# How long to charge AA NiMH?

Discussion in 'Digital Photography' started by Ed Wicks, Dec 28, 2006.

1. ### Ed WicksGuest

I have a Sony BC-CS1 charger. Apparently the charging light never goes
out.
How long should I charge a AA NiMH battery? Thx. Ed

Ed Wicks, Dec 28, 2006

2. ### CgiorgioGuest

Until it is fully charged. When charged with a fairly high current (greater
than about 1/4th of capacity), NiMH batteries start to heat up, (most of the
energy is converted to heat) and the terminal voltage drops slightly. Decent
chargers will detect this state for every single cell and either switch the
charge current off or switch to trickle charge for maintaining the charge. I
do not know about the Sony BC-CS1. For example the better models from GP and
others work fine this way. As a rule of thumb, a fully discharged new NiMH
battery can be charged with 1.4 times its capacity before overcharging sets
in.

Cgiorgio, Dec 28, 2006

3. ### Bob SalomonGuest

Until it is charged. How long does that take?

Take the mAh rating of the battery. Add 30% and divide that by the
chargers output. That is how long it takes that charger to recharge that
battery if that cell is fully uncharged.

Bob Salomon, Dec 28, 2006
4. ### =?iso-8859-1?B?bWlubmVz+HR0aQ==?=Guest

The NiMH batteries are known for high tolerance to overcharging. If you
charge your battery for 8 hours, it will definetely be charged (there
are no other chargers which are made for charging for longer).

=?iso-8859-1?B?bWlubmVz+HR0aQ==?=, Dec 28, 2006
5. ### MardonGuest

Close but not quite. The "Operating Instructions" sheet packed with my
Sony BCG-34HUF charger specify a 15 minute charge time for 2500mA NH-AA
batteries. The charging output is specified as 7.5 amps at 5.6 volts DC.
You formula calculates 26 minutes. If you deduct the 30% instead of adding
it, the formula calculates 14 minutes. Did you mean to say "add 30%" or
should it have been "subtract 30%" or does this particular charger just

Mardon, Dec 28, 2006
6. ### Ken WeitzelGuest

Hi...

That charger is little more than a trickle charger; it charges at only
70 mil's. So if you were charging 2500 mA batteries from "empty" to
completely topped up, it would take a little more than two days to finish.

Take care.

Ken

Ken Weitzel, Dec 28, 2006
7. ### ASAARGuest

How long have you waited for the light to go out? Some old
chargers have lights that don't indicate when the batteries have
reached a full charge, and use the light only to indicate that the
charger is powered on and is still charging, even if the batteries
have been charged so long that they're now being overcharged.

A figure can't be given for that charger unless you know the
actual capacity of the AA batteries. The BC-CS1 is an old, *slow*
charger that took 13 hours to charge 1,750mAh AA batteries. If you
have the original AA cells that came with the charger, they're
probably pretty old and have lost a lot of their original capacity.
So if by now they're providing only 1,200mAh, they'd charge quicker,
taking about 9 hours to reach full charge. But if you buy new
2,700mAh batteries, they would need just over 20 hours to complete
charging. Such old chargers usually had timer cutoffs, so that they
might stop charging after about 15 hours, and the 2,700mAh batteries
wouldn't fully charge with only a single charge cycle. If the
charger's charging light went out at 15 hours, the 2,700mAh
batteries would only be 75% charged.

You should probably get a new "smart" charger, one that knows when
the batteries are fully charged and stops charging at that point,
instead of continuing to charge until a time limit is reached.
They'll be able to deal with modern high capacity NiMH cells, and
depending on the model, can fully charge AA cells in less than a
couple of hours. There are some very high current chargers that can
fully charge AA cells in as little as 15 minutes, but the NiMH
batteries would probably last longer if you get a gentler charger
that takes an hour or more to finish charging.

ASAAR, Dec 28, 2006
8. ### Paul AllenGuest

Here's the summary I wrote up back in 2002, including a link to
definitive backup material by Marc Venis and a handy Java charging
time calculator:

http://paulngail1.home.comcast.net/batteries.html

Charging time does matter. If your charger is not one of the newer
automatic types, you must know the charging current and the capacity
of your batteries in order to calculate the approximate time to a
full charge. If your charger does not have its charging current
listed on its label, throw it out and buy a better one.

Paul Allen

Paul Allen, Dec 28, 2006
9. ### Just Another Digital FanGuest

LOL I got my first N-Mh AA 2600mA cells last week and only have an old
Uniross charger which states it charges at 150 mA.

I make that around 24.6 hours for a full charge! Must get a modern
charger....

The rule of thumb I used with slow charge Ni-Cd's is capacity/10 for 14
hours.

Just Another Digital Fan, Dec 28, 2006
10. ### Freedom55Guest

I have 2500 NiMH batteries which take 27 hours to charge in my 140mA
charger.

Ron

--
And it really doesn't matter if
I'm wrong I'm right
Where I belong I'm right
Where I belong.

Lennon & McCartney

Freedom55, Dec 28, 2006
11. ### George E. CawthonGuest

Check with Sony. Chargers are all different, some
are 15 minute chargers, others 1 hour, some are
set for much longer times, some are adjustable.
If the batteries are more than just warm and start
getting hot, they are still being charged so take
them out.

George E. Cawthon, Dec 28, 2006
12. ### Stephen HenningGuest

This unit is rated at 165 mA per cell.

Most AA NiMH cells are rated at around 2800 mAH. Hence, a complete
recharge would take about 17 hours. Most charges heat the cells up
until they go on trickle charge, so if the cells start cooling down,
that should mean they are charged.

Stephen Henning, Dec 29, 2006
13. ### Just Another Digital FanGuest

While it is true that 2800/165 = 17 hours this would result in a semi
charged battery. The losses due to heat etc would need to be added to
give a correct charging period and would be more like 24 hours for a
full charge.

Just Another Digital Fan, Dec 29, 2006
14. ### Ed WicksGuest

Thanks to all for the education on this subject. I went out and bought
a new charger and new batteries, and all is well. Thx again.
Ed Wicks

Ed Wicks, Dec 31, 2006