Off the charger NiMH voltages?

[bgiddins]

I'm charging up a bunch of NiMH AA cells at the moment - over the years I've built a collection of various generic cells - and the charger I have is a bit cheap - slow to charge and doesn't cut out consistently.

I have to pull them off because the charger won't consistently cut out when the cells are fully charged and they start to get hot - so tonight I've been monitoring the temps and have measured the voltages with a multimeter, and I've noticed that they're at 1.39V - but aren't hot like they've been in the past when left unattended. Nominally they are a 1.2V cell - but I'm presuming that's under load - at what voltage should I pull them from the charger given it's inadequacies?

 

[zipplet]

That is not a problem. I use the Lacrosse BC-900 charger which displays the terminal voltage - and it can terminate at voltages as high as 1.5V depending on the cell.

I don't know what voltage you should pull them at (SilverFox may be able to chime in!), what I'd do is monitor both the voltage and temperature. If they start to warm up and the voltage is above 1.4V or so, then it might be time to pull them.

 

[MrAl]

Hi there,


Another way to tell is to find out how much current your
charger puts out when charging a various number of cells.
You can use this number to approximate the time it should
take to charge your cells.
For example, if your charger puts out 2 amps and your cell
is 2000mAh (2Ah) then you should pull them off after about
1.1 times one hour, or about 66 minutes.
The formula for the time is this:

Time=1.1*CellCapacity/ChargerAmps
where
CellCapacity is in Ampere hours (1000 times mAh), and
ChargerAmps is the chargers amps in Amperes

Sometimes the charger amps varies with the number of cells
so you have to check that too.

This all assumes you deplete the cells all the way down or
at least somewhat close to that.

Another example...

Your charger puts out 1 amp, and your cells are 2500mAh.
2500mAh is 2.5 Ah, so
time=1.1*2.5/1
which equals
2.75 hours, which is 2 hours and 45 minutes.


You can also use temperature, but you have to measure all the cells.
Here is a graph showing the minus delta V and the temperature of the
cells. The temperature rises slowly but then suddenly starts to rise
much more quickly, which means the cells are charged.