Battery Banks with Different Ah and Discharge Rate

[jledwards0117]

I currently have a bank of 3 paralleled 12V 206ah batteries, each with a 100ah discharge rate. I want to add to my capacity by adding 3 12V 300ah batteries, each with a 200ah discharge rate. All batteries have a 12.8 nominal voltage.

While the current bank is in a traditional battery to battery parallel connection, I am also planning to change this and have each of the batteries, new and old, be connected directly to a Lynx Distributor/Lynx Shunt setup.

Any concerns with this proposed configuration, owing to the differing capacities/discharge rates?

 

[Zwy]

Discharge will be rated at the lowest discharge rating on the BMS's, in your case 100A discharge. It should not be a problem as any loads will be split between the 6 batteries in parallel. A 200A load would be just over 30A per battery. The biggest problem will be keeping the 2 banks in balance. Cable size and length will need to be the exact same. This is important.

Different capacities will not have any effect.

 

[stromy]

The recommendation is to only connect together LiFePo4 batteries with the same type, model, capacity, age (some say even batch). And they should share a BMS. So if you use different types/capacities of batteries, use a separate inverter for each set.
If you connect different batteries, you have a big chance of balancing not working, and of uneven usage. That will stress some of your cells into a reduced lifetime; or frequent protection events.
But anecdotally some people are connecting different batteries, and have no problems.

 

[JJJJ]

If you look at some of the older threads you will see that there are others who have combined different capacities in parallel and things have worked fine provided they are the same voltage. One member has been off grid for years and has contributed considerably to this forum over the years.

On the other hand with a DIY battery same capacity, manufacturer and resistance is strongly recommended within each battery.

 

[Zwy]

The recommendation is to only connect together LiFePo4 batteries with the same type, model, capacity, age (some say even batch). And they should share a BMS. So if you use different types/capacities of batteries, use a separate inverter for each set.
If you connect different batteries, you have a big chance of balancing not working, and of uneven usage. That will stress some of your cells into a reduced lifetime; or frequent protection events.
But anecdotally some people are connecting different batteries, and have no problems.

I have 4 batteries paralled inside the cabinet of different models of cells and capacities plus different ages. https://diysolarforum.com/threads/the-big-blue-box.55099/ All are connected using a solid main copper busbar and all busbars/cables connecting batteries to main busbar are the same material and length.

If what you claim is true, then I would never have balance. However, I posted this the other day and this setup has been in operation for some time.

Voltage is the same across an entire bank if there is not any extra resistance. Resistance comes from connections, wires, busbars. If resistance is the same for each battery connected to a main busbar, the entire bank will maintain balance. It doesn't matter on charge or discharge as long as resistance is the same. This is why it is important to have cables the same length. You can't have one pair of cables 2 feet long and the other 10 feet long. The resistance in the cables creates a voltage imbalance.

There are finer points to attaching batteries to main busbars and there is a thread where it is explained why/where to attach a bank of four batteries to ensure the bank remains in balance. In my case, I have four batteries, the 4/0 negative cable is attached to the midpoint between batteries 3&4 on the main negative busbar, while the 4/0 positive cable is attached between batteries 1&2.

 

[Ellcon123]

Parallel any capacity as long as same chemistry and cell count. They will balance themselves, if required, when they fully charge. Many older threads on this topic.

 

[jledwards0117]

Thanks to everyone for the replies. Great information, as usual.

What about my plan to connect the 3 older and 3 new batteries using Lynx Distributors? My thoughts are to run equal length cables from each battery to one of two Lynx distrubutors linked together and joined to a Lynx Shunt and then to another Lynx Distributor for the loads and chargers. The left-to-right configuration would be: Dist1 (batteries) -- Dist2 (batteries) -- Shunt -- Dist3 (load/chargers).

Any suggestions? For example, on the order to connect the batteries to the distributors?

Thanks in advance for all your advice and expertise.

Jim

 

[Buddha_]

Parallel any capacity as long as same chemistry and cell count. They will balance themselves, if required, when they fully charge. Many older threads on this topic.

Even with different BMS?

 

[JJJJ]

Yes. I have two batteries with one type of BMS and a third with another. They are working well together

 

[Ellcon123]

I have the same brand of BMS but different versions. The batteries only 'respond' to voltage. At the low and high ends of the voltage/soc curve they will self balance. I've used 2 batteries for a few years and added a 3rd at the start of this year. Never touch them.