Ni-cad chargers for Nimh

[nigelb]

Hi

NiMH's really don't like getting hot, nor do they like being charged fast.

If you charge an AA NiMH at say 0.5A (assuming its maybe 2Ah+ capacity) from flat, it should stay cold (in fact will self cool) until its almost fully charged, and will then start to heat up. Never keep charging NiMH's that are getting hot - something is wrong.

The peak detection of a nicad was easy, as you charged it the voltage across a cell actually goes down when its fully charged. With NiMH's, the voltage stops going up as fast, so you need a different circuit to detect the peak.

Its a complicated area, today my money would be on a charger that charges each cell on its own, pref will discharge each cell too, and expect to replace the cells every 1-2 years.

Oh, and if you depend on the cells, write the date first used on them in indelible pen, they all look the same otherwise.

Nige

 

[ColinJ]

I don't know what happened to my reply - I probably previewed it and then forgot to post it...

I bought the MAHA (POWEREX) MH-C204F, but I think that model might be discontinued now.

 

[jimboalee]

Please read the document pdf I posted earlier.

Eveready-Energiser state their recommendations.

The charger in Crackl'e post charges at 0.33C and has a timer. This won't fry batteries, but is commercial charger.

NEVER, NEVER fully discharge a NiMh cell like you did your NiCd cells. Garmins etc have a battery level indicator which goes to nothing when the batteries are 1.15V each. This is only just past the cell's 'mid-point voltage' and no where near 3/4 discharged!!!

NiMh cells NEED at least 1.0C for 20 - 30 minutes to 'Kick' them. Then reduce to 0.1C for the remaining time.

www.panasonic.com/industrial/battery/oem/chem/nicmet/

Here's some more info.
The pdf link '~ charging methods ~' is down the page with some other pdf links.

 

[nigelb]

Hi

Have read the http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf now (should have done before, apologies).

I note it says when a cell is fully charged, further charging causes a rapid temp increase, and that you can safely discharge each cell to 0.9V.

A NiMH cell that is almost fully charged and then subjected to 0.33C for a full cycle (as if flat) may well fry imho.

I've never heard of "kicking" a NiMH cell in normal use. If a cell goes into deep discharge (say put away and forgotten for a year), then charging it hard might wake it up (I've tended to cycle "soft" cells 3 or more times, but at maybe 0.1C, which has worked for me with nicads, never tried it with NiMH's).

For reference, I use nicads, nimhs and lipos in model aircraft, where the loss of a cell can mean a model lost at best, at worst someone hurt when it hits them. (many different shaped cells, from single cell 140mAh lipo "packs" to nicad power cells, short and dumpy and capable of sourcing 20A+ in normal use).

Anyway, the good news is there are now cheap peak detecting chargers for nimh's that do charge each cell independently.

Nige

 

[jimboalee]

NiMh like to be used regularly. Discharged and recharged immediatley. If they've been lay around for a few months, a trickle of 0.05C for a week will gently bring them back.
The term 'kick' is poetic licence for the high rate at the beginning of the recharge procedure.