Battery Monitor.Please help me understand what I'm seeing.

[Jeff in Ferndale Wa]

I just installed two brand new 6 Volt golf cart batteries that I purchased from Les Schwab. They are 220 AH each
I installed a battery monitor that I had on my previous trailer,but never took the time to understand what it was telling me other than voltage.
I have not used the trailer yet since installation, only tested the CPAP.
Pictures of monitor attached.
First pic is resting,recently disconnected from TV after a 40 mile drive. only thing operating at the time is the fridge running on propane,and I think the onboard propane monitor.
Second pic: fridge & CPAP without humidifier
Third Pic: fridge & CPAP w humidifier.
My questions:
voltage measures voltage
current measures amp being drawn
power measures watts being drawn
what does energy measure?
How can I tell from these numbers how long my batteries will last?
I know everyone recommends not taking the voltage down more than 50%
What voltage represents 50%?
I have a 2000W portable generator. If my batteries get down to 50% how long will I have to run the generator with the trailer connected to recharge the batteries?I understand the generator is not large enough to run the AC and/or microwave. I bought it only to recharge the batteries.

 

[Mark_K5LXP]

My questions:
voltage measures voltage
current measures amp being drawn
power measures watts being drawn

"Check"

what does energy measure?

Power over time. How many watts were cumulatively drawn since reset. Same as the kWh meter on the side of your house.

How can I tell from these numbers how long my batteries will last?

If you reset the energy counter (done with the menu button) it will count kWh drawn from your batteries. Amp hours would be a more direct value than watt hours but you can get there from here with simple math.

A 220Ah 12V battery has 2640 Wh of capacity. So if you start out with fully charged batteries and reset the monitor to '0' you should be able to use 2640 Wh of power before your batteries are fully discharged.

A few exceptions will influence whether you get that number, notably the amount of current you're drawing compared to the amount the battery specification was defined, but that's the theoretical number you could get.

At the end of the day, your batteries are "empty" when they've reached 10.5V under load, at whatever level of current you're drawing at that moment. If you're running a heavy load like a microwave with an inverter, you will reach 10.5V fairly quickly and may only get 1/2 of the Ah a given battery is rated for. If you draw a very tepid load like just a few lights you might get a bit more than rated spec. It's an easy benchmark to observe, 10.5V is 'done' irrespective of load and watching that rate of voltage decline during your use can be a pretty good indicator of remaining time left.

"Fancy" battery monitors take all these variables into account and will display a fairly accurate run time and capacity remaining based on your usage and historical battery performance. I find that information curious and sometimes interesting but it doesn't change the performance of the batteries so I kinda don't care. Watch your delivered Wh and voltage, and that will tell you all you need to know.

I know everyone recommends not taking the voltage down more than 50%

Not everyone. Batteries are specified to 0% capacity. You can use as much of the capacity you paid for as you want.

What voltage represents 50%?

That depends if the battery is idle, or being discharged. A battery that's idle the 50% point is about 12.2 V. One being discharged the 50% point is about 11.6V.

If my batteries get down to 50% how long will I have to run the generator with the trailer connected to recharge the batteries?

You didn't state what converter you have that's doing the actual charging. That is one variable, how many amps it will deliver to the batteries until they reach about 80%. The remaining 20% of the charge is a function of time and intelligence of the charger. I would allocate at least 3 hours for this stage.

A hypothetical example:
220Ah batteries
45 Amp converter
50% charge
Amount of energy to restore: 110Ah
60% of that is bulk, so 66 Ah @ 45A = 1.47 hours
40% of that is absorb, so 44Ah @ 45A to float ~3 hours
Total 4.47 hours minimum

Mark B.
Albuquerque, NM

 

[Jeff in Ferndale Wa]

Excellent reply Mark!
"A 220Ah 12V battery has 2640 Wh of capacity. So if you start out with fully charged batteries and reset the monitor to '0' you should be able to use 2640 Wh of power before your batteries are fully discharged."
Does this mean two 6v with 220Ah each will give me 5240 Wh?
Do I inderstand Wh correctly to mean that if I have 5240 Wh capacity and I use 100 watts/hour,in threory I could go 52.4 hours before battery would be completely discharged?

"If you're running a heavy load like a microwave with an inverter, you will reach 10.5V fairly quickly and may only get 1/2 of the Ah a given battery is rated for. If you draw a very tepid load like just a few lights you might get a bit more than rated spec."
I don't plan to run more than the fridge (on propane) and CPAP w humidifier,and a few interior lights (sparingly). I may test to see how much the furnace fan draws and use that occcassionally if I can afford the power. I suppose the water pump will run now and then and add the the draw. If I add an inverter it will only be to run a TV.

"Not everyone. Batteries are specified to 0% capacity. You can use as much of the capacity you paid for as you want."
Will running to 0% reduce the life of the batteries more so than discharging to 50%?

"You didn't state what converter you have that's doing the actual charging. That is one variable, how many amps it will deliver to the batteries until they reach about 80%. The remaining 20% of the charge is a function of time and intelligence of the charger. I would allocate at least 3 hours for this stage."
It's in a brand new(Feb 2021) Cascade travel trailer.I will look at it and see if I can find any specs.

Thank you very much,this helps a lot

 

[Larry N.]

Does this mean two 6v with 220Ah each will give me 5240 Wh?

Yes and no, but no the way you are thinking. Each 6V battery has 220 Ah at 6V DC. Combined in series they give you 220 Ah at 12V DC. That's likely how you have them set up. Only if you had the pair wired in series would you get that 5240, but it would only be at 6V DC, not at 12V DC.

Either way, 6V * 220 A == 1320 W, so a pair will give you 2640 W. If you wire them in series you still get 2640 W divided by 12V == 220 Amps.

 

[Jeff in Ferndale Wa]

Yes and no, but no the way you are thinking. Each 6V battery has 220 Ah at 6V DC. Combined in series they give you 220 Ah at 12V DC. That's likely how you have them set up. Only if you had the pair wired in series would you get that 5240, but it would only be at 6V DC, not at 12V DC.

Either way, 6V * 220 A == 1320 W, so a pair will give you 2640 W. If you wire them in series you still get 2640 W divided by 12V == 220 Amps.

I understand,thanks
so if I have 2640Wh capacity and I use 100 watts/hour,in threory I could go 26.4 hours before battery would be completely discharged?

 

[Mark_K5LXP]

if I have 2640Wh capacity and I use 100 watts/hour,in threory I could go 26.4 hours

That's how the math works and since 100W is very near the spec current for rated capacity, all other things being equal (starting charge level, temperature, battery age/condition, etc etc) you will get 26.4 hours. This doesn't have to be theoretical, try it at your home with the load(s) you expect to run.

I may test to see how much the furnace fan draws and use that occcassionally if I can afford the power.

That is a dandy idea. Take the continuous draw of the motor, then divide it by the time it actually runs during a convenient span of time like an hour, at the ambient outdoor and indoor temperature you might expect. Example, right around 30F my furnace runs about 20 minutes every hour to keep it 68F inside. It draws 7 amps when running, so 7A / 3 is 2.34Ah I need to allocate to furnace operation. With a 220Ah battery that would amount to 94 hours of furnace operating time.

I suppose the water pump will run now and then and add the the draw. If I add an inverter it will only be to run a TV.

You can do the same exercise with the power draw of the pump but they run so briefly it's something you don't usually have to account for it. The TV can be a factor though if it's something that's on for hours at a time.

Will running to 0% reduce the life of the batteries more so than discharging to 50%?\

The amount of service life difference between 40% and 80% DOD is a few percent. All batteries have a finite amount of power they will deliver in their lifetime, so it comes down to how quickly one uses up what's there before they fail. Many will cite the manufacturer cycle life chart showing ever decreasing cycle life vs depth of discharge, totally ignoring the ever increasing amount of power delivered during those deeper discharges. Unless you literally discharge your battery several times a week, week after week, year after year it's not possible to wear a battery out before it will ultimately expire. Maybe using up batteries is not the prime objective and overall I don't think it is, but the belief that only running down to 50% is preserving them in some way is a bit of a misnomer, because even down to 80% DOD (100% capacity) it's unlikely one will wear through a set. So "preserving" them is only assuring you're not using the capacity they have and you paid for, only to recycle them partially used when they age out making them more, not less costly for the life you've used. There are other operational and logistical reasons to ensure some reserve of power capacity during the course of normal use but I wouldn't count longevity among them.

Mark B.
Albuquerque, NM

 

[DonTom]

A battery that's idle the 50% point is about 12.2 V. One being discharged the 50% point is about 11.6V.

Unless a lith battery. Mine is spec'ed at 12.8 volts when 95% discharged. But also 100% charged is at a higher voltage. Near 15V, IIRC.

-Don- Reno, NV

 

[Gary RV_Wizard]

so if I have 2640Wh capacity and I use 100 watts/hour,in threory I could go 26.4 hours before battery would be completely discharged?

Right, but that's where the mathematical theory departs from the real world. Lead-acid batteries don't discharge at a consistent rate, so the total capacity is less at a high amp rate discharge than at a low one. And since the voltage falls as the capacity is depleted, the amps needed for a device to perform adequately change (or alternately, the device doesn't perform as expected). It gets complicated in a hurry once a lead-acid battery gets below about 50%.

 

[Mark_K5LXP]

Gary's 100% correct but will point out that's a system level consideration. The battery is what it is, and if there's a need to run a specific device then that's up to the user to accommodate whatever power profile it requires. You can use SOC to accommodate that but that's not a battery problem. I wouldn't call it "complicated", I've used the "bottom half" of batteries for decades and it all depends on just how accurate you want to be in your run time estimations. I've found for my casual RV use it requires zero thought or effort as I don't operate much in the zone where Peukert (the "complicated" part) becomes a significant factor. Which by itself is a system level choice, made when the batteries were selected for the target application.

Mark B.
Albuquerque, NM

 

[Kirk]

Only if you had the pair wired in series would you get that 5240, but it would only be at 6V DC, not at 12V DC.

Should have said, wired in parallel.

 

[Jeff in Ferndale Wa]

Thanks for the replies guys!
I have a week long stay coming up on Sep11 in a state park with no hookups,so will be able to test this all out.
All this info is a big help,and I actually think I inderstand!

 

[Isaac-1]

My take on the whole 50% vs 80% depth of discharge, for typical weekend campers discharge to 80% and don't worry about shortening the battery life, though always recharge it after a discharge cycle, sure this means the batteries will not last as long and you may be replacing them every 3-5 years instead of every 6-7 years, but hey we are only talking a couple of hundred dollars, vs getting those extra amp hours of use out of them when you need it, without having to add a larger battery bank.

 

[Larry N.]

Should have said, wired in parallel.

You're right -- good catch. Thanks. Where's that eyetooth?

 

[creativepart]

Thanks for the replies guys!
I have a week long stay coming up on Sep11 in a state park with no hookups,so will be able to test this all out.
All this info is a big help,and I actually think I inderstand!

Yes, you'll be able to test this out... but without a way to charge your batteries in that State Park when (OK, If) you run out of power you will be pretty unhappy.

Do you have a portable generator for recharging? Because it is required to dry camp for 7 days.

If you have 240 ah and you want to keep the discharge to around 50% that means you have 120 ah available and for a week - that is 17 ah PER DAY or 0.7 amps per hour. Which is pretty much not using any power all day for 7 days.

Even if you decide to take them down to 20% State of Charge, which will sulfate the plates in the battery quite a bit, you only have 27 ah available per day or 1.2 amps available per hour in the day. Forget about running the furnace at night. Even setting it at 60 degrees you'll be out of battery by day 2 if it's in the 40's outside.

If you are used to not turning on any lights or using any appliance other than your RV fridge running on Propane who, knows it might be possible. But I'd guess you'll be out of power by day 4 if not much sooner. Hope I'm wrong.

 

[Mark_K5LXP]

Everyone has different power needs and there's no way to guess if he will go 2 days or 20 days on the storage he has because I'm not sure even he knows what his draws will be. I just got back from a 3 day off grid trip in my class A (with 2 GC-2's) and was sitting at about 60%, taking no pains to economize with heat, TV, lights, whatever. I thought to myself I bet I could make it a week at this rate. Until there's some estimated or actual usage data there's no way to guess. When I camped with a trailer I was set up to charge from the tow vehicle so I never really worried about running out. I would favor a few driveway campouts to learn about power use first but nothing like cutting the cord and venturing into the great unknown.

Mark B.
Albuquerque, NM

 

[Jeff in Ferndale Wa]

Yes, you'll be able to test this out... but without a way to charge your batteries in that State Park when (OK, If) you run out of power you will be pretty unhappy.

Do you have a portable generator for recharging? Because it is required to dry camp for 7 days.

If you have 240 ah and you want to keep the discharge to around 50% that means you have 120 ah available and for a week - that is 17 ah PER DAY or 0.7 amps per hour. Which is pretty much not using any power all day for 7 days.

Even if you decide to take them down to 20% State of Charge, which will sulfate the plates in the battery quite a bit, you only have 27 ah available per day or 1.2 amps available per hour in the day. Forget about running the furnace at night. Even setting it at 60 degrees you'll be out of battery by day 2 if it's in the 40's outside.

If you are used to not turning on any lights or using any appliance other than your RV fridge running on Propane who, knows it might be possible. But I'd guess you'll be out of power by day 4 if not much sooner. Hope I'm wrong.

In my original post I stated I have a 2000w portable generator that I purchased for the sole purpose of recharging the batteries.
I don't plan to run more than the fridge (on propane) and CPAP w humidifier,and a few interior lights (sparingly).
CPAP will be my biggest single draw,but I rarely sleep more than 6 or 7 hours a night.
I may test to see how much the furnace fan draws and use that occcassionally if I can afford the power. I suppose the water pump will run now and then and add the the draw.
Temps should only be down to the 50's at night,and we generally leave the furnace off at night anyway unless it's freezing temps.
I do have a propane heater that I can use instead of the furnace,but that also will be off at night.We have plenty of blankets.
I have an inverter to power the TV if I want,but I need to run a power supply from the battery for it before I can use it. Not sure if I will get to that before this trip or not,so I don't plan for that to be a factor this time.

From what I've learned on this thread,I think I will probably have to recharge on the third day. Or,I might recharge for a couple hours each day to be safe. I will look to see how much I use after the first night.
My biggest concern is keeping my CPAP going with the humidifier.
Our last trip out a week or so ago I woke up in the middle of the night to a dead battery,which prompted me to go out last week and buy the two 6 volts.
That led to installing the battery monitor that I had on my last trailer,and then try to learn how to use it.
I had it on my last trailer for a couple years, but only went without hookups for one or two nights.

 

[creativepart]

Sorry I missed your comment about having a portable generator. That's all you need to run any test of any of your various electrical loads - because you have a means of recharging.

When I camped with a trailer I was set up to charge from the tow vehicle so I never really worried about running out.

Recharging a travel trailer's battery bank by plugging into the tow vehicle is not the slam dunk one might expect. First, not much more than 10 amps can come from the TV's alternator and the wiring is too thin and the run to long to deliver all of it to your TT batteries. Now, if you're towing the TT for 250 miles then yes, you'll get significant charging.

But if you think you can connect the TT in the campground, plug in the TT's power cord and let your TV idle for 45 mins and be recharged I think you will be disappointed.

 

[Matt_C]

Jeff,
You have a great collection of good answers here, but your battery monitor is not a really good device in that it does not report state of charge. This is not easy to determine by terminal voltage without unloading the system and letting it sit until the battery bank stabilizes.
Were I you, I would just count on running your little generator a couple or three hours every day and keeping track of what your monitor tells you. If it seems like you are losing ground, just run longer the next day. You should soon develop an understanding of what the house bank needs.
Matt

 

[Jeff in Ferndale Wa]

Jeff,
You have a great collection of good answers here, but your battery monitor is not a really good device in that it does not report state of charge. This is not easy to determine by terminal voltage without unloading the system and letting it sit until the battery bank stabilizes.

Matt, Above Mark gave this reply to my question:
What voltage represents 50%?

"That depends if the battery is idle, or being discharged. A battery that's idle the 50% point is about 12.2 V. One being discharged the 50% point is about 11.6V."

I most likely will always have the fridge on,and that appears to draw .82 amps.Would that not be enough of a discharge where I could use 11.6v as a guideline for 50% discharge?
Later on he said 10.5 v is empty regardless of load

 

[Mark_K5LXP]

Recharging a travel trailer's battery bank by plugging into the tow vehicle is not the slam dunk one might expect.

That's when charging directly from 12V. When I needed a quick charge I had an inexpensive inverter running at the truck and a 30A converter at the trailer. It was a small house battery in my popup so a decent bulk charge could be achieved in less than an hour, usually a half hour. Yes, that has all the trappings of idling a hundred HP engine to provide half a horse of energy but not having to carry a generator, deal with fuel and listening to a generator drone on in a quiet campground - priceless.

Mark B.
Albuquerque, NM