Thanks so much guys, I'm a tech head and doing this for fun primarily, EE/electronics is my weakest area so what better stuff to "attack" to fill the gaps. Feels good to be part of a community where we listen, help and encourage eachother!
An 8S battery will only be as good as the weakest cell (+ any help from balancers).
Yep, clear on that. I just think that with having to jump thru extra hoops to charge when the batteries have been depleted will only magnify the effect of the weakest cell(s). Yea I can keep charging each battery at 12V individually, it's just a pain, and they will keep getting out of wack if I charge @ 24V instead thru both at the same time.
Do you have any info on the voltage of the lowest cell (likely the cause of first batter cutout) ?
If you can provide any info on this it will help make a call on what is best.
Not yet, if I decide to proceed with this project that info will surface. I do have "fully charged" voltages (although I'm redoing right now with 14.2V charging), the "superstar" battery is at 13.7v after at least 30 minutes of rest, the "lesser of the two" battery is @ 13.5v (which is not bad actually, just unmatched).
If you have 4 good cells and 4 mediocre cells, ranging them as 2x 4S batteries, will best cells in one, worst cells in the other, is not a bad solution either.
So that's what I got now, 2x 4S (I'm pretty sure there's 4 cells in each box), but yeah I could further optimize I guess.
Most people who have been doing solar for a while pretty much all started on 12v systems. Mainly because 12v is familiar and easy to play and learn with. You won't get shocked by touching the + and - terminals with your bare hands on a 12v battery. It's easy to go to 24v. A lot of car/RV equipment run on 12v or 24v. And it doesn't cost that much. If you don't ever plan to expand beyond 4 batteries total, then you'll probably do just fine. But ask anyone here, it's addictive. You want to keep adding more and more. More batteries. More solar panels. Bigger inverter. A second or third inverter. Just more and more and more!
Totally. I did make some embarrassing mistakes which could have been dangerous (to me at least) on a charged 48V system. This is the primary reason I'm on 24V for the time being (and the fact that 36V doesn't seem popular, with few products — I'd be on 36V if it were).
But most people who started out at 12v, included me, regret not going to 48v sooner. I wasted a lot of money adding more 12v batteries and then switching to 24v equipment, and in the end finally decided that 48v is the way to go. So all the 12v batteries, 24v batteries, 24v inverter, etc are pretty much useless for a 48v system. Yes, you can put 4 12v batteries in series to make a 48v battery, but it is not ideal.
Totally, trying to learn as much as I can, and get experience and develop safe practices, on the 24V system before building my own 48V at some point w/ prismatic cells.
Now I have a 48v system and very satisfied with my solar setup. I also charge my Tesla and a Nissan Leaf with it.
I just charged my Model Y LR last night off this system until one of the batteries cut out! It worked great, and I will be repeating the test with some better cables w/ less voltage drop (due to having a 13A extension cord, I was limited to 12A). Inverter thankfully has a 120V setting so hoping I can push 15A at least (mobile connector will do 16A on proper circuit & voltage). Still like watching water boil considering I have a Wall Connector which will charge the Model Y @ 11.7kW+ on a good day (read: middle of the night when I'll get upwards of 245V @ 48A because there's little load on the grid), but seeing that juice come out of my very first off-grid setup is exciting nonetheless!
We suggest that your battery packs are "Native" to the rest of your gear... Meaning if you are running 24V Inverter then use 24V Battery Packs. Many folks do bodge together 2x 12V in series to make 24V but inevitably they hit problems that only escalate over time... There is an extensive group of those here... Batteries in Series can fail & most terribly in the worst cases.
Yea, I'm hitting some of these problems early on. I knew I would, I researched it, but still decided to take this path as my first adventure to learn. As someone who likes to optimize, it's definitely hard to even think that they're not native
Can you change those two 12V into a 24V ? YES
It would require removing the cells from the 4S packs and set that up in 8S config (see one of my diagrams below)
Replace the BMS with an 8S 150A BMS *** It depends on the cells and what they can handle... We would need to know what cells are within those 12V packs. If cells are 100AH cells then a 100A BMS is needed.
What kind of BMS should I be looking at? I would think each battery has 4 150Ah cells since each is 12.8V 150Ah, unless there are some small cells and it's 2P4S — I hope this isn't the case as I don't want to string together 16 cells for a 24V battery.
Unfortunately, the 12V steel boxes won't be of much use.
Bummer, they're really nice cases. Though transporting the battery with a modified case using the original cases would likely prove problematic.
Something to Ponder: The general recommendation is to NOT Exceed 250A Draw from your battery system for a single inverter system.
12V X 250A = 3000W or 3000W ÷ 120V = 120V/25A (240V/12.5A) *
24V X 250A = 6000W or 6000W ÷ 120V = 120V/50A (240V/25A) *
48V X 250A = 12,000W or 12,000W ÷ 120V = 120V/100A (240V/50A) *
Got it. Right now I have a 3000W 24V inverter that will do 120V and it ran great yesterday with my Tesla Model Y as a test load, almost 3 hours @ ~1450W continuous. I'm hoping to be able to repeat the test (this is primarily a test of the inverter, with which so far I am impressed) with slightly higher continuous draw closer to ~1800W. At 1450W the draw from the 24V battery was between 60.9A and 62.3A, as voltage dropped.
