Maximum charging voltage of FP3 batteries

Edit by @AlbertJP: These posts have been split off from the following topic: FP3 Battery inflated

I haven’t spoken to Fairphone but my battery reaches 4.4v when fully charged, (100%), which I try to avoid, the recommendation is 4.2v ???

Smart chargers, like smart people are often dumb and get it wrong.

The slow charge is only to stop a quick overload, leaving it on all night can create a slow overload.

The issue is more to do with the smart Power Control Chip than the charger. The PC Chip may continue to charge the battery. It communicates with the charger if it can, to decide what voltage to request dependent upon battery voltage.

It could be that the Qualcomm chip isn’t working optimally, ask Fairphone if that could be the case.

But would hot weather do that to a battery? That would mean the battery has to come with a warning in which climates it can be used, and I don’t think I heard of this before. This has to be warranty case.

When reading this as an electronics engineer I can not help myself, I have to leave a comment :wink:

Just as a reminder, which is valid for LiPo and LiIon-batteries:
The thing that really causes stress to these kind of batteries are situations when they are very full on charge or very low on charge.

The best idea for a long living battery are “flat cycles”: Avoid charging them more than 80%, avoid discharging them lower than 20%.
Or, the very best would be to cycle them just in a range between 40% and 60%.
Of course, this is just theoretical and nothing that people will do in daily life :wink:

To keep it short: The longer this kind of battery is kept next to an extreme level (high or low), the sooner it will die.

Your method - waiting for the discharge reaching almost 0% and then recharge it - is valid for the old-style-batteries (NiCd, NiMh, etc.), mostly to avoid the memory effect which is not a problem on LiPo and LiIon.

So my general suggestion would be to reconsider the way you are charging the battery in order to keep it working as long as possible. (*)
But of course, even in the way you are doing it, it should not bloat this way after you are just using it since October 2019. So, yes, ask for a replacement.

And please be very careful with bloated batteries! If a LiPo or LiIon battery feels sick and not comfortable, it sometimes tends to put that feeling to its owner - an experience you really don’t want to have :wink:

(*) For example I am using a bluetooth-speaker every day, charging it in flat cycles (around 30% - 100%, using it immediately after charging, so it stays not at 100% for a longer period of time). The battery is still working perfect after (I guess: more or less than) 12 years now.

Not really. All of these PMICs, containing the charging circuitry, are doing it in CC/CV-mode (the usual way these batteries are charged; limited constant current first, then decreasing current when the voltage reached the target value).
Both the PMIC in the phone and the protection circuit in the battery protect it from overvoltage (which would lead to fire and explosion VERY easily).

So, keeping the phone on charge all the time will not lead to “slow overload”. It will lead to a situation where the battery is at 100% all the time, in an extreme level, which leads to faster wearout.


Yes my poor use of words possibly, however your use of the words ‘faster wearout’ only happens for a reason.

My point is that charging at 4.4V, which is what is the norm on my FP3, is high and so prolonged charging at that voltage is effectively over-changing.

I am not aware of the charging algorithm and if there is either a float voltage of 4.4v or 4.3v etc or if there is an occasional toping up charge…

Image courtesy of

However given that you are an electronics engineer maybe you know how the Qualcomm Power Management works ~ I have yet to find details.

All these techniques - including USB-PD - in general are using higher voltages for the sake of lower currents. This way the PMIC generates less heat (which is a big issue in phones) and can provide more power in less time to the battery.
Just in a nutshell, things are a bit more complex in reality.

But I don’t think this is an issue in this case, as the thread starter mentioned he is not using Qualcomm Quick Charge, but standard charging methods instead (he calls it “slow charge mode”). Which is a good idea if he wants to keep the battery healthy for an extended period of time.

Usually the end-of-charge-voltage with most of these batteries is at 4.3 Volts. This can differ slightly by the used cathod material.
And don’t forget: In the battery is an additional circuitry included (Battery Protection Circuit) which will disconnect the battery from the terminals in case of over-voltage, under-voltage or over-current. This is realized by a FET, which has a “diode forward voltage” between drain and source.

Means: If you are measuring 4,4 volts on the terminals of the battery while charging, this is not effectively the voltage that is used. There is another circuit in the housing of the battery, which - by design - lowers the voltage a little bit.

We are getting a bit deeper than the thread starter wants it to be, I guess :wink:

But there are several (not just one) safety precautions done for charging in a phone. You can be very sure, that your battery is not over-charged, even you are measuring 4,4 volts at the terminal.
Maybe this is due to the mentioned voltage drop over the protection circuit or simply because they used a different cathod material.


I’m just bring the 4.4v issue to the front as that is what the onboard Service test shows ~ though to be fair it showed 4.399 last time :slight_smile:

Thanks OH! and I use a dumb charger USB2 5v

By the way as this is technically a cell not a battery could you point me to a reference regarding " another circuit in the housing of the battery, which - by design - lowers the voltage a little bit."

I still have a problem with the wording ‘over charge’ given the 4.4v I see


Lithium batteries can be safely charged to 4.1 V or 4.2 V/cell, but no higher. Overcharging causes damage to the battery and creates a safety hazard, including fire danger. A battery protection circuit should be used to prevent this.

Also when you say used ?? The issue is not about the battery/cell output but the standing voltage of 4.4 maintained by consistent charging.

Ah! maybe you are referring to something like this


And although the circuitry may limit the charge voltage the input and output are connected so both will be 4.4v

This number depends very much on the battery chemistry. There are a variety of electrolyte and cathode materials, and both have changed since the page you quoted was written in 2015. If the manufacturer of the cells in the FP3 has specified that 4.4V is the maximum charge voltage, there is no reason not to use it.


Yes I’ve just taken the battery out and it says
Limited Charging Voltage 4.4v

Thanks for your patience.

Do-it-yourself manuals like the one you linked are generally written for 18650 cells, with a liquid electrolyte and NMC cathode. The FP3 battery has a gel polymer electrolyte, and I don’t know what cathode it has. (The anode should be graphite in either case.)

For an 18650, there are many internet sources indicating 4.2V is the maximum.


This discussion is getting very off-topic now and will not help the thread starter in any way. (*) Just to answer very quick:

Right. But as most people don’t know the difference, i used the word “battery”. I can change to the correct terms as you wish.

This voltage is then connected on the battery pack including the battery protection circuit, not on the cell.

No, they are not. Look at the bottom part of your circuit. You can see the FETs, where the gate is controlled by the IC. And then consider the voltage drop between source and drain (which should be very, very low, but I don’t know the datasheet of the used component in battery pack of the FP3) and other effects.

And as @AlbertJP already mentioned: You don’t know about the exact materials used in this cell, so you don’t know about the exact charge voltage. If the manufacturer specified it to 4.4V, there is no reason to doubt it.

If you want, you can learn about the basics of charging by just getting some cheap modules from Ebay:

  • Get a module with TP4056, which is doing correct CC/CV charging.
  • Get a battery protection unit (often called BMS [Battery Management System], even when just used with one cell and no balancing is needed).
  • Get a standard 18650 without an included protection unit.

This way you can measure a lot and see how things work. But please be careful and always respect the limits of the cell.

For the thread starter: (*)
Just ask the support to get a replacement :wink:

(*) Not valid anymore as this topic has been split into another thread.


Thanks for this discussion and some good advice. It is indeed a problem that so many people continue to treat Li-based batteries the way they did NiCd. Not a problem for the battery manufacturers of course (nor indeed for those device manufacturers who make it difficult to change the battery, as most of them do nowadays) but definitely a problem for the planet.
One remark to @ontheair : I find that it’s quite possible and not difficult to keep the charge between, maybe not 40 and 60, but certainly 35% and 65%, most of the time. As I’ve said in another post, I have a timeswitch set to 40 min. controlling a pretty standard USB charger. All I have to do when I get home in the evening, if needed, is plug in the cable and push a button. All my Li-based devices get the same treatment and I’ve never had to replace a battery since adopting this approach.
I applaud your 12 year old Bluetooth speaker :stuck_out_tongue:


According to BatteryBot App, mine does too.

Much has been written about how to charge the battery (keep between 20 and 80 is what I’ve read in many other places, too), but I would believe however you use your phone, except for dis/charging the battery every day maybe, it should not inflate like this - given the phone is on the market for <2years (? guessing…) only. I’d rather assume a defect battery here.

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