Car battery(ies) as power source?

Let's say I want to power, say, a couple laptops and some wireless networking gear without a generator. In the field for, say, an 8-hour day of fairly intensive use. The kind of stuff that typically runs on a wall wart plugged into a 110v outlet, but that will probably draw less than 200w all told.

Would a solid car battery hooked up to an appropriately-sized inverter work and last all day? Rather than going 12vDC > 110vAC > 19vDC, is there a way to skip the middle step?

Any and all input is appreciated. Thanks.

Quite a bit of that equipment actually runs on DC and the wall warts convert the AC into DC. You'd be better off getting some 12v based power supply adapters for them and avoid the inverter.

Many laptops have car adapters which plugs into a cigarette lighter and the wireless equipment typically use 12v or 5v and a very small amount of amperage (could even power them using a solar panel and a small rechargeable battery pack.

Unfortunately one of the disadvantages of DC is that it is not very easy to change the voltage. It is much easier with AC to change the voltage level. But most of the devices probably use DC so you could probably hook them up to the battery.

One thing you need to find out is the power consumption. Car batteries have a rating amp hours that is how many hours that can supply 12v at 1amp, usually around 100amp hours or so. Since P=IV you can figure out how many hours the battery will last by dividing the amp hours by your current demand. 100amp hours means 20 hours at 5amps 10hours at 10amps etc..

Deep Cycle Marine batteries would be better than car batteries.

Yes, start with deep cycle batteries. Ours are quite large and heavy, but they will power a laptop and sensor all day (probably two) even on a cold day. We have used an AC inverter to provide power for a PA, laptop, and router. Now we use a 1000W portable Honda generator for that. We also have computers/sensors that require power. The sensors need 12-18v DC, and the computers anywhere from 12v to 19v. DC/DC converters that take 12V input and are switchable for output voltage power all of that. Everything is fused. The converters also have a USB charge port that is handy for charging a bar code scanner. I can't find the exact converter now, but I bought these from PowerStream.

I'm in the process of adapting Raspberry Pi 2 computers to run the sensor applications. These only cost $35 and draw much less power (< 1 amp). Hopefully this will allow us to ditch the heavy lead-acid batteries for much lighter lithium-based ones.

The PA is the biggest power hog. If someone cranks up the amp and is gabby, it draws a lot of current. Everything else is more steady and predictable.

If you're actually going to go out and buy a battery, get a deep cycle. That said I'd think a car battery wouldn't work for 200 watts(plus inverter losses) at 8 hours. A typical group 34 cranking battery is around 45 AH, an inverter is 90+% efficient, so your total pull should be under 20 amps(200/12=16.7amps), so around 3 hours, tops. 3 or 4 wired in parallel (jumper cables would work) would do it though.

edit: You can easily monitor the usage with a cheapo/free HF multimeter wired inline with the battery(on 10 amp mode), though the included leads are not up to the task of carrying much more than 1 amp, so you'll need to make up a set of heavy duty leads or just solder some 14 gauge or better wires in. Though with these revised numbers you'd probably fry it in short order, so an actual panel mount ammeter or a killawatt on the AC plug of the inverter(guess the inverter loss) would probably be the cheapest solution.

Double edit: messed up the calculations pretty bad there, corrections in bold.

Kenny_McCormic wrote: If you're actually going to go out and buy a battery, get a deep cycle. That said I'd think a car battery would work for 200 watts(plus inverter losses) at 8 hours. A typical group 34 cranking battery is around 45 AH, an inverter is 90+% efficient, so your total pull should be under 2 amps(200/120=1.67amps), so around 24 hours runtime. edit: You can easily monitor the usage with a cheapo/free HF multimeter wired inline with the battery(on 10 amp mode), though the included leads are not up to the task of carrying much more than 1 amp, so you'll need to make up a set of heavy duty leads or just solder some 14 gauge or better wires in.

200W at 12V is 17amps so at 45AH that is 3 hours not 24. it is 1.67amps at 120 volts but thats after the inverter. before the inverter you still need to supply 120*1.67=200W at 12 volts so 17 amps.

Also there are clamp on meters which are really nice for measuring current. No physical connection with leads

In reply to iadr:

Yeah most car batteries are rated in CCA and don't include AH. I used an example 100 amp hours because it makes calculations easy but that is fairly large. I have a solar battery that is 110 amp hours and it is 60+ pounds

just remember that things with rechargeable batteries do -not- like the square sine wave that inverters put into the power.

If.. I was going to follow your route... I think I would also invest in a cheapish solar array to help extend battery life

In reply to fritzsch:

Oops, major brain fart there, corrected. HF does sell cheap clamp meters, but none of them do DC amps that I know of, only AC.

A guy who writes an electronics blog in my local newspaper just had an article today about a portable mini-charger called a jHub Elite:

http://www.startribune.com/entertainment/tv/296245731.html

http://www.amazon.com/Modern-Portable%C2%AE-jHub-Elite-Smartphones/dp/B00SI8YNTK

It's not very expensive and may do what you're looking for.

All right, thanks, guys. It sounds like a gennie is the way to go. I thought it would be simpler.

Kenny_McCormic wrote: In reply to fritzsch: Oops, major brain fart there, corrected. HF does sell cheap clamp meters, but none of them do DC amps that I know of, only AC.

Yeah we use a nice AC/DC Fluke clamp on meter for my job. It makes checking current so easy. Pricey for sure though

Duke wrote: All right, thanks, guys. It sounds like a gennie is the way to go. I thought it would be simpler.

Yeah, this is pretty much what those little Honda inverter generators were built for.

mad_machine wrote: just remember that things with rechargeable batteries do -not- like the square sine wave that inverters put into the power.

Get a higher quality inverter that isn't a square sine wave. Anything rated for electronics should do it.

I run things off of an inverter instead of a direct DC-DC conversion because the times I want to run off of a 12V battery also sometimes coincide with a running engine. The voltage in an automotive system is very noisy. Electronics that expect a nice clean DC voltage tend to throw fits when exposed to alternator ripple and ignition noise and relay/solenoid spikes and other fun stuff. The inverter does a handy job of steamrolling all that noise.