I have two, possibly three 48V-nominal 50Ah Lithium Iron Phosphate battery packs used for E-bikes (16S cells, 2.5kWh capacity each). The packs have BMS circuitry which limits discharge to 50A (through the 10Ga output leads) and also limits charging to 5A (via the thinner 18Ga leads).

I wish to discharge two or three of these packs in parallel to power a 48V sinewave inverter which would then charge my plug-in Prius car at a continuous maximum rate of 1350W (120VAC) for up to 5 hours. The Prius battery pack accepts a charge of about 6kWh when fully depleted.

Question: Is there any danger/concern in paralleling these packs for a full discharge cycle? If I only parallel two packs instead of three the combined capacity of the pair will not be enough to fully charge the car, so one pack's BMS will shutdown first, leaving the other pack to supply the full current of roughly 30A to the inverter. The question is, will the remaining active pack backfeed current into the one that's shut down? I plan to test this with a much lower-current load to start out...

  • \$\begingroup\$ That depends entirely on how the embedded control system is set up. If one pack is sitting lower than the other two it will have current going into it if there is a path to do so. Do you have any links with more info on the pack in question? \$\endgroup\$ – alphasierra May 3 '17 at 22:40
  • \$\begingroup\$ "one pack's BMS will shutdown first, leaving the other pack to supply the full current" - why would one shut down first? \$\endgroup\$ – Bruce Abbott May 4 '17 at 4:10
  • \$\begingroup\$ I don't have any detailed info on the packs other than the stated BMS disconnect is at 42 volts (low voltage cutoff). These packs are "generic" so information is scarce. Due to slight differences in battery capacity between two packs, one pack's BMS would reach low voltage cutoff first. \$\endgroup\$ – Eugene Beer May 5 '17 at 1:55
  • \$\begingroup\$ To clarify, with 2 packs the total capacity of 5kWh is not sufficient to completely charge my car which requires 6kWh. So, the BMS in each pack will terminate output current when their respective battery voltage falls below 42V. I'll just have to do some experimentation to determine BMS functionality with respect to 1) when BMS cuts off output current, can input current still flow to the battery cells through those same battery leads (from a paralleled battery) 2) How much backfeed current \$\endgroup\$ – Eugene Beer May 5 '17 at 2:31
  • \$\begingroup\$ could possibly flow, if one battery were discharged to 42V and the other paralleled battery were still at, say, 46V or more? 3). If my car were only partially discharged and it needed only, say, 3kWh to top it off, at that point the car would stop drawing current from my LiFePo4 batteries and, potentially, one of the two batteries could backfeed as much as 50A into the other paralleled battery. It boils down to "what is the internal resistance of these batteries" at various SOC points? \$\endgroup\$ – Eugene Beer May 5 '17 at 2:32

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