Using 2 inverters for reliability

[ShirBlackspots]

I always thought it was a bad idea to mix batteries of differing capacities. Battery 9 has capacities ranging from 2200mAh to 2950mAh.

What inspired me to even start this project was that an cheap $300 HP laptop bought in 2014 refused to charge the batteries back in 2021. I break the battery pack apart, and they were four Samsung 18650's (ICR-18650-28A) in 2800mAh capacity, measuring between 2300 and 2400mAh with my battery charger (Its a cheap Chinese charger, so I think the mAh counting is a bit inaccurate, because it measures my 5000mAh BAK batteries as being around 4500mAh). I guess the battery BMS refused to allow a charge when the battery sat at a 0% state of charge for an extended period of time.

The replacement battery pack lasted only a year before the BMS in the battery pack refused to charge it (the laptop sat for months at a time not getting used). These batteries are some unknown brand, labelled as SZNS HO6N042FQ, 3.7V - 8.14Wh, 3 of them (they're supposed to be 2200mAh, basically these). Not sure what happened to the 4th.

Then there was the battery pack from my Milwaukee drill, that I took apart (I no longer had the drill, it stopped working). These are 9 Sony 1600mAh 10A 18650's. The battery I no longer have, I dented the top side trying to break the plastic holder together before I realized the internal battery pack holder had bolts holding it together, and since I didn't have any torx screwdrivers at the time, I used an appropriately sized drill bit to get the bolt head off.

I can probably make a simple 12.6V battery pack using these. I do have a simple 3S BMS (make a 3S5P battery for 100Wh), and a combined 16 of the 18650's (plus 4 in the newer HP laptop battery pack)

Also, a rather expensive mistake, I overheated a DeWalt 20V, 10Ah battery pack (these things are $200), using it with the drill. Two cells went open and would no longer hold a charge (no smoke came from the pack - but you could smell something). These are Samsung 50S, 25A continuous, 45A max discharge, 5000mAh batteries. I looked high and low for these batteries, and couldn't find them anywhere for sale. If I did find them, they were $20 a piece. That's why I went with the BAK batteries. They can be found on 18650 Battery Store for $4.50 now (on sale, normally $6.99).

So, in all, for the 18650's...
6 - 2400mAh
9 - 1600mAh

For the 21700's
7 - 5000mAh (Just tested one, and its completely dead, below 2.2V, the rest are good)

 

[OffGridInTheCity]

I always thought it was a bad idea to mix batteries of differing capacities. Battery 9 has capacities ranging from 2200mAh to 2950mAh

The short answer is 'it's OK' but we're way off base for this thread. Building batteries with individual, 2nd hand cells is a genre of it's own and not really mainstream for this forum. Suggest looking at youtube channels such as early @jehugarcia, early @HBPowerwall, and DIY Tech & Repairs (old but relevant for 18650) as some starting places with good/correct info - they (and few others) are what I based my work on.

 

[TK1979]

Yes. Each inverter powers it's own distribution panel (with regular breakers) out to circuits. So each inverter powers a different set of circuits. However, the panels are cross connected so that by flipping breakers either panel can power the other. So a single inverter can power both distribution panels - and all the circuits - if need be. Of course if all loads are on at the same time it will overload a single inverter (that's why I have 2 inverters) but that can be managed by serializing use or disabling some of the circuits (turning breakers off) during the emergency period.
View attachment 204309

This is brilliant!

 

[Hedges]

This is my battery. That's an 8AWG wire there on the main negative side of the battery. I'm using 0.1 x 10mm nickel strips that are double stacked. The interconnection between + and +/- and - are two strips of .1x5mm. I'm using 0.1 strip because my spot welder was having difficulty making consistent spot welds with 0.2 strips. I am completely redoing the strips, pulling off the 0.2 strips (if I can) to put the 0.1 strips on.

Solid strips tying those cells together.
Concern would be a shorted cell getting force-fed current from all the others.
A known or anticipated problem, and strips with individual fusing per cell is available.

LiNMC 18650 in 7S build idea

Ok, I have a TON of 18650’s and the typical 6S Tesla uses is a weird voltage, 7S puts it near inline for my 24V inverters…29.4V max, and 21V minimum I ordered a pair of 7S BMS boards to tinker with when hey arrive, I’ll post results.

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