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

On 12/26/2012 5:52 AM, wrote:


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.

I've never tried any of these. I've read reviews that suggest that
HF sells cells that were swept off the floor at the place that dug
rejects out of someone else's dumpster.

I do have some experience with drills bought from HF.
Those batteries were uniformly horrible. And I bought some HF
closeout tool batteries with the intention of using the cells elsewhere.
No go...all crap...six cells for 99-cents and it still was a bad deal.
I was never able to quick charge them and slow charging only worked a few
times.

There exist real rechargeable C cells...
http://www.batteryspace.com/nimh-rec...tton-tabs.aspx
is the first google hit. Looks like just over $4 each. Have no opinion
on that brand. Twice the cost of HF. But more than 3x the RATED
capacity, and I'd bet $$$ that they're actually usable in a real
application.

Most end-user grade C-size rechargeable batteries are a smaller,
often much smaller cell embedded
inside. You can tell by the weight.
At 1500 mAh, if you don't need the form factor, you're probably better
off with a quality
AA cell.

On Wed, 26 Dec 2012 08:52:11 -0500, wrote:

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.

Links would be helpful.

http://www.harborfreight.com/pack-of-2-nimh-rechargeable-c-batteries-90149.html
Those are AA cells wrapped in excess cardboard in order to fit in a C
cell package. You can tell with a magnet. If it sticks to the
outside of the cell, it's a real C cell and I'm wrong. If it doesn't
stick, it's cardboard. Welcome to Harbor Fright.

http://www.harborfreight.com/pack-of-2-high-capacity-nimh-rechargeable-c-batteries-97864.html
2500 ma-hr is probably a real C cell. However, it's a the low end of
the capacity scale for an NiMH C cell, which should be 3000 to 5000
mah-hr. I would be concerned about quality.

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

Between the changing capacity with age, the unknown state of charge,
and the rather odd cell capacity specification, you have a moving
target.

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

Forget everything you know about charging NiCd and NiMH. Lithium Ion
charging is very different and difficult. To do it right, you need a
coulomb counter fuel gauge, that counts electrons going in and out,
and adjusts the charge accordingly. You might be able to get away
with something less (as the RC model people suggest). Light reading:
http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries
Note that a warm and fully charged Li-Ion battery self-deteriorates.
Spend some time reading here before diving in:
http://www.candlepowerforums.com/vb/forum.php
http://www.candlepowerforums.com/vb/forumdisplay.php?107-Smoke-and-Fire-Hot-Cells-and-Close-Calls-The-dangerous-side-of-batteries
How to make a 18650 battery and flashlight into an accidental rocket:
http://www.candlepowerforums.com/vb/showthread.php?280909-Ultrafire-18650-3000mA-exploded
I've had an Ultrafire (cheap brand available on eBay) go bad on me,
but not explode. That's because there's a mess of electronics under
the negative terminal in the battery that's suppose to prevent shorts
and bangs.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Wed, 26 Dec 2012 09:11:27 -0800, Jeff Liebermann
wrote:

On Wed, 26 Dec 2012 08:52:11 -0500, wrote:

(snip)

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

Forget everything you know about charging NiCd and NiMH. Lithium Ion
charging is very different and difficult. To do it right, you need a
coulomb counter fuel gauge, that counts electrons going in and out,
and adjusts the charge accordingly.

Not necessarily difficult, but a world away from Ni-xx. The primary
ingredients for Li-Ion charging are a current-limited constant voltage
source that has good voltage control. Near enough is not near good
enough. We have used bench PSU's on occasion as part of out testing.

While John is reading up on the material you linked, he could also
check out the available documentation from some charger-controller
chip makers. Back half a lifetime ago I designed a commercial charger
using the MAX1737, and the data sheet included a lot of good info for
(Li-Ion) beginners on care and feeding requirements,

http://datasheets.maximintegrated.com/en/ds/MAX1737.pdf