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billrector · 03-25-2018, 07:21 AM

Hi,

Installed a Victron 712 battery monitor recently and am on my first trip with it. I am currently sitting at 86% according to the battery monitor but my voltage is only 12.36. Does that make sense? I would think the depletion % would be more. I am using the stock Interstate batteries and saw various amp-hour capacity ratings on one when I was setting up the system. Currently, I’m using 168 Ah for my setup.

Thanks,
Bill

Adventure.AS · 03-25-2018, 07:39 AM

Your reading may be fairly close. 12.4 volts is about 80%.

When you programmed the BMV-712 what amp hour capacity did you use? Perhaps your older batteries don't hold their rated capacity any more. Temperature is also a factor, have you accounted for that?

billrector · 03-25-2018, 10:20 AM

Quote:

Originally Posted by Adventure.AS

Your reading may be fairly close. 12.4 volts is about 80%.

When you programmed the BMV-712 what amp hour capacity did you use? Perhaps your older batteries don't hold their rated capacity any more. Temperature is also a factor, have you accounted for that?

Batteries are brand new....just replaced under warranty by Interstate. I’m using 168 Ah for the two batteries.

**wulfraat** · 03-25-2018, 10:29 AM

What temperature is it outside?

Lower temperature causes a lower voltage reading even though there is higher capacity available.

That’s the beauty of the bmv- if wired correctly it measures all loads and energy to/from the battery to provide an accurate guage of current capacity.

billrector · 03-25-2018, 10:30 AM

In FL at the moment so temps were in the high 60’s when I looked this morning.

**wulfraat** · 03-25-2018, 10:38 AM

Did you ensure that all negative side of the loads in the trailer are wired to the shunt? Inverter, propane detector, sub woofer, etc?

billrector · 03-26-2018, 06:45 AM

Quote:

Originally Posted by wulfraat

Did you ensure that all negative side of the loads in the trailer are wired to the shunt? Inverter, propane detector, sub woofer, etc?

I believe so! I basically spliced the two negative battery cables that go from the battery box to the trailer...one for the house system and one for the inverter. I also retired the Zamp connection to be on the load side of the shunt.

wcrockett · 03-26-2018, 12:26 PM

How many Amp-hrs does the meter show consumed since that last discharge? Also, I think you may need to discharge and charge the batteries a few times for the Victron meter to calibrate.

**Rocinante** · 03-26-2018, 01:42 PM

Have not set up a BMV yet, but is that 168Ah configured as total at full charge or available at full charge? That number makes sense for total, but with lead-acid batteries you can only use half that value.

SilverHouseDreams · 03-27-2018, 12:01 PM

Quote:

Originally Posted by Rocinante

Have not set up a BMV yet, but is that 168Ah configured as total at full charge or available at full charge? That number makes sense for total, but with lead-acid batteries you can only use half that value.

I'm going by memory from when I set mine up last fall, there is a 2nd configuration element that corresponds to % of capacity that is available hidden in a configuration menu for a relay. By default, IIRC, it is ~50% as I couldn't figure out why it was so far off for my 100Ah lithium battery...and then realized that the relay % sets the consumption curves.

Paprika · 03-27-2018, 01:30 PM

The Victron BMV-712 manual says this:

"Battery capacity (preferably use the 20 hour capacity rating (C20))"

In other words, for a new battery, use the battery's rated amp-hour capacity. (For an older battery, the rule of thumb I've heard is to deduct 8-10% for every year of use.)

**Rocinante** · 03-27-2018, 01:34 PM

Ok, so if I had a BMV712 right now, I'd set it for 200 AH, because I have a pair of Group 27 flooded lead-acid batteries. I imagine that somewhere I'd also tell it that I could only use 50% of that capacity. Cool.

billrector · 03-27-2018, 06:56 PM

Quote:

Originally Posted by Rocinante

Have not set up a BMV yet, but is that 168Ah configured as total at full charge or available at full charge? That number makes sense for total, but with lead-acid batteries you can only use half that value.

Agree....168Ah is the total. When it gets to 50% it must be charged. I was just surprised that it was showing 80+% at 12.4 volts.

billrector · 03-27-2018, 07:01 PM

Quote:

Originally Posted by SilverHouseDreams

I'm going by memory from when I set mine up last fall, there is a 2nd configuration element that corresponds to % of capacity that is available hidden in a configuration menu for a relay. By default, IIRC, it is ~50% as I couldn't figure out why it was so far off for my 100Ah lithium battery...and then realized that the relay % sets the consumption curves.

I’m not sure I understand this?? Why would the meter need to know that I can only use a % of the total capacity? If I am using wet cell batteries and they should be recharged by 50% capacity remaining, what difference does that make to the monitor?

**Rocinante** · 03-27-2018, 07:25 PM

Yeah, that's why I was asking about total capacity vs. usable capacity. There must be a place to specify either available capacity of a fully charged system, or the type of battery so the system can do the math (e.g. use no more than 50% of rated amp-hours.).

My system today is so poorly wired (e.g. factory installed voltage meter on the See Level II display and dealer-installed voltage meter on the Zamp display, neither of which agrees with the other) that I have very little confidence in our batteries' voltage reading unless I go out to the battery box and attach a volt meter to the terminals. Can't wait to upgrade to something much better next month! BMV-712 Smart will be part of that solution.

billrector · 03-27-2018, 08:09 PM

Quote:

Originally Posted by Rocinante

Yeah, that's why I was asking about total capacity vs. usable capacity. There must be a place to specify either available capacity of a fully charged system, or the type of battery so the system can do the math (e.g. use no more than 50% of rated amp-hours.).

My system today is so poorly wired (e.g. factory installed voltage meter on the See Level II display and dealer-installed voltage meter on the Zamp display, neither of which agrees with the other) that I have very little confidence in our batteries' voltage reading unless I go out to the battery box and attach a volt meter to the terminals. Can't wait to upgrade to something much better next month! BMV-712 Smart will be part of that solution.

Yes, part of the setup is to configure your total amp hours of battery capacity.

Paprika · 03-27-2018, 08:36 PM

Bear in mind that 50% is not a magic number. Battery lifetime declines in a steady slope as depth of discharge increases. In other words, the deeper you discharge your batteries on a regular basis, the fewer total charge/discharge cycles you'll get from them. It's not a case of "I'm OK down to 50%, but below that I'm causing damage." "Usable capacity" isn't 50% or any other percent; it's what you choose to do in the trade-off between depth of discharge and how many years you want to get from your batteries.

**Rocinante** · 03-27-2018, 09:38 PM

Seems a valid point. Yet another reason why I'm happy to switch from lead-acid batteries to LiFePO4 (e.g. never really knowing how much available capacity I have in my battery bank).

SilverHouseDreams · 03-27-2018, 10:39 PM

Part of the reason the system “needs” to know these capacity numbers is to actually report % left, or to calculate time to “zero”. If you don’t want the system to know this data, what purpose does the BMV serve at all?

It actually tracks amps in and out of the battery, so it actually has accurate capacity info far beyond a voltage read out...which is entirely useless as anything more than a guesstimate if you don’t allow the battery to rest with _no load_ for a period of time. Not overly practical.

billrector · 03-29-2018, 05:51 AM

Hi,
Yes, I understand the need to know your Ah capacity. What I was questioning was the need, and the ability, to input the 50% depletion number. You can enter an alarm for low voltage which I have done. I have had that alarm go off when I was using the power jack, but it went off when I stopped using the jack. I went back and read the instructions again and all they tell you to do is enter the total capacity, in Ahs, of your battery bank. The total Ah are not easy to figure out on the stock Airstream. There are many formulas online that yield slightly different results. I went with a guy’s recommendation on this forum which seemed a little high to me, but who knows?