diy solar

diy solar

Mixed parallel batteries

J

JJJJ

Aspiring apprentice
Joined
Feb 25, 2021
Messages
1,105
When I entered this group, I debated the server vs diy type batteries.

I am finishing up the second diy pack. Both are 302 AH.

From what I have learned, different capacities in serial are forbidden, but readings suggest that if there are differences in parallel that it is OK.

The reason I ask is that I am considering adding a EG4 200 AH battery to the mix.

If I read things correctly, this should be OK. Am I correct?

Thanks
 
Yep. Quite a bit on the topic.

I found this interesting from Steve S:

"IF packs are greater than 100AH apart then you will have issues"

Given that, I might tighten the range a little on the 302s if I do this.
 
I have just such situations and able to observe directly through the entire operation. I have two packs of 175 & three 280's..

Charging: from 10% SOC to about 85% SOC charging happens quite as expected but after that things change as the Smaller Packs start to get higher in voltage (3.425+) as the smaller packs are still getting charged the deviations begin of course. But the larger packs start taking more Amps as the smaller ones increase resistance (being full) and start reducing amps taken. IF not carefully "tweaked" the smaller packs will start to cutoff as deviation increases as they are still getting BULK/ABSORB because the larger pack resistance (EndAmps / TailCurrent) has not been reached yet, as soon as that occurs then Float can begin with (Constant Voltage / Variable Current) which will then decrease.

Discharging: from 100% SOC there are no issues till you start to reach 20% and lower as now the Smaller Packs reach their lower state of SOC, they will start to pull from the larger packs across the parallel bus. In effect, the Larger Packs will attempt to keep the smaller ones at the same Voltage Level. This occurs naturally with Parallel Banks. The weakest / Smallest battery packs essentially rules as a limiter IF NOT ADDRESSED.

Addressed: that depends on the BMS configuration and its abilities.

Hope this is coherent & helpful (just started Mugga #1 after a really rough night...
 
Steve,

Thank you very much for your insight. Your knowledge on this topic greatly exceeds my understanding.

Given what you shared, might it be prudent to isolate the different capacities (by turning off the other) and running periodic full cycles to help reset things?

Perhaps this is far off the track, but I was wondering.

Thank you again for your help.
 
I have a 50ah pack in parallel with a 280ah pack and am getting ready to parallel in a 220ah pack. All the same chemistry. I place parallel jumpers at every point I can to keep them all at the same state of charge.
 
I am curious to why you are adding a third pack that is not the same size? Why not just get the same sized batteries?

I remember @Steve_S explained his reasons in an earlier post.

I do have two 25 ah battery packs. One is pretty closely matched cells in capacity, about 2%, the other is about 10%. Both batteries are set up in parallel and are lithium cells. I’ve been doing using this one to three times a week for about 1 kwh to 2kwh at a sitting with no issues. That mismatch was caused by me purchasing cells from two different vendors. Done over again, I would of ordered the number of cells I needed from the original vendor cause I knew that I was going to end up ordering 16 cells and not just 12. The original vendor ran out of cells and got ones whose specs were the same, but performed better than the first.

I have a bunch of DIY stuff that is a hodge podge of add ons and I’m trying to get away from that.
 
I started with 50ah battleborns and those couldn't even start my inverter without going into lockout. I added a prismatic 280ah pack and now I am jumping to cylindrical for a more hardy pack. The cylindrical pack will be removable to run a race car and cover starting, lighting, and ignition without the weight of an alternator. But the vast majority of the time all 3 packs will be in parallel running our house.
 
One of the batteries from a group buy was damaged during shipping. I obtained a spare but the specs were slightly different. I was intrigued with the possibility of purchasing from a US supplier so I don’t have to share my cc overseas. I don’t use PayPal, long story.
 
What is the best known practice of connecting these? A fuse and switch for each parallel string?

Or only switch on every string and a large T-class fuse rated for the total combined after the combination node.
 
Having the ability to isolate and take each battery bank off line is a great idea in case maintenance is needed.
 
Fuse every battery.
newbostonconst said:
Just did a quick search for a link to point you to what I am talking about.
https://www.amazon.com/Blue-Sea-Sys...locphy=9016899&hvtargid=pla-571733390118&th=1
Click to expand...
That fuse has a 3000 amp interrupt. There’s probably enough juice behind the battery to arc across a tripped circuit breaker.

On this page, Midnite has some Circuit Breakers that have 20,000 amps or higher interupt, which should be good

Category Products - MidNite Solar's

MidNite Solar Category Products.
www.midnitesolar.com

I’m not a fan of circuit breakers for large batteries. They sound good, but after you match a circuit breaker with a decent interrupt rating to an enclosure, it takes up a bit of room. A Class T fuse is not small, six inches with the holder.

Asides Class T, others use MBRF:

MRBF Terminal Fuse - 100A - Blue Sea Systems

Space-saving ignition protected fuse for 30 to 300 Amp loads. Must use with Terminal Fuse Block.
www.bluesea.com www.bluesea.com
 
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