Battery Cooling PSA

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Well-known member
May 20, 2016
The Valley of the Sun
Just wanted to offer some info, in case it saves other people some headaches.

If it seems like your battery is having difficulty cooling down - noticing the compressor cycling a lot during charging or pulling power far longer than normal during charging - have your coolant pumps and filter checked.

The coolant filter is located in a one-way section of hose right after the battery cooling pump but before the battery and charger module. If any pump gets damaged, it can fill this filter with debris that will impede flow. If the coolant pump for the battery loop fails, no coolant will flow as it is the only one that can move coolant through that section.

The vehicle will also intermix warm motor loop coolant to bring the batteries above 50F before charging, or run a PTC heater if there is no warmed motor coolant available and both would be similarly impacted. The PTC heater would be able to convect heat slowly since it it located lower than the batteries but it would be fighting radiant loss from all the battery surfaces and nearly useless.

Similarly, the chiller that would cool the battery when it's at or above 97F would be providing no benefit since the cold coolant would be stuck "outside" the loop. Even worse, the power for the AC compressor is being skimmed from the on-board charger output during charging, meaning that running it is actually adding more thermal energy into the battery loop. This heat would also convect upward since the battery charger is located at the lowest point in the battery loop. Though some will be making it's way toward the outlet end and some will be going up toward the battery.

In my case I can get the battery over 97F and get the compressor cycling but, despite the line to the chiller block being frosted over, the lines to the pump and filter are warm to the touch. Since the pump is turning on and the solenoid that engages the chiller is triggering, the only remaining explanation is a clogged coolant filter. Even in the cold of winter, I can get my battery hot enough to trigger reduced performance or "stop now safely" warnings but no DTC. Really disappointed that there wasn't some sort of flow check or at least a diagnostic condition where turning the chiller on and not getting a reduction in temp would trigger a fault.

Hell, after dealing with this for a long while, I ended up with a refrigerant leak at the chiller, likely because it was not getting warm coolant flow to offset the continuous duty cycle of the compressor and getting far colder than expected. When that happened, the compressor would try to cycle on, fail to reach working pressure, then shut off and try again 3-4 times a minute for hours. Sadly, that was happening while I was out of town and had left the car plugged in (as it tells you to do so to help keep it cool).

I've been putting off deeper testing of the CHAdeMO mod until I get this sorted out. Now if only the coolant filters weren't like $250.

One other thing. I was speaking with a battery/charger engineer I work with and he noted that Wide-Open-Throttle events are one of the most damaging for Focus battery packs. It's not like they will explode, but frequent WOT use will cause them to degrade far faster than they would otherwise. He drove his around the Silicon valley area but has a lead foot and lost about 15% of his battery in about 3 years. It explains why my ~4 year old focus was down 20-25% when I bought it. A year after I got my new pack it was down ~1%. It's been about a year now, so I'll be checking again soon.
Interesting fact about driving at WOT and battery degradation. When I had my 2015, I would often find that I would be on the speed limiter, else I would get run over in the carpool lane (obviously when there was no traffic, which was quite frequent when driving in to work in the morning early). When this would happen, I would usually just push the pedal to the floor and keep it there, even though I knew I could use much less pedal and maintain the same speed. In fact, I used to joke that the FFE was one of the few cars that you could regularly "floor" from a standing start and achieve the speed limiter in normal driving! When I sold my car at 38,500 miles, I had about 85-90% battery remaining, by my rough calculations.