Charging NimH batteries while in a device.

[Fritz]

Ok, to start out, I have no idea what I'm doing. Now that' I've gotten
that out of the way, here is the plan.

I just got a remote control called the OneForAll Kameleon. It's an
awesome remote with one problem, it's batteries die in 1-2 weeks because
of it's backlight feature which automatically turns on whenever you
think about touching the controller. I don't want to give it up
however, so I am going to see if I can mod it.

It takes 3 AAA batteries and I am planning on putting rechargeables
inside. I then will then cut into it slightly and add metal contacts
which link inside of the remote to the battery poles. I'm going to
build a cradle for it that has matching contacts to deliver a charge
(think cordless phone). Using 3 1.2V NiMHs with 700ma capacity I know I
want to be charging at 70ma. I plan on just leaving the controller
there whenever I'm not using the entertainment center.

Ok, here is where I point out my total lack of knowledge. I only can
find AC to 4.5V DC adapters that output at 700ma, this seems like a bit
much (I'm thinking exploding batteries). I know I need to knock it down
to 70ma, I just don't know how. Please help me, and feel free to just
make fun of me for being an idiot too.

Thanks,
->Fritz

 

[Evert]

Hi,
Do you have already got some answers?
I have the same problem with a Sunwave SRC-2010 remote control. The
remote has a 5V connection plug so I like to charge nimh batteries
with a ordinary power supply.

Any thoughts if this can harm the remote?

Evert

 

[Form@C]

Ok, to start out, I have no idea what I'm doing. Now that' I've gotten
that out of the way, here is the plan.

I just got a remote control called the OneForAll Kameleon. It's an
awesome remote with one problem, it's batteries die in 1-2 weeks because
of it's backlight feature which automatically turns on whenever you think
about touching the controller. I don't want to give it up however, so I
am going to see if I can mod it.

It takes 3 AAA batteries and I am planning on putting rechargeables
inside. I then will then cut into it slightly and add metal contacts
which link inside of the remote to the battery poles. I'm going to build
a cradle for it that has matching contacts to deliver a charge (think
cordless phone). Using 3 1.2V NiMHs with 700ma capacity I know I want to
be charging at 70ma. I plan on just leaving the controller there whenever
I'm not using the entertainment center.

Ok, here is where I point out my total lack of knowledge. I only can find
AC to 4.5V DC adapters that output at 700ma, this seems like a bit much
(I'm thinking exploding batteries). I know I need to knock it down to
70ma, I just don't know how. Please help me, and feel free to just make
fun of me for being an idiot too.

Charging NiMh cells is not straightforward. They use a constant current
charger (similar to nicads). If you use a simple supply (like in el-cheapo
chargers) you MUST limit the current to a maximum of 1/10th of the cell's
capacity as you pointed out. Unfortunately you can never obtain a full
charge in this way due to the cell chemistry. The correct way to charge
them is to use a constant current source and monitor the voltage across
the cell. In addition you should use a cut-off timer to prevent
overcharging in the event of a failing cell. High speed chargers (1h or
less) often also monitor the cell temperature for each cell. There are
specialised ICs to handle the charging procedure to make it easier!

In addition to all that, you must cut off the discharge when the cell
voltage drops to about 1v per cell to prevent possible reverse charging
from other cells during the discharge process. This means that it is
usually bad practice to power things like torches or motor-driven devices
from NiMh cells as the cutoff circuitry doesn't usually exist.

Personally, I would not recommend embedding NiMh cells into anything
unless at least the discharge monitoring and temperature sensing equipment
is also embedded with it.

You may hit another more serious problem though, the cell voltage. The
controller will be expecting at least 4.5v (1.5v x3) throughout most of
the battery life and you intend only putting 3.6v (1.2v x3) onto it. It
really needs 4 rechargable cells to replace 3 zinc-carbons. It is quite
possible that it won't work at all, or that the cell life will be very
short indeed. At the very least, try it without modifying the unit!