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Electric Stunt => Gettin all AMP'ed up! => Topic started by: Motorman on July 09, 2022, 03:59:05 PM
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Anyone have experience charging plane batteries off your car battery. I know it's bad but how much can you get away with and still start the car?
Motorman 8)
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So let's see, we are going to charge a 24v battery from a 12v battery. I have seen RC guys do it but they started the car to recharge the battery after. I think the answer is more batteries.
Ken
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You can use older pack that don't fly well for the power supply. When I see packs with a low cell at the end of flight, or have packs that don't like flight discharge, I use them on a parallel board to charge, or top, packs at the field. Not all chargers take 24v input if using 6S packs. A bit on that here
https://stunthanger.com/smf/gettin-all-amp'ed-up!/battery-discharger/
I usually have and take enough batteries to avoid charging at the field. But, I will take batteries at storage or charged to about 80% for travel to a contest. Then I can charge at the field. Three packs or 6S in series work well for power and, in series, have a low discharge rate. If the power supply is lower voltage, or the car, the charger step up will provide the voltage, but at the cost of more amp usage. A good reason not to do much with the car battery.
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I'll answer your question. You can do it with most DC chargers. I've re-charged 4 times 5S 3,000Mah packs without a problem. Probably could go more but I didn't. You can always run the car if scared about it. An emergency portable starter battery for insurance is helpful.
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I'll answer your question. You can do it with most DC chargers. I've re-charged 4 times 5S 3,000Mah packs without a problem. Probably could go more but I didn't. You can always run the car if scared about it. An emergency portable starter battery for insurance is helpful.
All true!!! #^
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I've been planning on doing this myself. After reading Christ's post, I'm thinking that if I kept a voltmeter connected to the battery when I was charging, I could get an idea of whether I'm getting into the danger zone by taking out too much juice.
Looking at typical car battery capacities (around 50 amp-hours), and knowing that putting 2500mAh (2.5 amp-hour) into a 5-cell battery should take around 5 amp-hours (because -- physics; boosting the 12V to the 21V that the pack needs means the current in has to be over 21/12 of the charge current).
That's actually getting into scary territory: you're using 20 amp-hours to charge your batteries (if you're charging them fully). Car batteries are designed to deliver lots of current, not to be discharged deeply. So pulling 40% of your car batteries charge out and then asking it to start your car is -- well, you can hope the tow truck driver is good looking, but when does that happen?
So -- now that I've done the analysis, I'll definitely use a voltmeter. Or I'll run the car every second time I charge, or I'll adopt Fred's method.
Edit: Or, after commenting on this thread (https://stunthanger.com/smf/gettin-all-amp'ed-up!/solar-charging-system/) and thinking about it some more, I'll use Fred's "old dead flight battery" method or I'll buy a deep-discharge lead-acid battery and stash it in the trunk.
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Ok, so a couple of 4 cells should be ok.
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The other problem with a car battery is the actual capacity. How many amps are really in that full battery. Try your 4 batteries at home and check that car battery voltage after. How well does it start the car? I had my battery tested and it was "good." Shortly after, it failed to start the car after 2 charges of 6S batteries. Fortunately, I was not alone at the field. I tested it at home by using it to charge a battery and it was all but dead. Then took about 2 amps from my charger set to lead acid to have it be "full" and shut off. I started using old lipo's as a source after that. As Tim noted, car batteries are not deep cycle and don't do well or last long with low voltage.
Most chargers can cycle your car battery, set to appropriate discharge voltage, and give you an idea of the amps that they hold.
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The other problem with a car battery is the actual capacity. How many amps are really in that full battery. Try your 4 batteries at home and check that car battery voltage after. How well does it start the car? I had my battery tested and it was "good." Shortly after, it failed to start the car after 2 charges of 6S batteries. ...
This is, I think, closely related both to why car batteries seem to die suddenly, and why they seem to die in the winter. They either work, start your car, and you have no clue that they were just about dead, or they're near death, you ask a bit more from them than usual, and then they're really dead.
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I did it when I first started flying electrics until I needed a boost, never did it again, bought a cheap 1000 watt 2 stroke gen/inverter and a power supply.
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How about a deep cycle motorcycle battery like - MIGHTY MAX YTX 14 Ah GEL. I was thinking one could make a charging station with this type battery, have several packs charged to say 80% then top them off at the field as needed. This would eliminate having fully charged packs, getting to the field and find out you not going to fly them then having to discharge to storage. Don't know how many packs could be topped off but the MAX could be topped off and not be a problem.
Best, DennisT
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I have charge from my car battery since going electric in 2016. I normally charge at home then additonal flights max out on charging 4 batteries, 2 at a time. I have a large vehicle with a large capacity battery. I have not needed a jump yet!
I will add I was landing with roughly 33% capicty left in the pack
Tom