E-gpeden
Senior Heliman
Location: Lake Louise, Alberta, Canada
| Hello again Auger,
You're welcome... Reading over what I wrote, I figure that I should add some math, practical examples and work-arounds - sorry
As an example of what a non-maniac electric flier might lug around, I thought it would be worth looking at a single 55AH deep cycle battery and how one might manage a day's flying with 24 cells. For 32 cells, use 3/4 the computed charging capacity.
A deep cycle battery cannot handle big currents all the way down to zero capacity without dropping the voltage too low for the charger. Good chargers like my Orbit Mikrolader Pro have an adjustable low supply voltage cut-off. It is realistic to expect only 35 amp-hours out of the battery before the charger pulls its own plug at 10.5 volts under load. This is OK, because it is better not to run the battery all the way down. My charger puts out a maximum of 8 amps, but with 24 cells it is "only" about 7.4 amps (varies throuhout charge, not constant). This is at a peak of about 39 volts applied to the pack, which means that it is using 288 volt-amps (watts) peak. So the 12 volt battery must put out 288/12 = 24 amps to supply the charger its power! Factor in inefficiencies and it is even higher.
As far as guessing total charging capacity of a deep cycle batery goes, it is useful to think in terms of vollt*amp*hours (watt-hours). A 24 cell pack of 2.4AH (2400mah) cells requires at least 24 x 2.4AH x 1.2V = 69 volt-amp hours to charge, but due to inefficiencies it is more like 80 VAH. The 12 volt, 55AH battery can supply 55 x 12 = 660 volt-amp-hours theoretically, but like I mentioned before, it's more like 35AH x 12 V = 420 volt-amp-hours. Therefore the battery will have enough poop to charge a 24 cell pack 420/80 = 5.25 times. This more or less jives with what I get.
The work-arounds are:
a) arrive with 3 previously charged packs, using your charger powered by a DC power supply at home. Although re-peaked packs don't perform quite as well, just pop them on the charger right before flight and they will suck in another 200 mah or so. This allows one to bang off the first 3 flights pretty quickly before the 30 minute charging cycles begin. At that point you have one charging, one flying, and one cooling.
b) lower the charging rate as the charging battery nears depletion. This will reduce the sag in voltage and allow more consumption. Our Nicads like 6 amps or more for best performance though.
c) cheat by using your car battery to finish off the last charge.
d) tell the spectator that you'd be happy to do one more flight for them if you could just hook up to their car battery (done this a few times to finish a charge, hehe).
e) buy a bigger charging battery You can get 100+ AH batteries and lug one around in the back of your pick-up. Me? I want ones with absorbed electrolyte for safety reasons (van)
Using the above techniques allows me to get about 8 fights a day off the one battery. So,,, you can do quite fine with a single deep cycle charging battery. I think that with pre-charging each of your 2 packs, you could get 5-6 flights on a Joker or 7-8 on a 24 cell LOGO 20. I didn't want to turn you off, but just make you aware that your car battery would not do the job - it would die a quick death
At a recent fun fly I drained my two Optima charging batteries on each of two days, which meant that I was having lots of fun
Cheers!
Cheers!
Glen |
