Something to Consider Regarding Battery Degradation

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Well-known member
Feb 22, 2019
We know that our high voltage lithium batteries are sensitive to temperature, particularly during charging. So a good battery thermal management system is key to its longevity.

I was browsing through the 2014 service manual and came across this diagram.


This is the cooling/heating loop for the batteries.

Points of interest are numbers 10 through 18.
Briefly, temperature adjusted coolant flows through 10 from left to right.
It then flows to 14 where it is split into two paths.
One path, 13, takes the coolant to the lower battery pack, 18, where the coolant loops through the pack, and returns back to 13.
The other path, 15, takes the coolant up to 16, and then up some more to the upper battery pack, 17, where the coolant loops through the pack, and returns back to 16, and down to 15.
The two paths rejoin at 12, where the coolant that has passed through both packs goes through the charger, 11, from right to left.

In this system, the packs are not cooled in series, but rather in parallel. Parallel cooling is used by other manufacturers lithium battery packs, but what makes Fords different is that there is a pressure difference between the packs because the coolant has to flow uphill to the upper pack.

Ok, so what does this mean? At 14, the amount of pressure necessary to pump the coolant up to and through the upper pack is greater than to do the same for the lower pack. This means that the flowrate to the upper pack is less than that for the lower pack. Cooling and heating works better for the lower pack than for the upper pack.

If you were to have air bubbles in your coolant, they would have a tendency to hang around in the upper pack with its lower flowrate, while being pushed through the lower pack with its higher flowrate. Now let say you get a lot of air bubbles, and maybe because the battery pack gets warm, it makes those air bubbles expand. It follows that the pressure starts to increase in the upper pack, and this pressure restricts the coolant from being pumped up there.

The lower pressure path is for the coolant to go to the lower pack.

So to make a long story short, the upper pack will not get proper thermal management, while the lower pack will.

The solution is to make sure that there are no air bubbles in the coolant loop, and if any air bubbles are in the upper pack, they are removed.
It might be that the Ford Engineers put a restrictor on the lower pack so that the pressure to move coolant through it is the same as for the upper pack. But maybe they didn't.

I was thinking that possibly when the 2012 and 2013 year vehicles were being made, they did not do a proper vacuum air purge from the coolant loops, and over time, this affected thermal management of the upper pack, and caused overall battery degradation.

Additionally, if any year vehicle had a breach in the coolant loops and air got in, and that air was not properly purged out, this same situation could occur. I did a vacuum air purge of my 2017, just for shits and grins, a bit of air came out, and dropped the level in the degas tank. To my knowledge no service had been done to the thermal management system.

Like I said, something to consider.
Interesting posts for sure. I've got an engineering background, so it all floats my boat. Thanks for posting that. (I've had Bentley-publishing company Service Manuals for BMWs and other makes/models but am too cheap to spring for it on the Ford Focus Electric.)

Comments: Besides the trapped air issue you mentioned, the heat transfer engineers would have sized the coolant tubes, pumps, pressure gradients, etc., properly to get the right amount of heat control (adjustments) in the right places. Whether or not its the upper or lower battery masses, both were validation tested to work right; that is, to have heat controlled properly. If mass flow rates are different in diff parts of the battery system, it was on purpose, is my point.

The issue with the trapped air would have been anticipated too one might think. Although its a failure condition to be sure. Any engineer worth their salary would have thought of that.

Having all that watery coolant running through the battery, what could go wrong?!

Makes me think Nissan had the right idea on the 1st generation Leaf thermal management system using air to cool & heat the packs. I know they had trouble with them keeping up with the heat in Arizona & other super hot places, yet an engineer can just increase the size of the cooling fans a bit to get more mass flow rates (and hence heat injection or removal rates go up). ..... Nice to avoid all that liquid flowing through battery packs for obvious reasons, as in when they leak!
------ For example, TSB 20-2004 cites Ford's discovery of coolant leaks and they issued the TSB to lead repair technicians in that direction. I have a '16 Focus Electric in the shop now for battery and/or connector cable issues, so it could be the liquid junk leaking on to the cells inside there. Battery failure at Air cooling looks better and better as the best approach. Just run stronger fans.
Correcting my own last post, it turns out 2nd generation Nissan Leafs still don't use liquid cooled batteries. They are still air cooled. I think they don't want liquid coolant leaks ruining the battery and requiring warranty repairs. Ford has warranty repair issues over internal battery coolant leaks going on now as mentioned.

Some owners of Ford Focus Electric cars like ours have reported dangerous on-road "stop safely Now" messages and being stranded in traffic. I like Nissan's approach using air cooling. To beat Death Valley or AZ heat, bigger fans, more powerful fans, air conditioning (reverse heat pumps like the cabin uses) when needed, and/or bigger cooling conduction paths (fins, metal mounts, etc.) all would make for a leak-free, liquid-free system as smart Nissan engineers have chosen. for a nice summary, a little flawed but good summary.
Except that the majority of the SSN errors were not caused by coolant leaks. They were software errors. Once Ford had a patch (or two or three LOL) most of the SSN's went away.

Just about everyone else in the industry uses active cooling with coolant except the first generation Leaf. Especially if your modern BEV has fast charging capability. In that case simple fans aren't enough.

I've fast charged my Bolt (and a rental Bolt)--immediately upon plugging in just about every fan and pump kicks on in the Bolt to handle the heat load it knows is coming.

Personally my opinion is simply that Ford (and contractors) just made the FFE too complicated. Too many modules all talking to each other; all needing to be in a specific state for the car to operate correctly. If any one module had a bug or software problem the whole house of cards would come crashing down. This is likely due to the fact that it was their first fully electric car so they put sensors on everything and checks everywhere.
jmuellar, Nissan still doesn't use liquid according to sources I've seen on the subject.
Not just bigger fans, but more fins/metal conduction paths and air conditioning are being used now apparently. (Air conditioning is already present in these vehicles for the cabin, so it doesn't increase parts count to get some to the batteries, only sizing.)
One less failure mode (ask Ford about TSB 20-2004).

I know others pump a lot of liquid through the high voltage cells. Nissan gets my vote as long as they increase conduction paths & mass flow rates.
Well that is part of it: Larger fins also means the battery is bigger and there isn't much room where the batteries are under the floor.

Using coolant is an effective way to get the heat away from the batteries while still having a compact package.

Besides it isn't like car companies haven't been using coolant for, say, hundreds of years to cool things down..