Best practice for protecting battery when running DC applications directly...

[Jeremiah]

I wasn't sure if the BMS forum was the right place to post this or not so feel free to move it admins.
So I know Will is now recommending to NOT use low voltage disconnects in conjunction with Dc-AC inverters.
I plan on running several DC fans and lights directly from my battery and I'd rather not count on the BMS to do the disconnect if voltage gets too low.
Seems what I've gleamed from everyone is it's best to leave the BMS protections as a last resort (i.e. inverter fails to shut off etc).
What is the best practice in my situation?

48v system, Hybrid inverter, + direct DC stuff going on.
I'm fairly budget conscious so please nothing over $50. I plan to draw < 500watts DC. I've got several ideas on how to handle this but would rather know what the best practice is.
Thanks in advance!
-Jeremiah

 

[gnubie]

What model inverter are you looking at? It may have a low voltage relay set that it closes to kick off a generator. If it has a NC contact, ie when the inverter things volts are good the NC pair passes current, you could use that to control your lights and fans. Failing that you could always get an ebay voltage controlled switch.

 

[Jeremiah]

What model inverter are you looking at? It may have a low voltage relay set that it closes to kick off a generator. If it has a NC contact, ie when the inverter things volts are good the NC pair passes current, you could use that to control your lights and fans. Failing that you could always get an ebay voltage controlled switch.

I bought a 6kw Hybrid MustPower.
It does have AGS to kick off a generator; and I'll have it set to fall back on mains.
you're saying I could key into that to throw a relay to cut power to my DC appliances?
I like the idea of the main inverter making the decision instead of spreading the brains out to multiple systems.

 

[John Frum]

@Jeremiah a

I wasn't sure if the BMS forum was the right place to post this or not so feel free to move it admins.
So I know Will is now recommending to NOT use low voltage disconnects in conjunction with Dc-AC inverters.
I plan on running several DC fans and lights directly from my battery and I'd rather not count on the BMS to do the disconnect if voltage gets too low.
Seems what I've gleamed from everyone is it's best to leave the BMS protections as a last resort (i.e. inverter fails to shut off etc).
What is the best practice in my situation?

48v system, Hybrid inverter, + direct DC stuff going on.
I'm fairly budget conscious so please nothing over $50. I plan to draw < 500watts DC. I've got several ideas on how to handle this but would rather know what the best practice is.
Thanks in advance!
-Jeremiah

The 48 volts bit makes that tricky.
The discrete low voltage disconnects are generally 12 and/or 24 volts.
Its not ideal but if you are using a buck converter you could put an lvd like a victron battery protect on the 12 volt side.

 

[John Frum]

I found this https://www.poweringthenetwork.com/lvd-wall-mount/
disconnects pretty low though.

 

[gnubie]

I bought a 6kw Hybrid MustPower.
It does have AGS to kick off a generator; and I'll have it set to fall back on mains.
you're saying I could key into that to throw a relay to cut power to my DC appliances?
I like the idea of the main inverter making the decision instead of spreading the brains out to multiple systems.

Yes, quite possibly.

I downloaded the manual but it's not really clear what the AGS output gets up to. I'd check it to see if there are volts coming out or if it's intended for dry (no power) connection to the generator controller, ie the generator controller supplies voltage into the connector and the inverter merely switches it. If there is just one contact set and it closes when the generator is to be started I would slave another relay off it and use that relay's NC (normally closed) contact set to supply your DC loads. That way when the inverter closes it's contacts the slave relay pulls in and disconnects the load.

The risk here would be that if the control board in the inverter actually turns off due to excessively low battery voltage your own loads would be reconnected. There's ways around that with using the AGS to control things, but cross that bridge when we get to it. First step is to see how the AGS actually works.

I can do up some simple diagrams showing how it'd work for the case where the inverter is supplying volts out of the socket itself, and if not.

 

[Dzl]

Hi Jeremiah, unless I'm misunderstanding you (quite possible), I think that you may have misunderstood Will.

So I know Will is now recommending to NOT use low voltage disconnects in conjunction with Dc-AC inverters.

The issue isn't using a Battery Protect or similar device in conjunction with an inverter, the problem is using it inline with an inverter. If you use it just for DC loads, you are using it as intended. There are two things that you don't want subject the Battery protect to (1) reverse current (2) "loads with capacitors such as inverters."

See here (manual)
See here (explanation)

I plan on running several DC fans and lights directly from my battery and I'd rather not count on the BMS to do the disconnect if voltage gets too low.

This sounds like specifically what the Victron Battery Protect is designed to do: Low voltage disconnect for DC devices.

If I'm misunderstanding your goal, question, or scenario, I apologize.

 

[Jeremiah]

Hi Jeremiah, unless I'm misunderstanding you (quite possible), I think that you may have misunderstood Will.
This sounds like specifically what the Victron Battery Protect is designed to do: Low voltage disconnect for DC devices.
If I'm misunderstanding your goal, question, or scenario, I apologize.

You're correct, I just had that tidbit in my my mind from some video months back but I didn't know why.
Thanks. I will say whenever I see the word "Victron" I put my hand on my wallet to protect it.
Yeah a quick ebay search reveals I'd have to shell out $150 + shipping to Philippines for a 48v Victron this is not within my budget.
I appreciate the input though.

I'm leaning towards @gnubie 's solution to listen in on the generator automatic start line.
I think I can do a pretty cheap and safe solution that way.

I downloaded the manual but it's not really clear what the AGS output gets up to.

Hey above and beyond thanks! I also read through it last night and didn't find any details on that signal.

I'd check it to see if there are volts coming out or if it's intended for dry (no power) connection to the generator controller, ie the generator controller supplies voltage into the connector and the inverter merely switches it. If there is just one contact set and it closes when the generator is to be started I would slave another relay off it and use that relay's NC (normally closed) contact set to supply your DC loads. That way when the inverter closes it's contacts the slave relay pulls in and disconnects the load.

dynamite

The risk here would be that if the control board in the inverter actually turns off due to excessively low battery voltage your own loads would be reconnected. There's ways around that with using the AGS to control things, but cross that bridge when we get to it. First step is to see how the AGS actually works.

I'm thinking this shouldn't be an issue as I will be using mains for the inverter's "backup power." So when the battery is too low (I'll be setting this to 20% SOC), the inverter will draw power. In the unlikely case (though certainly possible here) that the battery is low and there is a brownout, I think I'm OK falling back on the BMS to disconnect when things get too low. DO you think this sounds reasonable?

I can do up some simple diagrams showing how it'd work for the case where the inverter is supplying volts out of the socket itself, and if not.

I appreciate it; I'll probably just go overkill and use a simple PIC; haha the non-EE brute force approach.

Thanks everyone, I'll post an update / pics when my stuff arrives.

 

[gnubie]

Yes, if the inverter control board did power down and reconnect your DC loads the BMS would eventually step in and disconnect when the battery voltage got too low.

 

[FilterGuy]

What model inverter are you looking at? It may have a low voltage relay set that it closes to kick off a generator. If it has a NC contact, ie when the inverter things volts are good the NC pair passes current, you could use that to control your lights and fans. Failing that you could always get an ebay voltage controlled switch.

That's brilliant!