Parallel "enable" signals for BP and Multi

DIY
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1

I am preparing to install a DIY LFP battery. I have a dry contact that signals “allow to charge.” Both the BP and the MP can be controlled by a set of dry contacts. The question is, can those controls be paralleled? Since they are controlled by dry contacts, they must emit some sort of signal (B+, Grnd, etc) and I worry that they may interact with each other.

It’s not too hard to have my BMS control a two pole relay or a opto-isolator, but it would be cleaner to not have the extra complexity.

2

Sort of related, or maybe the “answer.” In order for the BP or MP to “know” that the contacts have been bridged (ie, the switch is closed), it’s likely that one of the pins has been “set.” Either set high (12V) or set low (ground) or possibly some internal operating voltage, say 3 or 5V. If that’s the case, the “signal” being given to the MP or BP is either “low pin taken high” or “high pin taken low.” If that’s how it’s working, it should be possible to signal “charge enable” with a single wire – either a voltage, or more likely, taking the appropriate pin to ground.

Anyone know what these devices are looking for? How do they know that the pins have been shorted?

3

Well, I decided to dig in and see. I only have the MP here now, so I can’t test the BP until later this month when I have one to play with. But I played with the MP (hey, the worst that can happen is my 2 year old 3kW inverter smokes, right?).

Here’s what I learned.

  • Both the Charge Enable and Discharge Enable appear to behave identically.
  • The pin labeled + is at about 6V, give or take.
  • The pin labeled - is directly connected to battery negative (a few ohms at most).
  • Taking the + pin to ground is the same as shorting it to the negative pin (makes sense, since that is ground!).
  • The current flow when shorted is around 1mA, so it is purely a signal, not an operating current. Probably triggers some logic ICs.

This means that:

  • I can use ground anywhere, including at my BMS location. I’ll have other stuff there (LED’s, switches, buzzers, etc so I’ll have ground available)
  • I only need to run a wire from the + pin back to the BMS. I can then pull it to ground via most any sort of device – a FET, a transistor, an optoisolator, or a mechanical relay. For two controls, this halves the number of wires (it’s only 2 feet, but still less complexity).
  • Any remote switch that is not bidirectional needs to allow current flow from the + pin to the - pin. So a concern for most solid state devices, but not a concern for relays or some optoisolators.

I’ll come back and update this when I have a BP in hand. I hope it is identical (a hot pin pulled to ground).