Lifepo4 parallel with 18650

Creatura_n · May 1, 2022

Hey guys.
I have a question.
I have a 16s lifepo4 320A with a daly bms 200A 48v
I want to add some more 'juice' to my battery and i want to add in parallel a battery bank of 18650, to be more specificaly a 14s 18650 100A 48v battery bank with his own dalt bms of 120A.
Do you think it will work? Its there any way that it can affect something?
I know that there will be different voltage and the 14s bank wont get 100% charge

timselectric · May 1, 2022

It will work.
The two batteries will balance with each other.
And each, will do their part.

Creatura_n · May 1, 2022

Does the bms will be affected?

timselectric · May 2, 2022

The BMS will still only monitor its own battery cells.
You're just adding more storage capacity.
The 18650 bank, will be connected after the BMS.

newbostonconst · May 2, 2022

You shouldn't parallel a 14s with a 16s LifePo4.....the 14s will be overcharged most of the time....the 16s will be drained.

But if the 14s is Li-ion it will likely be ok....

14 * 3.2 = 44.8 LifePo4
16 * 3.2 = 51.2 LifePo4
14 * 3.75 = 52.5 Li-ion

Usable charge span of a LifePo4 cell is from 3.0 volts to 3.5 so
14s usable span is 42.0 volts to 49.0 volts
16s usable span is 48.0 volts to 56.0 volts

Usable charge span of a Li-ion cell is from 3.5 volts to 4.0 so
14s Li-ion span is 49.0 volts to 56.0 volts

Is the 14s still LiFePo4 or Li-ion? Are you making the pack?

timselectric · May 2, 2022

newbostonconst said:
You can't parallel a 14s with a 16s.....the 14s will be overcharged....

14 * 3.2 = 44.8
16 * 3.2 = 51.2
Difference of 6.4 volts per battery....
6.4 volts divided by 14 is .45 volts per cell of over or under charge..... that is to much and won't work.

18650 nominal voltage is 3.7 volts.
3.7 x 14 = 51.8
3.2 x 16 = 51.2
.05 difference per cell.
Close enough

newbostonconst · May 2, 2022

timselectric said:
18650 nominal voltage is 3.7 volts.
3.7 x 14 = 51.8
3.2 x 16 = 51.2
.05 difference per cell.
Close enough

I realized he was using Li-ion and was editing my post while you posted...but there are LifePo4 18650 batteries.

I included both chemistries so people don't get confused.

steviep19 · May 2, 2022

I used to parallel an 18650 pack with LifePO4. It does work, but you'll end up barely using the 18650 pack, or trusting your LifePO4 BMS to disconnect at high voltage.

Fully charged 18650 is --- 58.8v
Fully charged lifepo4 is --- 57.6 if you batteries are in perfect balance but bulk of your power is up till 54.4v (3.4v per cell)

I found connecting all my electronics to the lifepo4 pack then using a DC to DC like this to charge the 18650 pack, then reversing it to discharge back worked best. In my case it was an 8kwh 18650 build.

I used a pair of these, and used a relay to either activate the charging/discharging one.

https://www.amazon.com/gp/product/B07TX51XQG/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Went from barely using 1/2 the power to nearly the full 8kwh. As an added benefit, you can discharge the 18650 bank at a constant rate, and less often, as typically the cycle count is less on e an 18650 cell.

timselectric · May 2, 2022

Just keep the charge and discharge parameters for the LiFePo4 battery. The two batteries will share the power, according to their percentage of the total capacity.

Zuikkis · Nov 24, 2022

Bumping this old thread as I'm in a same boat.

My current energy storage is built from Think City EV lithium batteries. They were originally 12S but the voltage was slightly too low for my Deye inverter so I changed the configuration to 14S. I currently charge to 57.4V which is about 4.1V per cell.

Now I'm thinking of building a 16S Lifepo4 pack in parallel with this pack. 57.4V/16 = 3.587V which is perhaps a bit too high for daily charging? Perhaps I could use a trick mentioned above, set Lifepo4 BMS high voltage cutoff to 55V or so.. I assume BMS will reconnect when voltage is back to legal levels.

Lower end is a bit problematic also. These Think cells have really weird voltage curve:

They claim the chemistry is NMC but I have never seen NMC go so low? I have pretty much verified this curve with my own measurements. Currently I have been discharging these to 3V, which still leaves about 20% unused. But 3.0V*14 = 42V which is too low for 16S Lifepo4 (2.625V per cell).

Perhaps Lifepo4 BMS could cut off the low end too?

timselectric · Nov 24, 2022

Zuikkis said:
Perhaps Lifepo4 BMS could cut off the low end too?

Yes, it could be. But I wouldn't want my BMS doing this daily. (Wouldn't want to wear out last line of defense) It would be safer to set the charge and discharge to stay inside the safe range of both batteries. It will cost some capacity. But that's just the way it is when mixing different types of batteries.

Zuikkis · Nov 24, 2022

Thanks for your thoughts!

I've been trying to Google what is the real "safe voltage range" of LiFePO4?

I know most of the capacity is between 3.0-3.4V and there's usually no point going further, but how much worse it is to use 2.9-3.5V or 2.85-3.55V?

If I loose 10% or 20% battery life it still might be worth it..

Zuikkis · Nov 24, 2022

Huh, just found this, from Wikipedia. I guess I don't want to go to the red zone.

timselectric · Nov 24, 2022

I run mine between 2.6v and 3.55v .
But most would recommend 2.9v to 3.4v.
But you need to get above 3.45v occasionally for the cells to balance. Maybe a few times a month, at least.

Zuikkis · Nov 24, 2022

I found a damn good page explaining pitfalls of LiFePO4 charging:

Charging Marine Lithium Battery Banks | Nordkyn Design

nordkyndesign.com

The main point being.. Any voltage above 3.4V can overcharge the battery if held for too long. When above 3.4V and charge amps drop below certain limit, charging must be stopped and must not be restarted until battery has been discharged enough.

I think this means if you have LiFEPO4 + LiPO in parallel, you can't charge above 3.4V at all. If you charge to 3.5V (per LiFePO4 cell) for example, then cut off charging, there's still the same high voltage present in LiPO which will now flow to LiFePO4 and continue to (over)charge it..

3.4V*16 = 54.4V, and 54.4V/14 = 3.88V.. which means 14S LiPO really can't charge enough. 13S LiPO would be much closer actually!

However the BMS over voltage cutoff trick should work. Or not necessarily BMS, you could make a custom device that monitors voltage and current going into LiFePO4 pack and decides when is correct time to cut off. I have a Raspberry Pi monitoring my other pack anyway, would be reasonably easy to add this functionality.

timselectric · Nov 25, 2022

Overcharging of LiFePo4 prismatic cells begins at 3.65v .
Once mine reach 3.55v , they are held there for the rest of the day. There is no current flowing, they are just kept at full and balanced.

Zuikkis · Nov 25, 2022

timselectric said:
Overcharging of LiFePo4 prismatic cells begins at 3.65v .
Once mine reach 3.55v , they are held there for the rest of the day. There is no current flowing, they are just kept at full and balanced.

Read the link above.

I'm pretty sure he's correct.

LiFePO4 will reach 100% SOC even at 3.4V, if held there long enough. If charging voltage is not lowered after that, it starts to overcharge. But it might be hours (or days?) even at 3.55V before it really reaches 100%, so it probably works for you.

timselectric · Nov 25, 2022

It will take much longer to reach 100% at 3.4v.
3.4v is the 100% resting voltage.
3.65v is the 100% charging voltage.
But there's no need to get that high. Capacity gain is minimal in the knees.
But you need to get into the upper knee for balancing, this does help with Capacity.

ata2k6 · May 30, 2024

Hi all. sorry for reviving this topic. Same dilemma here, I have a built 14s 340 Ah NMC battery pack. I would like to add some 10kWh storage, but I'm too lazy to build another one. I was wondering, if 17S LifePo4 wouldn't be a better match as 16S for 14s NMC?

I mean 17S 100% SOC (3.4V/cell) for LifePo4 would be 57.8V, which would be 4.12V for 14S NMC. Same goes for nominal voltage of 3.2V per cell, 54.4V, equalling to 3.88V/ cell for NMC.

Regards,

Lifepo4 parallel with 18650

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