Thundersky 60ah LFP batteries

As I understand yes, Lifepo4's do lose capacity and peak voltage at lower temps. The TS web site likely has graph for this. How do you like the TS-60's? Is your bike on evalbum? or ?

Phil

Yeh i encounter increased voltage sag at lower temperatures with my LFP40s.
normally when i pull 40-50A continuous, i get 57v or there abouts, but when theyre cold its more like 55.xv

sounds like a nice bike, luv to hear more.

Matt

Yup, just like the windshield wipers in a blizzard, the cold affects the batteries, and things slow down.

I've just become too lazy / coddled to ride in the cold. And, I'd like to add a request for more details of your conversion!

Thanks.

LiFePO4's do seem to be affected by cold temperatures to a greater degree than lead-acid. I have been running my 48 volt LFP40 pack since mid-summer and into the occasional sub freezing temperatures we are now getting. A normal resting voltage for a pack at about 80% SOC is about 52.8 volts (3.30 VPC) At 90 amps (2.25c) discharge rate and 80% SOC, I have observed about 49 volts at (3.06 VPC) at 90F (33C), 45-46 volts at 45F (8-10C) and about 43.5 to 44 volts (2.75 VPC) at about 27-30F (-2 to -3C). At this trend, I'll be hitting the 2.5 volt per cell limit, even at 80% SOC, at about 15F (-10C). But I've gotten too old to ride at such temperatures - it's usually snowing anyway.

As far as range, it is hard to tell since I rarely push the range of the pack, but it appears that range will not be impacted too much if one slows down to keep the voltage a safe margin above 2.5 volts per cell as the pack discharges. I would consider at least a voltmeter with 0.1 volt resolution that can be read while riding to be pretty much mandatory.

Which brings up another issue - whether the 2.5 volt limit is as all-encompassing as the manufacturer's imply. It is hard to believe that a fully charges cell could be damaged just because it's voltage sags below 2.5 volts at the high end of permissible discharge rate and cold temperatures. Recall that with lead acid cells the minimum damaging voltage is a function of discharge rate. On page 28 of the Thundersky manual, a table specifies a minimum voltage of 1.5 volts at -35C (hard to believe it would be usable at that temperatures), reverting to 2.5 volts above some unspecified voltag above -35C. So the actual mimimum voltage is probably interpolatable - although the upper point for the interpolation is unclear.

Which brings up another issue - whether the 2.5 volt limit is as all-encompassing as the manufacturer's imply. It is hard to believe that a fully charges cell could be damaged just because it's voltage sags below 2.5 volts at the high end of permissible discharge rate and cold temperatures. Recall that with lead acid cells the minimum damaging voltage is a function of discharge rate. On page 28 of the Thundersky manual, a table specifies a minimum voltage of 1.5 volts at -35C (hard to believe it would be usable at that temperatures), reverting to 2.5 volts above some unspecified voltag above -35C. So the actual mimimum voltage is probably interpolatable - although the upper point for the interpolation is unclear.

What you are seeing is the external voltage from the battery under load, and this is less than the cell voltage (less by the current times the battery ESR). The ESR increases with lower temeprature so you see a lower external voltage at the same current when it is colder.

However, I don't think the cell intenally is at 2.5V so you are not damaging the cell (if the no load voltage is 2.5V, then you are too low). If it is cold and you get 2.5V under load, what does the cell measure with the load removed?

I haven't actually hit 2.5 volts per cell. But the open circuit cell voltage is affected very little by temperature. In cool weather and more than 50% discharged the voltage will still rise under no load, after about a minute, to at be at least 3.2 volts per cell.

I don't know if I agree with your interpretation that the 2.5 volt limit refers to open-circuit voltage. It certainly doesn't work that way with lead acid cells where the open circuit voltage is a poor indication of state of charge.

The solution would be for Thundersky to publish performance charts like the ones usually available for lead acid cells - where a specific minimum voltage is specified for specific ranges of rates of discharge. But, as expected by their their Chinese origins, no such data will be forthcoming - Chinese manufacturers produce all this high-tech stuff, but seem to be clueless as to what the English word "specifications" means, and how they are presented. they also don't know what "quality control" or "quality assurance" means.

Except for the cold weather performance, I am very happy with my Thundersky cells, but my feeling is that the quality of this product is more accident than design.

I agree the Chinese do a terrible job with their specifications. I am only trying to "guess" how these cells really behave based on my experience with Li-ion (but they are different). And you are probably correct that OC voltage resting is a poor indicator of SOC; I am only trying to descibe the cell under load and who knows for sure...

"I agree the Chinese do a terrible job with their specifications."

Yeah. I see two extremes. Either the specifications are sketchy and ambiguous, or they are rigorous but unrealistic. Replacement contactor relays for my scooter came with a spec sheet which, the translation help from my Chinese boss, specified things like the relay had to be mounted within one degree of vertical, was not to be used at elevations above 2000 meters (what would elevation have to do with anything?) or below 0C.

Either way, the specifications seem to be written to prevent any warranty claims rather than inform the user of the suitability of the predict for an application.