Best way to hook up 60 1.2V cells in 24V? (first post!)

[yaemes]

I've been lurking for a few weeks.

My 24V eBike is going to be a bit ridiculous. Thanks to an earlier project I have 60 superb 1.2V NiMH cells. I plan on putting them in a removable case in the triangle and every time I want to charge them I'll have to individually take them out of the case and charge each cell in a normal NiMH charger.

My first question is this: Is it better to make 20 1.2V cells hooked up in series, each made of 3 batteries in parallel OR make 3 parallel 24V cells each made of 20 batteries hooked up in series?

 

[dnmun]

series first, then parallel. but you don't wanna take them apart all the time, why not go with 2 36V strings in parallel and then get one schottky diode chip with the dual legs to tie them together and then charge each one separately to get charger endpoint control? or use 2 chargers at the same time which you can do if the 2 strings are isolated from each other by the schottky diodes.

 

[yaemes]

Make a 24V pack of 20 cells in series then hook 3 packs together in parallel?

Also that diode idea is smart. But that means charging 30 batteries in series, and I don't have a 36v NiMH charger and charging them individually makes them last the longest. I like the idea of a pack I could just charge all at once but if I have to take them out to make them last longer, that's an annoyance I wouldn't mind putting up with.

Thanks for the advice!

 

[vanilla ice]

You're not supposed to charge nickel batteries in parallel. IIRC I've read a few people have reported success, but its dangerous. Put as many in series as you want, 20s1p is fine, charging will just be 3x times a pain the neck because of the 3 strings.

 

[dnmun]

you never mentioned the size of the cells either, but you should invest ina 36V smart nicad chrger and build 2 30S strings tied together through a single double schottky diode. there is no other way to really do it. you can run without the schottky but the low pack will draw down the hot pack, which the schottky can block either way if you use the twin schottky input, single output, in the TO 220 package. but you give up the power lost in the schottky.

 

[yaemes]

Thanks for the responses. I am learning a lot!

They are 2300mAh Energizer AA batteries.

Being able to charge in series is great (I had no idea you could do that) but I still want a 24V setup.

I think I will set it up so I can take the battery container out of the bike, take it inside and flip a (30 amp rated) switch which would disconnect the three 24V packs. Then I would charge each pack one after another using this 24V NiMH charger. Eventually I would buy more of the same charger so I could charge all packs at once.

If anyone has anymore ideas I am wide open to them Especially relating to diodes!

 

[vanilla ice]

I think those would be rated like 0.5C or less.. may not be suitable for bike use!

 

[yaemes]

The motors at full power would take 20 amps and you're saying the cells would only sustain .5C (1.15 amps), but there are three cells so that makes it 2.35 amps max sustained... That seems grossly underpowered.... this is not very good... how come there are other companies that have NiMH batteries for eBikes?

 

[biohazardman]

AA cells are very small so you can't pull much out of them. Other companies make much larger batteries that can be discharged at a higher rate. Larger format batteries are usually better. You need to check the specs to find the max discharge rate before you buy. It's best not to run them at their max though as the useful life of the batt will be shortened considerably. NiCds on the left in the pic. A link to a slightly battery than the one you speak of has lots of info.

http://data.energizer.com/PDFs/CH15.pdf

 

[dnmun]

if you wanna go cheap and build your own outa nimh, then go buy some of the lots of 4.5Ah nimh on ebay. they sell flats of 40-50 cells at a time in ebay auctions so that would be a better size to start with, but you would still have to run 2 parallel strings to get 9Ah wide, kinda the minimum it takes to get the current through the motor.

 

[yaemes]

You linked to a 950mAh NiCd, I have 2300mAh NiMH. Thanks for showing me data.energizer.com though

I do not know much about motors but I can guess that it wont be doing its max all the time. Probably only half. So half of 20A is 10A which is equal to 1.45C when referring to to entire battery pack. Still too much current to feel safe. It would also equate to about 45 mins of runtime, not bad, but with a cost to long term battery health.

This is highly theoretical now. Maybe I'll make the pack then bust out the ammeter on a 500W motor I already have. Maybe I'll buy more NiMH cells.

This is the datasheet for my batteries.

 

[Drunkskunk]

..... not bad, but with a cost to long term battery health.

You might want to reconsider that as "Short term battery health".

Quoting from that datasheet, :
Rated Capacity: 2300 mAh* at 21°C (70°F)* Based on 460 mA (0.2C rate) continuous discharge to 1.0 volts.

You'll be commiting Cellocide, murdering those AAs in 2 or 3 uses.
I've had some experiance with them, and there are some older posts on the RC Groups forums about them. 0.5c is about the max they can handle for repeated cycles. They aren't even that good for transmitters. Mine died in about 6 weeks in a fairly low powerred transmitter.

60 cells in a 24 Volt pack would be 3 strings of 20, and make a 24 volt, 6.9Ah pack capable of 0.5C, or 3.45Amps continous draw. Thats 82.5 watts it can provide, in theory, continous. Its just not enough.

 

[vanilla ice]

Big difference between a 2300mah a123 string and a 2300mah AA string.. the ah rating is only a small part of the story with bats.