Using 20-cell Li-ion pack and 24-cell LiFePo4 pack in parallel. Feasible?

Hi there. Long time reader, first time poster. I intend to connect and use a 20-cell Li-ion pack and a 24-cell LiFePo4 pack in parallel.

I've got a 24-cell LiFePo4 battery pack rated for 76.8V, 56Ah as the main pack in my electric moped. Recently I assembled a 20-cell Li-ion pack from some 18650 cells. It's rated for 74V, 30Ah, and I intend to make it my auxiliary pack.

In the quest for more speed, I upgraded my moped with a motor-controller combo from Mobipus with a rated peak output of 7.2kW. Thus, during full throttle acceleration, current draw from the battery pack is approximately 100A. Voltage drops on either the main LiFePo4 or auxiliary Li-ion packs are significant (~8V) but tolerable.

Still, more speed and harder acceleration is desired. The idea of connecting both packs in parallel popped into my head. In theory, the larger combined capacity of both packs would lead to less voltage drop, spreading the load across both packs and extending their lifespan.

Fully charged, the LiFePo4 pack measures at 86V, and the Li-ion pack measures at 84V. The plan is to use the main LiFePo4 pack for a while, then connect both packs when the main pack reaches 84V.

The cutoff voltages of the LiFePo4 and Li-ion packs are ~67V and ~64V respectively. I intend to leave a small safety margin, and disconnect the packs at 70V.

I realize that the chemistry of both packs are different, and my experience in this field is limited. Would any issues crop up if both packs are connected and discharged in parallel?

Thanks!

Mainly, disconnect them from each other during charging, and don't connect the charger ports together during discharge. (assuming that charge and discharge ports are separate--if they are the same port on both packs, it doesn't matter).

amberwolf said:

Mainly, disconnect them from each other during charging, and don't connect the charger ports together during discharge. (assuming that charge and discharge ports are separate--if they are the same port on both packs, it doesn't matter).

Click to expand...

Ah yes, that's some good advice. The charge/discharge ports are on the same ports on the BMS to allow for regenerative braking.

Then I don't see any significant issue; there have been a number of discussions about paralleling different chemistries, and the consensus has generally been that as long as the full and average voltages are around the same, they should work out ok.

The pack with the higher C-rate (lower internal resistance) will supply most of the current; the other one will then "recharge" that one, if it ends up stable at a higher voltage than the lower Ri pack.

Agree, Ive done similar things myself. In my specific case, I had to ride the lipo by itself for about a mile, then connect them when both were at the same voltage. Then at the very end of the ride, if I discharged both enough, the lipo would be at risk of overdischarge, while there was just a tiny bit left in the lifepo4

I just had to pay a bit of attention at the connect up, and if I got really deep into the discharge.

There isn't much energy or capacity above 3.5v in lifepo4 or 84 volts. The 24s will charge higher but will drop to it flatter discharge curve . As li-on losses voltage faster in discharge. But together? Let us know how well it works over time real life as I see this as a good combo for marriage 20s and 24s. One 56ah and one 30ah is a head scratcher. It would be nice to have sense wires or bt bms for monitoring.? ? Still scratching.
What lifepo4 and 18650 cells

999zip999 said:

There isn't much energy or capacity above 3.5v in lifepo4 or 84 volts. The 24s will charge higher but will drop to it flatter discharge curve . As li-on losses voltage faster in discharge. But together? Let us know how well it works over time real life as I see this as a good combo for marriage 20s and 24s. One 56ah and one 30ah is a head scratcher. It would be nice to have sense wires or bt bms for monitoring.? ? Still scratching.
What lifepo4 and 18650 cells

Click to expand...

The BMSs of both packs have BT connectivity, so I can monitor the cell voltages on my phone.
The main LiFePo4 pack was assembled using 24 56Ah hardcased cells. They are used cells, and I presume they were sourced from an electric bus (Many of those here in Mainland China). They're not the best, but they're cheap cheap cheap.
The auxiliary Li-ion pack was assembled using 240 18650 cells in a 20s12p configuration. It consists of brand new FST-EC2500 cells, each rated at 2500mAh and 8A discharge, sourced locally from Taobao.

Sounds like 120 lbs of battery ?. Should take care of sag. If I wish one battery to be bigger it would be lifepo4 because of its flatter discharge curve.