from: http://www.eoss.org/pubs/faqloon.htm
Only one type of battery will do: lithium. Nothing that is available on the commercial market approaches lithium's power per pound. We normally purchase military surplus lithium battery packs. They contain 10 D cells in a series/parallel arrangement that produces 15 volts. The packs come apart very easily into two 5 cell strings. The D cells themselves are connected with welded tabs that can be easily cut and soldered into whatever voltage and amp/hour combination you need. The D cells are 3 volts each, so 3 will give you 9 volts, 4 will give 12 and 5 cells will give 15 volts. The cells in the battery packs will typically be out of date, but lithium cells have an excellent shelf life and we've never been disappointed. You can expect 6 AMP hours from each 3 volt D cell.
I connected a single 3 volt lithium D cell to a 10 Ohm resistor, put a Radio Shack RS-232 VOM (lent by Don, K9LYE) across it and let my computer log its progress. The voltage "rose" slightly for the first half hour to 2.81 volts and it then stayed there for 22 hours before it started to drop! It didn't drop to 2.5 volts until 24 hours after starting the test. I put a 5 ohm resistor across a second D cell and the voltage stayed at 2.78 volts for 12 hours. That's 6000+ mah per cell! A 5 D-cell pack will provide about 3.3 watts for 24 hours or 6 watts for 12 hours. You really need about 12 hours of capacity. That gives you four to six hours for the flight and another six hours to recover the payload when it lands.
Here are our results for various resistances across a single 3 volt lithium D-cell:
Life Watts Watts
Peak Peak in per per
Ohms Volts Amps Hours cell 5 cells Notes
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10 2.8 .28 24 .78 3.9 (Lifetime measured to 2.5 V)
5 2.78 .55 12 1.53 7.6 " "
3.3 2.75 .82 8 2.25 11.2 " "
2.5 2.71 1.08 6 2.92 14.6 " "
0.77 2.46 3.20 2:14 7.87 39.4 (Lifetime measured to 2.0 V)
(That 0.77 ohm load was a mistake. I thought I had 4 10 ohm / 10 watt resistors in parallel. One of them turned out to be 1 ohm. Boy, did it get hot!)
We order our lithium batteries from:
S & G Electronics
618 S. 62nd St.
Philadelphia, PA 19143
Phone (voice): 215 474-7663
Prices run around $15.00 per pack of 10 cells.
S&G accepts phone orders, but not credit cards. They will ship C.O.D., however. S&G is a real mom and pop outfit and very nice people to do business with.
LiPo (nominally 3.7V per cell) 3S is 11.1V
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Charge at 1C maximum.
Max voltage at charge for 1 cell: 4.20 (Use 4.18 to be cautious. Don't exceed 4.20 even by .01 volts!)
For storage at 40-60%: 3.85
Voltage down at 20%: 3.75
Voltage down at 10%: 3.70
Do not go below: 3.50
Standalone Cell Balancer for use with a non-balancing charger:
LiFe For Flight - Motor Packs (nominally 3.3V per cell) 4S is 13.2V
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http://www.espritmodel.com/a123-2300mah-battery-packs-lifepo4.aspx
Charge at 4.3C maximum.
Max voltage at charge for 1 cell: 3.60
For storage at 40-60%:
Voltage down at 50%: 3.3
Voltage down at 10%:
Do not go below: 2.50 (ultimate cut-off at 2.0)
4S Pack Weight: 330.0g (11.6oz.)
4S Pack size: 70x55x31mm
4s Pack Capacity: 2300mAh
One Cell: Dimensions 26mm(1.02") dia. x 66.5mm(2.6") ht., Cell weight 70g/2.5oz.
Power Supply
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Power Max - 12V outputs can be connected in series for 24VDC.
MeanWell SE-1000-24
Parallel Charge Board
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http://www.progressiverc.com Parallel Charge Board
Charger
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iCharger - all chemistry, 3010B