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

The friendliest place on the web for anyone with an RV or an interest in RVing!
If you have answers, please help by responding to the unanswered posts.

Mark_K5LXP

Well-known member
Joined
Nov 17, 2018
Posts
1,492
Location
Albuquerque, NM
This is not easy to determine by terminal voltage without unloading the system and letting it sit until the battery bank stabilizes.
How do you think the fancy battery monitors do it? By measuring current and voltage, and referencing a set of constants and sometimes past cycle history. If it's a battery you "know", just seeing voltage and current tells you all you need. Even one that's an unknown you can read within 10%, which for most purposes is plenty accurate enough.

Mark B.
Albuquerque, NM
 

Isaac-1

Well-known member
Joined
Dec 3, 2016
Posts
5,044
Location
SW Louisiana
What kind of refrigerator is it, .82 amps seems high for the control board on most RV refrigerators
 

Mark_K5LXP

Well-known member
Joined
Nov 17, 2018
Posts
1,492
Location
Albuquerque, NM
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
Yes, that amount of current will reflect the state of charge but it's important to be clear, state of charge at that current rate. So if you're seeing what would be say, 50% SOC at your refrigerator load you know you can count on the amp hours the batteries have at that amp rate. If at that moment you turned something else on that increases this current you'll see the voltage diminish a bit, which will be the "new" SOC at that current rate. If you don't have that load on for very long and use up many Ah before you turn it off, you'll actually see that voltage rise again to the SOC of the previous level. This is an artifact of what is an internal equivalent resistance of the battery which is specific to the battery, temperature, age and other subtleties. Fancy battery monitors compute remaining capacity and runtime by watching the voltage and current along with some other data but it's something you can calculate on your own if you're so motivated. Most folks just buy the monitor though...

Mark B.
Albuquerque, NM
 

Jeff in Ferndale Wa

Well-known member
Joined
Mar 9, 2014
Posts
677
Location
Ferndale, Washington
What kind of refrigerator is it, .82 amps seems high for the control board on most RV refrigerators
It's a Dometic,new in February this year.
That .82 amps would include any residual draw I may have in the trailer.
The fridge and possibly the propane detector are the only things I think I had running when I took the pic in the original post.
I suppose the monitor itself draws a little.
And now that I think about it, there are little led lights on the USB charging ports throughout the trailer. Probably minute amounts,but all add up maybe could all add up to .82 amps?
 

Mark_K5LXP

Well-known member
Joined
Nov 17, 2018
Posts
1,492
Location
Albuquerque, NM
Yep. Can't recall how that battery monitor decides what unit to use, if it's a menu option or if it just automagically decides when to go from Wh to kWh. I guess you'll find out.

Edit: The downside of this particular monitor is that it only counts Wh "out" and not "in". Seems it'd be simple enough to do a reset after every charge but not every charge will be 100% so for those cycles inbetween it becomes somewhat a guess as to what's gone in and what's left to go out. I almost exclusively use voltage and current to do a mental SOC and knowing my capacity by empirical testing I can make a pretty quick WAG for time to empty but I get it would be a lot simpler for most to have a monitor that does it for you.

Mark B.
Albuquerque, NM
 
Last edited:

Jeff in Ferndale Wa

Well-known member
Joined
Mar 9, 2014
Posts
677
Location
Ferndale, Washington
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
I was able to determine that my converter is three stage, 55 amp
 

Matt_C

Well-known member
Joined
Mar 4, 2019
Posts
489
Location
SE - Mich
Fancy battery monitors compute remaining capacity and runtime by watching the voltage and current along with some other data but it's something you can calculate on your own if you're so motivated.
Mark,
Sorry, I did not track this thread.
You really can't calculate this. You could if you sat there with a clipboard or included a recording device to log the current on a time basis. The real battery monitors integrate the current over time to totalize the actual power use from the bank. They are also great because you can use them to recover the bank to the level you need to make it to the next time when recovery is practical.
Matt
 

Mark_K5LXP

Well-known member
Joined
Nov 17, 2018
Posts
1,492
Location
Albuquerque, NM
You really can't calculate this
Not sure if 'you' is first or third person. I can do it, likely most people won't, or don't care. I get your point about integration but that's for computing power used, which is useful and interesting but does not contribute to determining energy remaining at likely arbitrary delivery rates. For that you can only go by the empirically derived peukert constant, current and voltage which makes that determination a pretty straightforward estimation on the fly.

Mark B.
Albuquerque, NM
 

Matt_C

Well-known member
Joined
Mar 4, 2019
Posts
489
Location
SE - Mich
Mark,
I can and have done this, but I have the instrumentation to do it. How you could do it without and integrating system, I have no idea. And yes, the system I have does include the Peukert exponent in its calculations. I did have to do two experiment to establish the Peukert value for the new bank, but the rest us all in the instrument. The required rated are determined by your system and are hardly arbitrary. My coach has many things that are non-constant loads like the potable pump, fans that are only on demand and the electric refrigeration. Because I don't like to abuse my house bank, I have both my own developed instrumentation that is cumbersome and requires interpretation and a real battery monitor. I like that much better than having the dinette full of fancy and expensive stuff.
Matt
 

Mark_K5LXP

Well-known member
Joined
Nov 17, 2018
Posts
1,492
Location
Albuquerque, NM
Integration is used to return cumulative Ah used which is fine if you want that data. But that information can't tell you remaining Ah because you don't know at what rate to predict it at unless you know in advance what rates you'll be using and for how long. In an RV that can be all over the map from an LED light to a microwave on an inverter, hence my term arbitrary. Even a nice monitor with lots of historical and environmental data to work with can only predict remaining Ah and time based on the current rate.

Mark B.
Albuquerque, NM
 

Willowflowage

Well-known member
Joined
Jan 22, 2018
Posts
58
Location
Ladysmith Wis
I have about your setup but bought 200 watts of solar. I deploy them on the ground with a 50 ft cord to get the best sun possible. Also need a charge controller too. This even on some cloudy days has me leaving the generator at home. Been out as long as 3 weeks and could go longer. If a lot of extra sun I use a DC charger for my 18volt tool batteries that I use for a chain saw, lights, radio, and USB devices. This buys me time on a lot of cloudy days.
But it looks like the cpac sucks over 8 amps which I couldn't support reliably every day so you may be married to your genny.
 

Gary RV_Wizard

Site Team
Joined
Feb 2, 2005
Posts
74,620
Location
At our Silver Springs FL home
But that information can't tell you remaining Ah because you don't know at what rate to predict it at unless you know in advance what rates you'll be using and for how long. In an RV that can be all over the map from an LED light to a microwave on an inverter, hence my term arbitrary.
I think that is a very key point and that variability makes a mockery of any attempt to make precise projections of things like required bank size or time between charges. We make educated guesses and apply some uplifts to cover what we don't know or cannot predict.

That doesn't mean we should not collect accurate data becasue that data is how our guesses end up being "educated" rather than wildass guesses.
 

Jeff in Ferndale Wa

Well-known member
Joined
Mar 9, 2014
Posts
677
Location
Ferndale, Washington
I'm home now after seven nights in a state park with no hookups.
Outside temps ranged from 70s to 50's
I kept detailed notes, but in a nutshell:
Started out when first set up on 1st day volt meter read 12.6x volts with a light load. .8 amps
Over seven days, ran the generator about 6 or 7 hours total, not really on a set schedule, just when I felt like it was not disturbing anyone..More or less every other day for a few hours.
Would not have been a problem to run more, just would rather not unless necessary..
When we woke up on the last day meter read 10.9xvolts with the same light load

Ran CPAP as necessary every night. Ran furnace as needed,shutting off when not in trailer,and setting at 60 at night.Propane fridge and residual chargers,always on
Most evenings spent outdoors with lights mostly off, but one stormy evening had the gang inside,furnace on, lights on, and music playing.

So now thinking about solar. I've read a bit about solar, and have some ideas, but wondering which direction to go.

I'm thinking starting with a 100w portable panel. Many places we stay here in the NW,there may be trees,or something, blocking the roof, so portable would better be able to find the sun. Unless I go big and cover the roof with panels!
I think with what I'm doing now,it will be sufficient, but if things work right ,I will want to expand.
Wife already is talking about watching TV and occasional microwave use.
But I don't want to go all in and find myself not using it.

Some questions:
If I were to buy a 100w portable panel,could that be converted to a permanant panel later?
I know I need a controller,should I spend the money up front for a better controller? Or go with the 20a most seem to come with and upgrade later if needs be?
I think buy the 100w panel now with the best controller,and add down the road if/when I think it's necessary?
The specs on the microwave that I could find say "1350 watts max output". My generator is a 2000w max inverter generator. I tried to shut down anything else that runs 110 except the onboard converter in the trailer charging tha batteries.
I tried the microwave several times and it kicked the circuit breaker on the generator. Shouldn't that generator run the microwave?

Another question,for those of you that have big solar arrays on your roof, what does that do for roof maintenance down the road??
 

Ex-Calif

Well-known member
Joined
May 15, 2020
Posts
1,784
For me solar isn't a "test the waters" situation. We know solar works. We can predict how much energy we can harvest and if I were going to install it on my RV I would not consider less than 400W and a decent MPPT controller.

This is enough solar to "make a difference" and based on your real world experience so far could unshackle you from the generator use that you had. 100W is certainly not "nothing" but tome 100S is one step above battery maintainer solar.

RV roofs are a great piece of landscape to install panels in a non-intrusive way and a lot of companies package 400W systems.
 

Mark_K5LXP

Well-known member
Joined
Nov 17, 2018
Posts
1,492
Location
Albuquerque, NM
Neat that you got to exercise the system and see cause and effect. Now a discussion of the minutia can happen.

When we woke up on the last day meter read 10.9xvolts with the same light load

I don't think you could've cut that much closer. Congratulations.

So now thinking about solar... here in the NW,there may be trees,or something, blocking the roof, so portable would better be able to find the sun.

Correct of course. Consider though there's the issue of schlepping the equipment out, setting it up and packing it back up every trip. And, anytime you leave the site so that others don't consider your unattended equipment a donation. So portable works, but has a diminished convenience factor associated with it.

I do both. I have a roof mounted panel and a portable panel in the basement. Frankly even with both running I'm still in deficit but it's "better than nothing". What I like about the portable panel is it gives me a power option away from the camper and campsite if I want to, which is something I do with my radio hobby. I can run my equipment at a "better" site than the campsite and not have to schlep a bunch of batteries or a generator around. So for me, having both is the solution.

I don't want to go all in and find myself not using it.

Nothing says you have to go all in. Get a small panel, even a harbor fright special that you can try out and see for a while. I have a number of roll-up and fold-up panels I keep around that I may only use once in a while but handy when the need arises. If you find this is solution is "it", then you can decide what features you will want "all in".

I know I need a controller,

Nope. For bulk charging which is what you're doing, no controller is needed. They're handy for reporting state of charge and other operating parameters but not necessary for field use. A controller is needed when a system is being run unattended, and when batteries reach full charge and need to have a managed charge profile. Neither of those describes a portable panel setup. I don't own a single charge controller.

My generator is a 2000w ... shut down anything else that runs 110 except the onboard converter...
I tried the microwave several times and it kicked the circuit breaker on the generator.


I don't recall your converter but let's say it's 45 amps. That's about 500W input at 120V. Let's assume you were charging your batteries full tilt so you have a 500W base load on the generator, then you kick on the microwave. Says it's 1350W, which is a total of 1850W but maybe the microwave has a surge greater than 1350W for the first few seconds it's on. Quite plausible that surge was enough to trip the breaker. Something to try is to run the microwave alone on the generator, then turn on the converter and see if it holds. Or, just know that you're right on the edge in that scenario and only run one or the other on the generator. It's not like you'll be running the microwave for hours and hours, so turning off the converter for a few minutes isn't going to make much difference overall.

those of you that have big solar arrays on your roof, what does that do for roof maintenance down the road?

For me anyway it's a permanent "cover" on the roof. There is some attention that needs to be paid to the mounting points and wiring but it's minimal. I don't give what's underneath the panels a second thought.

Mark B.
Albuquerque, NM
 

Jeff in Ferndale Wa

Well-known member
Joined
Mar 9, 2014
Posts
677
Location
Ferndale, Washington
For me solar isn't a "test the waters" situation. We know solar works. We can predict how much energy we can harvest and if I were going to install it on my RV I would not consider less than 400W and a decent MPPT controller.
Except for the fact that I don't know yet how much we will actually need.
I am trying to be as minimal as possible, but wife and I don't always agree on needs. For example, I don't think we should ever need to use a microwave, but she doesn't see it that way. I am ok with watching tv on tablets or phone, but she would like to have tv,and I would have liked to watch a football game one of the days we were out.
We are not full time,and this was the first time EVER that we stayed in one place for an entire week. Most times we are weekenders,or if on a trip ,we often move every 2-3 days. In this casewe could have made it easily for 2 nights without charging.
I will look at the usage numbers I recorded and try to figure out what I need now for what I did last week and go with that with the idea of adding more later if necessary.
So,I'm hoping I can build as I go.
 
Last edited:

Jeff in Ferndale Wa

Well-known member
Joined
Mar 9, 2014
Posts
677
Location
Ferndale, Washington
I don't think you could've cut that much closer. Congratulations.
I could have run the generator a bit more the previous day, but was looking to see how far I could go.
I don't think I ran long enough any of the days to get to a full charge.
And, anytime you leave the site so that others don't consider your unattended equipment a donation.
Do any of the portable panels have a place to attach a security cable? I realize that will deter only the honest thief.
I don't recall your converter but let's say it's 45 amps. That's about 500W input at 120V. Let's assume you were charging your batteries full tilt so you have a 500W base load on the generator, then you kick on the microwave. Says it's 1350W, which is a total of 1850W but maybe the microwave has a surge greater than 1350W for the first few seconds it's on.
It's 55 amps,so that would be 660 watts,correct?
My generator rated wattage is 1600w, 2000w surge. Now I can easily see how it would be to kick it out.
I have not checked, but there must be a switch to turn to converter off?
 
Last edited:

Gary RV_Wizard

Site Team
Joined
Feb 2, 2005
Posts
74,620
Location
At our Silver Springs FL home
Sound practical advice from Mark!

Microwaves are rated for their output power, e.g. 1000W or 1200W of cooking energy. To produce that much microwave energy requires more input electric watts, typically about 50% more. That's why most residential microwaves don't exceed a 1200W rating - it takes 1700-1800W to produce it and that's all you can safely draw from a single 15A outlet.

I just checked my almost new kitchen countertop microwave, rated for 900W of microwave power. It draws 1350W from the outlet.
 
Last edited:
Top Bottom