Mixing lipo with lifepo4 in parallel

[steviep19]

In February I purchased one of those 24v 200ah BYD lifepo4 batteries and it's been working great. I get about 150ah out of it.

I recently found a deal on Craigslist for some sonata 5300 batteries at $1 each, which are basically just two 18650's in parallel. I ended up buying 420 of them, and creating a 7s60p pack.

I've tried a couple different configurations.

1. I left the charge controller set for lifepo4. This works but the lipo batteries only get charged up to 27.2v 3.9v per cell and that's not high enough for my liking.

2. I set the charge controllers to 29.4, which charged the lipo batteries up nice and high, but too high for the lifepo4 battery. I had to disconnect the lifepo4 battery once the voltages got too high, or the BMS's over voltage protection would trip. This seems to work OK but I don't like pushing the old BYD batteries too hard.

3. This is the current way I'm doing it, and it seems to be the best, but I'd like some input. I have the Charge Controllers set for lifepo4, but once the voltage starts to get high enough, I use relay's to disconnect the lipo bank, and charge it using this DC-to-DC charger https://www.amazon.com/gp/product/B07N6BJWXH This way the voltage stays under the limit for the lifepo4 bank, and I can charge the lipo bank up to around 4.2. Once the sun goes down, the relay's reconnect the lipo bank, and it basicly keeps the lifepo4 bank floating until the voltage starts to fall withing it's usable range, then they discharge together.

I haven't found much on the internet about mixing lipo and lifepo4, and figured if anyone has done it, it would be here.

ps. both banks are protected by separate BMS's just in case.

 

[fopoku2k2]

I'm also planning on mixing my lipo Nissan leaf batteries with lifepo4 prismatic cell batteries. I would love to see what others think

 

[Diysolar123]

these are very different battery chemistry' with completely different voltage operating profiles.
that they each have a BMS is great, but remember that a BMS is your last line of battery defense if something goes "wrong"; a 50millivolt calibration error with your bms and that battery bank is damaged.

charged lipo battery voltages are "deadly" to lifepo4 cells.
low lifepo4 battery voltages are "deadly" to lipo cells.
lifepo4 will settle to around 3.3v/cell when charged, lipo will settle at 3.7v (this is damaging to the lifepo4).

since most of the "capacity" of a lipo is from 4.1v down to 3.8v...you literaly have a "dead" lipo before you even get into the safe voltage range for the lifepo4!!!

I think the reason you do not see much on this is, well, its a really bad idea.
That said...you can always try anything for fun, as long as you are comfortable with "bad things happening"...

You could, with some convoluted diode/fet arrays make sure the batteries NEVER cross feed each other I guess. You would need to have a seperate charging cicuit for each because their operating voltage ranges are seriously mismatched. You would then need load management logic so you would be pulling all your load current from the lipo bank first until it is almost completely dead, then switch over and start pulling load current from the lifepo4 bank while disconnecting the lipo bank.
so you have your 7s lipo pack with absolute limits of 26.11 to 29.4 voltage.
and you have your 8s lifepo4 which will want to sit at 25 to 26.4ish; but you could keep them in the "lower life range" and float at a very unhealthy 3.6v/cell or 29.2v.
There is "overlap" if you are willing to let each battery type live in the "shortened life" range..
dangerously dead lipo would be at 26.11v (7*3.73v/cell)
dangerously overcharged lifepo4 would be at 29.2v (8*3.65v/cell)

If "anything" goes wrong, your lifepo4 gets destroyed from overvoltage, or your lipo gets destroyed from undervoltage; or your BMS's will perform a disconnect, hopefully, but you could end up with a dangerous cross battery current surge when the BMS decides to "reconnect" based on its reconnect hysteresis.

 

[JoeHam]

I think the reason you do not see much on this is, well, its a really bad idea.

Couldn’t agree more.

 

[steviep19]

diysolar123 & Joeham,

I just realized number 3 was not what I ended up doing, and I never posted an update I ended up completely isolating the two types.

In 3, the lithium-ion bank is charged separately from the lifepo4 bank by a dc to dc converter, then discharged via another dc-dc converter. Neither bank is ever in parallel.

The Spec sheet on these Sonata 5300 cells shows usable al the way down to around 3.25v power cell at .2C

​

https://www.batteryspace.com/prod-specs/9266.pdf

I've been using this setup for over 8 months now, and it's been working great. Since they cell's charge and discharge separately from the main battery banks, they always charge at a constant 25a then taper off to 4.1v, then discharge at a constant 20a then taper down until 3.25. The BMS's are only there for safety, and balancing at the top.

The reason I liked this setup and posted it was because.
1. The lithium-ion batteries can charge/discharge separately from the lifepo4 bank at CC then CV.
2. The lithium-ion bank can act as a reserve 300 ah, as it's controlled via relay when to activate.
3. Lithium-ion cells have a shorter lifecycle, and I can cycle these only when necessary. They're cycled 1/5 as often as the lifepo4 bank.

If fopoku2k2, did this in a similar manner, I see no reason why it would not work. This could be good solution for some of these EV packs that do not operate at ideal voltages using step-up/down dc to dc converters.

 

[JoeHam]

Nicely done.

Isolated works, mixed.......... not so much. ?