how should i charge this battery?

Discussion in 'Electronic Basics' started by [email protected], Jul 6, 2005.

1. Guest

I have very little experience in electronics and only know the basic
math equations and funtion of all the basic components from resistors
to transistors. i have just started reading my first book on beam
electronics and need to know how to charge a battery pack. I took two
cordless phone batteries(one 3.6v 400mah the other 3.6v 600mah) and
wired them in paralel to get a 3.6 1000mah battery pack (is that
right?). What is the perfect voltage and amperage for charging this
pack? If I wired the pack in series, then how many mah would I have?
How would i charge this new pack?

2. Tom BiasiGuest

Hi,
You have NiCads. Nominal 1.2 volts per cell. A rule of thumb (nice and safe)
is to charge at 1/10 the mAH rating for 10 Hrs.
At that rate the NiCad temperature usually does not need to be monitored. In
practice you really won't get 1000 mAH out of the parallel pack but you will
get more than 400 mAH.
The voltage could be about 1.4 volts per cell. When charging in series it is
really desired that the battery packs be all the same type and rating.
Series by definition passes the same current through all cells.
If you ask me I would charge the packs separately. I would use 4.2 volts
per pack and limit the maximum current to about 40 mA. Do you have a way to
control the current? A discharged battery will draw more current than a
partially charged battery but keep the maximum to about 40 mA and charge
over night. This is not the optimum charging for the batteries but will do
the job.
Regards,
Tom

3. Bob MonsenGuest

Wiring them in parallel isn't a very good solution, unless you use
diodes to prevent one from running current through the other.
Unfortunately, that will waste a bit of the output voltage.

A better solution would be a larger pack, since it would probably be
lighter too. You can get 1700mAH 1.2V nimh batteries for a buck each,
and wire 4 together.

look for

HR-AAU overstock w/TABS

It'll be lighter, and you won't have to worry about cross-charging.

to overcharge it, or you'll kill the nicads. You can tell when you are
overcharging it, because the cells heat up quickly.

--
Regards,
Bob Monsen

If a little knowledge is dangerous, where is the man who has
so much as to be out of danger?
Thomas Henry Huxley, 1877

4. ehsjrGuest

You need to charge them separately. It's simple & cheap.
Get a 12V DC wallwart, 2 LM317 voltage regulators and 2
resistors. Wire it up like this:

---------
+12V ----+-----in| LM317 |out------+---Batt1--+
| --------- | |
| | | |
| +---/\/\/\-----+ |
| R1 |
| |
| |
| --------- |
+-----in| LM317 |out------+---Batt2--+
--------- | |
| | |
+---/\/\/\-----+ |
R2 |
Neg----------------------------------------------+

Batt1 is the 400 mAh pack, batt2 is the 600 mAh pack.
R1 is a 33 ohm resistor. R2 is a 24 ohm resistor.

Be *sure* to connect batt1 to the 33 ohm resistor
and batt2 to the 24 ohm resistor. Note that the
batteries are *not* in parallel or series. The
negative side of both batteries goes to the negative
from the wallwart. The positive side of each battery
goes to its own LM317/resistor combination.
Charge for 14 hours.

Do not use these batteries in series. Doing so could
damage the 400 mAh pack.

Ed