The current iteration of the mjbots power_dist board released back in the summer of 2020 is pretty useful. It pre-charges the input, provides a soft switch, and gives you a bunch of output connectors to make wiring easier.

r3.1 Limitations

However, this version did have some limitations and potential problems. The first is that the pre-charge method it uses, a simple on/off pre-charge resistor, is unable to support a wide range of supply voltages. Either the resistor has a low value, in which case large input voltages will cause thermal failure, or for larger values, it isn’t able to actually pre-charge the bus sufficiently before engaging the primary MOSFET.

Secondly, it switches the negative rail. As pointed out in the documentation and by numerous YouTube commenters, if you are not careful, this can result in magic smoke being released if ground on the output and input is connected in any way.

Third, the protection afforded by the board is relatively limited. It merely performs the pre-charge function. It is still possible for short circuits or over-voltage events to result in damage to either downstream circuitry, or the upstream battery.

Fourth, the quiescent current is larger than I would like. At around 2-3 mA, it isn’t that large, but it means you can’t leave a battery connected for more than a day or so. Even worse, some BMS see that quiescent load as something they need to remain active for, which reduces standby battery life more substantially.

Looking forward

Given those shortcomings, I wanted to see if I could do better for the next revision. In the next several posts I’ll walk through my design process.