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I am planning a device consisting of a load, a battery, and an external power source.

The idea is that the battery powers the system until the external power is plugged in -- then, the external source provides the power and charges the battery.

I am working with a 8.2V battery pack, and would like to add an auto charging circuit to this battery, such that it start charging when the battery has been discharged a certain amount, and stops charging when the battery is fully charged.

I would like to know what is a good way to accomplish this, and what voltage source/level I should be using to charge the battery safely.

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Please provide FAR MORE detail.
Do you have a specific battery in mind?
What is your application?

If 8.2= 8.2V it implies 2 x 4.15V cells = (probably) two x Lithium Ion (LiIon) cells in series. If so, use of a LiIon charger or a LiIon charger IC is required.

2 x LiIon in series should never be exposed to more than 8.4V but 8.2V is safer.
BUT there can be quite a lot more than that to LiIon charging.
Please provide details - What mAh, cell type, use, ...?

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  • \$\begingroup\$ actually its two nokia's battery Li-Ion 1100 mAh connected in series each writen on it 3.7 but when measure the two batteries in series it gives 8.2 volt Lithium cell \$\endgroup\$ – alone Sep 4 '12 at 17:58
  • \$\begingroup\$ my application is micro controller using PIC but i need it to keep working all the time and in case the main power supply goes off the system use the battery as back up for power and when the power return back it charges the battery and also provides the pic and all the system \$\endgroup\$ – alone Sep 4 '12 at 18:14

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