Bob,

thanks for the info. I will try that.

Andre=A0



________________________________
From: Bob Blick <bobblick@ftml.net>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
Sent: Friday, November 11, 2011 9:21 AM
Subject: Re: [EE]: battery charge resistor value ?

Hi Andre,

Nicad batteries can take up to 1/10 C continuous current. Nimh(except
for a few specialized versions) can't take continuous charging at that
rate, you will be lucky to get 6 months from most. But if you keep it to
1/200 or 1/100 C you will be OK if the batteries charge at all with such
low current(they must be quality cells).

Assuming 2.9 volts fully charged and .5 volts diode drop, that is 3.4
volts. The resistor voltage is 5 - 3.4 =3D 1.6 volts. 1500 mah nimh
battery charged at 1/100 C is 15 ma, so the resistor is 1.6 / .015 =3D 106
ohms. Resistor power is low, 1.6 * .015 =3D .024 watts.

So use 100 ohms and see how it goes.

Or as a fun exercise, replace the diode with an LED and substitute the
higher LED voltage into the calculations. Red is about 1.6 volts.

Friendly regards,

Bob


On Friday, November 11, 2011 7:57 AM, "Andre Abelian"=A0 wrote:
> Hi all,
>=20
> 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. the schematic is
> simple "slow charge"
>=20
>=20
> -+5------A--diode-K---resistor---------Bat+
> 5v =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 =A0 =A0 =A0 =A0 =A0 =A02.4v=A0rechargeable=A0=A0
>=20
> --------GND----------------------------Bat-=A0
>=20
> A=3D anode
> K =3D cathode=A0
>=20
>=20
> since there is no auto shut down etc I want to use it as continuously
> slow charge but not going to=A0
> over charge the battery. my board run time takes 5 ma and when wireless
> is 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
> going to happen
> and no thing will blow or burn?=A0

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