DC bus / DC-link capacitance value for ODrive S1 (needed for external precharge design)

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1

Hi ODrive team,

I’m building a custom BMS around a TI BQ76952, controlling a single main battery contactor (TE KILOVAC EV200 series) that feeds three ODrive S1 boards, each driving its own motor. Since I’m switching with a mechanical contactor rather than FETs, I need to add an external precharge circuit (resistor + small relay) ahead of the main contactor to avoid inrush current damaging the contactor contacts when it first closes onto the discharged DC bus.

To size the precharge resistor and timing correctly, I need to know the DC bus / DC-link capacitance on the ODrive S1 (in µF). This isn’t listed in the public datasheet’s specification table.

Could someone confirm:

  1. The DC-link capacitance value (µF) on the ODrive S1 board
  2. Whether this value differs across hardware revisions of the S1
  3. Any recommended precharge guidance TI/ODrive has published for this board, if it exists

My system details, in case relevant:

  • Pack voltage: 22.2V (6s Lipo Battery Pack)
  • Three ODrive S1 boards on independent branches, each drawing up to 80A max
  • Main contactor: single 300A-class unit ahead of all three branches

Thanks in advance — happy to share my precharge design back with the community once finalized, in case it’s useful for others doing multi-ODrive BMS integrations.

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Hi! Thank you very much for putting so much consideration into precharge / soft-start, that’s a great practice!

S1 bulk capacitance is around 328uF. It won’t significantly differ across revisions; I believe we did a short production run around 400uF but it’ll never be higher than that.

The only consideration regarding precharge is that the S1 has a quiescent draw of around a watt – so at 22V that’s about 50mA. So if you use e.g. a 10-ohm precharge resistor, you’ll have a precharge-ending voltage of around 22V - (50mA * 10ohm * 3) = 22V - 1.5V = 20.5V. When the contactor engages, you’ll have a very brief inrush event as the remaining 1.5V charges. But I really don’t have any concerns about that – it’s mostly if someone uses e.g. a 100ohm precharge resistor.

As a question – would a first-party ODrive soft-start/precharge/bus-switch product be of use to you? It’s something I may or may not be tinkering around with in my free time :slight_smile: