FP3 Battery Charging Basics

Charging the Fairphone Battery

I’ve written this to aid those who are querying the charge rates as problems have occurred with ‘slow charging’ that in cases indicated a charge may take 15 hours.

I have asked FP for software criteria to assess specific problems and am waiting for a reply

Energy efficiency of Li Ion batteries.

The following refers to the ratio between the power to charge the battery from the source to the power the battery.

In the Fairphone system loses are incurred in the following: (AC or DC adapters), (Cable wire size) and the (Charging unit in the phone) this may be 10%.

Further loses are taken by the internal reactions in the battery and more so if the battery gets warm as the warmth is power going to the battery which isn’t charging it. Figures for the battery charging efficiency can be at best 90%.

See Car Battery Efficiencies

Battery Charge/Discharge Efficiency
Li-ion 80% - 90%

These figure are 10 years old and efficiency may be far better?

For simple arithmetic I am going to ignore all losses , adjustments can be made later. I am also ignoring that as the battery nears a full charge the rate of charge will drop.

The FP3+ battery I have is rated as 3000mAh or 3Ah, to charge this requires for example 1A for 3 hours or 3A for 1 hour.

The rate of charge is controlled by the onboard power chip that decides the rate to charge and controls this by it’s perception of what is available.

The power chip may decide to charge at a maximum rate of 2A this would take 1.5 hours to charge, remember this would be if the efficiency was 100% wheraeas it is more likely to be 90%

However older USB cables cannot efficiently carry 2A and may actually burn out and cause a fire, so the power chip will test the cable to see if it is IF3 compliant. If it is it will try and communicate with the AC or DC supply adapter. If the adapter is also IF3 compliant it knows the supply voltage can be adjusted and will instruct the supply to up the voltage.

Uping the supply voltage.

Lets go back a decade or so. USB A cables were designed to carry a max of 0.5A at 5.1V. Now 5.1V is enough to run the power unit and give out 4.4V, the charging voltage for the battery, at 0.5A, but a 3Ah battery is now going to need 6 hours of charging, and more!

The IF3 system allows up to 20V at 0.9A to be delivered to the phones power unit. The power unit can then convert that to 4.4V at 4.5A

The FP unit only wants a max of 2.5A given the battery size and given the 4.5A available will charge at its fastest rate and take, with losses maybe 1.5hours to charge.

If the battery takes longer and the battery is new and healthy it is most likely to be a problem with the supply which could be either the AC/DC unit not being IF3 compliant or the cable not being IF3.

Other options would be a faulty power unit in the phone which is either inefficient or unable to communicate IF3 protocol due to faulty hardware or software…

Although software may be considered the root of the problem it is the least likely as it the most stable and common feature though it could be the software doesn’t recognise an obscure or outdated IF3 data transfer.

There are issues about a battery being very cold but if it is above 10C then it is not an issue.

My qualifications for the above is that I am a fully qualified radio electrician and have lived off-grid with solar panels etc for some 40 years, but I make as lot of mistakes in my unconventional lifestyle with my extreme philosophical thinking, so please feel free to criticise the above to improve it.

The best I get is a charge rate of 1% in 90 seconds which takes me 2.5hours to charge, more often 5 hours . As I charge directly from a USB source that provides 5.1V at 2.4A max I have work to do to understand how to up the charging rate. As there is enough power to charge in 1.5h, I have a ticket with FP to inquire how the phone’s power unit chooses to reduce the charge rate to some 1A. I imagine it is an IF3 issue that cannot cope with a 5.1v static supply. I will update this when I have more info.

Have fun, I am adding a second post that has links to relevant hardware and software parameters.

This is just a list of links:

  1. Information on the differences between USB ports for charging, includes information on supply and cables.

How USB Charging Works, or How to Avoid Blowing Up Your Phone - ExtremeTech.

  1. Please see this post on problems and dangers of charging at very low temperatures (below 0C)
  1. Information on usbC pin connections ~ a properly manufactured usbC cable can be inserted either way.


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