backchannel: tempting fate by contradicting Will Prowse

This post is about Will’s YT video “The Rules of LiFePO4: The 3 Most Common Causes of Failure and General Guidelines for Long Term Use”

backstory

I’ve been watching Will’s videos since he was, in effect, a homeless kid living in a ratty RV. He’s smart, honest, and deserves every bit of success he’s attained.

That doesn’t mean he’s always right. His lack of formal education and formal reasoning can sometimes result in his making claims that do not follow from the evidence.

The point in of this post is to identify where he loses his way or oversteps the evidence.

overview

@0:22 these cells are managed by a Battery Management System

Most are. Some are not.

@1:10 “No BMS = Destroyed Cells”

I understand the point he’s making, but cells can be destroyed with a BMS in place (even by the BMS), and cells can be fine without a BMS. It depends on how the owner treats the battery. To be fair, he does say that it’s possible to kill the cells with a BMS present.

I would argue that one way to get maximal longevity from LFP is to behave as if there were no BMS present.

1st cause - overdischarge due to storage

Agreed. If there are any loads present (even internal) the cells can be pulled very low.

2nd cause - Excessive Cell Imbalance

Cell imbalance itself doesn’t hurt the battery, assuming they aren’t so out of whack they are triggering overvolt/undervolt protections. It will affect apparent capacity since those protections will be triggered earlier in actual use. But that’s temporary; get the cells back in balance and it’s no harm, no foul.

@3:48 All you need to know to keep your cells balanced is you need to charge them up to 100% and keep it there every few months

Well, I’d say say “charge them up to the balancing voltage setpoint listed in the BMS documentation”, as there is no benefit to charging to higher voltage.

Or, better yet, treat the bank in such a way that it doesn’t get wildly imbalanced in the first place.

@3:54 people take this a step too far and they think they should charge up to 90% and not trigger the balance circuit and get a bunch of active balancers and they think their battery will last forever

This kind of statement is an example of sloppy thinking and/or expression. Let’s break it down.

@3:54 people take this a step too far and they think they should charge up to 90%

• SoC is not inherently tied to voltage except at the extremes.

and not trigger the balance circuit

• Passive balancing circuits are not triggered by state of charge; they are typically triggered by minimum cell voltage and voltage delta setpoints
• in some BMS the owner can tweak those setpoints to their liking

and get a bunch of active balancers and they think their battery will last forever

Really? “Bunch of active balancers” and “last forever”?

that is absolutely false

Yes, the straw man you just conjured is absolutely false.

@4:34 with LiFePO4 [charging to <100%] is completely unnecessary.

Nothing is necessary. But the literature is very clear that running in the “middle ~80%” greatly extends the life of LiFePO4, other factors held equal.

But again Will is conflating voltage and state of charge. The bank can be fully charged at voltages much lower than those that typically trigger passive balancing.

@5:00 the increased degradation rate you guys are so scared about is completely unnecessary. Also, people are scared to drop down to zero percent because they’re going to hurt their batteries. But again that is false. With LFP you can charge to 100% and down to 0% every single day without any issue.

You certainly can do that. Without any issue? The datasheet for my cells suggests otherwise. I encourage anyone interested in the topic to obtain and read the datasheets for their own cells.

@7:45 the best temperature is 70 degrees

Most cell datasheets specify 25deg C, which is about 77def F.

third cause: high ambient temperatures

While ambient temps are an important factor, here and elsewhere in the video Will suggests that heat from excessive C-rates is what’s damaging cells. According to the literature the C-Rate is implicated in electrolyte degradation and lithium plating. Blasting to 100% and holding high voltage appears to be quite bad for the chemistry. Again, refer to your datasheet.

conclusion

@10:06 as long as you follow my rules you’ll be set. Keep it charged. Really that’s the number 1 thing keep it charged.

If Will is the entity servicing your battery’s warranty then by all means follow his rules. If you want maximal life from your LFP cells then read the datasheet and base your use on that information.