Parallel SPIM08HP 3.7V bank with LFP 3.2V Bank? Separate BMS.

Madprofessor

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My first battery builds and I might have an issue.

I got four 24V 24Ah SPIM08HP 3.7V battery banks. Each is 7S3P. I am going to set them up into a 48V pack with a 14S BMS to be 14S6P.

I also have 16 LFP 3.2V cells and a 16S BMS for a 200Ah 48V pack.

Can I have these two packs in parallel on the SCC? Do I need two different SSCs? I saw an old thread that went back and forth but, beyond nomenclature, it never really had a conclusive statement. Will the BMS on each take care of any charge voltage difference?

I see that they have different real life voltages in packs. Do I parallel and charge at the lowest acceptable voltage? Or, am I hosed and wasted money on the two differnt types of packs? I started with a toe in the water and fell into the battery pool.

I appreciate your attention to this noob. I will listen humbly.
 

DThames

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Nov 22, 2019
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You are talking about adding only 50 ah to a 200 ah bank, so with the concerns of mixed chemistry, I would think twice. I would do it if I needed the peak amps. I would not want either BMS to ever shut off because when it comes back on, if the batteries have different voltages (very likely) you might see a very large current. You would want to keep your charge voltage below the upper limit of the lower voltage pack and the discharge voltage higher than the higher of the two packs. I did some testing with charge and discharge of these two type of chemistries. The Li-ion will deliver more of the power in the higher state of charge, then the LiFePo4 will dominate, then on the lower end the Li-ion, will pick back up, as I recall. This will work the Li-ion a bit harder, I do believe.
 

Madprofessor

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OK, rats. Then I messed up out of the gate. I hate wasting money, but I hate blowing stuff up too. Thanks!

I will have to find separate uses for these then. C'est la vie.
 

sunshine_eggo

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My first battery builds and I might have an issue.

I got four 24V 24Ah SPIM08HP 3.7V battery banks. Each is 7S3P. I am going to set them up into a 48V pack with a 14S BMS to be 14S6P.

I also have 16 LFP 3.2V cells and a 16S BMS for a 200Ah 48V pack.

Can I have these two packs in parallel on the SCC?

If the two packs are in parallel, they are essentially the same battery. The idea that you need different MPPT can't apply.

3.7V and 3.2v chemistry behave very differently. 3.7V chemistry has a strong voltage to SoC correlation, and they don't settle much from their charge voltage, plus they have a wider range.

If you're not careful, the 3.7V chemistry will be slowly trickling the LFP and holding it at elevated voltage, which isn't good for it.

14S 3.7V working is 42-58.8V
16S 32.V working is 40-58.4V

HOWEVER, anything above 54.4V (3.4V/LFP cell) is going to "float" the LFP at a higher voltage than recommended. You could stretch that to 3.45V/cell (55.2V) if you need more capacity.

54.4V/14S = 3.89V/cell - a very healthy peak voltage for 3.7V chemistry, BUT you're probably losing the top 30% of your capacity.

At 55.2V, that puts you at 3.94V/cell on the 3.7V chemistry - still a very friendly voltage, but still losing at least 20-25% of the upper portion of your 3.7V capacity.

LFP at 3.4-3.45V/cell, you can easily get 95%+ SoC, BUT your charge time will likely be 5+ hours even if starting at 0.5C.


Do I need two different SSCs?

see above.

I saw an old thread that went back and forth but, beyond nomenclature, it never really had a conclusive statement. Will the BMS on each take care of any charge voltage difference?

No. since they're in parallel, they are always at the same voltage. Period.


I see that they have different real life voltages in packs. Do I parallel and charge at the lowest acceptable voltage? Or, am I hosed and wasted money on the two differnt types of packs? I started with a toe in the water and fell into the battery pool.

While not optimal, paralleling the two batteries and bulk/absorption/boost charging to 55.2V and floating at 54.4V should get you 95% the capacity of the LFP packs and likely 75-85% of the 3.7V capacity.
 

Madprofessor

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Feb 24, 2022
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Thanks for the input folks. I promise not to let the smoke out.

It looks like my girl friend is getting a small solar set-up! (Perhaps in a steel box)
 
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