NiMH batteries, worth trying?

I see the NiMH C and D cells, 9 AH, 10 AH are cheap, so I am wondering if it is worth building up a pack and trying it. Have read some of the posts and looked at the specs. Doesn't look like they typically spec NiMH at high discharge rates. So I expect they don't do well at high discharge, but my riding habits are only high discharge for the very end of the ride. So my question is, has anyone had good luck with NiMH? I am specifically thinking of building a 9 AH 36 volt pack for an Iacocca bike.

Thanks,

Fred

fredtr said:

I see the NiMH C and D cells, 9 AH, 10 AH are cheap, so I am wondering if it is worth building up a pack and trying it. Have read some of the posts and looked at the specs. Doesn't look like they typically spec NiMH at high discharge rates. So I expect they don't do well at high discharge, but my riding habits are only high discharge for the very end of the ride. So my question is, has anyone had good luck with NiMH? I am specifically thinking of building a 9 AH 36 volt pack for an Iacocca bike.

Thanks,

Fred

Click to expand...

Your high discharge rate at end of ride will cause an increase of temp in low cost cells, and delay the charger turning on, and when it does, it will be charging a warm pack. In the summer this will cause problems that you need to compensate for, like charging on a timer in the middle of the night.


The lower the voltage to power ratio, or put differently- the lower the current demand-pak or otherwise, the better NIMH performs.

I have worked with and used NIMH for quite a few years. If you are going to go 24v on a 400w peak motor with low cost cells, you will need to run 2 paks in parallel.

But if you want to avoid that, try running at least a 48v system. this will give you 500 watts at ~10 amps, easier on the cells. A 72v system with a 8 amp current limit would be nice, with two 36 volt chargers. that would last a long time.

Good chargers are also important, and knowing what you are doing.


there has been lots written here about NIMH, and if the cost is going down, some good price/performance can be be had. but understand that you have to keep the current down and you have to keep them from getting too warm , during charge, discharge or storage.

I met a fellow in Orange county this winter, he had 30 ah of NIMH on his 24v ebike. It was going on its 7th year. But he had packs in parallel and he used 1/12C over nite charging , he did not use the delta temp over time charging, just the overnite trickle on a timer. So it can be done.

Due some serches on NIMH, and be sure to design you system for minimum current and tempertures for true low cost cost of operation.

your Iacocaa ebike will Low voltage trip on cheap NIMH voltage sag unless you give it two packs in parallel.

my two watts worth

d

I think those don't put out the ah they claim. Even if you keep the amps way down. At least that has been the consensus. Which ones are you looking at buying?

Hi Deardancer, thank-you for the response. Can you recommend a NiMH cell size and manufacturer(s).

Thanks,

Fred

Was thinking C size 9 ah on ebay, like these, or what's the name? Tenergy?

http://cgi.ebay.com/18-pcs-Rechargeable-C-Battery-9000-mAh-Ni-MH-1-2v-/140396550564?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item20b04959a4

fredtr said:

Hi Deardancer, thank-you for the response. Can you recommend a NiMH cell size and manufacturer(s).

Thanks,

Fred

Click to expand...

sorry, but I have not kept up with the top NIMH suppliers. SANFORD was good, as is still the wrecked Prius packs if you can find them.

I have never heard of a NIMH pack working well on a Iacocca 24 or 36v. If one did, it would be a miracle, expensive, or both.

Bite the bullet, go to an external extender SLA, or nicad, or Lith - ion or iron.

OR, Since you have a Brushed motor, buy a 48v controller and you will be in easier territory for NIMH.

d

Hummm, those cells smell fishy to me. I haven't been following NiMH technology in the last few years so maybe they made huge leaps in capacity, but 9000 mAh out of a "C" cell seems very optimistic to say the least. 9000 mAh is something you would see with D cells, not C cells.

If they really do 9000mAh, then I`d worry about their internal resistance.

I did run NiMH on one of my old bikes. My pack was made out of 40 SAFT 8500 mAh cells. Those were expensive cells that had fairly low internal resistance. I was able to ride my bike at 20amps with them it they would only get warm.

Now that I've used LiPo with its low cost, low weight and virtually no discharge current limit, I would never go back...

Do a search for "Tenergy nimh capacity" here and you will see reports showing 55% to 65% of claimed capacity.. the ebay stuff maybe even worse. It can work, you just don't get whats printed on the wrapper. Also the sub-C are usually the ones you want for high drain applications like bikes. The C and D are the ones for low drain only, but there may be a few exceptions..

I appreciate all the feedback. Am using nicad's on another bike and am happy with it. The Iaccoca bike is 36 volts, was going to try to get it someplace where I only spend a couple of weeks a year, so it is difficult to justify the cost of the newer technologies.

If anyone can recommend NiMH cells that do work and are a good value, would be willing to give it a try.

Nicad is the ideal chemistry for a bike that gets ridden one month a year. Leave em sitting around dead for a year, charge em up, and good as new.

Thanks, good advice. I see a good price on C nicad's, is it worth it to use sub C's?

Thanks,

Fred

NIMH .. Dont do it unless you want to see 60% capacity and crappy C rates and voltage sag from hell. Tenergy pack suck balls. Ive put their high rate C cells through their paces and they are no good.

Good LiPo is significantly cheaper than good NiMH. It seems backwards but it's true. I wanted to do nickel until I realized lithium was actually cheaper :wink:

If you can recommend a LiPO source, that would be great.

Thanks,

Fred

Hobbyking.com is pretty much THE source for cheap lithium-polymer batteries, and they also have chargers, wire, plugs, motors, and controllers for cheap.

As fizzit says, check HobbyKing.

You can build a 36V / 10Ah pack for 160$ using 4 of these:

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=6499

And this is a real 10ah capacity. Current limit would be 150A so no issues with the Iacocca bike - voltage sag is minimal.

Of course there are downsides:

- Shipping is expensive with HK
- Charging is more difficult (RC charger / power supply)
- You need to get a BMS or be very careful with monitoring high voltage / low voltage
- If you mess up real bad, the consequences can be real bad (although LFP demonstrated the tolerance is much higher than was generally thought)

If you decide to go LiPo, I strongly suggest reading those threads:

http://endless-sphere.com/forums/viewtopic.php?f=14&t=9170

http://endless-sphere.com/forums/viewtopic.php?f=14&t=10817

Thanks everyone, appreciate the helpful information. As a result I scrapped the NiMH idea. Have bought some more nicad packs from a surplus company. Will try in the Iaccoca but not a big deal if they don't work out. Looks like longer term will buy the packs recommended from HK, been interested in trying LiPO.

Thanks again,

Fred