two sets of lifepo4's and BMS what to do????

[Panamaszr12]

I amms learning ton;s but still need more help so here goes
I just received my second set of CALB CA100's I was going 12v but changed my mind and went with 24v
So I found out after ordering the first set of 100ah batteries I needed a second set of 200Ah
I was able to connect with the manufacturer in china and got a killer deal on the 200's (they are on their way now) yes the slow boat lol
My question is do I need two BMS's a 100 & a 200 or could I go with a 300A bms????
Any help here would be great

 

[Craig]

How are you going to pair the 100's. With the 200's?

 

[Panamaszr12]

I'm running them in parallel 16 cells
8 100ah & 8 200ah

 

[Panamaszr12]

oh someone may ask more detail
I'm doing an off grid system in a RV I'm building
I used a calculator on line that determined my needs and load for my area, this is the reason for the second set

 

[Craig]

If you are running them in parallel you will need 2 BMS one fir each set. Unless you parallel the 100 with the 200 first then put in series. So you would have 8 300 Ah cells put into series and then you would need the proper size BMS fir your equipment. Based on charge or discharge loads not battery size

 

[Panamaszr12]

I'm confused. Are you saying to combine them as one battery unit. if so I'd have 16 batteries, meaning each 100ah is connected to a 200 ah to get the 300 ah your talking about???

 

[Panamaszr12]

I found where I can get two BMS's at a good price 24v, 7-20s, 320 Amps, with bluetooth to monitor them. So I wtionsill go with the two BMS's and avoid any complications. LOL

 

[Ampster]

I think the suggestion of @Craig makes the most sense electrically. I don't know if they are physically the same height and distance between poles to allow pairing one 100 Ahr with one 200 Ahr into a single pack.
Otherwise if you have two packs the currents between the 100 Ahr 24v pack and the 200 Ahr 24v pack will need to be taken into consideration. Are your discharge rates high or are you slowly discharging them over time?

 

[Panamaszr12]

Awe I didnt even think about the height issue. I'll have to look at that. My initial guess is they are of different sizes based on their capacity. As for loads, my highest discharge rate will be 1300W coffee maker that will only run 10min, and if I run my tv/sound system, and lighting that is next in line at 800W max. Everything else will be running on DC

 

[Steve_S]

Mixing different capacity cells into one pack will fail. A 100AH cell will reach full well ahead of a 200AH cell if they are discharged. Plus a whole pile of other issues.

Build 100AH packs & 200AH packs. They CAN be put in parallel and they will share the discharge load & charge amps. The real trick is to ensure they are properly paralleled and that the BMS' for each pack is setup properly. I answered a similar question earlier, which answers some of your questions as well. https://diysolarforum.com/threads/c...ry-modules-with-sperate-bms.9381/#post-100959

 

[Panamaszr12]

cool Thank you for the info I'll read it now.

 

[Ampster]

Mixing different capacity cells into one pack will fail. A 100AH cell will reach full well ahead of a 200AH cell if they are discharged. Plus a whole pile of other issues.

My understanding is the voltage will be the same as they charge and discharge because they are in parallel. Yes there will be currents between the larger cell and the smaller cell or the current will be shared at the common buss bar. I think the biggest factor influencing that will be a possible voltage sag from a high current drain.. The biggest issue I see is the physical size of the cells. In retrospect he could have ordered twice the number of 100 Ahr cells and buddied them in groups of three. It is too late for that now.
Assuming you buy my argument about equal voltage, what other issues do you see? How are those issues different at the cell level than at the pack level?

 

[Ampster]

I answered a similar question earlier, which answers some of your questions as well.

That was a good discussion of the pack issues. My earlier questions to you may be moot if the cells are physically different. I don't want that to detract from finding the OP a solution.

 

[Panamaszr12]

This has been a learning process for me. I started looking to use a 12v system so I ordered 4 100ah batteries. Then I learned I'd be better off with a 24v system. So I ordered 4 more 100ah batteries, same kind/place/manufacture. Then I learned about the solar calculator, and was told I needed more storage, ugh. so I found a killer deal on 8 200ah batteries. It was what I could afford, otherwise I would of ordered 16 and sold the original 8 100 ah batteries.
now I have contemplated selling the originlal 100ah batteries and just ordering another 8 of the 200ah batteries. Seeing the price they go for on ebay I could easily sell them for less than what I can get the 200ah for, but once again I'd be waiting another month or so to get them.
I will have 800W at 37v of solar to charge from, and my RV alternator to run on low light days. I may even consider getting a generator to. This vehicle will primarily be off grid, but I do have a system that is shore power compatible as well.
With all that said what are any and all thoughts. As for timing I am in the process of building my rv from a uhaul box truck. It is slow going as I had a tripple fusion on my back a year ago last december. I am only able to work a max of three hours a day, and am not near as fast as I was before surgery, double ugh.
My goal is to have it built to the point where I don't have to do any outside work like I currently am. I'm installing the insulation (4'x12'x3'' foam) yes I got a killer deal thats why the 3'' thickness lol, and doing the wiring and plumbing at the same time. I'm doing one wall ceiling or floor at a time, as my uhaul is the storage area because of the size of the insulation. Once that is installed I will have a lot more room to work.

 

[Panamaszr12]

That was a good discussion of the pack issues. My earlier questions to you may be moot if the cells are physically different. I don't want that to detract from finding the OP a solution.

I just checked, the two cells are of different dimensions. all calb cells
100ah (L*W*H):*67*215.5 (mm)
200ah (L*W*H):183*72*280mm

 

[Ampster]

With all that said what are any and all thoughts.

I think upgrading to 200 Ahr cells is the best long term move. At the price of Lithium today the extra capacity is small compared to the possible increase in cycles. That is a belief that is hotly debated so I won't offer it as an opinion based on fact.

I have always favored a single pack with one BMS but I have the grid as backup. There is some good logic for having redundant packs of the same size in an RV situation.