# From Li-Ion to Li-Po battery: design a new battery pack for old mobile phone

I have an old working mobile phone. The battery, a compatible 3.6V 500mAh Li-Ion is almost dead (the original one was 800mAh). Is it possible replace the battery pack with a 1200mAh or so Li-Po battery?

I attached the pictures of the Li-Ion battery pack

The original phone battery charger has an output of 7.5V 880mAh

The most important aspect is the charging voltage. You must never charge a Lithium battery (Ion or polymer) above the voltage at which it is rated to charge. So the charging voltage must be the same or below for the new package.

The big question is what is the charging voltage of the charger in the phone. What you stated is the battery charger output of 7.5V @ 880mAh is unlikely to be the charger itself. There is likely a charger IC inside the mobile phone. Perhaps you can measure this by measuring the voltage of the pins of the battery with the battery inserted. For 1 cell this should be 4.1V, 4.2V, or 4.35V more than likely.

As for current, usually don't charge battery above 1C (where C is nominal capacity). So make sure the battery capacity is larger than the old battery. Check the datasheet of whatever battery you buy about this.

Unless you use the original battery, your solution is unlikely to prove optimum, as far as extracting the maximum amount of charge, but this shouldn't be an issue.

As I mentioned above, looking now at your picture, it seems U2 is the charger IC. It'll be good for you to obtain its markings and perhaps the datasheet.

• Sure thing. Hopefully you're a couple measurements away from getting it running. – Gustavo Litovsky Feb 18 '13 at 23:21
• Well, about the Li-Ion battery I have now, all that I know is that the battery is a 3.6V Li-Ion (500mAh). There are some codes on the Li-Ion as you can see in the pictures but they took me to nothing. Can I suppose the charge voltage is 4.1V like other Li-Ion battery? – user19149 Feb 18 '13 at 23:47
• BTW that's not a big problem if I can effectively measure the charge voltage while the charger is charging the battery trough the phone, right? I will be able to answer to this question later. My doubts are on the circuitry of the battery. As you can see there are 4 contacts (those thinner, in the middle of the second picture, the other bigger 4 contacts at the end of the PCB are unutilized by the phone). They make contact with 4 pin inside the phone when the battery is plugged in. I have to find a way to get measurements while the battery is inserted... – user19149 Feb 18 '13 at 23:48
• About battery capacity my intention is to find the larger Li-Po that can fit inside the original plastic case, in order to insert the new battery pack inside the phone. Due space limits I think I have to stick to 1200/1300mAh. "Unless you use the original battery, your solution is unlikely to prove optimum, as far as extracting the maximum amount of charge, but this shouldn't be an issue." Sorry, I didn't understand this passage. English it's not my native language. What did you mean? U2 IC seems to be marked as: 58201 *9H – user19149 Feb 18 '13 at 23:48
• It was nitpicking really. The battery capacity is actually dependent on the amount of current you draw as well as how much you charged it. I was actually meaning that it might be possible that you won't be able to fully charge the battery or perhaps draw too much current, unless you design it with the battery at the same voltage. – Gustavo Litovsky Feb 18 '13 at 23:51