# Need help with Battery Math

I thought I hit the jackpot at work last week when a shipment of new batteries was defective and they were going to toss them all out. I grabbed one and tested it. The cells were fine but the built-in charging circuit was bad. So, now I have about 250 of these cells and I’m trying to figure out if I have enough to convert a motorcycle. I did the math as well as I could but I don’t think I have enough build something practical. Will you guys do the math and tell me what, if anything, you could build with these.

Here are the specs.
248 Sanyo NiMH cells @ 1.2v
They were originally in 9.6v packs @ 2.9Ah

I was hoping for at least a 72v set-up with 50-60mph, 20-40 miles. I’m coming up with less than a kWh @ 72v tho. I have no experience stringing battery chains together. Let me know what you guys think.

Thanks
Steve.

quick and dirty. wired in series, (+ to -, + to -), add volts together. wired in parallel, (- -, + +), add the amp hours together.
to get 36 volts, you need 30 of your cells in series. 30 cells in parallel would give you 10.8 amp hours. 60 cells, enough for a e-bike hub.

OK here is what you can do, pretty simple because of the 248 cells you have.

Wire 62 of them in series. This will give you roughly 74.4 volts fully charged at 2.9 AH. Then make 3 more identical strigs with the remaining batteries. You will now have 4 strings at 74.4 volts. Now wire the 4 strings in parallel.

Each series string is rated at 74 volts @ 2.9 AH, so 4 in parrallel will give you 74 volts at 11.6 Amp Hour capacity. The watt-hour capacity = Volts x Amp-hours x .8 = 74 x 11.6 x .8 = 686 Watt-hours.

FWIW it does not matter how you arrange the batteries, with 248 of them you will always get 686 watt-hours. Example if you were to wire all 248 in parallel = 1.2 volts x (248 x 2.9 AH) .8 = 686 watt-hours. For power it always add rather in parallel or series.

What is your constant of .8 ?

[QUOTE=ccbreder;6443]What is your constant of .8 ?[/QUOTE]Inefficiency of the battery in either charge or discharge cyle. .8 might even be to kind depending on the discharge rate. When a manufacture specifies a Amp-Hour rating, it is at a specific discharge rate like 8, 12, or 20 hours.

Example; Trojan specifies the T-105’s at 225 Amp Hours at a 20 hour discharge rate meaning it will deliver 11.25 amps for 20 hours or 225/20 = 11.25. However that same exact battery is rated 185 Amp-Hours at the 5 hour discharge rate, 185 AH / 5 hours = 37 Amps. So in this case you would derate by .82. Now if we were to use the 1-hour rate of 225 Amps, the derate factor drops to .6.