2015-08-11 12.43.24There are some very nice, high current bistable relays avalible for e-vehicles (and other high-current applications – check www.ev-power.eu). They are usually very good and have low power consumption due to fact, that they consume power only on state-change and not all the way during power-on phase.
Such relays usually have 2 coils with connected middle point, where first coil is used to turn the relay on and second coil to turn relay off. Usually such relays have 3 contacts for controlling, though most of such relays also offer on/off switching using only one coil (any), when reversing polarity to obtain on/off function.

bistableHowever, BMSs usually don’t provide two pulse outputs to drive bistable relay, but only one output, which is meant to control normal relay. Therefore we cannot replace ordinary relay with bistable. I came around this issue, when I needed to use bistable relay to lower power consumption of the system, which originally used standard 100 A “car” relay.

After some thinking and testing, following solution was found.

bistabilni driver

When applying power to J1, the capacitor C1 is charged and this pulse is enough to turn relay on. As voltage on T1 gate is high (R2 can also be omitted, if Uin is lower then Vgs of P-FET), which means that T1 is closed (does not conduct) when power is applied.

When power on J1 is removed, voltage on T1 gate drops and opens the T1. This way, capacitor is discharged through T1 and relay in the opposite current-flow direction, which turns relay off.


  • You can use also other P-FETs – I used this one just for a reason, that I had it in stock. However, check Vds and Vgs, if they meet your voltages!
  • Voltage drop on D2 must be high enough, that voltage on R1/R2 joint is same (or higher) then after D2. If not, T1 will be partially open during power is applied and will heat significantly! (for 12 V relay, you can ommit R2 and D1)
  • If using higher voltages (i. e. 48 V system), you must ensure, that Vgs will not be higher then Vgs allowed for your P-FET. You can also use lower capacity capacitor C1.
  • You can use this schematics as a reference only! You do it on your own risk!