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EarthX 680 puffed

Just a guess here but typically mosfets fail to the shorted state. It would be interesting to see if the BMS is allowing excessive charging current to the cells, thus causing them to overheat, thus causing the cells to puff, due to at least one failed mosfet in the BMS.
 
Usable capacity vs lifespan

The lifespan of lithium batteries including LiFePO4 is mainly affected by temperature, depth-of-discharge and storage state-of-charge. While the maximum voltage EarthX lists is 14.6v (~100% SOC at 25C), this is not the ideal charge/storage voltage for long lifespan.

The datasheet provides a few lifespan examples:
4000 cycles with 20% DOD @ 1C discharge, 25C
2000 cycles with 80% DOD @ 10C discharge, 25C

In the ideal case, the charge voltage would be regulated such that the current into the battery drops to zero between 80-90% SOC with charging disabled entirely if the battery is at or below 0C. This would require temperature sensing, coulomb counting, and a battery model to accurately estimate SOC.

My suggestions:
1.) Drop the maximum regulated alternator voltage to 13.8v which should correspond to ~80% SOC at 25C, and select a battery model sized to handle emergency landing with all desired systems active in the case of an alternator failure, accounting for the 20% reduction in capacity.
2.) Buy the vented style and keep it in the cabin. If the temperature is ever at or below zero, heat the battery/cabin before starting the engine or before enabling battery charging.

Here's an article with more detailed info.
https://batteryuniversity.com/index.php/learn/article/how_to_prolong_lithium_based_batteries
 
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I suspect that like me, many of us are somewhere on the left-side of the Dunning-Kruger Effect curve when it comes to "lithium" or "lithium ion" batteries.

Tesla, for example, apparently uses lithium-nickel-cobalt-aluminum (NCA) chemistry, whereas EarthX uses Lithium Iron Phosphate (LiFePO4).

Differences? Pros/cons? Not sure, except that knowing that all "lithium" batteries do not behave the same.
 
LiFePO4 batts seem to have the common "Lithium-x" constraints- avoid depleting below 40% and charging to over 80-some percent to get maximum life cycles.

Avoid hitting them hard with charge or drain when cold or hot, keep them away from extreme temps.

Can give 2000 design charge/discharge cycles- but I doubt GA use sees all of that.

They may not have the highest charge density, but they are lightweight per amp hour.

If they offgas or thermally discharge, flames over 1000 degrees are not a failure mode.

I'm an AGM replacement away from a 900VNT if the battery could have a limp mode rather than A BMS voting it off my island in the sky.
 
My suggestions:
1.) Drop the maximum regulated alternator voltage to 13.8v which should correspond to ~80% SOC at 25C, and select a battery model sized to handle emergency landing with all desired systems active in the case of an alternator failure, accounting for the 20% reduction in capacity.


Depending on how the BMS balances the cell voltage, will all the cells get charged at 13.8v?

If the BMS only limits max voltage of each cell, you could have a situation of 3.6v + 3.6v + 3.6v + 3.0v The last cell not getting charged at all.

If the BMS is smart enough to keep the cell voltages even at all time, you would get 3.45v for all cells.
 
Now that we know mosfets control the connection to the battery, seems a fairly trivial task for EarthX (or someone) to make the BMS such that it limits and tailors charging for max life regardless of potential alternator output. Could use PWM to achieve that. Seems strange that EX puts that limit on alternator output based on battery size, when the BMS could be smart enough to limit that itself. The mosfets in the photo are good for 100V and 180A each, so pretty stout little guys.

Same in the discharge regime: BMS could limit output based on battery internal temp or charge level. Also, since they already have a connection for a fault indicator, could have a 2nd connection to override the BMS on the discharge side if it comes to a flight-critical need for every last electron.
 
As I mentioned in another post their new 4amp standby battery has that capability and restricts charging levels. Perhaps that capability will expand to the starting batteries.
 
The ETX manual mentions passive balancing, so cell imbalance isn't much of a concern. Series-connected LiFePO4 cells stay quite well balanced on their own when in the middle of their SOC.

I suppose they could implement SOC limiting if they added an internal DC-DC buck with a separate charging terminal for the alternator. With only two terminals the BMS would need to disconnect the battery above some setpoint, but then all systems would be exposed to the questionable voltage regulation capability of the alternator without the benefit of the battery as a buffer.

It's much easier and more efficient to have the battery always connected (except to prevent fire) while ensuring it's in a voltage range to maximize life.
 
The mosfets in the photo are good for 100V and 180A each, so pretty stout little guys.

The current ratings are not real world without substantial heat sinking and proper PCB trace sizing. A typical electric golf cart controller that can do 400A will have 50 or so mosfets paralleled and attached to some massive blocks of aluminum. Typical inrush currents of a permanent magnet starter (the worst case) is around 350A.
 
Blast Tube

I installed a blast tube to the battery that was located closer to the middle of the firewall and was giving me higher temp readings. (Installed thermocouples behind the batteries to monitor the battery temperature just in case) Before installing the blast tubes was seeing temperatures in the 140 F range and once the battery light starting blinking indicating a high temperature warning. I'm probably going to add a second blast tube to the other battery just in case, it's simple and cheap. I've also started using "both" batteries for start-up to reduce the amp surge to the battery that was doing the starting. My B&C alternator and external regulator never exceeded 60 amps.
 
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