I love the idea.
Perhaps it’s not strictly related but I would also gladly welcome a better battery in a few years time, fitting FP5 and using all the newest tech one hears about 
#dreams
My thoughts behind it is the recycle ability of it, plus if a phone is intended to last a long time, which these are, a lot of us users are going to end up with spare batteries over time, if the battery design is different in say the FP6 and we upgrade, at least we can still make use of them for the new phone as a powerbank, but if the battery design stays the same, then havng a dual charger / powerbank we got spare batteries for both. :).
I don’t think so. Most users will only order a new battery if the old one is at the end of it‘s live cycle. Although I can understand your use case, a fair powerbank would make more sense in my eyes, than an external battery charger or bulky case for phone batteries.
There’s an interesting item named “SHIFTkeys 2” which I found at another fair phone manufacturer. It’s still in it’s pre-order phase but first operational samples are finished already and presented on youtube. It can hold up to two of their handset batteries in separate internal charging bays in case one would like to have more than just one main battery.
Also for their previous phone model they have designed a pogo pin adapter to fit the latest battery model into the older phones to prevent owners to end up without fresh replacement batteries for their aged phones.
Interesting design as I think.
Some of you mentioned using spare batteries as a backup. If you have any advice for me on transportation, would you mind sharing in a dedicated topic? Many thanks!
Sorry, what you say is incorrect. Samsung has produced and marketed far more widely the Samsung Galaxy Xcover series of phones. Companies buy them and issue them out on work shifts to employees. At the end of the shift the supervisor whips out the batteries and replaces them in bulk to the next shift. Look at the charger Samsung Galaxy XCover 5 4-Slot Battery Charger for UK – AIOTSolution
It may be overkill for most private owners, but it gives the context.
I have used a sliding pin charger that has a usb power input. The current limit is quite low, so it is unlikely to overheat a battery. It seems to work very well with a small solar panel. It might be advisable to glue the sliders when the positions are established if only using for one make of battery. However, I vary my batteries: sometimes charging Fairphone 4 and sometimes Samsung Xcover 5 and the positioning is slightly different. The reason I like having a spare battery is because I have been caught out on an emergency and the original battery died. My phone does not come back to life instantly when plugging in a powerbank - it seems to want 5% or more before it will switch on.
Here’s one i made earlier:
Quite a simple bit of kit, i’ll upload the files if anyone wants. Only thing about this chip is it charges to 4.2v and the fp4 battery is rated at 4.4v, so it will only charge it to 85%. Arguably better in the long run anyway and I just happened to have the IC lying around so that was what I used.
