NiMh (nickel metal hydride) battery conversion

Danielson · Aug 19, 2008

Hi all, I have been working on and am in the process of coverting my FE400 to start using 12 4200mah sub c NiMh cells as used in radio controlled cars. They spin the motor nice an quick and it fires into life much better. My question to you is how many volts is it safe to put down the white wire into the CDI via the start button. A fully charged pack has around 17 volts and it starts without the white wire connected hot or cold but when the battery runs down to say 14 V it won't fire up as good. Is there a resistor that I can put in the wire to make the voltage safe ? I did read of another user running 2 x 12V batterys = 24v not sure what his configuration was. The other bonus is the weight as the total pack only weighs 850 grams against the 2.5kg sealed lead acid. The downfall is it could go bang with a constatnt charge going into the battery pack so a on off charge switch will be installed.
Cheers Dan

ned37 · Aug 19, 2008

The downfall is it could go bang with a constant charge going into the battery pack

they generally just get real hot and melt everything in sight rather than popping. (ask me how i know)
its a good thing in theory, but not in practice without a lot of work and your nimh bats weren't designed to put out the amps required to consistently crank that starter.

Danielson · Aug 19, 2008

they generally just get real hot and melt everything in sight rather than popping.

Your probably right about them not likely to go bang and yes they do get very hot when overcharged which is why I need to put a switch in the circuit so I don't have a constant charge going into them.

As for the nimh not being suitable for the electric start I would tend to disagree. I raced rc cars for years and the demands placed upon the batterys is huge when running very low 6 turn motors etc. It is possible for them to be drained from fully charged to flat in less than 5 minutes and from the testing I have done so far they start the motor with ease. I had them sitting around and they don't owe me much so I will just install them and see how they go over a period of time.
Regards Dan

BundyBear · Aug 20, 2008

Sounds like a simple & effective idea but I suspect there is some reason why every featherweight rocket out dosen't use NiMh or NiCd batteries. Buggered if I know what it might be but? Can't be cost - that'd be comparable to a Pb battery.

I've not had much experience tinkering with NiMh's but I think they are similar to the older technology NiCd's without their memory problems. I don't think delivery current would be a problem with these either. The biggest risk with NiCd's is accidentally shorting them out - a fairly small cell can deliver shitloads of current into a suitably low resistance.

NiCd's (& I think NiMh's?) like to be charged by a constant current source. You have to be careful when going over the 1 hour charge rate and REALLY careful about overheating when going over 1/4 hr rate. You can leave then connected indefinitely at less than 16 hr rate without risking overcharging.

Would it be worth fitting a temperature sensor into the battery pack with some other 'lectronics to either warn of high temp or even to automatically switch to a lower charge rate?

I wouldn't be too worried about an explosion either. It's only a pack of hot cells mounted just below your nuts??? Seriously though, at worst one cell might split & pop but it wouldn't be a huge bang. One of the existing lead batteries ruptured before in my bike anyway.

I'd be a lot more cautious if you were going to try LiPo batteries - they got a bad rep for going bang.

Another that might be good to try is the new LiFe batteries. There is a bloke who has sent me lots of info on them for use in electric vehicles, but I don't know how they'd handle start duty. From his descrition so far the sun shines out of them!!

Make sure you post back & tell us how it goes.

Danielson · Aug 22, 2008

O.K I have the nimh battery made up and installed and it starts the bike great everytime until it runs down. I have wired it up to receive charge from the regulator rectifier and it appears to get very little. I have searched other posts here and this is what I have. Yellow from stator goes to yellow dot on reg rec, red - white dot goes to battery +, brown to earth. Without white wire connected to the battery I can get 11 V dc at high rpm, idle is about 7 v dc, if I hook a 12 V globe in series from the white wire to earth it powers the globe but only with about 3v at idle 7 V at around 2000 rpm then drops back to 3 V when you past that point.
Tried another reg rec unit off a 501 with same result. I get 40 + AC Volts from stator yellow to earth. It appears to me that the regulator rectifiers limiting the current way to much to charge the battery properly. I will try a reg rec off a jap bike when I get a chance.
Any other ideas ?
Dan

BundyBear · Aug 23, 2008

Yuo said earlier that a full charged pack is 17V. The purpose of the regulator is to limit the charge voltage to a bit less than 15V. With a standard regulator you should not not have enough to charge your pack.

There are also a few of threads that talk about how the regulator works & why a meter across its output reads so low. Check the electrical forums.

I'd suggest either using less cells - but the starter won't spin as fast, or modifying or making a custom regulator with a higher voltage - but you'll likely blow globes

Danielson · Aug 23, 2008

Yuo said earlier that a full charged pack is 17V. The purpose of the regulator is to limit the charge voltage to a bit less than 15V.

The packs voltage has dropped to 14volts and still no increase in it mwith motor running. I realise the regulator won't allow it to get get fully charged but if I can maintain a charge of 14.5 + volts it will still start the bike. Once the pack drops into the low 14s it is below its useful power level.

Danielson · Aug 24, 2008

Just a thought, I have one of the yellow wires from the stator hard wired to the headlight with the square regulator tapped into the wiring to regulate the headlight ac voltage. Can the headlight working full time rob the other yellow ac wire of the voltage I need to charge the battery ?
Dan

BundyBear · Aug 24, 2008

The two yellow wires are connected to separate windings in the stator. Their outputs should be independent of each other.

Danielson · Aug 25, 2008

After speaking with one of the stator rewinders here I measured the amps of one of the yellow stator output wires today and they are only .15 of an amp or 150mah. I disconnected the regulator and ran the yellow ac wire through a simple rectifier off an old jap trail bike then straight to the battery. I went for a 3 hour ride and it started off the button every time. I still used the switch to turn off charge and only turned it on after dropping the bike on an awkward hill several times. Each time I dropped it

it took a fair amount of cranking to get started again and the battery held up well. I would say for sure that my lead acid battery would have only cranked it half the amount of time and without as much ooomph before being flat. I will post some pics of the setup when time permits.
Cheers Dan

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