Need a really big laptop battery!
February 12, 2010 12:02 PM   Subscribe

How can I get a really really big rechargable "laptop battery"? Thinking 12-24 hours worth of charge on a laptop that can drain a high-cap battery in 1 - 1.5 hours.

I thought the best way to go would be to grab a car battery and an inverter, but I've read that car batteries are excellent at high-output rates but don't actually have a large capacity. Most estimates say the car battery would last about an hour.

A vehicle would be nearby; intended use is camping (and a lot of travel but not a fixed location). I've heard that some of the solar cells are getting good enough to power a laptop, but in the middle of the night I need the power to be available for hours on end, so what would the solar cells store into?

Is there some sort of big beefy computer-centric battery pack I can buy, that doesn't rely on a wall outlet?

Bonus points if it can also be recharged while my vehicle is in motion.
posted by weasel to Technology (14 answers total) 5 users marked this as a favorite
The inverter would be perfect for just this situation - you're not running off the battery when the car is in motion; you're running off the alternator. Don't idle all damn day whilst playing WoW (alternators are seriously stressed out when used at full load at idle), but other than that you should be fine.

I used to have an old (1998) honker of a laptop and an inverter that I used all across the 50 states.
posted by notsnot at 12:13 PM on February 12, 2010

Response by poster: But in the middle of the night at the campsite, I don't want to have the car running for 12 hours while I work :) We might go days with the engine off, hence the need for a big battery.
posted by weasel at 12:16 PM on February 12, 2010

Car batteries are designed to give maximum amperage for a short period of time, to run the starter motor that starts your engine. You need a deep cycle battery, which is designed to deliver power over a longer period of time and can be discharged more completely without ruining the battery.
posted by electroboy at 12:18 PM on February 12, 2010

You can get external li-ion batteries like this one or if you search around you can find others with varying capacities and price points. Or you could get one of those jumpstart-battery-inverter combos like this but it's less efficient if you're running battery power through an inverter and a power supply versus just getting the correct DC voltage in the first place.
posted by GuyZero at 12:19 PM on February 12, 2010

There are so called "gelled batteries" or "gel cells" available. They are used in the wheelchairs, so they should do. If you are able to come with an adapter, it should be possible to use those in any free time application by choosing the one with appropriate specifications. They are slow to recharge, but otherwise great.

posted by Doggiebreath at 12:24 PM on February 12, 2010

Best answer: You just need a high capacity battery and a DC/DC converter (any power brick for your laptop that can plug into a cigarette lighter is fine. Don't use an inverter (converting 12V DC to 110V AC) and then a normal power brick (converting 110V AC to ~20V DC) as you'll lose energy in the conversion back and forth. If weight isn't an issue, then your best bet is lead-acid (like a car battery) that is designed to hold a lot of energy. You want a deep-cycle battery and you have a lot of options, but batteries intended for RV or marine use (and not for starting) are the easiest to find.

Find out the watt-hour (Wh) capacity of your laptop battery (it may be listed on it, or it may have an amp-hour (Ah) rating, which you multiply by the voltage to get the Wh rating). Find a deep-cycle battery with a Wh rating that is whatever multiple of the Wh rating of your laptop battery that you want (e.g. 12 times the Wh rating to last 12 times as long). Grab a connector that will allow you to connect your cigarette lighter adaptor to the battery.

You can try just connecting the battery into your cigarette lighter for charging, but you aren't likely to be happy with the results (either you'll blow a fuse or your battery will charge very slowly). It would be better to visit an RV supply store and get a proper charging setup. You'll also want a 110V AC charger for when you have access to the grid (charging your battery by burning gas in your vehicles' engine is quite inefficient).
posted by ssg at 12:31 PM on February 12, 2010

"I thought the best way to go would be to grab a car battery and an inverter, but I've read that car batteries are excellent at high-output rates but don't actually have a large capacity."

Starter batteries don't like to be discharged more than about 10%. There cousins the deep cycle battery handle deep discharging to 80-90% without failing.

My laptop has a 14V, 63 Wh battery. Your standard golf cart battery has ~200Ahs at 6V or ~1200 Wh. In theory 200 times the life. But the voltage is wrong so I'd need either two of them (if my laptop operated on that, many are pretty forgiving about voltage) or an inverter.

Ideally you'd want to avoid using a DC-DC inverter because efficiencies are low. Better to string as many 1.5V cells (you can buy lead acid deep cycle cells pretty easily) together as you need to get to your laptop's voltage requirements. For car camping use I'd stick with lead acid because they are much more forgiving of over charging and they are cheap. Their drawback is weight but you are letting the car carry it around for you.

If you've got a truck there may be a ready made kit available to install dual batteries under the hood. The second battery can be configured to run your inverter with no risk of killing your starter battery. Available power is such that regular driving around can keep the power battery charged. This is the set up I had in my 4X4 with an added configuration to run my winch off both batteries.
posted by Mitheral at 12:37 PM on February 12, 2010

Even without a deep cycle battery, you should be able to get a few hours with a plain-jane car battery. My guesstimate is 5-10 hours easy, even with the overhead of a 12v to 120vAC converter.

Barring that you can just buy a decent portable UPS, charge it at home, and drag it to the camp site. For the price of car battery you can get this little UPS.
posted by damn dirty ape at 12:43 PM on February 12, 2010 [1 favorite]

Response by poster: A-ha! I'm starting to learn the lingo.

My two laptops are rated at 90W on the charging bricks, and the batteries are 85Wh. Any cheap deep-cycle battery seems to be able to power everything I need just fine (with an inverter).

now the only question is how to recharge it... :/ I wonder if there's a way to jack a cigarette lighter back into the battery pack?
posted by weasel at 1:23 PM on February 12, 2010

You might want to look into something like the Xantrex Powerpack - according to the spec sheet it'll power a 30 Watt load for 14 hours. Also, you can piggy back additional deep cycle batteries onto it to increase it's capacity. It's designed to recharge from either AC or DC so you could run the car to charge it up when you needed to.
posted by macfly at 1:58 PM on February 12, 2010

now the only question is how to recharge it... :/ I wonder if there's a way to jack a cigarette lighter back into the battery pack?

As ssg noted above, the cigarette lighter can't handle recharging a big battery in any practical sort of way. If you want to use your car to do the recharging then you'll essentially need to hook it to the car's battery with fat cables, as if you were jump-starting something. RVs do this with the battery that runs the lights, water pump and such when the RV is parked. You'd probably want to incorporate a battery isolator so that the engine doesn't use the deep cycle battery for starting, and so that your computer doesn't suck down too much of the car battery's charge.
posted by jon1270 at 2:15 PM on February 12, 2010

Brunton makes a number of external batteries that can both charge and discharge over a lighter-style power jack. They're expensive. Not sure if even the biggest one would do what you need though.
posted by adamrice at 2:38 PM on February 12, 2010

Get yourself a marine deep-cycle battery and then a DC-DC converter to replace your power supply. A direct DC-DC converter will be more efficient than going from DC to AC back to DC. You should be able to get about 20 hours out of a standard deep-cycle marine battery.

See this Askme conversation:
posted by JackFlash at 2:38 PM on February 12, 2010

There cousins the deep cycle battery handle deep discharging to 80-90% without failing.

However it will last longer if you don't discharge it past 50% capacity. In the long run, you'll save money by just getting a larger battery.

Here's a good FAQ on lead acid batteries.
posted by aubilenon at 8:05 PM on February 12, 2010

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