I am using Panasonic NCR18650B protected cells. They fit perfectly in both of my Devterms and give me about 4 hours of use. I ordered them in April shortly after placing my pre-order for the Devterm and pickings were slim then. I had a tough time finding anyone that had stock.
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That’s interesting, do you happen to have a pair of calipers to measure the diameter? according to a NCR18650B datasheet I found the approx dia is 18.9mm but that would be too large to fit in the compartment. Along similar lines nightcore states the approx dia of the NL1834 3400mAh batteries I have is 18.7mm, but in reality it’s much closer to 18.4mm and yet they still don’t fit in the compartment. Confusing 
You are lucky, I was able to find my calipers. They are mesuring 0.725 Inches (18.41 mm) in diameter. I am not sure what to make of that if the datasheet says elsewise. I orderd them from www.ordtronics.com.
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Ok I feel completely stupid. I was actually able to get the batt cover fully closed on my devterm with the nightcore NL1834. From @packmule43 measurement I figure it must be possible. You sort of have to fiddle with it and make sure the batteries are seated as deeply as possible. It’s an extremely tight fit but it works. Anyway happy how this worked out 
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Glad to hear that you got it worked out.
This is a misunderstanding.
These are to protect the DevTerm not the battery.
Also how does the devterm would “protect” your battery when they are not in?
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I purchased Samsung 25R 18650 2500mAh 20A Battery from the https://www.18650batterystore.com
they are not button tops
2500mAh, 18.33mm x 64.85mm…(Not protected!) I bought them due to their dimensions - 18.33 mm slightly larger dimensions are available - but to get protected cells you get to a diameter of ~18.6 which causes the door to not close - I purchased an external charger and also cell protectors for storage.
Panasonic NCR: * Model: NCR18650B
- Size: 18650
- Style: Button Top
- Chemistry: NCR
- Nominal Capacity: 3400mAh
- Continuous Discharge Rating: 4.9A
- Nominal Voltage: 3.6v
-
Protected: Yes, PCB <—Protected!
- Rechargeable: Yes
- Approximate Dimensions: 18.90mm x 69.41mm ← 18.90 mm diameter!
- Samsung Model: INR18650-35E
- Size: 18650
- Style: Button Top
- Nominal Capacity: 3500mAh
- Continuous Discharge Rating: 8A (2 Mosfets / Protection cut-off 10A)
- Nominal Voltage: 3.6v
- Protected: Yes
- Rechargeable: Yes
- Approximate Dimensions: 18.65mm x 69.25mm ← little smaller…
I just ordered the Samsung types - since they have a slightly smaller diameter and are protected - I will continue to charge my unprotected cells in my nifty charger till I get the protected cells - based on what I can tell protection is a current limiter in case the cells are exposed to a direct short…
This is more dangerous than charging them in the DevTerm itself. The only time unprotected cells are at risk is when they’re not installed in a device; when they’re in the DevTerm, they’re protected by the DevTerm’s battery management system.
Instead of taking them out and charging them in an external charger, just connect a USB charger to the DevTerm’s USB Type-C port and they’ll charge in-device - and there’s no risk you’ll accidentally short them while you’re popping them in and out.
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There is also the risk of getting the polarity wrong… I have a burn scar to testify to that - incident with a 20 volt cordless drill pack and buck boost controller converter …
I bought some NiteCore NL1832 (3200mAh protected) from nitecore.co.uk who I believe are an official UK distributor.
Batteries fit fine into the case, I’m not sure getting them out will be an easy task (length is a tight fit) but I didn’t have any troubles fitting the cover back over the batteries.
As for charge / run time I’m not sure at the moment as I want to be around when charging for the first few times in case of a fault.
These Samsung are snug length wise as well cover just closes - and they came mostly discharged put them in a smart charger before installing them in the unit.
I purchased Orbtronic 18650 3500mAh Protected (Panasonic-Sanyo) Best 18650 Battery For Flashlight 3500mAh (Free Battery Case) Orbtronic
Dimensions are good
- Diameter (mm): 18.6 mm (+/- 0.03)
- Height (mm): 68.9 mm (+/- 0.03)
I had no issues with fit and was able to charge them fully from within the DevTerm. I have yet to test battery life.
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These are are on the upper end of how big you can squeeze in - but also the upper end of power. The Samsung ones I settled on are slightly longer and larger and the cover just closes. (but with less capacity…) Enjoy!
beeing in Germany I bought this Panasonic 18650 Li-Ion 3500mA Akku günstig kaufen | Akkushop
it is a Panasonic 18650 Li-Ion cell with 3500mAh (protected),
-Nominal Voltage: 3.7V
-Continuous Discharge Rating: max. 10A
-Dimensions are exellent 68.66x18,4mm +/- 0.2mm
…the price is pretty high
While testing battery life it looks like the Orbtronic 18650 3500mAh Protected (Panasonic-Sanyo) is not good after all. The DevTerm just randomly shuts off after some time. The batteries had between a 70 to 80 percent charge. The green power light is faintly on but screen just goes off. The power stays on fine for hours with batteries if USB-C power is plugged in so my guess is the batteries can’t supply enough power.
Just ordered a couple of the Nitecore NL1834
I am sorry to hear that, I have not had that experience. I also used these cells in a external charger / power pack to charge my phone. This gave the cells a few good discharge and charge cycles before I ever got my DevTerm. I have been getting 4 hours consistently with no issues. I don’t use the Thermal Printer below thirty percent battery because I have had the power drop out on me and the whole system shut off.
The protection on the cells is something orbtronic puts on themselves. The protection is there to limit current to prevent thermal runaway if the cells are shorted. It is possible that they limit the current to much for the DevTerm and I am just not pushing it that hard.
I am by no means a battery except it is just what I have been using.
I hope you have better luck with the Nitecore NL1834.
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@packmule43 That’s encouraging to hear. I just found my Nitecore D2 Charger so I will try a few charge and discharge cycles and see how that goes. I was a little surprised since I wasn’t pushing things hard with only a Terminal and a few Chromium tabs and the gearbox set to only run the A53 CPUs.
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