Li-Ion BMS

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EV power circuits

A design guide

Electric vehicles using a Li-Ion traction pack present some design challenges, among which are:

  • The Li Ion cells must be kept from dropping below 2.0 V, else they will be damaged; this can be a problem when the vehicle is parked, unplugged, for a long time
  • A BMS (Battery Management System) is required, that needs to be powered when the vehicle is on, and when the vehicle is plugged-in; it is not immediately obvious how to provide these power sources
  • If not using a 12 V auxiliary battery, the ignition conundrum becomes an issue: there's no way to turn the vehicle back on once it's been turned off

This page offers various power circuits that address these issues, including minimizing the idle current draw while the car is off and therefore maximizing the safe storage time.

Here are 30 different permutations to an EV circuit, based on which components are used.

  • Whether the charger has a 12 V output, and, if not, whether a separate 12 V power supply is used
  • Whether the charger includes an input for remote turn-off
  • Whether a 12 V auxiliary battery is used, and, if its is, how it's charged, and, if not, if a DC-DC with a low voltage cut-off is used, or a second, small DC-DC
BMS 12 V supply (VS)
when plugged in:

more info
Charger on/off

more info
BMS 12 V supply (VL)
when ignition on:

more info
more info
more info
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EV circuit

Electric vehicle power circuit.

The BMS requires 2 sources of 12 V power, which are combined with 2 diodes inside the BMS to power its electronics, while still enabling it to determine whether the vehicle is plugged in, on, or both:

  • VS - whenever the vehicle is plugged in
  • VL - whenever the vehicle is on

For simplicity, the following are not shown:

  • Fuses - proper fusing practices should be followed
  • Diodes on the output of chargers and DC-DC converters, should they be needed to prevent the batteries to be discharged when the chargers are off
  • Main contactor(s) and a precharge circuit to limit inrush current into the motor driver are not shown (they may be included in the motor driver)
  • Current back-off: some BMSs are able request that the battery current be gradually reduced when the battery is almost full or almost empty, and some chargers and motor controllers are able to abide to such request
  • Interlock function (to keep from driving off while plugged in); however, this can be provided by the BMS, which disables discharge if it is powered by both Vs and Vl.

© 2008~2018 Davide Andrea. All rights reserved, except where noted by CC mark. Page published on Nov 08 2009. Graphic design by morninglori