Then it’s definitely not a NTC thermistor.
It might be a ‘constant’ resistor (carbon, metal) which have a slight positive temperature coefficient, which mean it will be read as a temperature drop by the phone.
Since the coefficient is quite small relative to the NTC, even a very hot battery could show as a lower temperature. But I didn’t do any math around this.
However: I would not use it, unless the phone is in a steel sink with nothing combustible around it 
I use since today an original Huawei HB5N1H battery. Here is my test result. The battery works well. When charging, it was slightly warmer (40 °) but then cooled to 36 °. During the loading process I copied many data over the network and by the way phoned. No problem!
Does this battery fit very well in terms of thickness? Above it was stated that it is 5,6mm, which is 1,1 mm more than the original one. This may be a major disadvantage if the battery sooner or later tends to expand a bit…
See for yourself. The batteries are very similar in thickness.
Original FP1 battery on the right side.
Stefan: could you make sure that Daniel4 / Pippi etc in forum Fairphone1 maintenance -end, see this discussion and options and try to find some possible way to investigate an acceptable alternative for FP1 batteries, even as suggested have the FP1 users pre-order batteries to help with the financing. I know the the FP1 community would greatly appreciate this, as it better reflects the ideals/vision of sustainability of Fairphone. (also there appears to be some bad feelings/impressions of some users vs. Fairphone internal people, that is not justified and could be improved)
You can mention others by adding “@” before their names: @anon99326380 @Pippi
Links help others to see the context. For reference, @bxl_11 means this thread. I completely understand Fairphone’s point of view on this, as @anon99326380 explains it:
Yes, but it is saver if there is some space for the battery to expand, although these batteries for the FP1 are relatively weak.
Remember the problems Samsung created:
“When batteries are charged and discharged, chemical processes cause the lithium to migrate and the battery will mechanically swell. Any battery engineer will tell you that it’s necessary to leave some percentage of ceiling above the battery, 10% is a rough rule-of-thumb, and over time the battery will expand into that space. Our two-month old unit had no ceiling: the battery and adhesive was 5.2 mm thick, resting in a 5.2 mm deep pocket. There should have been a 0.5 mm ceiling. This is what mechanical engineers call line-to-line — and since it breaks such a basic rule, it must have been intentional. It is even possible that our unit was under pressure when we opened it.”*
(*https://www.instrumental.com/blog/2016/12/1/aggressive-design-caused-samsung-galaxy-note-7-battery-explosions)
That is why I was asking about how it fits in terms of height.
For reference, see the results of Samsung’s investigation in this Ars Technica article.
FP1 has indefinite space for the battery to expand (well, at least until the next room wall, if you are indoors). → Detachable back cover, not glued together.
I’d rather think, it may expand, because the cover on the back is just plastic… 
The cover on my FP1 is made from metal. I don’t know about yours though… 
Let’s stay on topic.
Habe mir jetzt bei Elektropapa noch einen Lifestock an vhbw- Batterien besorgt.
Ein seitlicher Platzhalter wird nicht benötigt, weil die Batterie anschlussseitig eine Raste hat, die ein seitliches Verschieben unmöglich macht. Die vhbw- Batterie ist ohnehin nur wenige Millimeter schmäler.
Die vhbw- Batterie ist allerdings etwa einen cm kürzer. Der Platzhalter besteht bei mir aus einem zusammengefalteten DIN A 7 Blatt. Damit sich das nicht auffaltet, habe ich Tesa drumgemacht. Bastelzeit etwa 5 Minuten.
Nicht so profimäßig wie das mit dem Neopren, erfüllt seinen Zweck jedoch genau so.
Einen Vorteil hat das gegenüber der Original- Batterie: Sie passt vollständig ins externe Ladegerät. Das ist wichtig, weil meine Mini- USB- Klinke einen Wackler hat. Wenn sich die Reservebatterien bewähren, lasse ich eine neue Mini- USB- Klinke einsetzen.
Einen Ladezyklus habe ich mittlerweile durch. Die Batterien kommen 64% geladen. Das Entladeverhalten (unterhalb 15%) ist gelichmäßiger als beim Original. Beim Original hat es von 15% nach 0% etwa 2 Minuten gedauert, von 100% auf 80% hingegen 6 Stunden.
Nach dem Vollladen zeigt das FP wieder 100% an. Die Kapazität ist naturgegeben geringer. Aber deshalb habe ich 6 Stück gekauft, die Teile sind auch leichter. Einwöchigen Touren in stromlose Gebiete (Kottische Alpen und weiter als 10 km Luftlinie zum Monviso) steht jetzt nix mehr im Weg.
Okay, all kidding aside: everyone should keep in mind that slightly thicker batteries as part of a DIY-solution may bare the small risk that expansion creates problems. Within a plastic body, this is may deform the body itself, but I am not so sure how the back cover of FP1, although being from metal and detachable, would behave.
I’m pretty sure the metal back cover would simply pop off, as it has with the bloated batteries before.
Thank you for the testing and advice. I can confirm that the Akku Huawei wie HB5N1H 1800mAh is working fine - cost 10 .- €, the community is a greater help than the Fairphone Company 
I have searched for this battery on Amazon.de and found it.
Yet I did opt for another one and not for the cheapest one. When I checked the description I choose one that at least mentions a protection against overloading or overheating (at 12,99 €):
The description of the vhbw battery at 8,99 € does not mention any such feature.
I have not checked the temperature readings, if they are working with my battery, but will do so soon (haven’t the phone on me right now).
But maybe there is no difference between those batteries and it’s just the advertising.
p.s. just forgot:
There is a “24 month replacement warranty for Akku-King rechargeable battery in case of defect!”. Of course that’s void, when using it in a FP1, but it shows at least some trust of the seller in the product.
by way of feedback, i got this 1500mAh version for all of 6 euro. had to do some fitting along the lines described above, after a few days seems to work fine, as in on a par with the original before it bloated:
https://www.ebay.de/sch/sis.html?_itemId=null&_nkw=ORIGINAL+HUAWEI+HB5N1H+AKKU+Ascend+G300+G312+G330+U8815+U8818U8730+T8288+HB5N1&_trksid=p2047675.m4100.l9146
i guess someone could sticky this thread, or even better just post a sticky with viable solutions in the forum.
thanks to everyone that helped!
I’m just installing this battery 
on my community FP.
It is a 1500 mAh battery found on the French FNAC website for 7,82€ including shipping. It arrived at 60% of charge this morning. I ordered it on monday 31 Jully, we are Wednesday 4 August.
The temper is said to be at 21 ° but in the room it is already upper.
A piece of cardboard is used at the bottom of the battery in the empty space.
Now the back cover fits well. It a long time since it didn’t fit anymore with my bloating battery.
I have no idea of any protection inside. I hope it is protected against overcharge. My phone is plugged most of the time as a wifi bridge for my computer.
Thanks @Stefan and @Micha2017 for the tricks.
More info latter.
I have ordered and fitted the vhbw battery shown by @Stefan above from this website on 31 July and it arrived (in Ireland) today 8 August: https://electropapa.com/en/catalog/product/view/id/11158/s/li-ion-akku-1800mah-3-7v-f-r-handy-smartphone-telefon-wie-hb5n1/?___from_store=de I found that the easiest filler I could produce was from a Stadler rasoplast eraser 526 B30 (https://www.staedtler.co.uk/en/products/pencils-accessories/erasers/rasoplast-526-b-eraser/) - easily cut to size. The battery came with 61% charge. Nothing very informative in this post! But I did wish to thank @Stefan @HermannMeuth and everyone else in this thread for your great help. Will let you know if there are any interesting developments.
Just wanted to share some experience with my Akku-King battery.
I have used it for some time now, sometimes wlan and bluetooth switched on all the time, somtimes just using the phone for the occasional call and for playing music. And I have just one sim-card installed.
When turning on all connections. the battery lasts about a day; when the phone is being in standby most of the time, the battery lasts longer than three days.
So the batterie should easyly last a working day and can be recharged over night.
Temperature control on the other hand must either be working fine or the measurement is not working at all, which is what I guess.
All the time the temperature is said to be 22-24 °C, which to me seems pretty unrealistic, even if the battery at no time felt hot or even warm.
It seems that the temperature measurement of those Huawei batteries is not compatible to Fairphone, as nearly all batteries in this thread report hardly any change in temperature. The only exception seems to be the original Huawei @Micha2017 has, although not the one @FrViPofm has installed.
Well, I just hope, that the protection against overcharging, overheating and short-circuit are working.
Samsung battery with working temperature sensor:
http://www.ppppiero.it/blog/a-new-battery-for-your-fairphone-1-howto/
