I recommend the following battery: XTAR 18650 3500mAh 3,6V-3,7V Li-Ion (protected). The cells fit perfectly.
| Capacity | 3500mAh |
|---|---|
| Minimum capacity | 3350mAh |
| Rated voltage | 3.6V - 3.7V |
| Charging end voltage | 4.20V ± 0.05V |
| max. discharge current | 10A |
| max. discharge current PCB | 10A |
| Discharge voltage | 2,50V |
| Protection | present PCB |
| Positive terminal | raised (button top) |
| Diameter | 18.4 mm ± 0.30 mm |
| Height | 68.80 mm ± 0.50 mm |
| Weight | 48 g ± 0.01 g |
| Charging method | CC-CV |
I bought Nitecore’s NL1823 cells. They fit fine. Not sure how much run time I’ll get yet. But I’m not going to be on my Devterm for hours at a time.
Nitecore is an amazing brand, I have couple of products from them which are not batteries but everything has top quality.
Those cells im sure are a good choice 
Careful though, lots of fake claiming to be Nitecore. (not talking about the one up because I don’t know , but in general, nitecore is one of the brand that have a lot of counterfeit on the market)
Use this link for the batteries, I purchased this exact model and it works perfectly
It also comes with a charger block and a flashlight, is about, it comes with 4 batteries and costs under $25
For clarification, i am not selling this but only saying that this is the battery i bought for my device and it works fine
Due to one of our main-issues, when batterypower is not strong enough to power the system without crashes under the charge of 60 % here is a german website with some helpful information. I translated it and made some comments or marks. (Akkusicherheit - Steam Monkey)
Capacity and discharge current: you have to consider what you expect from your battery. As a very rough rule of thumb, you can first ask yourself: Do I want a long runtime or rather a high performance (i.e. a lot of watts)?
If the cells are designed for a high capacity (more mAh, i.e. more stored energy) **
most of us are looking for 3000-3500 mAh**
, it is usually the case that they can withstand lower loads. If too much current were to flow out of such cells at once, they would heat up considerably and the chemistry of the batteries could be damaged immediately or in the long run. This could cause the cell to short circuit. A shorted lithium-ion battery can “explode,” outgas, or begin to burn.
Other types of cells are built to withstand high loads (i.e., lots of watts). In order for them to do this, the separators within the cell are thicker and the battery does not heat up as much under load. However, because of the thicker separators, the battery can store less energy - so the capacity is lower. **
This is propably the right direction to go**
So how do you know if you want more endurance or power from your batteries?
You need to know approximately how much power (watts) you need for the devterm.
In addition, you need to know at what residual voltage of the battery your battery carrier no longer works, so your battery is displayed as empty. With most battery carriers, this residual voltage is around 3.3 volts.
To calculate how much current is drawn from the battery of a regulated battery carrier (with one battery cell), a rough rule of thumb can be applied. This rule of thumb does not yet take into account all contingencies, such as voltage collapse under load, but it does give an approximate idea of the stresses to which the cell will be subjected.
You calculate:
Set power divided by the lowest voltage of the cell at which your battery carrier still works, so for example:
50W : 3.3V = 15.2 amps.
Since the battery carrier also consumes power itself and the voltage drops under load, an additional safety margin of about 30% should be added:
30% of 15.2 amps = 4.5 amps.
All in all, for a battery carrier with one battery, it can thus be roughly assumed that 15.2 + 4.5 = 19.7 amps of discharge current will flow at 50 watts of power.
A look at the data sheet of the cell tells you whether you are moving your battery with the targeted power in the green range.
Our main task is** ** we have to look for batteries with maximum mAh in relation to the necessary /minimal discharge current!
We need to know how much continuous current the devterm requires including the high peaks of powering (for example by using the printer) before choosing the powercells. On the other hand it should be at least a protection feature included, that is monitoring the temperature of the cells.
ive bought two nitecore 3500mah with 8a discharge 18650 batteries from taobao, it costs me RMB 266 yuan.
its longer 5mm than other brand batteries, so its hard to put it into devterm.
and it has more than 3.60 voltages after running more 5 hours to power off.
Length of battery is the same issue I have with the Samsung batteries - the battery protection feature adds a little length for the protection circuits. I bought two sets of batteries but as hard as they are to replace I just leave them in the DevTerm and charge using usb C.
I’m curious about the performance of these. I’m tempted by the price, but in the pictures they appear to be 5800mAh, which is way higher than I’ve seen anyone else using.
How are they working for you?
The performance with and without a battery is about the same I’m at work right now but I look over your question and look at my batteries to verify what you’re asking about
Got 2 Nitecore NL1832 from B&H photos, and DevTerm claims to have a battery life of ~10h (estimated by the applet, speed mode 1 in the gearbox (single core, up to 720Mhz)). They fit well in the case.
I’ve bought those same batteries XTAR 18650 3500mAh 3.6V - 3.7V Li-Ion battery (protected) | akkuteile.de and I have yet to see the power run out if I charge them in the DevTerm.
I also have bought a charger Keeppower L2 Plus - 2-Bay USB Li-Ion Charger with Display | akkuteile.de and for some reason if I charge them in the charger the DevTerm only detects 50% capacity.
Since I’m a total noob maybe I bought the wrong charger?
Do those protected cells you linked actually fit comfortably in the DevTerm?
I ordered a pair of unprotected Samsung INR 18650-35E since I was afraid the extra size of the protection circuit may be too much to fit.
If you’re asking me: No: they do fit very tightly and it’s awkward to put the battery cover in and they are quite hard to get out, but they do fit and work fine.
But I can’t say they fit “comfortably”, so I had to answer “No” instead of “Yes” to your specific question.
Next time I would probably buy them without the knob at the + pole.
I wish I could use these, but way too long
Built-in charging port 
I ended up purchasing two kinds of batteries before I received my Devterm A06. Although I wasn’t really sure of the risk, I bought unprotected batteries since it sounded like they’d fit better and would be ok as long as they were a reputable brand and I was careful in handling them.
Both pairs of these work fine for me and are of comparable size:
(currently sold out)
Both charge in the Devterm, and also in an external charger, although after testing both it seemed it was best to leave them in the Devterm and charge while I was using it. Though the external charger seems to work fine, I had a few times where it showed fully charged and then when the batteries were put in the Devterm it showed they were at least than full charge. Seems like the device may maintain some kind of memory of the battery and it could get confused if batteries are swapped out?
While they fit in the case fine, removing these (and I expect any batteries?) was difficult since there’s nothing to grab onto to get them out, and care is required. So I put some tape around the middle of them, and made a “tab” out of the excess tape so I have something to grab onto and pull them out. Though to be honest, I’ve left the Sanyo in for a week or so now, and just keep charging in the device when I’m using it, and disconnect when I’m done.
Doesn’t seem like it’s possible to charge batteries in the Devterm when the device is powered off?
For any future revision of the Devterm, it would be nice if a ribbon/string of some sort was added that could sit behind the batteries and allow for easy removal. But my tape trick works too. 
Oh, and I get about 4-5 hours of battery life out of them. (Mostly compiling and testing things – not heavy emulator/game use yet except for testing, and not really any printing either though I have tested it too.) I tend to charge up near to full when I use it, and then next time disconnect and let the batteries drain until a little under 10% and then plug it in again. I’m not sure how low I can go, but I’m not convinced the device stays on or is usable past about 5%, so I start charging at 10% again just to be safe. And if you’re printing anything, I’d say make sure to have at least 50% battery or more as printing really sucks the power.
Running LG HG2s here as they are my preferred 18650 cell. Does the DevTerm only actually charge the battery while the system is powered on?
Sadly, it seems so, at least on my A06.
Strange: My A-0604 charges fine while switched off. It’s my usual method of charging, in fact: I rarely have it plugged in while it’s switched on.
To double-check, I just plugged my partially-discharged DevTerm into my benchtop power supply: 2A power draw at 5.2V while powered off, which from prior experience will gradually drop to zero once the batteries are charged.
Does the yellow charging LED switch on? Maybe its just that it is of while charging when switched off…
