# How to design a circuit that charges 9v solar battery and discharging it later on ?

Discussion in 'The Projects Forum' started by Mu86neer, May 20, 2009.

1. ### Mu86neer Thread Starter Member

May 1, 2009
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i may not have been very accurate in my question, what i have done so far is that i designed a small solar power system using 18V panel,LM317 regulator ,charge controller as well as 9V rechargeable battery,, my problem is how to charge the battery and when it's full it cuts of the charging current to feed the load resistor???,,thank you

2. ### THE_RB AAC Fanatic!

Feb 11, 2008
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1,312
Don't use a load resistor? Just use 2 resistors on the LM317 to make a voltage regulator of 9v output then connect it to your battery. The battery can never overcharge. You might want to include a trimpot to give exact voltage adjustment depending on the battery type.

3. ### Wendy Moderator

Mar 24, 2008
21,351
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Uhhh, all batteries can overcharge, and it shortens their life. What kind of rechargable is it?

A good read on the subject...

The Battery University

4. ### Audioguru Expert

Dec 20, 2007
10,497
1,167
Some little 9V rechargeable batteries are six cells and others are 7 cells. They are nominally 7.2V and 8.4V. They are about 9V and 10.5V when fully charged but still on the charger.

They will overheat (and vent or blow up) if the charging current is not limited.
An old 7.2V Ni-Cad was charged at 12mA for 14 hours. A newer Ni-MH is charged at 15mA for 14 hours.

5. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,432
1,312
I won't argue that, but what I said was use a "regulated voltage" around 9v. Then it won't overcharge.

With the right voltage it's called a "float charger" where the current into the battery equals the self dishcarge current and the voltage is maintained. Like float systems on 13.8vdc, but i'm sure you knew that.

6. ### Wendy Moderator

Mar 24, 2008
21,351
2,919
The problem is, 9V batteries don't work that way. You have to adjust your voltage to the chemistry of the battery, it is critical. It's why I included the Battery University link.

The OP didn't include the kind of battery he was using, so we are left to guess. Betcha he/she never comes back.