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shdesigns
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reply to aurgathor

Re: effect of high charging current on NiMHs

100ma is a 1/2C charge rate. Not that high.

For a peak charger, that would be a normal rate. Too low and you can't detect the peak.

I suspect the 20/40ma rates are for dumb chargers.

I have some 180mah 9V NiMh batts and they are near useless. Don't store much and self discharge.

Those batts are 200mah. Not much better.

Check out ebay item 230847565106
Tenergy site seems to have older version on their site:
»www.tenergy.com/30593

Rated for 500mah. That's 2.5x as much and they won't self discharge.

I will be getting some for my DMM as I'm tired of buying 9v batts (forget to turn off and batt dead in 24 hours.)
--
Scott Henion

Embedded Systems Consultant,
SHDesigns home - DIY Welder

public

join:2002-01-19
Santa Clara, CA
said by shdesigns:

100ma is a 1/2C charge rate. Not that high.

Fast charging nimh quickly degrades capacity, in spite of what product ads claim
For best results use 0.1C charge current.


shdesigns
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said by public:

Fast charging nimh quickly degrades capacity, in spite of what product ads claim
For best results use 0.1C charge current.

The problem with that is NiMh do not like being overcharged. NiCads handle it much better. Preventing overchargeing is not easy on lower charge rates.

0.5C is not bad. Most smart charges charge at 1-5C (have seen 10C used). That will degrade them.
--
Scott Henion

Embedded Systems Consultant,
SHDesigns home - DIY Welder


aurgathor

join:2002-12-01
Lynnwood, WA
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1 edit
reply to shdesigns
Yeah, incorrect/outdated specs for the batteries were one possibility I was thinking of.

I did some more search on optimal charging rate for NiMH, and stumbled onto this:
»www.candlepowerforums.com/vb/sho···atteries
A very good thread, unfortunately, most of the cited material is now 404.

In any case, I concluded the following:
a) 0.5C - 1C is used partly because detecting completion is most reliable in that range (and also because the charging is reasonably fast)
b) if a charger is going much over 1C, the life and cycle time of the batteries may plummet
c) 0.1C may put in more charge with less stress, but it's much easier to miss completion at such a low rate, and overcharging is bad, even at such a low current

Looks to me NiMH charging is actually much more SW than HW, and so I may just try to roll my own charger when I get some spare time.
--
Wacky Races 2012!

public

join:2002-01-19
Santa Clara, CA
reply to shdesigns
said by shdesigns:

said by public:

Fast charging nimh quickly degrades capacity, in spite of what product ads claim
For best results use 0.1C charge current.

The problem with that is NiMh do not like being overcharged. NiCads handle it much better. Preventing overchargeing is not easy on lower charge rates.

0.5C is not bad. Most smart charges charge at 1-5C (have seen 10C used). That will degrade them.

If you charge 2500mAh AA cell at 1C that is 2.5A. It will be sizzling hot and quickly ruined.


shdesigns
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Yes, i do AA's at 750ma regardless of capacity. That is enough to peak charge. If I'm in a hurry i use 1.25A, again the 0.5C level.

I have seen 3000mah A cells charged at 10 amps. Then again they are designed for it and are discharged at 25+ amps.

public

join:2002-01-19
Santa Clara, CA
said by shdesigns:

I have seen 3000mah A cells charged at 10 amps. Then again they are designed for it and are discharged at 25+ amps.

Typical nimh aa cell will run hot when loaded at 1A. Which AA cells can supply 25A?


shdesigns
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said by public:

said by shdesigns:

I have seen 3000mah A cells charged at 10 amps. Then again they are designed for it and are discharged at 25+ amps.

Typical nimh aa cell will run hot when loaded at 1A. Which AA cells can supply 25A?

I did not say AA, I said A cells (often 5/4 A). Yes, they get hot enough when discharged you cant hold them in your hands very long. I remember they were Sanyo. Used in R/C plane, flight time of 6 to 8 minutes.
--
Scott Henion

Embedded Systems Consultant,
SHDesigns home - DIY Welder