Cheap nimh cells

throbscottle · Oct 29, 2012

I was looking at sub-c cells on eBay to rebuild 2 battery packs for my drill (15 cells per pack) and found these 6000mAH cells:
**broken link removed**

The vendor says they are made by Hi-Capacity Power Products (means nothing to me, but they do have a web site) Should I trust them? Or just go for more normal capacity cells?

I wait with bated breath...

TIA

dr pepper · Oct 29, 2012

I think its juct luck what you get from these places, they might be naff, but they might be respectable cells rebranded.

I got a battery pack for my pushbike lights, one originally designed for cameras and its great, at least within its capacity rating.

Re-stuffing battery packs is something I would do, just make sure there isnt a chip in there that tells the drill or whatever that the pack is now dead or tampered with preventing it from working.

throbscottle · Oct 29, 2012

It's a fairly cheap Draper drill (you get a carry case with drill, two batteries and charger, some very cheap drill bits and screw-bits. Cost me less than these cells cost) - the packs are just full of cardboard wrapped ni-cads of 1200mAH capacity, absolutely nothing special (but quite hard to find) - but it's given good service and looks like it will continue to do so.

dr pepper · Oct 29, 2012

Should be fine then, I've done a cordless strimmer which now works well, and started a dewalt drill but scrapped the battery when I found it was chipped, might be a business opportunity if you worked out how to re blow the chip.

ChrisP58 · Oct 29, 2012

One thing to keep in mind is that the charge termination detection method is different for NiCd and NiMh cells.

Charge termination of NiCd is usually done by -ΔV/Δt (drop in voltage / time)
Charge termination of NiMh is best done with ΔT/Δt (increase in Temperature / time)

Charging a NiMh pack with a NiCd charger can damage or, at least reduce the life cycle, of the NiMh pack.

For more info, see these two links:
How to charge NiCd cells
How to charge NiMH cells

throbscottle · Oct 30, 2012

Hmmm, interesting reading. The charger for the drill is a cheap fast charger which drops to trickle when the battery is full (and I suspect it's shortening battery life considerably - they are always quite warm when "cooked"). Looks like I'd have to turn it into a slow charger if I get these babies! (Which looks like a good idea anyway...)

jpanhalt · Oct 30, 2012

When powering high current devices with NiMH, you must consider the allowable discharge current for the cells you pick. NiMH have considerably less discharge capability than LiPo, NiCd, or lithium iron phosphate cells (e.g., A123, LiFe).

In fact, for ignitions (max drain a couple of amps) they work fine, but I don't know any modelers who use them for motive power at our field.

John

throbscottle · Oct 30, 2012

Hmm, interesting again. I'll have to check the drill's current draw. Oh wait, my meter only goes to 200mA. Darn it. Maybe I'll just get nicads anyway (humph!)

Thanks everyone. Once again, I can consider myself educated

jpanhalt · Oct 30, 2012

If you can find the Lithium-iron phosphate batteries, you will be happy. I read that A123 was in bankruptcy/reorganization. LiFe is another brand. They offer the voltage of lithium with high current and safety. Of course, if you use the same number of lithium cells (nominal 3 volts or so) as you had NiCd (1.2 volt) the drill will really "go" -- maybe in just a few seconds.

Can you still get NiCd's in the UK? They are a good battery and despite having lower labeled capacity than NiMH, they are capable of far more discharge current and rapid charging. You can easily charge a 1200 mAH SCR NiCd at 5A (i.e., 4C). Watch out for heating. They make nice hand warmers when we fly electrics in the Winter. NiCd's can have thermal run away. If shorted, they can catch on fire.

John

throbscottle · Oct 30, 2012

Yes, we can still get nicads in the UK. I'd never heard of LFP cells before - interesting!

Thanks

jpanhalt · Oct 30, 2012

The LFP cells are quite popular in portable hand tools in the US. In fact, the tool manufacturers bought so much of the production capacity that some modelers bought the tools just for the batteries inside. Now, they are becoming available as individual cells.

Get yourself a nice Bosch with fresh LiFePO4 batteries inside. That is the easiest route to happiness.

John

throbscottle · Oct 31, 2012

Lol - That's me told then

JimB · Oct 31, 2012

For NiMH batteries in the UK, have a look at these guys:

https://www.strikalite.co.uk/

I have used them several times and always have good service.

JimB

WTP Pepper · Oct 31, 2012

jpanhalt said:
If you can find the Lithium-iron phosphate batteries, you will be happy. I read that A123 was in bankruptcy/reorganization. LiFe is another brand. They offer the voltage of lithium with high current and safety. Of course, if you use the same number of lithium cells (nominal 3 volts or so) as you had NiCd (1.2 volt) the drill will really "go" -- maybe in just a few seconds.

Can you still get NiCd's in the UK? They are a good battery and despite having lower labeled capacity than NiMH, they are capable of far more discharge current and rapid charging. You can easily charge a 1200 mAH SCR NiCd at 5A (i.e., 4C). Watch out for heating. They make nice hand warmers when we fly electrics in the Winter. NiCd's can have thermal run away. If shorted, they can catch on fire.

John

I do not claim to understand the chemistry, but I believe that the charging of NiCads in endothermic and as a result can cool the cells below what you would expect from other chemistry in cells and that's why they can be charged at such a high currents. i.e. They don't heat up as others would under the same charging regimes.

jpanhalt · Oct 31, 2012

Source: https://www.mpoweruk.com/thermal.htm
Lead acid batteries are exothermic during charging and VRLA batteries are prone to thermal runaway (See below). NiMH cells are also exothermic during charging and as they approach full charge, the cell temperature can rise dramatically. Consequently, chargers for NiMH cells must be designed to sense this temperature rise and cut off the charger to prevent damage to the cells. By contrast Nickel based batteries with alkaline electrolytes (NiCads) and Lithium batteries are endothermic during charging. Nevertheless thermal runaway is still possible during charging with these batteries if they are subject to overcharging.
The thermochemistry of Lithium cells is slightly more complex, depending on the state of intercalation of the Lithium ions into the crystal lattice. During charging the reaction is initially endothermic then moving to slightly slightly exothermic during most of the charging cycle. During discharge the reaction is the reverse, initially exothermic then moving to slightly endothermic for most of the discharge cycle. In common with the other chemistries, the Joule heating effect is greater than the thermochemical effect so long as the cells remain within their design limits.

And here's a temperature curve for NiCd's

View attachment 68365

Be careful of the dotted line.

John

Cheap nimh cells

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