ORIGINAL: -pkh-
Your best options are:
1) Car battery. Monitor the car battery voltage and start it up occasionally to charge it up.
2) Deep cycle marine battery.
I know a few guys that use option 1. You'll probably figure out how many batts you can charge, or how long you can charge before you have to start it up and run it for a few minutes.
For option 2, try to get an amp/hr rating for the marine battery, convert that to Watt/hrs (12V x amp-hr rating), figure out how many Watt-hrs you need at the field for battery charging (nom batt pack voltage x total amp-hrs you want to charge), and try to get a battery with at least the Watt-hrs rating you need.
Example:
You want to recharge six 3S 2200mAh battery packs: Watt/hrs requred = 6 x 11.1V x 2.2Ah = 146.5 Watt-hrs
So you want to get a battery with at least 150 Watt-hrs of capacity = 150Watt-hrs/12V = 12.5 Amp-hrs
The charger will require some power, and there will be some power loss due to charger inefficiency, so you may want to add 10-20% to that Amp-hr rating to be sure you can charge that many batts.
Hope that helps!
pkh, he's talking about recharging a 10,000ma battery pack. With the associated power loss from the charger, even charging at 1C rate, which would take over an hour to recharge that pack, a standard car battery would only be good for a few hours at best. It's NEVER a good idea to use the car's alternator to recharge a low battery, it's not designed for that and will eventually result in a dead alternator. The voltage regulator will sense the low voltage condition, which results in a 'full field' condition, or close to it. The alternator can't run at full field charge rates for too long without damaging it, especially at idle.Also, it would take more than a few minutes of run time to recharge a weak battery. You will get a high surface charge initially, but to restore the battery to full capacity takes a lot longer.
If he were dealing with smaller packs, as you mentioned, the car battery would be just fine. Some of the guys Ifly with bring a car battery on a dolly to the field for charging, but all of them use smaller packs, 4000ma packs or less, or only recharge rx batteries. Every time you discharge a non deep cycle battery, it will never be able to be recharged to 100% capacity. Each time you discharge and recharge it, you lose some capacity until the point that the battery is no good. A deep cycle battery doesn't have this problem, and is designed to be discharged and recharged multiple times, but are more expensive.
Let's say for examle he's charging at 1C, figuring in a 20% power loss due to the charger, you're talking about a min 12 amp draw on the battery. To compare that to a car, a modern fuel injected car/truck uses about 7-10 amps with the key on and engine not running to power up the electrical systems (on average). The average passenger car battery is around 540 cranking amps on average. If you leave the key in the on position without starting the car, you will come out to a dead battery in 3-4 hours or so. I don't know about you guys, but Ican easily spend more than 3-4 hours at the airfield
.
crazyronnie, if you do decide to go the car battery route, please consider using a separate battery and not the battery in your car, and just recharge it at home. With the charge current you will be drawing, it would suck to get stuck at the field with a dead car battery, especially if no one is around to give you a jump.
I'm not talking out my ***** on this subject, it's what I do for a living. I'm an electronic engine management specialist for Ford. I deal with car electical systems daily.