> in my board I have 2 AA batterys that I want to use rechargeable battery =
instead and connect to
> 5 volt power coming from outside make it slow charge.
> since there is no auto shut down etc I want to use it as continuously slo=
w charge but not going to
> over charge the battery. my board run time takes 5 ma and when wireless i=
s on it takes 100ma
> 2.4v power is connected to boost circuit and output is 3.3v to pic.
> my question is what resistor value should I use that slow charging is goi=
ng to happen
> and no thing will blow or burn?

I'll assume NimH as NiCd are legal or discouraged due to ROHS issues
(due to Cadmium content.)

Modern NimH AA cells above about say 2000 mAh are not suitable for
trickle charging long term at even very very low rates. In the fight
to fit more active material in they have removed the ability for the
cell to recombine Hydrogen and Oxygen to reform water when the cell
"gases" when charged when already fully charged. Even a very very low
continuous rate will destroy them.

_____

If you use low capacity AA NimH cells - say below 1500 mAh and
preferably <=3D 1200 mAh, you will be able to "trickle charge them
continually at say /20 rate. So for 1200 mAh =3D 1200/20 =3D <=3D 60 mA

Fully charged voltage of a NimH cell at room temperature is a nominal 1.45V=
..
Charging them to somewhat less reduces their capacity but improves the
cycle life.
So say 1.3 to 1.35V per cell if you don't mind the loss in capacity.

If you use a 1 amp rated low voltage Schottky such an a 1N5817 you can
expect 0.3V diode drop at low currents like this. Going to even a
1N5819 increases that to say 0.4V at low current.

You can work out resistor value from the above BUT doing it this way
is a compromise.

________________

BETTER:

A substantial improvement at modest cost can be achieved by putting a
voltage clamp after the resistor and before the diode  - like a zener
but much sharper in action. If you set this to 2 x Vcell + 0.3 the
battery will completely stop charging  when this voltage is reached. I
used 0.3V for the Schottky diode as there will be about zero current
flow. So if Vclamp =3D 2 x 1.35 + 0.3 =3D 3 Volt the battery will be able
to be "trickle charged" at whatever rate desired and will cutoff when
charged.

A TL431 in TO92 package requires 2 resistors + the IC to form a clamp,
ON Semi TO92 180 C/W junction to ambient, 85 C/W junction to case.
http://www.onsemi.com/pub_link/Collateral/TL431-D.PDF


180 C/W and 100 mA max.
At say 20 mA Pd =3D 20mA x 3V =3D 60 mW
Temperature rise =3D 180 x 0.06 =3D 11 degrees C
50 mA =3D~ 30 degrees C.

Use fig 18 for up to say 50 ma. For higher currents add a jellybean
PNP and one more resistor.



         Russell McMahon
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.