14s or 15s 18650 Powerwall? - for MPP Solar MPI 10KW @ max 60V

Hello,

I am planing and actually building a powerwall with 18650s I got from old laptop and ebike batteries.

Actual state: I disassebled lots of both of them and harvested round about 5.000 cells, tested them and sorted them into 50mAh range, now ready to build up some packs.

I plan 240p or more exactly: 2x 120p in parallel for better maintenance.

Since month I plan to build a 14s system - because anybody does so when building a 48v system.

But yesterday I asked myself: why not 15s?
The inverter I ordered ( not arrived yet) is an MPP solar MPI 10KW which can handle 48v batteries.
In fact the charging voltage can be set up to 60v according to the manual.

What I've read here so far is that a usable single-cellvoltage range from 3.3 to 4.0 volts is recommandable.
So 15s x 4,0v equals exact 60v charging voltage.

15s would give me a bit more capacity then 14s and a better spreading of the current over more single cells.
my BMS could handle 15s too (14s - 24s and it's NOT a batrium)

Are there any negative points or something to pay special attention to?

And second: is this single-cellvoltage 3.3 - 4.0v OK or can I go lower without loosing cell lifetime?

Do 14s. It fits alot better. No 15s does not give you any more capacity and the inverter is best optimized for the 14s voltage range.


Going 15s you loose capacity you actually can use if you need one day since you cannot go to fully charged.

The larger range you go the more you loose in overall cycles yes. You have to decide wher eyou set that. Some go full some only go 3.8-3.6..... I have 4.0 as my max but can bump to 4.2 if i know i would need that exta one day. I have my normal cutoff at 3.4 but emergency is at 2.9.. With that said I RARELY hit below 3.85 since i have a big battery bank.

Generally speaking, it's usually better to not use an equipment at its Min/Max supported levels. Often, the efficiency and/or lifespan will drop towards those extremes. 14S is the safe and smart choice here.

in general I agree, but with inverters it's often the other way round - they work better in the range at their max limit.

OK but this is why I asked this question here.
I still have some time left till I will start building the packs,
but if no one can give some more pro arguements, I will go with 14s and a maximum voltage of 56v

When it comes to mpp do not max it.

I Still think that inverter works best at 14s. I have not tested efficiency but i know whats inside...

Stefanseiner said:

in general I agree, but with inverters it's often the other way round - they work better in the range at their max limit.

Click to expand...

All efficiency/voltage graphs I've seen so far indicate otherwise. The highest efficiency is reached around 40%~60% of max voltage, and then trails off a little when going higher.
https://www.researchgate.net/figure/Efficiency-curve-of-various-solar-inverters_fig8_258220930

Of course, this does not include losses in the cabling, which will be lower with higher voltage. So guess the sweetspot is probably somewhere in the 60%~90% region.