How to make sure you have enough battery

[Yoshimura]

We bought our first RV a few months ago for our next trip to Florida this winter. Most of the time, we will be bound docking but I'm wondering how I will be able to manage the battery capacity when we'll be off the grid for a night. Having the furnace shut down in the middle of the night because the battery is dead is probably not something we want to experiment with. I assume that after a while, you learn what you can expect from a 105 Ah battery but until then, how can I make sure I won't have bad surprises in the middle of the night?

At least, I will need to run the 12V fridge and the furnace for the night. Hopefully, we will also be able to watch the TV a few hours and take a shower.

Any tips?

By the way, we have an underhood generator if needed. And the only battery monitoring system is the RV voltmeter.

[booster]

Originally Posted by Yoshimura

We bought our first RV a few months ago for our next trip to Florida this winter. Most of the time, we will be bound docking but I'm wondering how I will be able to manage the battery capacity when we'll be off the grid for a night. Having the furnace shut down in the middle of the night because the battery is dead is probably not something we want to experiment with. I assume that after a while, you learn what you can expect Teen Killed In Crash Near Little FallsThe 15-year-old crashed his vehicle into a tree Saturday evening.from a 105 Ah battery but until then, how can I make sure I won't have bad surprises in the middle of the night?

At least, I will need to run the 12V fridge and the furnace for the night. Hopefully, we will also be able to watch the TV a few hours and take a shower.

Any tips?

By the way, we have an underhood generator if needed. And the only battery monitoring system is the RV voltmeter.

You had mentioned elsewhere, you were looking at a battery monitor, and that is by far the best way to keep track of the battery SOC.


If you have a full battery when you settle in for the night, you would have avout 80ah of usable power. Compressor frig in coolish weather will be 30-40ah per day, so less than 1/2 that overnight. Furnace we would need to what it is to get some idea.


Big thing is to keep the inverter off, assuming you can do that in your model as it will use power even if you aren't using 110v power. Run the engine if you run the micro on the inverter.



Hot water may or may not need power, TV/DVD will and likely need inverter on.


My guess would be you will be OK overnight, but need to charge the next day and unless you drive a long ways, it will likely be short of full, maybe 90%, same if coming off shore power. That should still be enough for the next night.


Best check would be to shut off everything you can and wait an hour before you check the voltage. If you are then under 12.0v, you are getting close to 20% SOC and running out of power.


Once you get the monitor, it will very easy to know, so if that can be done before you go, it would be well worth the effort. One 105ah battery with a compressor frig can be a challenge.


I went to the Roadtrek site and it appears you have the Suburban propane furnace, as do we. We used about 15-20ah overnight with it running in 40ish degree temps in the past with it, but we keep it pretty warm at 65* most nights. It showed propane water heater so none used there.


You are still looking at making it one night if you are close to full in the afternoon, I think.

[Yoshimura]

I really wanted to use a shunt but it seems it won't be possible to install it before we leave. It's already winter here. However, I think I will need to really consider using the kind of sensor you suggested me. Without a monitor, it seems that all you can do is hoping.

Our TV/DVD operate directly from 12V so I should be able to leave the inverter off.

Thanks!

[booster]

Originally Posted by Yoshimura

I really wanted to use a shunt but it seems it won't be possible to install it before we leave. It's already winter here. However, I think I will need to really consider using the kind of sensor you suggested me. Without a monitor, it seems that all you can do is hoping.

Our TV/DVD operate directly from 12V so I should be able to leave the inverter off.

Thanks!

Good on the inverter, that will save you capacity.


It will be very interesting to see what the battery monitor folks say about using the non shunt sensor. I don't think anyone has tried that to this point. If it is accurate enough, and compatible with electronics, it could really save folks a lot of work sometimes.



Good luck on your trip!

[Knit]

We commonly use 40-50 Ah at night running compressor fridge, lights, Cpap, heating, no TV. My guess is you probably be ok for one night at a time. But why not sleep in the van a few nights in the driveway and see how much you use?

I have a Victron monitor with the shunt, but testing even with just the battery level lights or a voltmeter should get you close to knowing how much you use.

[GallenH]

Knit: I'm about to install the Victron. I'm guessing that you're happy with it?
Yoshimura: Running an old 1997 PW with a single 100AH battery. Just finished camping at the CO River where we were stationary for 3 days (friends did the daily driving in their Jeep). Fridge is 3-way so we were on LP. Cold at night so ran Suburban furnace continually. Primitive battery level indicator dropped from full to 3/4 each night. 100W suitcase solar charged battery during day up to full. Will know more when I get accurate readings from Victron. We don't run TV. Use iPad to watch movies/TV series. Never ran RV engine.

Booster and Knit are spot on. Test and see what you use. From what I've learned on this forum, Booster's observation about 105AH and compressor fridge is a good guage: If I were running one, I would get another 100AH into the system. But who knows. Test her.

[Max58]

This is very interesting Info. We have a 2008 Roadtrek Agile. My friend just put some.very high end new batteries in his Sprinter ($3000+) but they hold a massive amount of amps. Not sure what id want to put in ours but the 200 amp stock batteries seem to only last about 24hrs without charging. I dont want to soend $3000 but would like to find a happy medium. I wonder if with solar and a new battery package we could run for 2-3 days without running engine or generator?
Thanks
Max

[mikes47]

Originally Posted by Yoshimura

By the way, we have an underhood generator if needed. And the only battery monitoring system is the RV voltmeter.

Have you considered "topping off" the battery before going to bed by running that underhood generator a while prior to turning in for the night?

[SteveJ]

Originally Posted by Max58

This is very interesting Info. We have a 2008 Roadtrek Agile. My friend just put some.very high end new batteries in his Sprinter ($3000+) but they hold a massive amount of amps. Not sure what id want to put in ours but the 200 amp stock batteries seem to only last about 24hrs without charging. I dont want to soend $3000 but would like to find a happy medium. I wonder if with solar and a new battery package we could run for 2-3 days without running engine or generator?
Thanks
Max

Do you have a monitor in place to let you know how much power you are consuming? You will have to determine if your current batteries are worn out or if you actually use that much power.

As a for instance, I use one 100 AH battery. I can usually make that last two days with some furnace run time in there. Three days for sure without, assuming no TV, which is normal. 150 watts of solar does fill it up nicely.

I do have an LP fridge, with circuit board. My parasitic drain runs about 8-9 watts with fridge in cooling mode and other monitors and things.

I do tend to be quite frugal with battery power. LED lighting, device charging and a bit of water heating and pumping time, that's about it. Maybe the electric blanket for 30 minutes, drawing about 3+ amps from time to time.

[Yoshimura]

Originally Posted by mikes47

Have you considered "topping off" the battery before going to bed by running that underhood generator a while prior to turning in for the night?

Yes, I will definitely do this until I have a better understanding of our consummation.

[booster]

Originally Posted by Yoshimura

Yes, I will definitely do this until I have a better understanding of our consummation.

I think a bit of terminology and charging realities might need some clarifying.


"Topping off the batteries" would seem to indicate taking them up to full to have maximum capacity available, which is fine except you really won't be able to do it practically by running the generator or van engine. The last 20-25% of capacity in a lead acid batteries is going to take something like 6-8 hours of charge time because the acceptance rate of the battery drops very quickly once you get around that 80% full mark. Nobody is really going to want to run their generator all day, every day, to get that tiny amount of power into the batteries.


This goes back to some other discussions and comments that may not have been adequately explained when referring to just how much capacity that 105ah battery would have for running things. Looking at the "normal" AGM setup and use patterns, it is very rare for the batteries to get totally full without doing some special things.


* It is normal for most of us to consider 20% SOC as the low end for discharging our batteries, as from there you are getting very close to killing them completely, which is obviously not good for you or the battery. This makes it really an 84ah battery when looking at usable power.


* Then you have consider a realistic estimate of just how full the battery will be when you go to discharge it. From what we have seen over the years, I think a reasonable estimate would be something like 85% SOC maximum if you have done some sort of short duration charging like running the generator or van engine. If you happen to have enough solar to cover your use and charge the batteries, you may get much higher than this in good sun as the solar will be able to charge a long enough time to totally fill the batteries (we see true 100% full from solar top off pretty regularly in our system)


* On top of all of this is the problem of a lot of the "smart" chargers aren't all that smart and will nearly always over or under charge batteries depending on the SOC when they start charging. They tend to use algorithms and timers to try to estimate when the battery will be full, but are rarely correct. To really know if a battery is charged full and it is time to go to float, you need to look at the actual amps the battery is accepting. Very few chargers are capable of doing this amp check and be settable to float transition based on those amps. It does appear to be much more common for chargers to undercharge than over charge, especially on deep discharge cycles, so typically many/most folks will come off of overnight shore power at something like 90-95% full tops, and maybe lower.


* Long drives on the alternator can get batteries totally full as there can be the necessary hours of charging done. Of course the batteries can also be overcharged if they were close to full when you started, so a disconnect is a good idea to prevent over charging, assuming you know when to use it.


* All of this stuff will become very, very, obvious once you get a battery monitor in place and get all the settings done properly (I think many of them are not set very accurately). You will be be able to have the monitor keep track of the batteries and tell you when they are truly full or not. You will also be able to see just how close, or not close, your charging equipment is getting to doing a good job of accurate charging under various conditions.


So in the real world of off grid power use, the batteries for lots of people are going to cycling in the 20%-80% range of SOC unless they have solar helping out quite a bit.



This makes that 105ah battery really have only 63ah in real world use, so a very large difference.


Note that this is only for lead acid batteries, wet or AGM, not lithium which have much different characteristics. Harry's hybrid systems of lithium and AGM is intended to address many of the above mentioned issues, and does it pretty well.

[Max58]

Great Info! I also need to get a battery monitor. I think my 200AH batteries are toast or my girlfriend just uses a lot of devices that require Batt.

[SteveJ]

Originally Posted by Max58

Great Info! I also need to get a battery monitor. I think my 200AH batteries are toast or my girlfriend just uses a lot of devices that require Batt.

I installed two of these. They only measure in one direction so two are required. They do share the same shunt. Directions included for wiring. Maybe not the best way to go but it seems to work fine. And is quite frugal.

https://amazon.com/gp/product/B01JOU...?ie=UTF8&psc=1

[Yoshimura]

Unfortunately, it won't be possible for me to install a battery monitor before we leave so I will have to rely on the coach voltmeter.

Considering the battery under a small load, when (at which voltage) should I start considering starting the generator to recharge it (50% DoD). When should I definitely stop using it (80% DoD)?

[booster]

Originally Posted by Yoshimura

Unfortunately, it won't be possible for me to install a battery monitor before we leave so I will have to rely on the coach voltmeter.

Considering the battery under a small load, when (at which voltage) should I start considering starting the generator to recharge it (50% DoD). When should I definitely stop using it (80% DoD)?

When you are actually charging the battery, you will have no way to know, for most of the cycle, how full the batteries are because the voltmeter will be seeing the charger voltage. When you shut the charger off, you will also need to wait for the surface charge to go down before the voltage will tell you very much.



You may be better off just to pick a time amount to run each day to replace what you use



You have an underhood generator, which is way overkill on capacity for the single battery, so it will likely give as much as the battery will take which will probably be in the 50 amp range, maybe a bit more. The issue with the oversize charging gets to be battery temperature so you might be best to do a couple of short runs each day. You will get a big surge right at the beginning of each run, which will speed up things. My guess would be 2 twenty minute runs a day would do it for you.


You will be able to tell pretty well if you ran enough from the voltmeter. If it keeps going lower on progressive mornings, you need to run more time. This will only work up the 80% SOC though as the rest is too slow changing, so you would never get higher than 80%, but would get back to that state each day.

[Yoshimura]

Thanks for the detailed explanation booster but here's my use cases...

Most of the time we'll be on the road or connected on shore power. However, it's very possible that once in while we rest a few hours somewhere in the afternoon (like in a rest area) of a complete night with no shore power. Again, this won't be the norm.

So we stop somewhere and start using the battery (we start the heather, or maybe the roof fan. Maybe we take a shower (water pump) or use the USB plug to use/recharge a iDevice. Or maybe the girlfriend make a cup of coffee but forget to start the engine). Considering this, I have no idea the state of charge when we stopped and I have no idea how many Ah we could have taken.

So I assume that the only solution to prevent us to damage the battery by completely discharging it would be to check the voltmeter while the battery is being used.

I assume that if I ever see something 10V, I should immediately stop using it. But what about something more conservative.

I found this chart but I don't know how representative is it. However, I see that under a light load (C/20 or 5A), I should consider starting the generator when I voltage drop below 12.3V (which is around 50% DoD). And under 12V, I should definitely start the generator (20% DoD).

Those numbers sounds right?

Thanks again!

[Bud]

Originally Posted by Yoshimura

Unfortunately, it won't be possible for me to install a battery monitor before we leave so I will have to rely on the coach voltmeter.

Considering the battery under a small load, when (at which voltage) should I start considering starting the generator to recharge it (50% DoD). When should I definitely stop using it (80% DoD)?

Hi Yoshimura,

All I have is old stuff with a battery volt meter. By old, a one stage charger, 2 group 27 Walmart deep cycle marine batteries, stock 2004 Express van alternator, no solar.........

Over time I have learned what the voltage should look like. I can have on the tv, satellite receiver, lights, then when I turn on the Fantastic Fan on high or furnace I can guess at what the voltage will read. The battery 'Sag' will become greater and can drop to 11.7 or 11.8. When the furnace or fan turns off I can check it again. Some of the learning occurs the next morning when I check the voltage after everything is turned off for awhile. I don't recall it ever being lower than 12 or 12.1 volts, usually 12.1 or more. If I'm staying another day and need more amps, I just start the van's engine to charge the batteries, NOT the LOUD ONAN. The stock alternator is also faster than the LOUD ONAN. I don't boondock enough consecutive days to have an interest in solar. I don't think I have ever gone more than 10 recharges without a full charge. I'm rarely under a 50% state of charge, maybe never.

I feel just fine NOT having a battery monitor after reading booster posts for quite awhile. Odd you say since booster is a battery monitor fan or is it freak? booster?

If I'm not understanding something correctly, please correct my thinking.

Thank you.

Bud

[Bud]

Originally Posted by Bud

Hi Yoshimura,

All I have is old stuff with a battery volt meter. By old, a one stage charger, 2 group 27 Walmart deep cycle marine batteries, stock 2004 Express van alternator, no solar.........

Over time I have learned what the voltage should look like. I can have on the tv, satellite receiver, lights, then when I turn on the Fantastic Fan on high or furnace I can guess at what the voltage will read. The battery 'Sag' will become greater and can drop to 11.7 or 11.8. When the furnace or fan turns off I can check it again. Some of the learning occurs the next morning when I check the voltage after everything is turned off for awhile. I don't recall it ever being lower than 12 or 12.1 volts, usually 12.1 or more. If I'm staying another day and need more amps, I just start the van's engine to charge the batteries, NOT the LOUD ONAN. The stock alternator is also faster than the LOUD ONAN. I don't boondock enough consecutive days to have an interest in solar. I don't think I have ever gone more than 10 recharges without a full charge. I'm rarely under a 50% state of charge, maybe never.

I feel just fine NOT having a battery monitor after reading booster posts for quite awhile. Odd you say since booster is a battery monitor fan or is it freak? booster?

If I'm not understanding something correctly, please correct my thinking.

Thank you.

Bud

Forgot to mention one more time that My Learning what voltages to expect under what loads did not happen overnight. It just took some time and paying attention to the voltage meter. Today I'm probably a little too cocky and smug about it, paying little attention to the voltage meter compared to when I started using one 3 years ago. The key for me was being able to predict what the voltage would be after the battery has not been used for several hours.

[booster]

Originally Posted by Yoshimura

Thanks for the detailed explanation booster but here's my use cases...

Most of the time we'll be on the road or connected on shore power. However, it's very possible that once in while we rest a few hours somewhere in the afternoon (like in a rest area) of a complete night with no shore power. Again, this won't be the norm.

So we stop somewhere and start using the battery (we start the heather, or maybe the roof fan. Maybe we take a shower (water pump) or use the USB plug to use/recharge a iDevice. Or maybe the girlfriend make a cup of coffee but forget to start the engine). Considering this, I have no idea the state of charge when we stopped and I have no idea how many Ah we could have taken.

So I assume that the only solution to prevent us to damage the battery by completely discharging it would be to check the voltmeter while the battery is being used.

I assume that if I ever see something 10V, I should immediately stop using it. But what about something more conservative.

I found this chart but I don't know how representative is it. However, I see that under a light load (C/20 or 5A), I should consider starting the generator when I voltage drop below 12.3V (which is around 50% DoD). And under 12V, I should definitely start the generator (20% DoD).

Those numbers sounds right?

Thanks again!

The chart is likely pretty good relatively accurate, I think it is the Lifeline one, maybe. I have tested the Lifeline version on our Lifeline batteries and it was pretty good.


The problem you will have is that you have to the amps being used at the time to be able to use the chart. The lines marked C/* are the current stated in battery capacity divided by hours, so C/100 would be 1% of battery capacity and so on. Without knowing the amps, you are out of luck, I think.

[booster]

Originally Posted by Bud

I feel just fine NOT having a battery monitor after reading booster posts for quite awhile. Odd you say since booster is a battery monitor fan or is it freak? booster?

If I'm not understanding something correctly, please correct my thinking.

Thank you.

Bud

I think I prefer to be called a "battery monitor crusader" It has a nice ring to it.



I don't want to be accused of calling Bud a dog , but the learning curve he is talking about is like house breaking a puppy. After a while you learn the signs of when to put the outside before they have an accident so it basically you that got trained, at least until the puppy finally catches on, not the dog.



As long as your loads are relatively small and relatively consistent, the learning curve is certainly doable over time. The better the understanding of how power use, charging, and such work, the easier it gets to do and probably more accurate at the same time. Until recently, almost every camper got along OK without a full battery monitor setup.



Even those of us that are big monitor fans get to the point, after a while, that we rarely look at them as we now from past history what is going to be there anyway. In our case, the solar is probably the most unpredictable thing we have, as it varies in AH recovery immensely.