I am installing an LFP bank with external controls. I have a limited number of signal wires to work with. I need the BMS to manage “Allow to Charge” to both a MP and a BP. Both of these have a 2-wire control signal (shorted is “on,” open is “off”). I’ve played a bit, and it seems that on the MP this can be accomplished by taking the “hot” terminal to ground, and the BP this can be accomplished by taking the “+” terminal high, or the “-” terminal low (the BP is fairly well documented, the BP is horribly documented).
My concern is that unless I use a relay or other isolator/multiplier in the signal wire, I need to treat them both the same. The BMS has a “dry contact” (basically, a toggle switch). Is it possible to control both devices by a single contact? Ideally, these devices would have similar internal logic – I could parallel them, taking both “-” terminals to one side of the switch, and both “+” terminals to the other side. But I don’t think the two devices will behave well if I connect them together like this.
Does my question even make sense?
Thanks!
Well, I’m not one to sit around. I’ve got an answer, and it ain’t easy.
First round was a failure. The standard assistant puts the “Allow to Discharge” on the Temp terminal, and the “Allow to Charge” on Aux 1. I found that I could connect the “+” on the MP to ground, and it would charge. I could connect the “-” on the BP to ground, and it would charge. Unfortunately, if I connected the BP “-” and MP “+” lines together (on one side of a switch), the BP would go active. Not going to work.
But, on a lark, not sure why, I tried the Temp (Allow to Discharge) and had the correct behavior. Huh. So I reran the assistant configuration, and put ATC on the Temp and ATD on Aux 1. Tied Temp “+” to the BP “-” and nothing happened (good). Then tied the lines to ground, and they both activated. Opened to ground, they both deactivated. So this seems to work. ATC MUST be set to the TEMP terminals, NOT the AUX. Who’d a thunk?
I’ve solved my issue (I hope), but thought I’d put it out there in case someone comes across my post with a similar issue.
VICTRON – why can’t you make your external controls fully compatible with each other (and make how to do it clear in the manuals too). I have 3 Victron devices I want to control. MPPT with that stupid adapter cord (that is fail ON, not fail OFF – which is why I had to get a BP), BP with “pull + high or pull - low,” and MP with “pull + low” (although that’s not documented – the manual says “take + to -” but a little digging finds that “-” is simply ground).