FP4: how to charge battery without phone

Hello, I want to buy the fairphone 4, there is still a problem for a removable battery, it is a pity that it does not sell a battery charging station, on ebay many sell this product, the problem is is that I do not know which one is compatible, which charging station should I buy in order to be able to charge a battery in parallel?


I have had similar thought with regards to having a backup battery. You can look here, but I don’t know how well it works External Battery Charger for FP3 - #2 by AnotherElk

Hi fonky Welcome to the forum

I have an issue with charging a second battery.

The battery and phone communicate and record the state of charge, the data of which is used to control the rate of charge. I’m not sure all this data is held in the battery which means that by swapping batteries it will interfere with the phones AI that manages the charging algorithm. This in turn may then effect battery capacity in the sort and the stats in the long term.

So you may want/have to calibrate the batteries now and then.

@amoun I’m not an expert by any means, but “record the state of charge” doesn’t make any sense to me. As far as I understand a battery’s charge state is simply determined by the voltage. So even batteries with different capacities should not be a problem. Since you not using batteries of different voltages.

I’m not so sure about that AI bums. I rather think the state of the battery is determined every time you charged a cycle. I’m not sure why this should confuse an AI. From what I understand replacing the battery does nothing of what you described

Battery State of Charge (Battery SoC) is calculated as an ongoing process by measuring the voltage and the current used. These two are multiplied to determine how many Watts are used. The wattage at various times (maybe every second) is then added to predict how long the battery will last at the current rate, which clearly changes.

This ‘rate’ is then used with other ‘data/rates’ to determine the approximate life cycle of the battery. This in turn will impact the charging rate as a battery that is depleted in terms of cycle life will be charged more carefully/slowly/less current. Hence the AI or algorithm that is used to charge the battery. Hence the ability to calibrate the AI for a battery as the data held may not be efficient or optimal.

On the voltage side alone. The phone uses Quallcom Q3 AI to communicate with the charger to decide what voltage can be used given a) the state of the battery (cycle degredation) or SoB and b) the SoC

So charging is not based on battery voltage but SoC

For example if a battery is under heavy load for a while then the voltage will be low but that does not mean the battery has a low SoC

So the AI has to work out what current, from Wattage used, the phone should be charged at.

The Q3 AI will, if connected to a Q£ compatible charger then decide what voltage to apply to the phone.

If the AI doesn’t communicate between phone and charger the default may well be 500mA at 5V ~ ie old PC USB so as not to charge the phone at a too high a rate.

The AI knows what is the best rate but if that cannot be communicated to the charger is does the default low, irrespective of the battery voltage.

A bit off topic and probably not quite how it works

See also


Just to sumarise.

The voltage of a battery drops immediately when a load is connected. The voltage will recover when the load is removed.

At the two moments just before removing the load and after the SoC charge is the same but the voltage isn’t. The AI allows the phone to be charged under any laod as it calculates the SoC and does not react to the voltage.

Can somebody confirm that he got a FP4 battery loading with an external charger and name which one it was? That would be great. Really sad there is no official one.

This one is a dealbreaker for me and keeping me from switching from my actual phone with replaceable battery (LG V20)

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