Making battery pack from cheap AA NiMH cells

[Hyena]

Hey guys,
I'm looking for a cheap option to upgrade the heavy and old SLAs I'm currently using.
I've got a 36v 500w golden motor kit but I plan up to mod the controller to run @ 48v. I'd rather go faster for a shorter distance
I use my bike more for fun than serious commuting and would be happy with a range of around 10km.

If I was in the US there's a few other options I'd probably go with, but being in Australia the shipping costs on batteries from OS is probably too high to entertain.
I came across THESE bulk NiMH AA batteries that are supposedly rated at 2800mAh. $98 for 48 seems pretty cheap, for under $200 AUD I could make a 48v 5.4 Ah pack. The only unknown is their C rating, which could be a limiting factor. Is there a rough minimum you'd expect from them ? I've seen some cells only rated at 2C so I guess I'd have to plan for that and anything else is a bonus. If that was the case, is it worth doing if it only puts out 10 amps at best, or should I give it a miss ?

The other thing i need to sort out is a charger. There's no need to balance charge NiMH packs is there ? I've seen 3 wire chargers around the place with the 3rd wire only being for a thermistor, so I'm assuming its ok.

Feel free to nip my ideas in the bud if I'm overlooking something fundamental

 

[voicecoils]

Hey guys,
I'm looking for a cheap option to upgrade the heavy and old SLAs I'm currently using.
I've got a 36v 500w golden motor kit but I plan up to mod the controller to run @ 48v. I'd rather go faster for a shorter distance
I use my bike more for fun than serious commuting and would be happy with a range of around 10km.

If I was in the US there's a few other options I'd probably go with, but being in Australia the shipping costs on batteries from OS is probably too high to entertain.
I came across THESE bulk NiMH AA batteries that are supposedly rated at 2800mAh. $98 for 48 seems pretty cheap, for under $200 AUD I could make a 48v 5.4 Ah pack. The only unknown is their C rating, which could be a limiting factor. Is there a rough minimum you'd expect from them ? I've seen some cells only rated at 2C so I guess I'd have to plan for that and anything else is a bonus. If that was the case, is it worth doing if it only puts out 10 amps at best, or should I give it a miss ?

The other thing i need to sort out is a charger. There's no need to balance charge NiMH packs is there ? I've seen 3 wire chargers around the place with the 3rd wire only being for a thermistor, so I'm assuming its ok.

Feel free to nip my ideas in the bud if I'm overlooking something fundamental

I've found that the rechargeable NiMH batteries I've bough (from dicksmith) have been pretty crap, even in my Wii remotes they don't last long, yet were supposed to be around 2ah. I'd be cautious about the capacity rating. Also, would you build a tab welder to construct the 96 cell pack?

What Ah rating do your current lead acid batteries have, and what do they weigh?

 

[powermed]

Consider it nipped in the bud...since at best you're only saving a few bucks it's hardly worth all the hassle, time, unhappy surprises and pain you will likely encounter. IMHO

 

[Hyena]

would you build a tab welder to construct the 96 cell pack?
What Ah rating do your current lead acid batteries have, and what do they weigh?

I was planning to use a few tubes, like poly pipe with the batteries inside and mounted around the frame, and solder where necessary.
If the current capacity is over rated though then its not worth the mucking around.

My SLAs are 6x 6v 10ah panasonics pulled from a UPS. I'm not sure what they weigh, but I estimate 1.4 metric tonnes :lol:
Prior to that I had 3x 7ah 12v packs from the ebay seller that currently has these listed
http://cgi.ebay.com.au/BULK-OFFER-30-UNIT-OF-12V-7AH-SEALED-LEAD-ACID-BATTERY_W0QQitemZ110307369682QQcmdZViewItem?hash=item110307369682&_trksid=p3911.c0.m14&_trkparms=66%3A2%7C65%3A1%7C39%3A1%7C240%3A1308

I dont know if it was the batteries or the charger that was stuffed when I bought mine, but they barely lasted a minute before LVC kicked in.
The fact that they're now selling a bulk load of them so cheap makes me think the batteries themselves are duds.
For around $4 each though I'm tempted to grab a handful to play with!

I'm also tempted to try my luck with picking up something like this cheap.
http://cgi.ebay.com.au/48V-10AH-LiFePO4-Electric-Scooter-E-Bike-Li-Fe-Battery_W0QQitemZ120324737803QQcmdZViewItem?hash=item120324737803&_trksid=p3911.c0.m14&_trkparms=66%3A2%7C65%3A13%7C39%3A1%7C240%3A1318
I know from posts here the build quality is pretty terrible and people have had issues, but even if I rebuild the pack myself and get another BMS hopefully I'd end up with a decent battery for the money. Or not... ?

 

[dogman dan]

Likely not. But I don't have any info on the seller. 10ah will not be enough to keep the c rate low enough for the pack to last. I also tried some 24 nimh tool packs I had laying around. I ran two at a time parallel, at 24v. They got scary hot since they discharged about 10 times faster than they could in a drill, taking me about one mile. If budget is that tight, just get another sla, and start saving for a real lifepo4 pack from somebody reputable. 4 PSI-BMI- Lifebatt type cells could be a viable option to add 12 v lifepo4. But for under 100 US, you wont find much worth your money besides sla.

 

[fechter]

The other problem with AA nimh batteries is you can't put them in parallel during charging. Each series string really needs to be isolated and charged separately or you can get extremely unbalanced charging currents resulting in a nasty cascading failure (exploding batteries).

If you only had a few strings, it wouldn't be too hard to isolate them with diodes or a connector/jumper arrangement.

 

[Hyena]

Thanks for the help guys.
I was lucky enough to pick up one of these batteries which I think will perform much better than the cheap alternatives I was chasing.

http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItem&item=140276575093

According to the seller it's supposedly rated at 2C with peaks up to 5C, although the inline fuse is is actually only 20a so I'm guessing I wont be pulling too much current out of it.
I'm tempted to throw a 30a fuse in but that's probably asking for trouble. I guess I'll see how I go.

It's so small and light, I just hope it performs well enough.

 

[pwbset]

Hmm. $0.70/wh for LiMn? That's a great price. Let us know how that pack works out for you.

 

[Hyena]

Yes they were certainly well priced! I'd get another one but unforunately the guy selling them has no more and can't get any more. He said he bought them from a wholesaler who was getting out of the e-bike business, but I haven't seen any around elsewhere anyway.

I took it for a ride today and only got around 10km range, but alot of that was hills with minimal assistance so if I pedalled more I would have gone alot further on the flat.
The BMS tripped several times on hills at very slow speed, so I'm not sure what the current cut out on it is. The 20a fuse didnt blow so it mustnt be very high which is a bit disappointing. I'm pretty happy with it though as a small and light pack, it certainly keeps the bike more light and nimble than having racks of SLAs on board.

 

[voicecoils]

Yes they were certainly well priced! I'd get another one but unforunately the guy selling them has no more and can't get any more. He said he bought them from a wholesaler who was getting out of the e-bike business, but I haven't seen any around elsewhere anyway.

I took it for a ride today and only got around 10km range, but alot of that was hills with minimal assistance so if I pedalled more I would have gone alot further on the flat.
The BMS tripped several times on hills at very slow speed, so I'm not sure what the current cut out on it is. The 20a fuse didnt blow so it mustnt be very high which is a bit disappointing. I'm pretty happy with it though as a small and light pack, it certainly keeps the bike more light and nimble than having racks of SLAs on board.

Nice deal! Is there a production date sticker on it somewhere? What's the packs voltage hot off the charger?

What specs does your controller have? If your using a 36v16a controller you probably wouldn't be tripping a 20a fuse.

For comparison, with my 36v10ah lifepo4 pack the farthest I've gone is 30km and I only used 5.7Ah.

 

[Hyena]

There's no details that I can make out from the stickers other than the voltage, unless anyone can anyone translate something meaningful from these?

At 190mm x 140mm x 68mm it seems to be physically smaller than other packs on ebay, and thats including the BMS under the shrink wrap and a layer of plastic or something to make it all stiff and square. So maybe its not actually 10ah ? Or the LiMn cells are smaller than those used in the LiFePO4s?
I'm going to take it for a 10km ride to my parents place tomorrow but I doubt I'll make the return trip on a single charge. I'll take my charger along given it probably won't perform that much better than todays tests.. Was your 30km trip with much pedalling ? The only pedalling I've been doing is a bit of assistance up hills where the bike wouldnt make it under its own steam.

Hot off the charger (well, maybe half an hour after it'd finished and I noticed it was done) its reading 40.5v. When the BMS shut it down after the first ride I unplugged it and tested the voltage and it was still reading reading 37-38v on my multimeter. When I plugged it back in though it doesnt power up the controller at all, no battery LED level read out and no sparks between the connector at all. I dont know why I'm able to measure a voltage with my meter yet it doesnt even try to power up the controller, which at rest could only be drawing a few ma. After a full charge though it happily runs again.
Is this normal BMS behaviour or are strange things afoot ?

I'm not sure what the controller is rated at. It has no stickers at all on it - it came as a 500w 36v golden motor kit and has a 30a inline fuse.

 

[pwbset]

Hot off the charger (well, maybe half an hour after it'd finished and I noticed it was done) its reading 40.5v.

Hmmm. 37v is nominal for a 10s LiMn, but 40.5v hot off the charger seems a little low for LiMn. My 10s LiMn string is usually around 41.8v freshly charged and they are older cells at that.