Three cell Lipo batteries for the FT-817 work really well, although not really sure about taking them on aircraft. The Lipos intended for use on RC Electric airplanes generally have a very low internal resistance and no internal protection against short circuit. My worry about the cells stems from witnessing a few incidents in radio controlled electric aircraft modelling, one of my other interests.
In my opinion, a better alternative for sensitive environments are the 18650 cells in a three cell holder for peace of mind. I did try these cells several years ago on the FT817, but cell capacity has increased since then if you select a reputable cell such as Panasonic or Samsung.
There are a lot of recycled 18650 cells on Ebay like the ones with the ****fire branding. An earlier post showed how poorly some of these cells performed. These cells are commonly recycled laptop batteries with a new label on them.
I had heard that the Samsung branded 18650 cells were good performers, so ordered 4 of the 3000mAh cells with a designator ICR18650-30B for $38 total. These cells have an inbuilt short circuit protection PTC and are supposed to be able to be short circuit indefinitely without rupturing according to the datasheet. The current limit is around 6 Amps as limited by an internal PTC, plenty enough for the FT817.
After arriving, they were put on the charger and straight away things didn’t look so good. For starters, the internal resistance was higher than the datasheet at around 150 mOhms when the datasheet said less than 120 mOhms. Also, they didn’t take a lot of charge in mAh to come up to 100% when the charger stated that they were initially 30% charged.
One other thing about these cells is that the datasheet says that they have a charged voltage of 4.3, not 4.2 like most other cells. This means that a standard Lion charger will not charge them to 100%.
A cell charged to 4.2V was then put on a constant current load of 700mA. Initially the voltage dropped fairly quickly, but then settled at around 3.6V for longer than I expected. After reaching 2.75V, the load was disconnected and at this point, the capacity discharged was 2500mAh, which wasn’t that bad I guess. I think that my charger is characterised for Lipo batteries and was incorrectly reporting the original charge level of 30%. The cell was charged up and the internal resistance this time was 96 mOhms, so maybe it takes a cycle or two for them to settle in.
The 3 cell holder had an XT60 DC and balance connector fitted.
Just to check it out on the FT817, it was run for about 4 hours and still had reasonable voltage. So far it looks pretty good.
For a charger, I found a multi chemistry one for less than $20 from Ebay. It has a selector switch for 1.2, 1.5, 3.6 and 4.2V. I tried it on NiMH batteries and Li-Ion types and it seemed to work fine. The charger terminated at exactly 4.2V for the Li-ion cells. Power input is 100-250VAC.
The_cells are shown in the FT-817. Although they fit by length and width, they are too high for the battery hatch to go back on. The cell holder is about 2-3mm higher. I left the hatch off and put the radio back the leather case.
After three activations in the UK recently, the cells held up well. One thing that is noticeable is that the cells drop fairly quickly down to 3.6V, but stay around this value for most of the discharge. The Lipo’s took a bit longer to drop to 3.6V.
For the most part in the future, the RC LiPo batteries will be used, but sometimes for sensitive environments, I will use these Li-ion cells.
Thought it was time to be a bit more systematic about characterising these cells. A cell was charged up to 4.2V (not 4.35V) and then discharged through a 0.66 Amp constant current load. The discharge was logged using an Arduino Mega and MS Excel using this great logger application. It didn’t take very long at all to hang it together and make it work. A couple of protection components were put around the analogue input 0 on the Arduino and then it was calibrated. See the graph below which confirms a little over 2.5Ah capacity on this load. If the cell was charged to 4.35 V as per the spec. sheet, it would be higher, but of course this is a non-standard voltage for most chargers.