BBQ! (Basic Battery Question)

Urlauber · 01-24-2022, 01:30 AM

BBQ AGM ETC

Very basic question - my Thor Rize has two group 31 AGM batteries (that's what the manual says).

What's the consensus about how low you can let voltage go? I've read different things, so what say you, oh experienced RVer, to prevent any serious stress to the batteries?

My BMPro display has shown them at 12.1 - 13.1V. There is no SOC gauge. The solar charger shows them full at around 12.8 fwiw. I want to double-check if the voltage display is correct.

So, when is it time to crank up the generator? And what happens if you don't (to the compressor fridge, for ex)?

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Bonus question:
I wouldn't by an RV as a power backup, but since it is now parked in front, in case of a power outage in the pueblo, would there be any issue to crank up the generator and plug the fridge, computer screen, aquarium pump into an RV outlet?

hbn7hj · 01-24-2022, 01:49 AM

Get an SOC battery monitor.

folivier · 01-24-2022, 02:58 AM

You really need to look at resting battery voltage. Compare the readings to a chart easily found by searching. But I agree a SOC gauge gives you much better information. Victron BMV-712 has bluetooth and is easy to install. The Victron Smart Shunt does what the 712 does but doesn't have the gauge so is cheaper. My last coach I installed the 712 but never looked at the gauge, preferred the BT app which had much more info.

Urlauber · 01-24-2022, 05:17 AM

Thanks for the pointers gents. Are there any such gauges that sync/pair with the BMPro/RV Master multiplex display?

At this point, though, I'd like to start with a simple multi-meter...

My understanding with these gauges is that YOU have to feed them the parameters of your battery, so knowing those would be a good point to start. Yes there are charts and numbers online, but they don't all agree. Some say don't let them go below 11V, some say 50%, some say discharge 80% for AGM is ok, some say measure at no load, some at 0.1C, some say rest it etc etc.

Maybe installing and setting up such a thing is simpler than I imagine it. Was just hoping there would be something that tells me, hey, if your battery shows 12.6V, and it is disconnected from loads, that means it's got 80% of its total capacity left, and I can discharge it down to 11.9V which would be 20% left.

GroupB · 01-24-2022, 02:29 PM

My answers to the 3 questions:

1. There is no consensus.

2. My bogie is to stay above 12.1 at no load, "to prevent any serious stress".

3. No knowledgeable answer.

booster · 01-24-2022, 03:44 PM

Voltage is not a very reliable method of knowing battery state of charge as it is influenced by too many things. Battery type, model and brand, battery age, is it in a "rested" state, are there any loads on it, etc

The battery monitor is a very good recommendation, IMO.

How far you take the batteries down is a personal choice. Many feel that going only to 50% down SOC is necessary, but I am not one of them. Battery life is shortened a bit by go the 80% regularly, but you gain 60% in capacity, so it seems like a very good trade off to me. With only two battery gp31 batteries having the extra capacity is probably a good thing, depending on your use and charging patterns.

The 12.1v is probably an OK voltage to go to if the loads are not very large on the batteries at the time and there haven't been any big loads on them for a few hours, but the monitor takes out all of that for you.

Bud · 01-24-2022, 04:08 PM

Quote:

Originally Posted by booster

Voltage is not a very reliable method of knowing battery state of charge as it is influenced by too many things. Battery type, model and brand, battery age, is it in a "rested" state, are there any loads on it, etc

The battery monitor is a very good recommendation, IMO.

How far you take the batteries down is a personal choice. Many feel that going only to 50% down SOC is necessary, but I am not one of them. Battery life is shortened a bit by go the 80% regularly, but you gain 60% in capacity, so it seems like a very good trade off to me. With only two battery gp31 batteries having the extra capacity is probably a good thing, depending on your use and charging patterns.

The 12.1v is probably an OK voltage to go to if the loads are not very large on the batteries at the time and there haven't been any big loads on them for a few hours, but the monitor takes out all of that for you.

As long as you Understand and Maintain your system. OR:

I've used battery voltage displays for about 6 years, successfully. The last batteries (5.5 years) were Walmart group 27 marine deep cycle batteries. The last four months two agm 100 amp batteries. I don't think that battery voltage meters are feasible UNLESS you are willing to learn the 'voltage sag' with your batteries and appliances.

ymmv

Bud

folivier · 01-24-2022, 04:28 PM

Here is a simple battery capacity chart. Simple enough to get an "idea" of your remaining 12 volt lead acid battery capacity. More accurate if you aren't using much battery.

Urlauber · 01-24-2022, 07:57 PM

Quote:

Originally Posted by booster

Voltage is not a very reliable method of knowing battery state of charge as it is influenced by too many things. Battery type, model and brand, battery age, is it in a "rested" state, are there any loads on it, etc

The battery monitor is a very good recommendation, IMO.

How far you take the batteries down is a personal choice. Many feel that going only to 50% down SOC is necessary, but I am not one of them. Battery life is shortened a bit by go the 80% regularly, but you gain 60% in capacity, so it seems like a very good trade off to me. With only two battery gp31 batteries having the extra capacity is probably a good thing, depending on your use and charging patterns.

The 12.1v is probably an OK voltage to go to if the loads are not very large on the batteries at the time and there haven't been any big loads on them for a few hours, but the monitor takes out all of that for you.

Thank you very much! What monitor would you suggest? I looked at Victron (expensive), Renogy (medium priced), and some cheaper Chinese knock-offs, and the reviews are pretty harsh for most of them. Thoughts on this one? What would I need to do to install this... batteries are underneath the car.

booster · 01-24-2022, 08:17 PM

Quote:

Originally Posted by Urlauber

Thank you very much! What monitor would you suggest? I looked at Victron (expensive), Renogy (medium priced), and some cheaper Chinese knock-offs, and the reviews are pretty harsh for most of them. Thoughts on this one? What would I need to do to install this... batteries are underneath the car.

With all battery monitors you need to tap into the negative battery cable and also connect the wires from a shunt if used to the unit display if used.

The one you show has a 60 amp limit which is pretty low considering most Roadtreks with AGM batteries will charge at 80 amps or higher. You can use a remote shunt with it however, the manual states.

The biggest issue I see with that unit is that you can't set the "full charge" parameters, so you are stuck with what they think you should use. This may or may not be best. Even the Victron default values will leave you at about 85% full because they think your charging equipment isn't capable of going fuller.

I like the Trimetric myself, although it it is a bit clunky in appearance and function, as it has everything you need and is very settable. Victron is similar but is a smaller screen and more button pushing to get what you want to see, but it does have Bluetooth so you can see it on your phone.

Urlauber · 01-25-2022, 12:56 AM

What are your thoughts on something like this... notice the glowing (maybe paid for, maybe not) reviews compared to the Victron, Renogy etc offers.
I do like the price, even if I had to buy two of them for complete monitoring of ins and outs...

Edit - OK, these don't provide a SOC display.

@Michael · 01-25-2022, 01:28 PM

Quote:

Originally Posted by Urlauber

What would I need to do to install this... batteries are underneath the car.

Make sure that you have a spot on the ground wires between the batteries and the rest of the coach where you can insert the shunt. In my case, the AGM coach batteries are grounded in more than one place, making a ground-side shunt worthless, as a significant fraction of the current is missed by the shunt.

I ended up buying a shunt that can monitor the hot side instead.

Urlauber · 01-25-2022, 03:59 PM

Quote:

Originally Posted by @Michael

Make sure that you have a spot on the ground wires between the batteries and the rest of the coach where you can insert the shunt. In my case, the AGM coach batteries are grounded in more than one place, making a ground-side shunt worthless, as a significant fraction of the current is missed by the shunt.

I ended up buying a shunt that can monitor the hot side instead.

Very good point, thank you! There are two 12V AGM batteries which I assume are running in parallel. I have been under the car a couple of times but still trying to understand how everything is cabled. There is a bunch of wires attached to one side of one battery, but I'll need to slide around and see what's on the other one (not easy with 3" ground clearance

)

tgregg · 01-27-2022, 03:41 PM

Quote:

Originally Posted by Urlauber

What are your thoughts on something like this... notice the glowing (maybe paid for, maybe not) reviews compared to the Victron, Renogy etc offers.
I do like the price, even if I had to buy two of them for complete monitoring of ins and outs...

Edit - OK, these don't provide a SOC display.

Victron makes one like this that handles up to 500a and is $9 cheaper

https://www.amazon.com/Victron-Smart.../dp/B0856PHNLX

hbn7hj · 01-27-2022, 04:09 PM

Preferring a display I use the surface mount Bogart Trimetric.

tgregg · 01-27-2022, 04:58 PM

Quote:

Originally Posted by hbn7hj

Preferring a display I use the surface mount Bogart Trimetric.

I prefer a display as well. I was just posting in response to OP's post about the high cost of Victron.

I had a Bogart Trimetric and it worked very well, but now I have a Victron 712s because in addition to a display and the bluetooth capability it has a relay that can be configured to respond to a variety of conditions. I use it to start and stop charging based on SOC.

themexicandoctor · 01-28-2022, 08:51 AM

Victron BMV 712 with Bluetooth & then come back on here & determine the final settings beyond default.

SteveJ · 01-28-2022, 02:56 PM

I've used this one for just over a year. It provides the info that makes living with 12 volts a lot easier. No tie in with my Victron stuff but otherwise it has worked well.

The light on it modulates on and off when charging so you may want to install where it won't be an irritation when sleeping. When discharging the light stays on making an excellent low current night light.

Not too difficult to set up, just go through the steps.

Less than $50 at the big A.

https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1

Urlauber · 01-28-2022, 03:45 PM

Quote:

Originally Posted by Urlauber

What are your thoughts on something like this... notice the glowing (maybe paid for, maybe not) reviews compared to the Victron, Renogy etc offers.
I do like the price, even if I had to buy two of them for complete monitoring of ins and outs...

Edit - OK, these don't provide a SOC display.

Quote:

Originally Posted by tgregg

Victron makes one like this that handles up to 500a and is $9 cheaper

https://www.amazon.com/Victron-Smart.../dp/B0856PHNLX

You probably did not click on my link... the SmartShunt is not cheaper, it's much more expensive than what I was asking about.

Urlauber · 01-28-2022, 03:49 PM

Quote:

Originally Posted by themexicandoctor

Victron BMV 712 with Bluetooth & then come back on here & determine the final settings beyond default.

Thank you, pics are always helpful to learn how things can be set up.
I decided to go with something much cheaper for now and use that to figure out how things can be set up and managed, before I pay some more serious money for a Victron etc.

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