Probably anything less than C/20 is bad for the cells. The information isn't easy to find.

Hi,

That's interesting. I did NiCd at very low charge rate just to keep them from self discharging. The key there though was to limit the charge time to a short period like 20 minutes just to keep them up. Cells lasted for years and were always ready.
I since then however switched to Lead Acid due to the higher capacity available for much lower cost. My NiCd cells cost me $40 USD and rated for about 2AHr, while the Lead acid cost me about $20 USD, and is rated for about 7.5AHr. A little heavier but i dont mind with that extra run time of almost 4 times the NiCd pack. The Lead acid is 12v too, so can be used for other things if needed.

 

Hello,

Read this page from the battery university about charching the NiMH:
http://batteryuniversity.com/learn/article/charging_nickel_metal_hydride

Bertus

 

Hello,

Read this page from the battery university about charching the NiMH:
http://batteryuniversity.com/learn/article/charging_nickel_metal_hydride

Bertus

Very correct. That being said many NiMH battery chargers do not monitor either voltage or temperature. They just supply a limited current at a low level and leave the rest up to the human. AAA and AA charged at the same rate with no regards for Amp-hour rating of the battery.
:-(
Just because it says it will charge the battery does not mean it will charge it correctly.

(edited to add ...)

In fact, on a quick search I can't find a "good" NiMH battery charger. If it says typical charge time is 8 to 12hours it is not charging at c/2 is it? If it says it will charge NiMH or NiCad does it just supply a current?
How can it charge at "c/2" if it doesn't say it is for some specific A-Hr rated battery?
I can see it doing different rated batteries, AA or AA if it monitors voltage and temperature, but it charges at what current?

Are there any "good" battery chargers out there at all?

(edited again to add ...)

Found one.

http://data.energizer.com/PDFs/ch15mn2.pdf

It specifically says it monitors voltage and temperature. List price, about $46.00 US. So I guess you get what you pay for.
Found on the Eveready web site and Amazon.

 

Hello,

Read this page from the battery university about charching the NiMH:
http://batteryuniversity.com/learn/article/charging_nickel_metal_hydride

Bertus

Even the good chargers say they trickle charge (after charging) at 20 mA for AAA or 50m mA for AA. So would it be safe to charge a 2300 mAh NiMH AA for 50 mA over 2 days, or indefinitely?

 

Hello,

I made a ICS1702 based reflex charger a very long time ago.





Bertus

 

Hello,

I made a ICS1702 based reflex charger a very long time ago.

View attachment 103274

View attachment 103275

Bertus

Sweet!

 

Hello again,

Here is a voltage/temperature profile i did when charging two NiMH cells a while back. I dont use any Energizer stuff anymore just the low self discharge NiMH cells from various manufacturers. Got burnt with Energizer products way too many times.

The top traces are the voltage curves, and you can see they dip down when the cells are charged (the so-called negative delta V or minus delta V).
The bottom traces are the cell temperatures and the ambient temperature. You can see some correlation between the voltage, charge state, and temperature. It's important to measure the ambient temperature in addition to the cell temperature to make sure it is the cell temperature that is changing and not just the ambient. It is also important to make sure the charge current is really constant if the algorithm is designed to detect minus delta V. Also notice that the minus delta V is a very small change which is more pronounced with higher constant charge current and less pronounced with lower constant charge current.

 

Hello again,

Here is a voltage/temperature profile i did when charging two NiMH cells a while back. I dont use any Energizer stuff anymore just the low self discharge NiMH cells from various manufacturers. Got burnt with Energizer products way too many times.

The top traces are the voltage curves, and you can see they dip down when the cells are charged (the so-called negative delta V or minus delta V).
The bottom traces are the cell temperatures and the ambient temperature. You can see some correlation between the voltage, charge state, and temperature. It's important to measure the ambient temperature in addition to the cell temperature to make sure it is the cell temperature that is changing and not just the ambient. It is also important to make sure the charge current is really constant if the algorithm is designed to detect minus delta V. Also notice that the minus delta V is a very small change which is more pronounced with higher constant charge current and less pronounced with lower constant charge current.

View attachment 103297

Good stuff, thanks.