Using a Car Battery to Power Laptop and Desktop Computers

In a previous post, I explored the possibility of using a car battery to recharge a laptop computer. This post summarizes several answers to the question of how to keep a desktop or laptop computer running when the lights go out. The conclusion is that, for some purposes, the best way of letting the user keep on working may be to keep a laptop computer synchronized with the desktop.

Previously: Powering a Laptop

The solution developed in the previous post was simple enough. The key ingredient was a car battery — more accurately, a deep cycle marine battery, because car batteries are not meant to run down very far, and sometimes a computer’s battery backup needs to do exactly that. The battery needed a recharger, and I would also need an inverter to convert the battery’s output into the kind of electricity needed by the laptop’s power adapter. (I vaguely recalled hearing about laptops that could run directly from a 12-volt external source, but mine was not one of those.)

The process described in the previous post was simple too. I would work at the laptop as usual. When the laptop’s battery ran down, I plugged the laptop’s power adapter into the inverter and connected the inverter to the deep cycle battery. Most times, I would continue working at the laptop while the deep cycle battery was recharging the laptop’s internal battery. Every now and then, on that ten-week camping trip, I would use a power outlet at a restaurant or gas station to plug in the recharger and recharge the marine battery.

On that trip, I kept a log of my computing hours. One time, I did not recharge the marine battery; I just kept using it until it was dead. From fully charged to fully dead, on that occasion I got a total of 85 hours of computing out of the deep cycle battery before I had to recharge it.

Powering a Desktop: Full UPS Options

I had owned several different kinds of uninterruptible power supply (UPS). The big ones were expensive, heavy, and relatively bulky, but were delightful when the power went out: just keep on working, for a half-hour or more, no worries. The smaller UPS units were less expensive, but not usually cheap. They were mostly just good for giving me a few minutes to make notes of what I was doing and then shut down the system in an orderly fashion, so as to avoid data loss and harm to hardware or software. Large or small, the typical UPS battery seemed to last a few years — depending, I guess, on how much it had been used — and then replacing it would cost enough to make me ask whether there was a better way. Which was precisely the situation that prompted me to consider the question once again, and to write this post.

This particular unit had run for more than three years before the battery died. At that point, the orthodox options were to replace the entire UPS with another one, or to keep the UPS and just replace its battery. Some units made it relatively easy to access the battery; some didn’t. This was a CyberPower 350SLG. My question: do you suppose they secured the battery compartment with a simple switch, like the ones that hold laptop batteries in place, or at least a simple Phillips screw? The answer: oh, hell no. They had to use a Torx screw. Fortunately, I had a Torx wrench, which I used at least once a year, for unnecessary errands like this one. But CyberPower wasn’t done. Once inside, I saw that the wires leading to the battery connectors were very short, so as to make it as difficult as possible to unplug them from the old battery and replug them onto a new one.

But, I admit, they could have made it even harder. They could have soldered the wires directly to the battery terminals. They could have installed a capacitor to shock me when I touched that Torx screw. So let’s give them that much. Credit where credit is due.

I intended to simply replace the battery. But now I had a new problem. I ordered a battery that the merchant advertised as a replacement for this UPS. The merchant was wrong; moreover, the merchant refused to give me credit. This was apparently a potential cost that should be factored in. So I was stuck with this battery and, given the expense of shipping heavy batteries, I did not see that I was going to net much from it on eBay. Besides, I still had the problem of needing a battery backup.

So at that point, there was the question of whether maybe I could use this somewhat larger incorrect replacement battery with my little CyberPower 350SLG. Since the replacement wouldn’t fit inside the 350SLG case, I would have to extend the wires and leave the battery sitting outside the UPS. My browsing suggested that this could work, and it would make it easier to replace the battery next time, maybe with something even larger.

But there were several problems. First, I discovered that electrical wire, and the electrical connectors needed to extend the battery wires, were not cheap. They weren’t really expensive, but somehow it did not seem right to spend $15 on that trivia — necessary, for me, since I lacked convenient access to a junkyard, where I might have gotten the connectors and wire from an old car — plus whatever I would have to pay for replacement batteries in the future, and still just have a rinky-dink little UPS. (Connectors seemed advisable: sources differed on whether soldering wires directly to the battery would harm it.) Knowledgeable people also suggested that I would want to go to the trouble of running the wires through holes that I would drill in the case (and would then sand down to remove sharp edges), and tying knots in those wires on the inside, so as to prevent me from ruining everything when, inevitably, something would get tangled in those extension wires, or would butt up against the external battery, and then I would accidentally stumble over it, or otherwise give it a good hard yank, and rip it all apart. A greater concern: knowledgeable people said that, to cut costs, the recharging circuit in the 350SLG might be designed only to recharge the 350SLG’s stock battery. Connecting a larger battery might burden that circuit, causing it to overheat and produce (a word that I was going to encounter repeatedly in this exploration): fire!

So there was the option of dumping the CyberPower 350SLG, buying another nice, big UPS, and then playing around with external battery alternatives (instead of buying another expensive replacement battery) a few years down the line, when the battery inside the big UPS wore out. But a long, old discussion (among others) led me to think that, unless I had specific information about the recharging circuit inside that big UPS, there could be the same overheating concern in that situation too. Apparently overheating could also be a concern if the UPS was not designed to carry a heavy load for longer than the amount of time allowed by its internal battery. Moreover, if my external battery was not the kind of battery anticipated by the UPS charging circuit, there was apparently the possibility that the UPS would damage it by trying to recharge it with the wrong voltage, or at the wrong pace. According to one report, another possibility was that, in some units, the recharging circuit would detect that the voltage was wrong, and would simply refuse to recharge the external battery. I saw reports from people who seemed to be doing OK with this kind of arrangement, but it was hard to tell what kinds of loads or temperatures their units might have been dealing with, and in some cases there did seem to be issues with battery life. For typical car batteries (but, some said, not for marine, sealed, gel, or maintenance-free batteries), there was also the concern that charging would generate hydrogen gas, resulting in fire! explosion!

Minimal UPS Options

People who lived in houses rather than apartments, or whose computer systems were operating in non-residential (e.g., rural, office park) locations, might be able to place their lead-acid batteries outside, or to connect fans and vents, so that hydrogen gas produced by car batteries during recharge would not gift them with a fiery cataclysm. Such ideal locations would also facilitate another option: a generator. A generator, usually requiring venting if not outdoor placement for its exhaust and its noise, would be the ideal solution for situations where the user needs a big alternate power source, or (due to e.g., weather emergency) needs power for an extended period.

As another approach, those who kept a laptop constantly synced with the desktop computer could have the alternative of simply continuing their work on the laptop, with an occasional trip to some generator-powered building for a recharge. Of course, they would still have to figure out how to power their router, and do whatever was necessary to get online as needed.

There were generators that would come on automatically in a power outage, available not merely for large businesses but apparently also for private homes. It seemed, at a glance, that prices for these might start at about $2,000. A humbler option would be to buy a UPS sufficient to keep the computer running for five or ten minutes, while the user ran outside, fired up a personal generator, and manually plugged the UPS into it via extension cord. One source cautioned that it might prove necessary to buy a larger-than-expected generator in order to supply sufficient power to satisfy the UPS, so that it would accept and rely on the generator’s input rather than draining its own internal battery. The issue there was presumably a matter of the stability of the incoming power, as distinct from its technical purity: according to an interesting SuperUser response, computer power supplies were generally designed to clean up dirty electrical power. So — for computers, at least, and hopefully also for UPS units — it was apparently not necessary to worry about whether the generator or other source produced pure sine wave output.

Another source said that the electronics in laptops are shielded from power fluctuations because the laptop uses the incoming power to charge its battery, and then runs the computer from the battery. Something like that was also feasible for desktop computers. APC offered a relatively non-technical explanation of different technical approaches to the circuitry inside a UPS unit. The concept of interest here was best captured by the “double conversion” (a/k/a “online”) approach: alternating current (AC) from the wall outlet goes through a “rectifier,” whose purpose is to convert AC into direct current (DC); and then that DC goes through an “inverter,” whose purpose is just the opposite, to convert DC into AC.

Why go to that trouble? Because, in that arrangement, you could connect a battery at the midpoint, as an alternative to the AC input. The inverter will be getting DC either way — from the wall socket, through the rectifier, if your local electrical utility is delivering power normally; or from the battery, if the wall socket is dead. Quality Power Solutions offered a brief explanation of the superiority of this double-conversion approach. Drawbacks apparently included increased power consumption and heat, due to the inefficiency of the two power conversions, as well as the greater expense for UPS components that would always be under load, never on standby. Amazon offered some double conversion UPS units for sale, apparently starting at around $450.

It seemed that a user could approximate this double-conversion scenario by simply connecting a battery charger (essentially, a rectifier) to a car battery; connecting the car battery to an inverter; and then plugging the computer into the inverter. As long as the power was coming from the wall socket normally, the battery charger would charge the battery; and if it was a reasonably intelligent charger, it would stop charging when the battery was fully charged. In all events, the computer would continue to be powered by the output from the inverter, whether the power source was the wall socket or the battery. But apparently it wasn’t that simple. One user described his homemade double-conversion UPS. Its components cost about $600, circa 2007, and evidently it ran well for five years, at which point he made a few upgrades. For those interested in trying the simple version of this double-conversion scenario, I saw that Amazon offered battery chargers at providing 12 amps of current or more (big enough, presumably, to power a desktop, though perhaps not continually), starting as low as $40.

The generator scenario made me think there might be another option, essentially an offline double-conversion system. It would go like this. Ordinarily, the user would have the computer plugged into a standard UPS. When the power fails, the user would unplug the UPS from the wall socket and plug it into the inverter, and would then connect the inverter to a marine battery. For a desktop computer, this would require the user to be present when the power went down, or to buy a UPS big enough to keep the computer running until s/he could return to the scene and make these connections. The marine battery could then run the computer and recharge the UPS until power was restored or the marine battery ran down. If the user had access to a generator-powered building, s/he might continue for some time with a cyclical arrangement: use one marine battery to power the computer; take a second marine battery to the building for recharging; come back at day’s end with the recharged battery, or immediately bring back a third battery that has finished recharging; and swap the charged and discharged batteries while the UPS continued to power the computer. Of course, marine batteries aren’t cheap. But two or three of them, used in this manner, could support desktop computing for an extended period. This approach would probably support laptop computing for months, in situations where an onsite generator or other laptop recharging source (e.g., solar; bicycle) would not be an option.

A revision to that scenario might involve recharging the marine battery by using jumper cables to connect it to a running automobile, idling in the driveway. That approach might not be wise or realistic if the recharge would take many hours, or would be too rapid for the marine battery, or if recharging that battery (a big one, especially) would place an excessive load on automotive electrical components. Perhaps that load would be reduced if the car was given at least 15 minutes to recharge the car’s battery (which would have just been used to start the car) before attaching the marine battery. There would also be the option of recharging a laptop via the car’s cigarette lighter, or possibly via an inverter clipped directly onto the car’s battery — again, done (at least sometimes) while the car was running, so as to avoid running down the car’s battery.


Given the expenses and complexities of some of these alternatives, not to mention the dangers of some of the do-it-yourself options, it seemed that efficiency might best be served by buying a basic UPS, just sufficient to permit orderly shutdown upon power outage, combined with a synced laptop — assuming the user did have a laptop running a compatible operating system. As described in my other post, in my own experience, a synced computer was useful, not only as an alternate workstation when the primary system was unavailable, but also for running tasks (e.g., backup, synchronization, spreadsheet calculation) that would have bogged down the main machine. In times of extended power outage, as I had seen, one marine battery was sufficient to keep a laptop running for many hours of computing.

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