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Thinking of switching from agm to prismatic

Yurtdweller

Solar Enthusiast
Joined
Nov 23, 2020
Messages
273
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USA
Hello. I have been off-gridding for years. I’ve been using AGMs, but my current 2pc bank is slowly dying and 250ah nameplate is really only producing about 60-80 ah. I’m considering going with 4x 180 prismatic, since (pre-covid) I travel to up to 5 different locations per year, and weight and space are issues. I am a sculptor/jeweler and I run a workshop full of tools, off a 2000w Inverter@12v. I would like to have 120ah per night and ideally double for bad weather. Eventually, I plan to go to 24v system, but I can’t buy the inverter right now. So, questions re lifepo4. If I buy 8 prismatics and configure for 12v, can I later reconfigure for 24v? Will I need a different bms for each configuration? If I buy 4 cells now, can I buy 4 more cells in a year, or would that be as bad as using a new and old agm together? I’ve lost my entire industry to covid, and am struggling financially, so I would prefer to scale up, if possible. One way or another, I need new batteries very soon. Thanks folks, and if there is a good source for prismatics near Houston, Tx, please let me know. #toddthecarver
 
Hello. I have been off-gridding for years. I’ve been using AGMs, but my current 2pc bank is slowly dying and 250ah nameplate is really only producing about 60-80 ah. I’m considering going with 4x 180 prismatic, since (pre-covid) I travel to up to 5 different locations per year, and weight and space are issues. I am a sculptor/jeweler and I run a workshop full of tools, off a 2000w Inverter@12v. I would like to have 120ah per night and ideally double for bad weather. Eventually, I plan to go to 24v system, but I can’t buy the inverter right now. So, questions re lifepo4. (1) If I buy 8 prismatics and configure for 12v, can I later reconfigure for 24v? Will I need a different bms for each configuration? (2) If I buy 4 cells now, can I buy 4 more cells in a year, or would that be as bad as using a new and old agm together? I’ve lost my entire industry to covid, and am struggling financially, so I would prefer to scale up, if possible. One way or another, I need new batteries very soon. Thanks folks, and if there is a good source for prismatics near Houston, Tx, please let me know. #toddthecarver

(1) Yes.
(2) Yes, but there is the risk associated with dissimilar performance. If you're not abusive to the cells, it should be minor if anything. While spendy, buying all 8 now would be preferred. If you stuck with 12V, it would be even less of an issue, because you could pair old with new on each cell group vs. having 4 new in series with 4 old.

Also consider the 280ah cells at the moment. I believe they are more cost effective.
 
why switch to 24v system... i thought about it but very little gains... i just ordered 8x 280ah cells and will just run in parallel. much larger i would understand to reduce cells in parallel .. helps and wire size and more solar on charge controler.. but need 24-12 convertor. so i stayed at 12
 
why switch to 24v system... i thought about it but very little gains... i just ordered 8x 280ah cells and will just run in parallel. much larger i would understand to reduce cells in parallel .. helps and wire size and more solar on charge controler.. but need 24-12 convertor. so i stayed at 12
With the batts I am using right now, it is hard to pull enough amps to run the inverter at peak. If I go to 24v, I won’t have to pull as many amps. Also, my current batts cannot accept many more amps, incoming, and if I switch to 24v, I can use the same cc, and double my panels. I have not fully researched the 280 prismatics, but I believe they get more amps in and out? A step down converter large enough to run my inverter would cost as much as a new inverter.
 
(1) Yes.
(2) Yes, but there is the risk associated with dissimilar performance. If you're not abusive to the cells, it should be minor if anything. While spendy, buying all 8 now would be preferred. If you stuck with 12V, it would be even less of an issue, because you could pair old with new on each cell group vs. having 4 new in series with 4 old.

Also consider the 280ah cells at the moment. I believe they are more cost effective.
Yes, I meant to say I was looking at the 280s. They seem like a screaming deal, even with adding the bms. And I think I’ll need a balancer, too? It’s just really hard for me to come up with enough dough to get 8 cells and $300 worth of hardware right now. I have not had a show(regular income) since March. I’m surviving selling on Etsy right now, but it’s $500 bucks one day, then nothing for a week, sometimes. I just don’t have $800 bucks lying around right now.
 
Do you have a volt meter? If you monitored your cells somewhat by manually checking them every once in a while and had a charge profile that kept them away from the bottom and top end you could get away with not needing a BMS. Balancers also aren't necessarily needed if you have some idea that the cells are staying close in voltage.
 
Yes, I meant to say I was looking at the 280s. They seem like a screaming deal, even with adding the bms. And I think I’ll need a balancer, too? It’s just really hard for me to come up with enough dough to get 8 cells and $300 worth of hardware right now. I have not had a show(regular income) since March. I’m surviving selling on Etsy right now, but it’s $500 bucks one day, then nothing for a week, sometimes. I just don’t have $800 bucks lying around right now.

You're likely having trouble delivering the current with ragged out batteries - they take a big voltage hit and crash.

Thinking budget-wise, if you went with 4X new 280Ah cells, they would have little trouble delivering 100A+. This is more capacity and current potential than you've ever had even when your original batteries were new. If you found it's not enough, you could stick with 12V and buy another 4 reconfiguring for 2P4S - 580Ah of 12V. This would also minimize issues with dissimilar performance as you'd have an old cell paired with a new cell on each group, (-)AB-AB-AB-AB(+) where A is one of the original cells and B is a new one. "AB" means an A and B in parallel with each other. The "-" is a series connection between pairs.

You could then get away with a single 4S BMS.
 
You're likely having trouble delivering the current with ragged out batteries - they take a big voltage hit and crash.

Thinking budget-wise, if you went with 4X new 280Ah cells, they would have little trouble delivering 100A+. This is more capacity and current potential than you've ever had even when your original batteries were new. If you found it's not enough, you could stick with 12V and buy another 4 reconfiguring for 2P4S - 580Ah of 12V. This would also minimize issues with dissimilar performance as you'd have an old cell paired with a new cell on each group, (-)AB-AB-AB-AB(+) where A is one of the original cells and B is a new one. "AB" means an A and B in parallel with each other. The "-" is a series connection between pairs.

You could then get away with a single 4S BMS.
Those were my thoughts re ‘scaling up’. That’s my favorite thing about building my system. I started with a single 100w panel and a cheap cc running my van battery, and just upgrading what I can as I go. And, yeah, I just realized that I won’t have the same limitations with fresh prismatics that I have with my old batts. I just watched Will’s video again on 4 cells and for the first time the one on 8 cells. Maybe when I sell my mc I’ll have enough to pull the trigger. 280ah should keep me happy for a while. (Every time I say something like that I come up with another reason to upgrade in a few months:0). I’d like to build a solar boat, and I think these cells would be perfect for that in a 48v config. I guess I need to get back to sculpting gold again:/
 
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