How to transfer AC loads from BMV relay output?

jdobush

I'm trying to squeeze a little extra out of my off-grid system. I have regular grid power available nearby, this is just a project in my garage. I'm using a Victron BMV-712 battery monitor that has a configurable relay that I can set up for state of charge or voltage levels. 

I haven't found any transfer switch that seems to have an external input for transferring loads. For example, I'm looking to add a load when the battery bank is fully charged, or close. My inverter is very limited on the transferring, and is voltage based, so it would kick out on a heavy load, then back in just because the solar charger is bringing up the Bulk voltage again. 

I found a similar product from amazon with configurable voltage levels, but it still won't know or wait to have a full absorb cycle. I have multiple needs here i guess, but if I can add/subtract a heavier load when I'm not in a full state of charge I can avoid the worst of this constant cycling.

BB.

Just plain double throw relays would work. Example of connection (not right or wrong--Just example):

Load = Common terminal
AC Mains = Normally Open
AC Inverter = Normally Closed

https://www.amazon.com/double-pole-throw-relay/s?k=double+pole+double+throw+relay

A typical coil based relay takes around 6 Watts--So that is extra load/wasted power--Could be an issue for smaller power systems (6 Watts * 24 hours per day = 144 Watt*Hours of load when coil is "on").

There are bi-stable relays (only use power to switch states).

And there are solid state relays (single throw mostly? On/Off) where you could add or remove a load...

Not sure exactly what you want (size of loads, is OK if AC power is "glitched to load" during switching, etc.)....

There are large/commercial PDU (power distribution units) used in computer rooms and such... Probably way more than you want--But it is out there:

https://www.eaton.com/content/dam/eaton/products/backup-power-ups-surge-it-power-distribution/power-distribution-for-it-equipment/eaton-power-distribution-handbook-MZ155002EN.pdf

-Bill

jdobush

Thanks for the advice! I guess I needed to dial back my searches for very specific products to component level. Makes a lot more sense now. 

It looks like my BMV and solar charge controller both have relay contacts but need an external power source. Good point on looking at the current draw too. I imagine if I set it up to be normally open, and closed only when I'm in a surplus situation that would avoid drawing from the batteries at rest.