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mbutts avatar image
mbutts asked

System battery flat, shouldn't the BMV712 relay prevent this?

I did my monthly visit my motorhome in storage. It has two 12V AGM batteries in parallel that are about a year old. When I arrived, the battery bank was discharged and the BMV-712 wasn't lit.

I ran the motorhome's generator (puts out about 30A) and the Victron app was showing an oscillating +0.2 A/-0.2 A line so I'm assuming they were too discharged to be detected by the motorhome's charger. The bank showed a SOC of 100% and voltage of 5.21 after two hours of generator time.

I have the relay setting at 77% to prevent this much of discharge while in storage. Shouldn't this setting cut off all power draws so the battery bank doesn't get this low?

BMV Battery MonitorRelaydischarge level
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Justin Cook avatar image
Justin Cook answered ·

@mbutts, think of the relay as a programmable signal wire... it can be programmed to send a signal based on various criteria such as low or high voltage or SOC, but it can't do anything on its own - that signal only does any good if there's a connected component that can do something with the signal (such as the Victron BatteryProtect).

One use for the relay-out would be to connect a high-current latching relay at the POS battery terminal and program the BMV to send a signal to open the relay (disconnecting the battery from the system) at a certain voltage; this would prevent parasitic draw from drawing the battery down beyond that voltage. One could also set a similar trigger based on SOC; the danger in an SOC-based setting is that -as mentioned previously- SOC calculation is based on current measured through the shunt, so anything not running through the shunt (say, a load connected directly to the battery, or battery self-discharge) doesn't count against the SOC calculation. Thus you could have a severely depleted battery that still shows as 100% SOC because the BMV can't measure what doesn't run through its shunt.

In your case, since the BMV had lost power, the SOC showed as 100% when you restarted it because the unit defaults to 100% when first powered on... so that bit isn't a mystery, at least.

The default 50% setting that you refer to does tell the BMV what the discharge floor of your batteries is, so the 50% setting is appropriate for a Pb battery that you don't want to run down past 50% capacity, but it's for alarm and calculation purposes - the BMV can't actually do anything about whether or not you discharge beyond that. It will alarm to let you know you shouldn't discharge further, but that's all it can do.

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mbutts avatar image mbutts commented ·

Thanks for the thoughtful and thorough answer. I've been frustrated with the device due to a lack of information. The setup instructions were exceedingly brief (one page pictogram), the manual, while complete, assumes knowledge of the device, and the Internet is chock-full of many opinions, not facts, on use of the BMV712.

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Justin Cook avatar image
Justin Cook answered ·

@mbutts, the BMV cannot prevent discharge of your batteries; the relay-out is designed to trigger an external control that, depending on what you have connected to it and how you have it set, may do any number of things... but preventing discharge is not one of them unless that relay out is triggering another relay that disconnects your batteries entirely from all parasitic loads... and even then, there's simply no preventing self-discharge of AGM batteries left sitting. Think of the relay-out as a switch... you can program it to do all kinds of things, but it doesn't do anything unless you hook something up to it.

The BMV is only a measurement device. It neither controls charging nor discharge of a battery bank. Please fully read and understand the user manual for more information on what the BMV does, how it functions, and what the relay outputs do and how they may be used.

EDIT: I will also mention that if your SOC is showing on the BMV as 100% while your batteries are fully discharged, then either the shunt has been incorrectly connected and is not, in fact, measuring all current in/out of the battery bank, or your batteries are self-discharging far more than they should be given their age and they should be tested. The BMV cannot, of course, measure self-discharge of a battery. So, first read the user manual, then check your connections and be certain that the shunt was installed correctly and the other parameters of the BMV programmed correctly, and then go have your batteries tested.

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mbutts avatar image
mbutts answered ·

@Justin Cook

Thanks for the speedy reply! I was looking for clarification on the relay feature since I found the manual's description of the relay function to be very vague. I made the leap in my mind that the reason the relay's factory setting was 50% was to prevent deep discharges.

I did sent a photo of my wiring to my local Victron sales rep last year & he said I had the shunt wired correctly.

I have to pull the batteries to charge them back at my home so I'll have them tested by the battery retailer while I have them out.

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