Mix-Matching Lithium Batteries

[Glennman]

I currently have 8, LifeBlue 100 ah batteries charged by my 4000 watts of solar. I would like to add more batteries, but I may want to utilize some other brand. The batteries I have are 12v and wired for a 24v system. Does mixing newer and/or different brands of batteries cause problems? Even if I go with the same brand, I'm concerned that mixing new with old may be an issue. I'm thinking that the internal battery management system might account for this (?). Of course I would stay with 12v wired for 24v either way. Any thoughts?

[Gary Hatt - Publisher BCM]

I have heard you should not do this, as the BMS may not match and may cause problems.

My 2 pesos worth.

[luvrbus]

Lion Energy told me not to mix the batteries shot my super buy down on 6-100 amp Lithium batteries, I bought the 6 anyways 100 bucks ea with only 512 cycles was a deal to good to pass up 

[Glennman]

So, if I want to add amp hours I have to junk the batteries I have? They only have about 40 cycles on them so far. That is very discouraging! Thanks anyway!

[luvrbus]

So, if I want to add amp hours I have to junk the batteries I have? They only have about 40 cycles on them so far. That is very discouraging! Thanks anyway!

They don't make anything simple for you it is about money

[KF6-BusFan]

I have to junk the batteries I have? They only have about 40 cycles on them so far

I would treat those as if they were new and not worry about it. 40 cycles is something like 1% of their lifespan; they are 99% young.

Or, put 40 cycles on the new batteries alone before you bring them into the combined system.

Make any series string have batteries from the same purchase. Parallel the new 24 volt strings in. Make sure the charging voltage is compatible with both strings.

If you want to double check, run the combined system down about 50%. Start a charge cycle and monitor the amperage going into the individual series strings. It should fall off roughly the same on each string, as each string fills up with charge. At some point one string will fall to a charge amperage of basically zero. If the other string falls to zero "shortly" after that, it should be fine. Probably 10-15 minutes of difference will be of no consequence.

That's the way it looks to  me. I actually have a 12 volt system with 2 batteries in parallel that are from the same maker but about 1 year apart. I guess I'll run this same test. I've never observed the individual charge currents, but now I'm curious.

[Glennman]

That makes sense BusFan. I would hate to make the purchase, then run the tests and have them fail! My plan was to incorporate some Renogy batteries into the system. Luvrbus says they have a good tech, customer service team, so maybe I'll ask them about it. I really need to pick up a good amperage tester as well. I used to have an old Ampprobe that I inherited from my dad. I haven't seen it for a while. I'll have to dig through my tools in the shop and see if I can find it. I need to learn to do this stuff anyway. Thanks.

[KF6-BusFan]

Yesterday, my first test went like this:

I have a 2 battery system. Both 12v, 300ah per the mfg. This particular battery model has been reported by other users as really only containing 280ah.

One has been in light service for about a year and has maybe 5 full cycles on it (from maybe 20 quarter cycles); the other has been in light service for about a month and might have 1 full cycle on it. Both have roughly the same manufacture date.

Prior to this, I have only run them separately.  To begin this test, I charged them each up fully, separately. Then I pulled about 28 amp-hours out of only battery 2. While preparing, I was running lights off battery 1, so it probably lost 2-4 amp-hours. This was intended to simulate two batteries of different ages.

Then I paralleled them and began a charge cycle. My inverter/charger is a Heart Interface "Freedom 25". Super old school. Set to 14.4 volts bulk charge, capable of supplying 100 amps.

The combination was pulling 65 amps off the charger. I measured a roughly equal flow into each battery. This continued for about 15 minutes. Then the combined amperage dropped to about 20 amps. I measured one battery was pulling those 20 amps and the other was pulling zero.

About 15 minutes after that, the charger entered a mode where it would stop charging for about 1 minute, then resume. I have seen this before, and I believe during this time it is letting the battery rest a little and then testing whether it "wants" some more charging. Each time it resumed, both batteries would pull charge amps for a few seconds, then the one would go to zero while the other continued to accept charge.

This cycle repeated for about 5-10 minutes and then the charger entered float mode because it decided the system was charged. The way my charger is programmed currently, if the charge current falls below 5.6 amps for 5 minutes, it decides the system is charged.

In short, this is the behavior I expected. I would have made more scientific measurements and records, but I only had the system ammeter (which showed the combined current, and is inside the bus) and one clamp-on ammeter (which I had to move between batteries, which are outside in a luggage bay); it was awkward and I was cold. I thought of it as more of a safety check to see whether it would do what I expected, and whether anything dangerous (heat, unexpected current flow, overvoltage, etc) would occur.

Next: I will repeat with the batteries at a higher imbalance amount.

[RichardEntrekin]

I would plan very carefully how I would wire 16 batteries in a series parallel configuration. Personally I am not nearly as concerned about the difference in age of the two groups of batteries as I am about wiring them correctly. In my unasked for opinion, you really need a robust bus bar for the pos and neg 24V rails, then exactly equal cable lengths from the series pairs to the bus bars. Because the lithium batts have so low internal resistance, they are way more sensitive to becoming unbalanced from battery to battery due to cable length(varying resistance)

You may have already addressed this in another post that I have not read. If so, my apologies.

[Glennman]

When I connected my original 8 batteries (series/parallel for 24 volts), I used all 4/0 cables and they are all the same length. The company that designed the system for me said that the equal length cables are good, but that they can be off a little (if necessary), as the BMS is supposedly going to manage any differences. They said that it is more important with lead/acid batteries, since there is no mechanism to compensate, and that people tend to apply old school standards for lead/acid that do not apply to BMS controlled lithium. At the time it made sense, but I went with exact lengths anyway. Better safe than sorry. I am waiting to make a move on adding (or not) batteries, since I am not going to be using the bus for a few months anyway. It's a lot of money to put out for sure. Plus, now that I can run my Pioneer mini split off the bus alternator, AND I recently added 2 more solar panels (giving me just under 4000 watts), I may not need additional batteries anyway. Thanks for the input