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leftcoastman35 avatar image
leftcoastman35 asked

Wiring an alarm with the VE.BUS BMS "load disconnect" wire?

We are trying to wire a simple alarm to the Ve.BUS BMS to alert us when there is a low cell voltage condition on our Smart Lithium batteries. Our system is 24v.


The Ve.BUS BMS "load disconnect" wire sends "battery voltage" or thereabouts when there is NO low cell voltage condition. When there is a low cell voltage condition, there is zero voltage on this wire.


Apologies as I'm not an engineer, but I do have basic wiring knowledge. How could I wire a simple 24v alarm such that it alerts me when there is a low cell voltage condition?


PS - we have tried to use the BP-220 but burned it up within a day on our boat as the loads are too high. I know this alarm isn't perfect, but it's better than what we have now, which is nothing.

BMS
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wkirby answered ·

Use the Pre-alarm terminal. From the datasheet:

Pre-alarm
The pre-alarm output is normally free floating and becomes high in case of imminent cell under voltage (default 3,1V/cell, adjustable on the battery between 2,85V and 3,15V per cell). Maximum current: 1A (not short circuit protected). The minimum delay between pre-alarm and load disconnect is 30 seconds.

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leftcoastman35 answered ·

I must have an older version of the VE.BUS BMS because I do not have the "pre-alarm" output.


There are only 4 outputs on the BMS:

battery -

battery +

charge disconnect

load disconnect.


Is there a simple relay I can use that would invert the load disconnect output?

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wkirby avatar image wkirby ♦♦ commented ·

In that case, yes, you can use a simple relay. Wire the coil to the "Load disconnect", don't forget to put a flyback diode in parallel with the coil otherwise the back EMF from the relay coil will damage the BMS.
Wire the 24V alarm via the NC (normally closed) contacts of the relay to the 24V supply.
This way, when the BMS "Load Disconnect" goes low, the relay will lose power and the NC contacts of the relay will close and the alarm will sound.

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