question

Probably more hassle than it is worth. Unless the 18650s are protected, you'll kill them with overdischarge.

To do it you need to add an isolated charging system for the 18650s, a charge/voltage sensing relay to switch them from charge to supply, isolated converter to drop the voltage. To keep costs down, I would goes you need to program an arduino or similar to control the relays.

Or split the DC circuits, drive some off the existing setup and switch some onto the 18650 setup. Then add a cheap 18650 charger.

Very messy either way.

Hi Mnonknz, Its very hard to do that on a 13 volt system but on a 48 volt bank it can work as a example see below

*** Edited you may get away with 5 x 3.2 cells and under charge them a bit - thats the only way I could see it working ******

- It can be done the 18650s cell pack voltage is around 3.7 volts per cell nominal and the LiFepo4 cell is around 3.2volt per cell nominal

so 16 cells in series x 3.2 =51.2 volts Nominal voltage LifePo4 min cell voltage 2v per cell or 32v high 4.2 or 58.8

14 cells x 3.7v in series = 51.8 volts Nominal voltage 18650s min Voltage 2.5v per cell or 35v high 3.65 or 58.4

so with two separate BMS systems one for the 18650s and one for the Lifepo4 banks you can control it.

Charging can be set as LifePo4 levels

even if the bank size capacity is different it will work - as both banks would be joined in parallel so there voltages would go up and down together - thus on the way down the larger bank would supply more amps than the smaller one as the Voltage (pressure) stays the same

the big thing then is each banks BMS must have a cutoff for low voltage the Lifepo4 2v min (I prefer 2.7)

- The 18650 min voltage setting as 2.5 min (I would set it at 2.7 as well)

Now of NOTE it is NOT best practice to do this - but it will work as long as you keep the TOTAL Banks nominal voltage level close. and Dont go over the min or max for each banks cells

Some will not like this and thats fine - but reality is you want to use both banks - yes one bank may not work as hard as the other and there are always risks - but the call is yours - Its better than throwing them away.

I have done it on one of my sites as the customer had a 1000 amp bank of 18650 cells 14 cells in series , and he wanted to combine another 1000 A/Hrs but these were 4 x Lifepo4 packs of 250 A/H each -

Pointless and detrimental.

Charging 18650 Li-NMC or NCA to only 3.65V is leaving a massive amount of capacity on the table meaning that pack will contribute very little; furthermore, the 18650 will DISCHARGE INTO the LFP just sitting there.

LFP voltage settles to 3.3-3.4V, and shouldn't be held above 3.40V unnecessarily, so the presence of the 18650 pack in parallel may be detrimental to the LFP.

18650 makes for junk 12V anyway. Decent for 24V and 48V in 7S and 14S, respectively.