Mixing new and old lithium LiFePo4 batteries

Osolemio

Hello everyone.

I wanted to know if anyone had experience or knowledge in regards mixing new and old lithium ion LiFePo4 batteries. I am considering an installation with 1 battery module from Pylontech or BYD (around 2.5 kWh) with the possibility of upgrading the system within a few years with more modules. 

I remembered that with lead acid mixing new and old was not a good idea, and I wondered if this would also be the case for lithium ion. Would it depend on the inverter / charger? Would some sort of balancing be required before installing the new module having used the old module already for a few years?

Thanks in advance!

Raj174

@Osolemio
Experience on this forum has shown that there is very little lose of capacity at 3 to 5 years so combining old and new should not be an issue in that time frame. For battery modules, it's necessary to make sure they are fully charged before installing in series/parallel connections. It is helpful to have access to individual module cells for monitoring during charging.

Rick  

Osolemio

Thanks for your answer @Raj174 . By access to individual module cells, do you mean for example that each module has a Modbus or CAN connection that allows monitoring variables and changing parameters of the module?

Raj174

@Osolemio
Yes, it is important to verify that all the cells in a module are achieving top balance, both initially and periodically while charging.

Rick

karrak

From my understanding it is not good to mix Lead Acid batteries of different ages because their charge efficiency decreases and the absorb time necessary to fully charge them increases with age so to fully charge the old batteries means the new batteries will be overcharged or if we fully charge the new batteries the old batteries are undercharged. Both these scenarios cut down the life of the battery as a whole.

With Lithium Ion batteries the coulomb/current charge efficiency of around 99% does not change much as they age and they do not  need any absorb time.

The one thing that LFP batteries do loose with time is capacity, if used conservatively I would say that this would be  around 1%-2% per annum.

Both the Pylontech and BYD "modules" have an inbuilt BMS which if wired up to external equipment as per the manufacturers instructions should protect the battery against any imbalance issues. I would ask Pylontech and BYD for their advice about adding new modules at a later date. I can't see why there would be any problem doing this.

Simon

Dave Angelini

The key question is how does the BMS interact with the other BMS as Simon says. LG Chem has a contactor that opens up the low battery bank to protect it from zero charge, they use one up and then use the next. Simplyphi does this bank internal for their parallel banks automatically.  Tesla also has the logic built-in for their model2. 

I don't think it would be good to mix internal to a bank different age batteries unless the BMS was designed for it. 

Osolemio

Great, thanks @Raj174, @karrak and @Dave Angelini for your comments, they are very helpful. I will also contact both manufacturers, as @karrak suggests, and see if there are any particularities with their products.