Need help! How to charge 24V 18Ah Nimh pack at 8A?

Hi - I am struggling to find a charger for the pack used in a load moving device.
The pack is Nimh, 24V, 18Ah SAFT cells. I tried using a Cellpro PowerLab 8. It charged the pack but overheated it in the process. I'm confused because it actually switched to trickle charge after 18Ah yet the pack was super hot - it actually melted the heat shrink outer pack wrap.

Anyway - we are trying to switch from NiCad to Nimh and were caught off guard by these charging issues.
Thanks for any advice etc.!!

Blast from the past! NiMh requires CC of about 0.5A for that last 90% SOC along with a thermal interrupt of some form or another? Did you ever charge the NiCd with the CellPro? I’m not familiar with that charger, is it an RC unit?

But 8A sounds like a LOT of charge current? I’ve got some nearly 10 year old 13Ah F cells and never charge them above 2A. Often just 1A if there’s no rush.

Thanks Ykick - problem is there is a hurry. These cart moving units are used in an industrial mgfg plant and they use up the 18Ah pack in like 3-4 hours so we want to always have a freshly charged pack ready to go on the charger.

What do mean is it an RC unit? The charger appears to be somewhat popular in the radio control realm if that's what you are asking.

I took a look and it’s a pretty good looking charger. Multi-Chemistry, etc. Yes, I would call it an RC Charger albeit a very serious one.

So your total series cell count is 20S? If so, the charger is seeing that correctly and not charging as 21S?

This is one of those situations that screams for High C rate Lithium that wouldn't mind 8A charge current but you got what you got.

Can you limit the capacity of the charger so that it doesn’t go into trickle or CC mode for any length of time? Most RC chargers can limit capacity.

For the frequency you’re cycling these packs you should be able to get to 90% SOC much quicker than waiting another hour or more for the CC phase to complete the last 10% along with the resulting heating. They do get hot near 100% SOC but wrap shouldn't melt.

Are you using temp sensor connected to the charger? IIRC, some NiMh profiles may rely on temperature for termination?

Frequent cycling is good for this chemistry though. NiMh self-discharge at an alarming rate IME and if stored for any length of time often need several cycles to return to full capacity.

Check the internets for the proper charge profile for NiCd & NiMH, but from memory you've got two basic choices for both: either charge at a low rate, such that the cells can dissipate it all as heat when fully charged without cooking themselves, or use a temperature sensor and use delta-T to terminate the charge when full (or they'll cook).

It sounds like you need the latter, but do check the exact requirements for the chemistry as it sounds like you risk ruining a lot of expensive packs. I'd try and dig out the SAFT datasheet.

Also note that once the cells have overheated themselves trying to dissipate the overcharge and are still being charged, creating more heat, they will probably never have the same capacity or current-delivery ability afterward (and it'll get worse each time this happens). Unfortunately I speak from experience on that one. :/

i'd love to have seen 18Ah cells back when I was using these on my ebikes. I think 8Ah was the biggest I saw then, an F-size cell IIRC. (by SAFT, too, I think). Since you can't parallel NiMH (at least not for charging) it would have simplified making longer-range packs....


As others have said, your first best bet is to find the datasheet for the cells from SAFT to see what the charge profile should be, max charge rate, etc. And also what the max allowable temperature is, so you can put in thermally-activated electrical cutoffs in multiple places inside the pack to prevent a fire (or even explosion if the cell vents aren't sufficient for outgassing) if the charger should ever fail to detect the delta-T point and continue charging anyway.

Well this project drags on...
I have tested charging the 18Ah pack which is 20S2P config BTW.
Used a Powerlab 8 and set charge to 8A. The pack went into an overcharge condition and got quite warm - it melted the outer pack shrink film.
Now I have a new pack with a thermistor planted in the centroid of the pack. How the heck do I use this as a backup to terminate the charge if the Powerlab misses the standard delta-V termination trigger?

Thanks for any suggestions!

bgarner said:

Well this project drags on...
I have tested charging the 18Ah pack which is 20S2P config BTW.
.......s!

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?? Where did these packs come from ? . Are they a commercial product ?
If so, what charger did the supplier provide ?
I seem to recall that charging NimH cells in parallel is a big No No !
http://www.powerstream.com/NiMH.htm

Charging:
This makes these cells and batteries difficult to charge in parallel. This is because you can't be sure that each cell or pack is the same impedance (or resistance), and so some will take more current than others even when they are full. This means that you need to use a separate charging circuit for each string in a parallel pack, or balance the current in some other way, for example by using resistors of such a resistance that it will dominate the current control.

The coulometric charging efficiency of nickel metal hydride batteries is typically 66%, meaning that you must put 150 amp hours into the battery for every 100 amp hours you get out. The faster you charge the worse this gets.

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amberwolf said:

i'd love to have seen 18Ah cells back when I was using these on my ebikes. I think 8Ah was the biggest I saw then, an F-size cell IIRC. (by SAFT, too, I think). ".......

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I have an Aprilia "Enjoy" Ebike original battery pack from the previous century (1998. :wink: ) that is a 24v 13 Ah made up of 20s, 1p Saft "F" cells.....so 13Ahr NimH cells from the previous century ! :lol:
.. And it still works !!