Swapping Sla to NiCd

[Geebee]

I am thinking about buying 5x24volt NiCd drill packs to wire in parralel for my Tongxin hub.
Questions are has anyone done something similar?
If I only charge at the 20 hour rate will I have an issue with parralel charging?
If charged at the 20 hour rate I should not have to worry about over charging by leaving on several hours to long?
Discharge will be a max of less than 2c so even the cheaper drill packs should survive?

Cheers all

 

[Ypedal]

I don't know if Justin has any left, but take the time to compare the cost of those packs vs pre-made packs. 8ah F cells handle 20 amps fairly well..

 

[Drunkskunk]

Running in parallel isn't a problem, Works great. But you can't charge Nicad or NiMH in parallel. they have a funny charging pattern were the voltage actualy drops lower just before full charge, and if (when) the parallel cells don't do it at exactly the same time, then the charger can be fooled into over charging them.

SLA and some Lithium cells can be charged in parallel.

 

[fechter]

I wish there was an easy way to charge those in parallel. I learned about parallel charging NiCds by having a rather large collection of them explode while charging. Razor sharp chunks of sheet metal were embedded in the wall and caustic electrolyte paste was spattered everywhere.

Yeehaw... makes me want to try it with lithiums

Anyway, the best bet would be to use separate chargers for each string, then put them in parallel for discharge. I should be possible to design a setup to charge them at the same time with one charger, but I'm afraid it might be a bit complicated. Diodes on each string with resistors across them for charging might be the cheap and easy route.

No problem on discharge. NiCd's can really pump out the amps.

 

[Geebee]

But does this apply with trickle charging?
Say 300 ma into a 6 to 8 amp hour pack?

 

[xyster]

But does this apply with trickle charging?
Say 300 ma into a 6 to 8 amp hour pack?

That's a really good question -- sounds on the face of it OK. You could try it with a couple cells and if it didn't work, go the diode route.

 

[fechter]

I'm not sure. As the first cell (string) approaches full charge and the terminal voltage drops, it could cause a 'runaway' current transfer from the less charged cells to the more charged cell (string).

If you had a big plug with enough pins, you could separate the strings during charging, but have them each fed by a diode and resistor to equalize the current during charge. Diodes would prevent any transfer between strings. Once charged, plug the battery into a mating plug that puts the strings in parallel (no diode losses).

I've heard of charging Nimh in parallel at low rates, so it might work with NiCd. I still wouldn't do it.

 

[xyster]

If you had a big plug with enough pins, you could separate the strings during charging,

Perfect job for a Lego-like block of powerpoles....