Mixing new and old lithium LiFePo4 batteries

Osolemio · December 2017

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 · December 2017

@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 · December 2017

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 · December 2017

@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 · December 2017

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 · December 2017

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 · December 2017

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.

heduino · October 2019

Hi. I know this post is a couple of years old. Can anyone comment on mixing brands of LIfePo4 batteries? I have an electric scooter with a battery pack of 60V, 60ah. This pack was mounted in 2013. 2 or 3 cells are degrading, and the BMS now turns the scooter off at about half the range as usual. The brand I used was Sinopolis but it seems they dont sell them anymore. Can I mix with a different brand? Also Old and new? thanks.

BB. · October 2019

Welcome to the forum Heduino,

I really do not have much in the way of details I can help you with.

My first suggestion is to replace the whole battery pack at once. If you have several cells that are already degrading after 6 years of usage, the others are not going to be far behind (in all likelihood). And an "unstable" Li Ion battery pack can be a risk for fire (really any battery bank with a mix of "good and bad" cells is at risk). And Li Ion fires are something you don't want to be around (besides being very hot and difficult to put out, they can emit very toxic fumes that can mess you and your home up for years, or even kill you).

The detailed answer is--It depends on the BMS design and ratings. If the BMS can actively balance individual cells, then a mix of old and new cells could be "managed" by the BMS (it will actively prevent over/under voltage of any individual cell). So, it should keep the risks down. If your BMS does not actively balance the cells, then old/new unmatched cells run the risk of out of voltage range cells at either/both end of charge and deep discharges. Many battery pack for hobbyists are built from "matched cells" so that once "balanced", they stay in balance through the entire charge/discharge cycle for (mostly) the life of the cells. With unmatched cells, and a passive BMS system (per string type), sets of unmatched cells are just not something that I would want to play with in my home/business.

And, many battery packs are spot welded electrical connections. You should not use a soldering iron to reconnect wiring as the heat from soldering can damage the cells and cell vents.

Also, a suggestion on trying another website that may have more detailed answers. Candlepowerforums has a lot of information on Li Ion battery "projects" and they may be able to help you with yours:

https://www.candlepowerforums.com/vb/forum.php

-Bill

ElectricLove · October 2019

There are ways to mitigate the issues but you should really not mix batteries that aren't built together and at the same time. You should only use "batched" batteries, this is true of all battery cells and it is especially critical and true of a Lithium installation. Lithium Iron Phosphate surely is known for its safety but they still contain a lot of energy and issues can become very big problems if you aren't careful and thoughtful on the front-end.

A battery that was built by a reputable/reliable company uses matched cells. Even within a single manufacturing batch cells are sorted into bins, like this: https://www.youtube.com/watch?v=KnOSmmNhwzc Internal Resistance (or Impedance) along with Voltage are carefully and precisely measured and battery cells from the same production run can have small variation, thus it is important to match them. Do you think if within a single manufacturing run with cells that have done the exact same thing their entire short lives (same storage, same shipping, same environment, same usage) manufacturers would still spend the time and money to sort them this way if using matched cells/batteries wasn't important?

It is very important to long-term performance and that is afterall the entire reason you are choosing Lithium in the first place, right? If you wanted just something that'll "work" and be cheap to get you going, you'd go with Lead Acid. No, you are being smart and using the best batteries you can so that you have a long-lasting and reliable system with as little maintenance and headache as possible! So don't cheap out on it and try to use mis-matched batteries this way.

My suggestion is to either find a supplier who can get you the capacity you want at a price you can live with for the lifetime of that pack (I always suggest Ohmmu, www.ohmmu.com/group31) OR just get a cheap FLA battery pack for the first few years and when you have enough money saved for the "ideal" pack then upgrade and get some money back by scrapping or selling off your FLA... I suppose a 3rd option is to get a Lithium pack, use it for a few years and then sell it and buy a larger lithium pack... In any case assume you are replacing the entire pack when you do the replacement, don't piece-meal a battery pack.

Now, for the part I said you can "mitigate the issues":

You can do batteries of all sorts of different chemistries and lifes, etc IF you have a proper way to manage them, not only within the pack (cell-to-cell within each string) but between packs (important if mixing age/chemistry). You need relays that can disconnect each pack from the system and ways for each pack to monitor during both charge and discharge and for each pack to be able to disconnect when it reaches certain limits (so one pack can disconnect when it has reached 100% while another stays connected to charge until it reaches it's own 100%... for example)... But this entire concept gets complicated to be certain...

Mixing new and old lithium LiFePo4 batteries

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