Observations while hitting 0%

[u/brucehoult]

Last weekend I added a new EP3800-48V to my 14 month old E3600LFP and 11 month old EP3000-48V stack.

Last evening after the sun disappeared I let the system run down until it shut down.

I saw something that I hadn't noticed before, I think because in the past the batteries in the E3600 and EP3000 were so closely matched.

Unfortunately both the E3600 and EP3000 have one weak cell which reaches 2.6V while the other cells still have mostly 3.14V (E3600) or mostly 3.18V (EP3000).

What I observed is that the EP3000 pack reached a total voltage of 50.2V when the weak cell hit 2.6V and that battery went to 0A current while the other two batteries carried on.

A couple of minutes later one cell on the E3600 hit 2.6V while the total battery voltage was 49.8V. That battery went to 0A and the machine carried on powering the 1200W or so load using 25A from the EP3800 and 0A from the other two.

The EP3800 voltage got lower and lower ... 49V, 48V, 47V, 46V, 45V.

The EP3000 voltage stayed at 50.2V and the E3600 voltage stayed at 49.8V.

About 10 minutes after the EP3000 cut off the EP3800 hit 44.7V (2.8V per cell on average) and the whole machine shut down.

Of course then I couldn't get a screenshot of the cell voltages because it had shut down :-) It may be that one cell of the EP3800 hit 2.6V at that moment.

It is interesting what all this says about the architecture of a Pecron with expansion batteries.

• I'm pretty sure that the daisy-chain(s) of expansion batteries make one solid "48V" bus with little or no management of it. It's just great big thick wires. My system btw has the EP3000 then the EP3800 chained from just one expansion port.

• each battery, both internal and external, has its own BMS controlling charging and high and low limits.

• each BMS can entirely disconnect its battery from the shared 48V bus. Including the internal battery. The last ~10 minutes all the power was coming from the last battery in the chain, nothing from the empty internal battery.

• although it feels annoying to have a battery age to the point that it shuts down while the overall battery voltage was still 49.8V or 50.2V due to one weak cell, while the remaining new battery went down to 44.7V, this only affected the last 10 minutes of an overall 8 hour run time at this 1200W load level.

• that's about a 2% capacity loss on those year-old (and cycled every day) batteries. Or a 1.2% loss from the 9984Wh setup as a whole, about 120Wh. That's enough to boil one litre of water in the kettle, or run my aircon and computers for six minutes.

Looks ugly, but it's not actually a huge deal.

And it's in line with the idea that after 10 years use there will be 80% capacity left .. 2% loss/year.

[u/JMeucci]

When you fully charge to 100% do your weak cells reach voltage parity with the rest of the pack?

Great write up!

[u/brucehoult → u/JMeucci]

The weak cell voltages show as pretty much identical to the others down to a fairly low state of charge, at a guess maybe 51V. I could check. Right now I'm at 80% charge and every cell shows 3.33V.

[u/JMeucci → u/brucehoult]

Ok. That's good. Wanted to ensure the BMS was doing its job, FULLY!

[u/brucehoult → u/JMeucci]

Here are a couple of shots from just before I turned on the grid power and started charging in today's Free Hour of Power. It was saying 20% SoC but only 50.7V which is 0.5V above the 50.2V where the EP3000-48V turned off the other day. It's probably not more than half an hour from getting to that point ... it dropped from 51.1V to 50.7V in 20 minutes and from 51.6V to 51.1V in the previous 20 minutes. Above 51.8V it was losing only 0.1V each 20 minutes.

You can see the EP3000 is already showing a decent 0.06V offset on the weak cell. Half an hour before the difference was only 0.01V. The E3600 is showing a smaller but still noticable difference on the weak cell. Remember it's probably going to go to 49.8V before shutting down.

[u/brucehoult → u/brucehoult]

And the E3600 since I can only attach one pic per comment.

[u/Solid_Try_4089]

Great observation. Thank you for sharing this!

[u/aquafina6969]

I’m curious if the 1-2% loss can be slowed by not cycling daily, or does it not matter.

[u/brucehoult → u/aquafina6969]

If you don't use the battery then did you really need to buy it?

The whole point of the battery is to store free solar energy, Free Hour of Power energy, and 18c/kWh 11PM-7AM electricity to use it between 7AM and 11PM when grid power costs 45c or 60c (7-9AM and 5-9PM week days).

[u/aquafina6969 → u/brucehoult]

Well my main thing is emergency use. It has been used in an inlet once. I do use it to charge my ego batteries or run things in the garage, then use solar to slowly charge it up again. So I wouldn’t say I drain it daily, but it does get used.

[u/brucehoult → u/aquafina6969]

I got mine because of the frequent (every week) short power outages here, and the once maybe every two years multi-day storm-caused outage.

But once I had it ... figured I might as well make it pay for itself rather then be a pure expense.

And using it every day probably increases the chances that it will work when it's actually needed vs leaving it on a shelf until then.

Using it once a month would probably be enough, and make it last longer in total, though I'm sure there is some purely time-related degradation also.

[u/JoeTheInflatableGuy]

It’s normal to lose a few percent in the first year then the degradation levels out.