On Tue, 25 Dec 2012 09:24:20 -0800, in sci.electronics.repair you 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.

Perhaps it might be useful if you would disclose the brand and model
number of these "C" cells? The typical "C" NiMH battery has a
capacity of 3000 to 5000 ma-hrs, not 1500, which sounds more like a
"AA" battery or possibly a sub-C size battery.

Hi Jeff,

These I bought at Harbor Freight. 1500mAh are Chicago Electric Power
Systems (in my battery pack) item # 90149, and the 2500mAh (I just bought) are
Thunderbolt Magnum item # 97864.

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.


The 1.4 factor is:
1 / 1.4 = 0.7 = 70% charging efficiency.
That's a reasonable number.

Using a timer is fine, if you're very careful. However, I guarantee
that it will shorten the life of your battery. The object of the
exercise is to charge the battery without going into overcharge.
That's easy with a new battery, where the aformentioned timer formula
works just fine. However, as the battery ages, it's capacity also
decreases. Charging at the same 0.1C rate for the same amount of time
is a guaranteed overcharge. I have some experience with that, having
successfully trashed some rather expensive radio battery packs with
timer controlled charging. In order to do it correctly, you would
need to ocassional measure or estimate the battery pack capacity, and
adjust the timer according to the reduced capacity. You would also
need to know the state of charge, or discharge the battery to a known
charge point.

http://www.buchmann.ca/article18-page1.asp
"Harmful overcharge can occur if a partially or
fully charged battery is charged with a fixed timer. The
same occurs if the battery has aged and can only hold 50
instead of 100 percent charge. Overcharge could occur
even though the NiMH battery feels cool to the touch."

I am aware of partially charged batteries and not to charge
them at the same length of time as charging fully discharged batteries.

In other words, to make the timer method work, you would need to know
the present capacity and state of charge of each battery at the time
of charge. If you keep detailed records and make accurate
measurements, that's possible. I heard a talk by someone from NBC who
did exactly that for maintaining the battery packs for portable
cameras and recorders. Serial numbers on every battery pack, regular
testing, and detailed records were required.

You made very good points regarding aging battery's capacity going down and
I will certainly keep that in mind.

I may consider rechargeable lithium batteries in the future, but I will do
some research first to learn about their issues.

Thanks again, John