that mean in practical terms?

I knew there was a way to work out all the math, at least if I had full information about the inverter and the adapter; but since I didn’t, and was short on time for research at the moment, I decided to just let the recharging proceed and then see where things stood. I had started charging the laptop from the Maxx at 6:45 AM.  I left the laptop running during this process, so I could keep an eye on it, and perhaps that would be typical:  I would probably want to continue working, and to be able to disconnect the adapter as soon as it was done, to minimize drain on the Maxx.  If I had been actually working on the laptop, I would have drained the Maxx more; if I had powered down the laptop, I would have drained it less.

I found that BatteryBar (discussed in the other post) provided useful information during the charging process.  Among other things, it reported charge rate and elapsed time.  At 9:45 AM, exactly three hours after hooking up the adapter, the charging was completed — the laptop reached 99.0%.  Consistent with the Lenovo battery-preserving mode, the laptop would not charge to 100%.  I disconnected everything and hibernated the laptop. Then I dragged the Maxx in its plastic case over by the closet where the Schumacher car battery charger was sitting.  I connected it to the battery and plugged in the AC cord.  The charger said 000.  It stayed like that for a minute.  I disconnected it from the Maxx.  It stayed at 000.  It was unresponsive to button presses.  I unplugged the AC cord, let it sit for a minute or so, and replugged it.  This time, it gave me 0.0.  I connected it to the Maxx.  It said 12.5V.  I punched the button to choose the fast charging mode.  It said 82%.

So a single laptop charge had drained the Maxx to nearly the point where it was when I brought it home from the store.  It appeared that I would get two laptop charges from it before I would need to recharge it.  Gone were my dreams of having five or ten laptop charges from a single Maxx charge.  Then again, I had actually gotten two charges at once:  I had a recharged battery that would run for another 3.5 hours, and the Maxx had also given me three hours of computing, for a total of 6.5 hours.

Further use revealed that that first impression was very mistaken.  I got an enormous amount of use out of a single deep cycle charge.  I am on the road right now with the battery, as of Sept. 27, 2013; I will do a more definitive revision of this post when things settle down.  But it appears I got about 85 hours (!) of laptop computing out of my last recharge of the Maxx, over a period from mid-August to the end of September.  The approach taken there was to charge up the Maxx and then use it to recharge the laptop, and then run from the laptop battery.  It took about two hours to recharge the laptop — which I would be using at the time — and then I could run for about three hours on the laptop’s battery.  So a single charge of the laptop from the deep cycle gave me about five hours of computing.  So the 85 hours of computing means that, as confirmed by a spreadsheet I am keeping, I got 17 laptop recharges out of one recharge of the Maxx.  I was able to use one deep cycle recharge for six weeks because I was also doing a lot of computing in public libraries and restaurants.  (The 85-hour figure is adjusted to indicate only deep cycle recharges; it does not include any computing facilitated by a direct wall outlet recharge.)  I have been able to recharge the deep cycle at a convenience store, at the home of someone who invited me in for dinner, and at the garage of a mechanic who did some work on my car.  Another post has more info on this camping trip.

One time, when trying to use the deep cycle battery to recharge the laptop, it would not charge.  The tooltip arising when I moused over the battery icon in my system tray said, “Plugged in, not charging.”  It would charge when the AC adapter was connected to a wall outlet; but it would not charge when the AC adapter was connected to a 12-volt inverter connected to the deep cycle battery.  Another post explores the solution to that problem.  Briefly (silly me), the problem was that I had drained the deep cycle battery completely, so it could not charge the laptop.  There was no difficulty in recharging the deep cycle from this completely discharged state.  I don’t know exactly how long the recharge took; I left it charging overnight.  [These remarks about the 85-hour run are an interim interjection.  The following paragraphs, written before August 1, will have to be revised to take account of these developments.]


There were still some unknowns, but at this point it seemed I could pause to reflect on the situation.  First, about the Maxx.  I did not know whether the charger’s 82% indication was accurate.  If it was, then I could get two or three 6.5-hour sessions out of a single charge of the Maxx, and would still remain above or at least not significantly below the recommended 50% mark for recharging.

The scenario, then, was that I would go to a place where I could recharge the Maxx and also the laptop; I would work on the laptop while recharging it at that location; and then I would go out into the field, with a fresh 3.5-hour charge on the laptop and with nearly 20 hours’ worth of laptop recharges on the Maxx.  That would be good for at least another day or two of semi-continuous computing, and the field time would be stretched to the extent that I was able to use other devices (e.g., eReader, DVR) (above) to do some of my work. Ideally, I would be using that recharging location to charge other devices simultaneously.  This would be easy enough in the case of other electronic devices that required AAA batteries:  I had an inexpensive little charger and a pocketful of rechargeable AAAs.  But for the laptop in particular, it would be helpful to make the most of that power outlet while I had it.

There were a couple of ways to increase my laptop power at that recharging location, assuming I would be staying there all day or overnight — long enough, that is, to recharge a significantly drained Maxx.  One was to buy extra laptop batteries and recharge them in the laptop, one after another.  But as noted above, batteries for my ThinkPad were not cheap.  They would be worth little whenever I did decide to sell the ThinkPad.  Another option was to buy an external battery; but the problem I had run into there (above) was that the best external batteries (i.e., those by Anker and Intocircuit) were not compatible with the ThinkPad.  Compatible external batteries would have the advantage, over the Maxx, of having matching voltage, thus not requiring the inverter and its extra drain.  Of course, compatible external batteries would also tend to be significantly lighter and more compact, since they would not use lead-acid cells.  They would also be safer.

Expressed in those terms, it appeared that the ThinkPad itself was a critical barrier to my plans.  I had not seriously considered getting rid of it, having invested money in its purchase and time in its configuration.  I had assumed that the battery project was more or less a sidelight, a technicality that I could resolve satisfactorily with some attention and a modest expenditure.  But now, as I grappled with the situation firsthand, it appeared that I might have been better advised to reconsider the whole hardware situation from the outset — to be prepared, that is, to jettison the ThinkPad if I found its battery options unappealing. Indeed, I could still do that.

The investment in the Maxx did still seem worthwhile.  Anything that could provide 20 hours of computing was worth taking seriously.  But what I was contemplating now was a vision of having multiple recharging processes underway at the same time, whenever I did arrive at a recharging location:  the Maxx and its Schumacher auto battery charger would be doing their thing; the laptop (and, ideally, its consumer-oriented AC power supply) would be capable of charging at least two affordable laptop batteries simultaneously; and the external brick, a model better than the Energizer, would be able to run the laptop for another half-dozen hours and, at the recharging station, would be recharging itself at the same time as the Maxx and the laptop.

I could stretch the scenario even further, without much additional expense, by adding more laptop batteries or possibly a second, less expensive deep cycle battery.  With such arrangements, accompanied by devolution of some computing tasks to smaller devices (e.g., eReaders, DVRs), it seemed plausible that I might drive off for at least several days, possibly the better part of a week, before having to worry about recharging. At this writing, laptops with significantly longer battery life were coming on the scene.  The problem was their price, typically in the $1,000+ area.  Besides, I liked my laptop.  Switching to a different model in its price range would take a lot of time for reconfiguration, and did not appear to offer a revolutionary change in the terms of the situation.  So for the time being, it seemed that I did have to work within the foregoing battery constraints.

7 thoughts on “Using a Car Battery to Recharge a Laptop

  1. Gary

    I came across this post looking for a solution allowing my wife to use her cpap machine while camping. After reading your interesting and intriguing musings, I only have one comment…

    Go Camping. Enjoy the nature. Leave the laptop at home.

  2. John

    Thanks Ray! I’m going to Wal-mart right now. I read your post and love this idea. I will be following your instructions. You are very kind for sharing. I am always in the field. I had a 9-cell battery but it no longer works. It brought 14 hours of computing life. Please keep us updated.

  3. pngai

    You should know that if your charger is putting 10 Amps into the battery, it will be drawing much less from the wall. If it were 100% efficient, it would be drawing 12V/120V as much current, or 1 Amp.


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