On 12/23/2012 1:53 PM, wrote:
Hi,
Since I created a NiMH battery pack using recently bought NiMH "C"
batteries, I am in search of knowledge for the best way to slow charge these
1500 MAH "C" cells in series. I prefer using a timer and a constant current
charger (I made) that supplies a constant 150MA.

There is a lot of information on the web, but one site says one thing,
and another says something else.

www.batteryprice.com/batterycareguide.aspx provided the kind of
information I was looking for:

(Battery Capacity in MHA/Charge Rate) X 1.4 = Time to charge (slow
charge). Note: Batteries "fully discharged" to 1V each cell.

In my case, 1500/150 = 10 X 1.4 = 14 hours. This seems logical
to me, but I wonder if anyone sees a flaw.

Thank You in advance, John


What does it say on the manufacturer's technical datasheet or charging
application note?

If they were NiCd, you'd have no problem.
The consensus appears to be that NiMH don't tolerate long-term overcharge.
Opinions vary on short-term overcharge. I expect you're doing about
the best you can do with simple schemes.

The difficulty is not in the equation. The difficulty is in determining
full discharge. How do you determine that?
Do you measure the voltage? under what conditions? OR do you just
charge it when the performance of your device drops? OR at random?
The range between 1.1V and 0.9V is huge depending on the load current
you use to test. And the device probably quits long before that.
I know because I have a computer hooked to a programmable power supply
and load fixture and I've tested it on a many different cells with widely
varying history.

One example application is the emergency flashlight.
You can't just charge it when it goes dark. Even in the best of conditions,
when it's starting to dim, you know you should charge it IMMEDIATELY...
but you've only got one more thing to do...so you do it.
More batteries are damaged from overdischarge than anything else.
For it's intended use, the emergency flashlight needs to be at FULL
charge all the time. So does your cell phone and your car battery and
your electric drill and...
Even low-drain applications are not immune. Do you want your alarm
clock to go dead at 4AM on the morning of your big presentation.

Bottom line is that you expect your device to perform for the entire
duration of the need...even when you have no idea how long that is.
That requirement often leads to systematic overcharge.

But:
I expect you're doing about the best you can do with simple schemes.

If you've done something like solder on the cells, you can expect
some variations in self-discharge and capacity. Some amount of overcharge
tends to keep them equalized.

Are we having fun yet?