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Laura Hoey avatar image
Laura Hoey asked

BMV-712 always reads "Infinity"

I connected the BMV-712 to a new 170ah LFP battery yesterday (BMV is about 1-year old, and worked last year on a Lead Acid set-up). Although it initially indicated "10d 10h" when the battery was disconnected from shore power, now it reads "Infinity". I have done a factory reset, but nothing changed. I disconnected the UTP cable, but nothing changed. The battery monitor also consistently reads .11v lower than a direct read at the battery (I had this same problem with the Lead Acid set-up). For example, in the attached image the reading is 13.69, but a direct read at the battery is 13.8. I also wonder why "Starter Battery" indicates a voltage amount when no starter battery is connected?

Are there any tricks I can try, or is it possible I have a bad monitor? Maybe a bad UTP? Please forgive the messiness of my set-up right now, it will get cleaned up once I isolate the problem.

BMV Battery Monitor
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2 Answers
wkirby avatar image
wkirby answered ·

Is the battery fully charged and then synchronized? If it is not synchronized, it does not know how full the battery is and therefore doesn't know how much time to go. So it shows infinity.
Also ensure that you have amended the battery capacity setting on the BMV. The SOC calculations rely on knowing the correct battery capacity.

For the Voltage discrepancy, remove the fuse from the inline fuse holder along the red wire. If you can read it's rating, it's probably 100mA. This fuse has an impedance of about 6 Ohms. This calculates to a Voltage drop of about the amount that you are seeing.
Set that fuse aside and install a 1A fuse. The Voltage readings will be super.

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Laura Hoey avatar image Laura Hoey commented ·

Thanks for your response, especially the inline fuse question (that was a real annoyance). I synced the battery last night when the voltage read 14.57v, which I consider fully charged for a LFP battery. It still continued to show Infinity. I may try bypassing the monitor all together, bring it up to full charge and then reconnect the monitor to see what happens.

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bora avatar image
bora answered ·

When I installed my BMV-712 I had the same problem, turned out I had the shunt wired up wrong. So a reversal of the -VE leads sorted the problem.

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Laura Hoey avatar image Laura Hoey commented ·

Thanks for your response. That's interesting.... I only have one lead connected, and it's to the B1 slot (closest to the UTP connection). If I try B2 I get no power. I may have to just ignore it, I never took the information that seriously anyway.

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Mark avatar image Mark ♦♦ Laura Hoey commented ·

@Bora is talking about the main NEGATIVE cable connected to the large M10 bolts on the shunt, not the positive cable connected to B1.

It's essential that the side of the shunt labeled as 'Battery Only' is connected directly to the battery negate terminal & that ALL loads & charge sources are connected to the other end labeled as 'Load / Charger'.

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Laura Hoey avatar image Laura Hoey Mark ♦♦ commented ·

I see, thanks for that clarification. Unfortunately (for me finding a solution), the wiring between the shunt and battery is correct. I'm beginning to wonder if the UTP cable became corrupted in some way.

Thanks for your input.

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Mark avatar image Mark ♦♦ Laura Hoey commented ·

So you have properly checked what @Bora suggested?

Can you post some photos including the PCB on the shunt?

The most likely cause for your issue in shunt installed the wrong way around.

Also, if you are charging or drawing no load or a very small load then it's normal to read infinity. Also make sure you have entered the correct battery capacity.

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Laura Hoey avatar image Laura Hoey Mark ♦♦ commented ·

Yep - here's a photo of the shunt. Negative lead from the trailer (disregard the red heat shrink) is connected to the 500A side of the shunt, and the 50mV side of the shunt is connected to the Negative side of the battery (and no other connections). B1 is connected to the Positive terminal, as is the Load lead from the Trailer. Shore power is disconnected, and has been since 4/23/2019. Trailer battery disconnect has been disconnected that entire time, too (it is a true disconnect). Battery was fully charged when shore power was disconnected on 4/23, and it has been slowly settling down (trying to ascertain where it's "resting" point is).

But, I think you may have answered my question/concern: If I am not drawing a load, even if the battery is not connected to shore power (or any other source of power) it will read Infinity. I don't recall this when I had it hooked up to a Lead Acid battery, but maybe when my wet cell was hooked up it was always either charging, or partially charged. I think you have answered my question!

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Mark avatar image Mark ♦♦ Laura Hoey commented ·

Yes the shunt wiring is the correct way around.

If your leaving the battery disconnected from all loads (just BMV connected) for an extended period then don't rely on SOC % as it doesn't consider battery internal discharge. Simply rely on the resting voltage instead during this period.

Also some wiring suggestions;

For the lowest resistance, cable terminals should be in direct contact with the shunt blocks, both washer & spring washer should be fitted above the terminal.

The 'load / charger' side of the shunt is still negative / '-' but I see it labeled as positive / '+' in the photo, this may cause confusion.

I dont know what your maximum load is (Amps), but the negative cable looks a little small if your running an inverter for example. Ensure wiring size is capable to handle the max load & also the fuse / circuit breaker rating. It also has to be sized to limit voltage drop over the length to an acceptable limit.

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Laura Hoey avatar image Laura Hoey Mark ♦♦ commented ·

Thanks for the suggestions!

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