BMS

[Pi Curio]

I'm looking into converting two of my 12V 100Ah batteries into a single 24V 100Ah battery and I have a few questions about it. First time trying to do something like this so would greatly appreciate your help.


My plan is to open two of my 12V battery cases and pull out the whole assembly, then remove the BMSs, connect the two cell packs into series, and wire everything back to a single 8S JK BMS.

1. I saw a couple of DIY lifepo4 builds and noticed that there were cell terminal busbars of equal length across the battery pack.
With that said, can I connect the two cell packs in series using the cable as shown in the image above?

2. Am I on the right track with this BMS for a 24V 100Ah LiFePO4 battery? Also not sure does it come with the main cables or if I need to buy these too.

 

[TacomaJoe]

You are on the right track. As noted, you want the jumper to closely match the other intra-cell jumpers. I like the 24V BMS idea. I'm not a fan of series connected batteries with separate BMSs.

 

[mikefitz]

The BMS will be supplied with the sence wires that connect to the cells but you will need to add the crimp terminations.
Exactly how to connect to the cells will be apparent once you open up the batteries. Be aware some batteries don't have bolted buss bars but have welded or soldered interconnections. Whilst this be a greater challenge it should still be possible.
Ensure the cable joining the two packs is substantial.

 

[HRTKD]

If the batteries are in good shape, consider selling them and starting from scratch.

Should I move up to 24v, I'll be looking at these BMS:

8 Cell BMS (24 Volt) – Overkill Solar

BMSs for 8 cell LiFePo4 battery assemblies.

overkillsolar.com

 

[Pi Curio]

You are on the right track. As noted, you want the jumper to closely match the other intra-cell jumpers. I like the 24V BMS idea. I'm not a fan of series connected batteries with separate BMSs.

Definitely, single BMS for 8 cells in series.

I already have one of the batteries that had BMS cutouts during charging due to cell imbalance, and now slowly bringing it up in voltage hoping the BMS will balance it out eventually.

One of the reasons why I'd like to get access to the cells for measurements, and later, be able to monitor them via BT.


Thanks for confirming the BMS choice is good for this project. This is the first time I had a look into their BMS datasheet and looked for one with 100A, 8 cells, and active balancing. The one I've linked seemed to have the highest active balancing current in the 100A continuous range and the lowest circuitry resistance in the chart but I had no way of knowing if am I on the right track.

Truly appreciated.

 

[Pi Curio]

The BMS will be supplied with the sence wires that connect to the cells but you will need to add the crimp terminations.
Exactly how to connect to the cells will be apparent once you open up the batteries. Be aware some batteries don't have bolted buss bars but have welded or soldered interconnections.

Interestingly enough, the image I've shared to showcase my plan was made from a screenshot from a teardown video of a single battery. With that said, these do have welded interconnections, I guess I'll just have to open one of the batteries to see what I got there to work with.

Whilst this be a greater challenge it should still be possible.
Ensure the cable joining the two packs is substantial.

Agree.

I think there might be several ways to go about connecting these two cell packs in series. Though, leaning towards using a busbar made out of the same material and drilling holes into existing ones to try and make the connection and ideally attain roughly the same distance from cell-to-cell terminals.

 

[Pi Curio]

If the batteries are in good shape, consider selling them and starting from scratch.

Should I move up to 24v, I'll be looking at these BMS:

8 Cell BMS (24 Volt) – Overkill Solar

BMSs for 8 cell LiFePo4 battery assemblies.

overkillsolar.com

Selling these two batteries would be the best route, just not sure if there are any buyers here. Guess I can put it up for sale for a week or two and see.

Would make it much easier to upgrade the storage capacity if I buy a 24V battery and just add another one in parallel later.

Thanks for the link.

 

[Pi Curio]

Update:

After much consideration, I ended up ordering the JK BMS for this project. Would probably go with the Overkill BMS but didn't find any near me.

I guess, that makes me fully committed now to this 12V off-the-shelf to a 24V smart battery conversion.

I did try to look up if anyone had done anything like this before for some clues, but no luck. It's probably due to a number of good reasons for why not to do it such as losing the product warranty etc.

The way I see it, it makes total sense to make my system a 24V and undo some of my rookie mistakes that were made when I started will solar and just didn't know a lot about a whole bunch of things I needed to account for in my specific installation at my place, the whole nine yards of it.
If anything, it did well for sharpening my senses in making a well-thought-out system design for given solar conditions. Or at least I'd like to think it did.

Plus, these LiFePO4 cells will probably last at least 10 years, after nearly 1000$ in for the batteries thus far, and knowing what I know about solar now, the perfectionist in me just won't have any of it in less than most optimized system design for the given installation potential

So, the BMS is on its way here. That being said, the plan is to use this time to learn more about how to wire and program this thing and find out what tools I'll need to make it all work.


It does look pretty straightforward, but I do need to see how it pans out in practice so I'll keep my expectations low for now.

But if it does work, and works really nice, things will be looking great for further expansion with just a bit more additional investment into batteries as opposed to starting from scratch with a higher voltage one.

Time will tell.


To be continued...

 

[Pi Curio]

Update:

After I received the BMS, I opened the battery case of one of the batteries to see what I got in there to work with.

To my surprise, the assembly was significantly better than what I've previously seen in a few teardown videos dating only a few months before the purchase of this battery. The cells look impeccable, checked the manufacturing dates, and all is good on that front. This just made everything an even more straightforward process than expected.


For now, I've decided to use the JK BMS for a 12V battery. The idea is to observe how the BMS and the cells behave during several cycles, charged up to 14.2V-14.4V, discharge under heavy load, and then repeat.

All in all, satisfied with the results thus far.