On Mon, Oct 17, 2011 at 9:18 PM, Jason White <
whitewaterssoftwareinfo@gmail.com> wrote:

> Hello, I am trying to design a simple battery charger. It is supposed to
> charge 4x AA (2000mAh) rechargeable batteries off of a 5 volt supply. Rig=
ht
> now what I have designed has an op-amp is tied to a 4.76 volt reference a=
nd
> the positive battery terminal. When the battery's voltage is below the
> reference it switches on a NPN transistor with a 35 ohm resistor at its
> emitter. When fully on the batteries will receive about 140mA. The charge=
r
> will continue to remain on until the voltage rises above the reference.
>
> I believe I'll be using a dual op amp, which leaves room for expansion.
>
> What do you suggest for fuses ? advice on resettable fuses, temperature
> sensors, and other safety features would be appreciated.
> What Is the recommended charge voltage cutoff ? (RTFM ?)
> Is the voltage reference method described a viable mean of charging
> batteries ?
> What other cheap/low part count methods are there to safely charge
> batteries
> ?
>
>
> Note: This is not a production design, it is meant to be simple and cheap=
..
> Though I don't want it to destroy the batteries or leave them uncharged.
>
>

You probably won't charge the NiMH batteries fully. Also, NiMH shouldn't be
charged with a constant-voltage method. You charge them with constant
current, and detect a -dV and/or +dt (rise in temperature). Also,
personally, I don't feel comfortable charging them in series like that sinc=
e
slight differences in capacities might cause odd charging behavior over
time. I always charge in parallel, via single channels. Also note that you
don't know exactly how much current is going through the transistor since
the B-E current vs C-E current properties change over temperature and amoun=
t
of amplification. Also, I don't think you /really/ need that capacitor
there.
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.