Urgent quick fix. Charging a FP3 battery with a FP4

Hi there,

I am looking for some technical advice regarding the difference between a fp3 battery and a FP4 battery pin assignment.
The longer description: we have a fp4 and a fp3. On the fp3 the usb port is broken and we can no longer charge the fp3 battery. As we are near the desert in Morocco, we can’t buy spare parts here and ordering is not an option as we don’t stay long enough in one place.
So I thought of making a connection between the pins of the charging port of the fp4 and the fp3 battery. But before I make something out of wire and tape, I would like to know if anyone has information if the batteries have a different pin connection as I want to avoid a short circuit or something like that.
Thanks for any help!!!

Do you mean the polarity of the batteries’ connectors? Those are indicated at the tops of their back sides (pictures from shop.fairphone.com, FP3 first / FP4 second – CLICK the pictures once or twice to enlarge):

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The inner pins could be thermal and ID. If you put the FP3 90° up on the FP4 the inner pins could be the wrong way if the FP3 battery and the FP4 battery have a different the same pin order. Without the inner pins it could be that the FP4 won’t start charging.

I would prefer another solution: take a charger for a camera and connect + and - and measure if it works. A powerbank with a small Liion battery, because then the charging current will be low, could be opend and used as a charger without the original battery.

I would charge outside a building, use a multimeter to measure the voltage and touch the battery if it gets hot.

Hi, thanks for your tips and answers, I think the main question is if I could charge it without the inner pins, or if I can connect them wrongly and damage some things. I guess as those are data pins, it should not damage the phone or the battery.

The idea of connecting a power bank directly is interesting, but I fear that the batterie will get too much power as I estimated that there is some charging logic between the usb port and the batterie within the FP.
Do you know of there is something to be aware of, he. A charging manager or current limiter, or is the current of the USB C port directly connected to the batterie poles?

The USB port (5V or higher) is not directly connected to the battery (~3,7V). The cut off voltage for charging Liion is about 4.2V, it depends on the chemicals of the battery. Some general Liiion chargers just load the battery up to 80% to make sure no problem occurs.

I cannot say anything about the inner ports. The schematics of the FP5 and its battery are published. Maybe that can help you for understanding. Any general Liion charger for single round cells would be quite good. Maybe you can buy or borrow it in the region.

Not sure if you already found a solution, but rather than trying to connect the battery to the other phone I would try to find one of these universal chargers. They only connect + and - to the battery
They surely will lower the life expectancy of the battery but at least you are not risking to damage the other phone.

Sorry for not really answering your question and in case my advice sounds dull, but does

really mean “broken” or is it just not working? In the latter case, did you try to thoroughly clean the USB port (maybe Morocco desert sand has entered there…?)?
Just asking… :wink:

Hi, thank you all for your great tips, it really helps me to solve the problem, even I am not there where I wanted to be. But it worked at least once. I connected a usb micro adapter with the plus and minus of the battery and could charge it a little. The connection was made with safety pins, tape and an orange twig. the usb cable was connected to a battery pack.

Unfortunately the usb port of the fp3 is really broken, some parts just fall out and a wire hangs out.
But with the charging method it could work if I improve it a little. The battery obviously does not need to have a connection on the inner two pins.

Thanks for all your help!