Victron BMV-712 questions

[billrector]

I’ve had this battery monitor installed for about a year now. This particular trip I’m having some questionable readings.

When I got to my first destination after driving for the entire day, I turned on the furnace and immediately got a low voltage alarm (12.1V) on the battery monitor which I thought was odd. I turned off the furnace and the monitor voltage jumped to approximately 12.5...which I also thought was odd. One disclaimer....I did add a cigarette style plug to the batteries so I could use a portable tire inflator. I know it is not on the proper side of the shunt but I plan on using this infrequently. I have since removed it.

Last night after being fully charged before dark through mostly solar, I again ran the furnace and got a low voltage alarm again but the % charged was still very high on the monitor?

I charged the batteries for only about an hour this morning using a generator and it again showed the batteries fully charged. The history shows that my maximum consumption was 47 Ah...which is way below the 85 or so Ah that would equate to a 50% discharge of my stock Interstate batteries. These batteries were replace under warrant about 6 months ago. I also have upgraded my converter to a Boondocking multistage converter.

So, I’m a bit lost as to why my battery voltage is so low so quickly after being fully charged and why the monitor still shows plenty of capacity when I hit this voltage alarm.

[billrector]

I continue to be confused by this. I arrived back to the trailer last night after a great day of solar activity. Batteries showed 100% and voltage was at 12.9V. Yesterday, I checked my setup in the Victron and my Ah setting for the batteries is correct at 168 Ah.

Upon arriving home, we turned on the inverter to watch a movie. A little later, the furnace turned on as it got well below freezing here last night. I watched the voltage drop to 12.4 but the % charged stayed around 90%. Later, I turned on the generator just to boost my chances of making it through the night. This morning batteries showed 11.84V but the % charged was 83%. This makes no sense.

The only possible explanation that I can come up with is that my batteries (which are maybe 6 months old) are NOT capable of 168 Ah. I take great care of these batteries and use a disconnect switch when in storage. So, either my batteries (or battery) are bad or the temperatures getting below freezing are greatly impacting the batteries??

[ROBERT CROSS]

IMO...the best place to start is a more capable battery pack.
2 100ah AGM's would be a good place to start for a reasonable amount of $$$.Our set lasted 11yrs.👍
Just replaced this year with 200ah of Battle Born Lithium and a PD adjustable converter.

I drew 138ah with the furnace alone over 12hrs.

Bob
🇺🇸

[daleyocum]

Assuming something is not drawing an excessive amount of current (check that on the battery monitor) that does likely mean your batteries aren’t up to the job. There really is no substitute for AGM or, better, Lithium.

[AirMiles]

The stock Interstates are not true deep cycle batteries, are aging quickly, and are not providing anywhere near their rated amp hours. https://marinehowto.com/programming-a-battery-monitor/
https://marinehowto.com/what-is-a-deep-cycle-battery/

[ROBERT CROSS]

Originally Posted by ROBERT CROSS

IMO...the best place to start is a more capable battery pack.
2 100ah AGM's would be a good place to start for a reasonable amount of $$$.Our set lasted 11yrs.👍
Just replaced this year with 200ah of Battle Born Lithium and a PD adjustable converter.

I drew 138ah with the furnace alone over 12hrs.

Bob
🇺🇸

oops correction...54ah. Brain lags behind fingers. 😂
10/14/18
9:30am 40*amb. furnace @68 Batt 12.8 -7.3a
4:26pm 40* 76% 12.96 - 6.7a. -39ah
6:15pm 74% 12.94 -43ah
9:55pm 39* 68% 12.87 -7.3 -54ah

Bob
🇺🇸

[JeffE]

The battery monitor should give you (at least) 3 values:

- AH used (capacity used/remaining)
- Current voltage
- Current amperage draw

As the load on a battery increases (drawing more amps from it), the voltage will decrease, and the current draw will increase. If a battery is at rest (no draw) and has been resting for awhile, then you can use the battery voltage as a rough indicator of the capacity remaining (but the actual numbers vary by battery make and type). But you can't use voltage as much of an indicator during discharge.

You can see this yourself -- look at the voltage when there's no load, then start up the furnace. Note the amperage draw and the voltage. Then turn off the furnace, turn on the inverter and put a load on it. Check the amperage and voltage. You'll see the pattern.

In your case, I suspect you have the low battery voltage alarm set too high -- the furnace is drawing current (I don't know how much, but anecdotally, I was told that a 200 Ah AGM bank won't support the furnace overnight on a 25' FC). So the battery voltage goes down.

Assuming you have lead-acid batteries (not lithium):

So the question is: what should you set the low battery alarm at? I don't have the answer. My old Trace SW2512 used a default of 11.8V, I believe.

If you're concerned about the condition of your batteries, I'd suggest fully charging them, then run something to draw them down to where the meter reports the state of charge to be, say 75%, then let them rest and look at the voltage. Compare this voltage to the manufacturer's stated rest voltage capacity graph (most publish one). If the graph shows 75%, you've got happy batteries. If it says 50%, then they don't have the capacity you expect -- they're either tired, or just started with less capacity than you expected.

An even more accurate test is to do the same test, but use a specific gravity hydrometer to measure the state of charge. This will work on flooded lead acid batteries, but not AGMs or gels. And it's virtually irrelevant for lithium chemistry.

Jeff

[uncle_bob]

Hi

Most likely cause of all this is a lose connection. It might be at a battery post, it could be elsewhere.

Second most likely cause could be the 712 not being wired directly to the battery post.

Number three would be only one of the two batteries being in the circuit ( = wire missing / disconnected between them)

Number 4 would be the shunt being in the wrong location.

=======

So let's *assume* none of that is the problem and just look at the batteries:

If you have a typical furnace (as if anything is ever typical in an AS) you should see it pulling about 8 A on the 712 when it kicks on. If you see something else then either you have a different furnace or a shunt issue. Ten hours at 8A will pretty much nuke the stock batteries. This is with full charge and at 70F. (yes there's more to follow ....)

The inverter when doing not much of anything ( running the TV and other basic nonsense) can pull upwards of 2A. Do that on top of the furnace and you are up around 10A or so. Now you have a bit less than 10 hours ....

If it's cold out, and your batteries are outdoors, they don't have "full" capacity. A typical number you see is 20% capacity loss when at freezing. Get down around zero F and you may be below 50% capacity. There are variations in battery chemistry so any numbers like this are a guess.

On top of that, all your voltages change at low temperature. The 12.6V that you *think* means full charge actually means "stop using" when it's cold out. ( = the voltages all shift higher when it's cold). Your charger needs to understand this stuff ( = have a temperature probe) so it can charge them to a higher voltage.

Combine less capacity with not getting fully charged and what happened may not be a big surprise. Just as with any "batteries go to zero" process, this is not good for your batteries.

Bob

[daleyocum]

Originally Posted by uncle_bob

Hi

Most likely cause of all this is a lose connection. It might be at a battery post, it could be elsewhere.

Second most likely cause could be the 712 not being wired directly to the battery post.

Number three would be only one of the two batteries being in the circuit ( = wire missing / disconnected between them)

Number 4 would be the shunt being in the wrong location.

=======

So let's *assume* none of that is the problem and just look at the batteries:

If you have a typical furnace (as if anything is ever typical in an AS) you should see it pulling about 8 A on the 712 when it kicks on. If you see something else then either you have a different furnace or a shunt issue. Ten hours at 8A will pretty much nuke the stock batteries. This is with full charge and at 70F. (yes there's more to follow ....)

The inverter when doing not much of anything ( running the TV and other basic nonsense) can pull upwards of 2A. Do that on top of the furnace and you are up around 10A or so. Now you have a bit less than 10 hours ....

If it's cold out, and your batteries are outdoors, they don't have "full" capacity. A typical number you see is 20% capacity loss when at freezing. Get down around zero F and you may be below 50% capacity. There are variations in battery chemistry so any numbers like this are a guess.

On top of that, all your voltages change at low temperature. The 12.6V that you *think* means full charge actually means "stop using" when it's cold out. ( = the voltages all shift higher when it's cold). Your charger needs to understand this stuff ( = have a temperature probe) so it can charge them to a higher voltage.

Combine less capacity with not getting fully charged and what happened may not be a big surprise. Just as with any "batteries go to zero" process, this is not good for your batteries.

Bob

Echoing what Bob said do check the terminals and battery posts for corrosion and clean them no matter what. That said you will likely be happier with more battery capacity on board.

[billrector]

I do see that I draw 8 amps on the 712 when the furnace is running. However the furnace does not run non stop....maybe just 50% of the time? So I would think that with stock batteries I could make it about 20 hours? I get no where near that. Connection to batteries is good and no corrosion. I also have 0 gauge cables between the batteries. All loads are wired to the load side of the shunt.

[KelloggKid]

A couple of random thoughts......

1) In a couple of places you stated that your batteries were fully charged. How did you determine this? An hour or so on a generator will not fully charge a battery that is down as far as you've described. I've been surprised at how long it takes, even on shore power. Look for the voltage to have stepped its way down to about 13.2 V and the current to be sitting at 0. That would indicate a fully charged set of batteries.

2) Forget the State of Charge indication on the meter. Even if you were able to get everything tweaked to where it is accurate one day it will be off the next. The most important things for you to watch are the battery voltage the current draw and the amp hours consumed. As others have pointed out, a large current draw such as the furnace will pull the voltage down so if you want an indicator of what the batteries state is wait until the current has dropped off (ie the furnace has shut off).

3) As others have also said, the Interstate batteries will probably never put out the amount of amp-hours you are expecting. That said, a lead acid battery needs to be fully cycled a number of times before they can put out their best performance. If your batteries are only 6 months old you may not have cycled them enough to get the best amp hour capacity. Especially if they weren't fully charged when you thought they were. I have two Group 24 Interstates in my trailer and have set my Victron battery capacity at 100 amp hours and while the SOC indicator is better than it was it is still off.

4) If you study the current draw over time you will come to learn what each item in your trailer consumes. Once you develop that awareness then you can adjust your lifestyle to maximize your battery life. For example, we have learned that the two swivel lights over our dinette used less power then the overhead ceiling lights so when we are boon docking we use them and keep the ceiling lights turned off.

5) If the shunt for your Victron system is installed inside the trailer, as most are, there is a possibility that your inverter didn't get wired into it. The ground circuit for all the loads in your trailer are all available internal to the trailer and easily wired into the shunt. The one exception is usually the inverter which on some models is wired directly to the batteries and the wires may not pass through where they are readily accessible. The inverter draws power even when turned off. More than you would think. You can easily test to see if the inverter is wired into the Victron system by watching the current draw on the Victron meter, turn the inverter on, and then turn on and off an AC load such as the TV, small fan, desk lamp, etc. You should see the current flow fluctuate as the AC device is turned on and off.

6) The Victron needs to have seen at least 10 full charge cycles to gain experience with your system. If you don't have the parameters set properly, it may not have synced. Look at the history data and see how many times it has "synced". If you aren't letting the batteries fully charge, or don't have the system set so that it knows when they are fully charged, then a sync will not occur.

[uncle_bob]

Originally Posted by billrector

I do see that I draw 8 amps on the 712 when the furnace is running. However the furnace does not run non stop....maybe just 50% of the time? So I would think that with stock batteries I could make it about 20 hours? I get no where near that. Connection to batteries is good and no corrosion. I also have 0 gauge cables between the batteries. All loads are wired to the load side of the shunt.

Hi

Ok, if the converter charger only gets the batteries to 13.6V, then they only are charged to about 60% of capacity when cold. That capacity is about 30% lower than at normal temperatures. Net result is you only *have* at most 40% of what you expect to have. That's with the batteries working fine.

Toss in the fact (mentioned above) that you need about 8 hours of generator run time to fully charge the batteries ( in this case get them to the 60% point) and you are even worse off .....

None of this is the batteries fault. It's just the way they work when they are cold. We rarely set things up to fully charge them when cold because most RV people pack it up when it starts staying below freezing ....

Since the 712 looks at current, it is not going to help you in this case. It will see the current taper off as part of the charge cycle. It then decides the battery is "full" and resets to max amp hours. It's not that the 712 is stupid, it is doing exactly what it is set up to do. The gotcha is that the charger is not ramping up to higher voltages like it needs to.

Bob

[ROBERT CROSS]

snip.."I have two Group 24 Interstates in my trailer and have set my Victron battery capacity at 100 amp hours and while the SOC indicator is better than it was it is still off."

Shouldn't it be 2x100ah=200ah. It is for our 2 BB's.
The SOC works very well for the BB's.
You could try setting at 198...see the SOC is more accurate.

Bob
🇺🇸

[KelloggKid]

Originally Posted by ROBERT CROSS

snip.."I have two Group 24 Interstates in my trailer and have set my Victron battery capacity at 100 amp hours and while the SOC indicator is better than it was it is still off."

Shouldn't it be 2x100ah=200ah. It is for our 2 BB's.
The SOC works very well for the BB's.
You could try setting at 198...see the SOC is more accurate.

Bob
🇺🇸

I started out with the assumption that my capacity was 2 X 80 = 160 so set the Victron capacity at 160. When my voltage got down to 12.3 the SOC reading was over 90%. I then changed the capacity setting to 140 and the SOC, while still high, was closer. I continued to increment the capacity setting downward and now have the capacity set at 100 and when my batteries get down to 12.3 the SOC is sitting at about 60%.

The SOC reading will never be accurate and I do not depend upon it. The interesting thing is the lack of correlation between the amp hours consumed, the capacity setting, and the SOC. Obviously when the Victron system calculates SOC it not only looks at how many amp hours have been consumed by also it factors in things like the rate at which the consumption occurred. A slow (low current) draw down will not effect the battery capacity to the same extent as a high current draw down.

As I stated earlier, the main benefit of the Victron for us has been teaching us how the various devices consume power and then using that knowledge to make good decisions as to their usage. As an example, after owning the trailer for a year we realized that we had never once used the inverter and were highly unlikely to ever do so. Having learned how much power it consumed even when turned off, I totally disconnected it which has had a positive effect on our battery capacity.

[uncle_bob]

Hi

If your 712 does not give reasonable numbers then:

1) It's broke
2) It's not wired right
3) It's not set up right
4) You are running at some crazy temperatures

Run under normal circumstances, the device gives very accurate numbers.

Bob

[waninae39]

my 1st 712 was defective
the vendor sent me a 2nd one

its working fine

[SilverHouseDreams]

Just a side question, many of these trailers have 3 sets of cables that go from the battery to the trailer. One set to run to the 12V distribution center, and one for the inverter. Is your inverter cabled through the BMV shunt for certain?

[ROBERT CROSS]

Originally Posted by uncle_bob

Hi

If your 712 does not give reasonable numbers then:

1) It's broke
2) It's not wired right
3) It's not set up right
4) You are running at some crazy temperatures

Run under normal circumstances, the device gives very accurate numbers.

Bob

Yes I'm not understanding either...if it's NEVER going to be accurate what's the point of having it?
SRM24's are only rated to supply 140min@25a draw.
A more capable battery in the loop would be a big improvement.

Bob
🇺🇸

[uncle_bob]

Originally Posted by ROBERT CROSS

Yes I'm not understanding either...if it's NEVER going to be accurate what's the point of having it?
SRM24's are only rated to supply 140min@25a draw.
A more capable battery in the loop would be a big improvement.

Bob
🇺🇸

Hi

Just to beat this dead horse one more time:

Starting from 140AH at 70F, you get 70AH usable before you should shut down (with full water in the batteries, fully charged ... ).

70 AH at 70F goes to <50AH at 32 F. It likely drops below 30AH at zero *if* it's fully charged.

At 32F when the battery is to the "don't use" point, it will be at 12.6V. Running it to 11.8V at 32 is no different than running it to 11.2V at normal temperatures.

12V at 32F is roughly the 10% capacity point on the battery. This is not good, you are in the "may damage" range.

A normal charger is trying to get the battery up to 12.6V. To charge when well below freezing it needs to target something like 13.4V as the finish point. The voltages to do this will quickly damage a battery in the middle of the summer.

It is likely (but never sure) that a cold battery only gets to about 75% charge level with a normal charger, even after hours when it's very cold.

We started at 30 to 50 AH in the cold. With only 75% charge we now are down to 15 to 25 AH. Run a furnace at 7 or 8A and 50% duty cycle for 8 hours and you will have either one dead flat.

It's not magic, it's just the way batteries work.

Going to bigger batteries will take your 15 to 25 up to ... umm ... maybe 35 AH (at most). That barely gets you through the night with the furnace.

Going to a temperature compensated converter / charger gets what you have back to maybe 50AH. You should get through the night that way.

====

What do others do?

Well, I have 200AH of lithium's under the sofa. That gives me 200AH usable capacity even when it's cold out. My furnace pulls way less power, so the combination does pretty well.

Bob

[ROBERT CROSS]

At 40* ambient, furnace set @ 68
after 12hrs...
9:55pm SOC 68% 12.87v -7.3a draw -54ah used.

Bob
🇺🇸