How long will house batteries last?

[MattyA]

Hi. I have a 2019 Forza 38w. Stock everything, no special batteries. If I have the inverter on and use the batteries to power only the residential fridge and charge a coupon of iPads using an AC socket, how long until the batteries hit the 11v cutoff? Worried I’ll go to sleep and have no house batteries when I wake up.

[gurroz]

MattyA, how many amp hours are your batteries? Are they lead acid or AGM? Do you have a battery bank? If so how many batteries? You should not be discharging lead acid or AGM below 50% or 12.06.v. If you have two lead acid batteries each 100 amp hours then you have 100 total amp hours at your disposal. If you take them down to 11.v you will destroy the batteries pretty quick. You should try and figure out the amp hours used by your frig and also take a look at the iPads usage.

[MattyA]

Thanks for the reply. It’s dark but I’ll check the battery amp hours tomorrow. It’s not in the manual, obviously I read that in detail. There are four sealed lead acid deep cycle batteries but I don’t know the stats on them. Nor do I know how much the fridge pulls or 3 iPads and 2 iPhones. I do know that the coach electrical system will disable the inverter if the batteries get too low so hopefully I shouldn’t be worried about taking them so low that it would damage them. Hopefully. This trip will be interesting and I’ll learn a lot.

Ps now I’m wondering how long it will take them to fully charge when I’m driving tomorrow. Or how long it would take to charge them when running the generator.

[MattyA]

Update. The I cannot get in at information from the four batteries other than that they’re wired in series. The bar that holds them in place covers up the information printed on top. Space to add two extras, through.

[DavidM]

They can't be wired in series, that would give 48V, they must be wired in parallel.

So in any case with the following assumptions, lets figure your battery life:

1. Four Group 27 batteries at about 80 Ah each.
2. That means 320 Ah total or 160 Ah usable.
3. You have a large, 120V residential fridge powered by an inverter.
4. That fridge will draw about 150 Ah of DC to the inverter daily.

So 160 Ah usable divided by 150 Ah daily gives a bit over one day of use before the batteries are discharged below 50%.

Your questions about time to recharge while driving and with the generator are more difficult to answer.

Lets assume that you have a 60 amp converter and it will be powered by your generator if running. You can charge from 50% to about 80% state of charge (SOC) at the full converter amperage, then it starts to drop rapidy. So you can get to 80% in .3*320/60 = 1.6 hr. The last 20% will take several more hours of charging.

Most RVers who recharge their batteries with a generator cycle them between 50% and 85%. But you do need to fully charge them up every 3-4 days, otherwise they will sulfate and lose capacity. A couple of hundred watts of solar panels is an ideal way to do this every day. It will add that last 50 Ahs of charge to your battery bank over a full day of solar charging.

Charging while driving is even more difficult to figure. Your chassis alternator (unlike the converter) has a fixed voltage output of 13.5-14.0 volts depending on the alternator. That won't charge very fast particularly as the SOC rises. As just a guess, if starting at 50% SOC your alternator may charge at a rate of 50A, slowly dropping as it nears 80%. So maybe 3 hours of driving to reach 80%. After that, like converter charging, it will take many more hours to get near 100%

The best way to know what is going on is install a shunt based battery monitor. That will tell you instantaneous amperage and cummulative amp hours in and out of the batteries. These rang from $200 for a very good Victron unit, to $80 for a good Renogy unit.

David

[creativepart]

They should be 4-100 amp hour batteries wired in parallel yielding 400 amp hours.

In my experience with the same equipment you should get approximately 10-hours overnight if you don’t take your batteries below 12.1v or 50% discharged.

Your Forza has an EC30 controller for your generator and you can set that to auto start the generator at 12.0.v for backup.

When dry camping we run the generator at night for dinner prep and TV about 90 min to two hours so we go to bed with topped up batteries. Then in the morning we turn on the generator during breakfast for an hour or so.

We have 300w of solar and that takes over during the day to keep the batteries charged until dinner. But any time we run the microwave or other large draw we’ll start the generator.

If your going to boondock your Forza the first thing to do is to add a shunt-based battery monitor to your RV. So, you’ll know the true state of charge in percentage of amp hours remaining rather than the RVs volt meter.

[creativepart]

One more thing about charging via the alternator or via the generator... plan on it taking much longer than you think to fully recharge your batteries. Plan on 4-plus hours from 50% discharged to fully charged or close to it.

Don’t routinely take your batteries below 12v or you may find them getting weaker on you every time you do. We try to keep ours above 12.2v at the lowest and after three years they are holding up well. But our solar cells keep them fully charged all the time we are not using the RV, too.

Also, I’ve found that just running the inverter without the fridge or anything else significant will drain your batteries to 12v in about 30 to 36 hrs. So be careful about leaving it on.

Your RV has enough parasitic loads to totally discharge your batteries with nothing on in less than a week. So, always turn off the two power disconnects in the stairwell when storing the RV even for short periods of time.

[gurroz]

I second the shunt based battery monitor. I use the Victron 712 which has bluetooth. My wife gets annoyed when I check the battery levels on my app, however, it has saved us several times. Most recently this past summer I was plugged in to shore power and noticed my battery was drawn down to 76%. I pulled out our portable solar panel topped off our battery bank every few says. I have also checked it and noticed something was left on that needs to be turned off.

[MattyA]

Awesome info. Many thanks everyone. Sounds like I’ll need to be very careful for the 2.5 days I’m without electrical hookup. Wish I had asked all this before I left for the trip since I’d have gotten the battery monitor. Argh!!

[creativepart]

To tell your battery charge state without a BMK you can use the volt meter in the RV. But you have to know how to read it.

The key is that the battery needs to be AT REST for about an hour. Especially, after any charging. So, after you turn off the generator the batteries will read much higher than actual for up to an hour. And, heavy usage will make them read lower.

So, in the morning after you've run the generator, turn it off and go for a walk. When you return before turning a bunch of things on check the volt meter inside the RV and see what the voltage is. That will be close to accurate.

Anytime you see your batteries showing more than ~12.7v you are not seeing a true number. So, when you see 13.2v don't think "I'm good to go, all charged up."

Here's a chart that will show how voltage compares in percent.

[MattyA]

Ok. Cool. Yeah, I see a lot of 13.1, 13.2. We’re heading into the national park tomorrow and I told the whole family that other than the fridge, no electrical to be used for 2 days other than when the generator is on!!!!

[creativepart]

Originally Posted by MattyA

Ok. Cool. Yeah, I see a lot of 13.1, 13.2.

Just remember you can have dead batteries and with the generator running your volt meter will show 13.X volts. So, don't go by that as anything other than confirmation that your battery charger is working.

This experience will show you why when you get home you're going to add a shunt battery monitor for future trips.

And, solar panels.

And, more batteries.

And, bigger batteries.

And, expensive Lithium Iron Phosphate batteries.

[DavidM]

Originally Posted by creativepart

Just remember you can have dead batteries and with the generator running your volt meter will show 13.X volts. So, don't go by that as anything other than confirmation that your battery charger is working.

This experience will show you why when you get home you're going to add a shunt battery monitor for future trips.

And, solar panels.

And, more batteries.

And, bigger batteries.

And, expensive Lithium Iron Phosphate batteries.

The curse of the residential fridge while dry camping- It takes more of everything to make it run longer.

David

[MattyA]

Originally Posted by creativepart

This experience will show you why when you get home you're going to add a shunt battery monitor for future trips.

And, solar panels.

And, more batteries.

And, bigger batteries.

And, expensive Lithium Iron Phosphate batteries.

Haha. In almost everything in my life that sentiment would be oh so true. But strangely I like my stock motorhome and can’t see myself doing too many upgrades. For sure if we get more into the off-grid stuff. But so far we almost always have at least 30 amp hookups, or even 20 amp which can keep the basics going ok. I will say that between a couple of Harvest Hosts we’ve visited and the like, it has become clear that there are some very unique places if you are willing to work without electric. I have used an RV with a propane-powered fridge before and if I had that I’d be much more confident about the electrical situation. But on the other hand, that fridge was half the size. You can’t have it all!!

[Wyatt]

Originally Posted by MattyA

Haha. In almost everything in my life that sentiment would be oh so true. But strangely I like my stock motorhome and can’t see myself doing too many upgrades. For sure if we get more into the off-grid stuff. But so far we almost always have at least 30 amp hookups, or even 20 amp which can keep the basics going ok. I will say that between a couple of Harvest Hosts we’ve visited and the like, it has become clear that there are some very unique places if you are willing to work without electric. I have used an RV with a propane-powered fridge before and if I had that I’d be much more confident about the electrical situation. But on the other hand, that fridge was half the size. You can’t have it all!!

Adding one note, that has been mentioned on many other threads but I don’t believe on this one, is that lithium ion batteries not only can be drawn down more, but the voltage remains more constant until the end.

[imnprsd]

You might find this story interesting:

This summer I decided to install a supplemental Victron 17A Bluetooth charger. I did this to replace (using) my Dimension Inverter/Charger for everyday charging. Why?

Well, the purpose of buying the Victron was to start using it to recharge my house batteries with the expressed purpose that doing so would extend the house battery useful life. You see, I have already gone through 2 sets of battery banks over 5 years, and I am hopping that if I used the Victron, with their more advanced battery charging algorithms, that it will lead to 3+ years of useful house battery use.

Did it it work? ...I don't know. I just did this in the Sept-2020 so you need to ask me in Sept-2024.

What I learned by monitoring the bluetooth display is that I was I burn about 80-100AH/day just to power my lights, fans, Intellitec EMS Panel /PMS Controls, Thermostat, hot water heater control board, and whatever other 12V sources that are running off my engine and house batteries.

Note: This was me spending a full day in my RV with 120V shore power connected to all my 120V-AC systems. I also have a Keyline VSR (battery combiner) so that when I am charging my house batteries... I am also charging my engine batteries to maintain them at 100% SOC.)

Specifically, I had no idea that this much 12v battery power was being used for 12V circuits and my interior lights! ...But as you can see in the Victron picture below, after 3.8 days I used 351AH. That's an average if 92A/day!

From this I conclude your, Inverter with internal 3-stage charger is always working to re-charge your house batteries when you are plugged into shore power.

==> So I am hopeful my "smarter" Victron Charger will in fact help to extend the 2.5years of average battery life I have got from my previous two battery banks over the last 5 years. ...And at $450-$550 per battery bank swap, this is another expense I did not expect. ...Worse, the last 20% of battery life is the worst. I.e., in the end I cannot even boondock over night without my inverter shutting off due to a weak battery bank.

Note: I always recharge at 40-50% SOC too, but I do run though a ton of overnight "cycles" on the road... when I spend the night in rest stops, Wallmart parking lots, or wherever I choose to "stealth camp."

OTHER POINT OF INTEREST

I drive my RV about 7K-10K Miles over a 3-5 month period. That means I am always driving somewhere with my 160A alternator (putting out 70-85A) in addition to 400W of solar power that is putting out 0-18A depending on the available sunshine. So when I am driving, my house batteries are always receiving a charge as best my alternator and solar MNPT controller can manage.

Now I understand that the house batteries will NOT accept a charge when it reaches 100%, but maybe this is not 100% accurate. Why? Well, each time I change my house batteries if find they are deformed on the sides.

...Bulging a bit would be an accurate description. ...And this is indicative of over charging, as they say. So it can't be 100% true that a battery will stop accepting a charge when it reaches 100% SOC or else I would not be seeing physical battery deformation when I replace them.

Of course, I am still trying to understand battery charging technology; but there seems to be some disconnect between the "theory" of battery charging and the "actural" results of battery charging, and useful battery life that don't all add up!

So I'm experimenting with this Victron supplemental charger -- and if my house batteries last more than 3 years I will declare this experiment a success.

For now, I hope I have explained to all of you, that even when you are plugged into shore power, your inverter/charger is still maintain up to 80A/day just to keep your lights on inside your coach and power all your other 12V monitoring systems and controls!

This means out of my 430AH house battery bank I use 20% of it for everyday use.

Note: I turned off my solar controller when I took these measurements. So that 92A/day measurement does not include any charge from my solar cells.

[MattyA]

Update in case anyone is interested. We stayed three days/nights without shore power. I don’t have any of the high-end monitors mentioned above but the coach control panel says that at the beginning of the night at 8pm (generator shutoff time) our battery was at 12.7 or 12.8. We ran pretty well nothing other than the fridge all night. Even turned off the water pump and water heater. The first night we left our extra blankets under the coach so turned on the furnace at 3am or so (propane but it has a blower so clearly consuming electricity). At 8am (generators allowed) we were at 11.9 and the panel was about to auto-cutoff the house batteries. The next two nights we cranked then furnace during the hour preceding generator cutoff and had extra blankets and so didn’t need the furnace overnight and the coach was showing house batteries at 12.0 at 8am. So lessons learned:
- 2 hours of generator in the morning and 2 hours at night will pretty well fill the batteries to allow for unsupported operation overnight
- The coach batteries (stock lead acid 4 batteries, assume 80 or 100 amp hours each), won’t allow for much more than an overnight
- It’s a PITA to be concerned with power management to this degree (ok, that’s a decidedly non-conservationist comment, I get it, sorry, not meaning that I want to be wasteful...)

Clearly lithium batteries that can be run down below 50% to almost 0% with no issue would be very advantageous, although wouldn’t solve the need for daily generator-running to charge-up. If I filled up my battery tray I could fit 6 of them at 100 amp hours each...a lot of safety margin but I’d still need to use the generator daily or close to it.

We stayed the next two nights outside the entrance of the national park with 50amp hookup and it was a lot easier. The campsite was not quite as nice (but close) and it was only about 10 mins more driving to get to the same places in the park was the in-park campsite. So in the future for national park trips I’ll seriously consider location and unless the in-park sites are significantly location-advantaged, I’ll tend towards staying at commercial campsites just outside the entrance.

[DavidM]

Realize that your reported voltages are skewed by recent charging or current draws. For battery voltage to be anywhere near useful, it needs to be measured with no charging or any current draw for at least one hour.

That means that your fully charged batteries probably weren't that good and that your morning voltages indicating 70% discharged also weren't that bad.

David

[creativepart]

Way to go Matty! Look at you. You learned so much more than any one here could possibly tell you before hand.

You might consider some Solar... for less than one Lithium battery you can add 400w or solar and that would always be a plus.

I say that, but I do hear folks that add solar and then add Lithium batteries say they don't find the solar as helpful anymore. Because Lithium batteries are so capable. But as long as folks are on AGMs the solar is a plus.

Pretty soon you'll get the dry camping/boondocking thing down and it won't be as stressful.

50 amp service is great. The thing to look for now, is deep woods National Forest campgrounds with 50-amp service. We go to one every year just 15 miles north of Durango and it's the best of all worlds. (There are more just like it all over you just have to find them.)

Here's a photo of where we go. Taken 6 years ago when we had a travel trailer. And, yes, they have 50 amp service there.

[gurroz]

MattyA, thanks for the update. I would not put a lot of stock on your coach monitors. If it were me, with no bus bar battery monitor, I would use a volt meter and check each battery. If you in fact did get down to 12v or below, you were damaging your batteries. In the future with no modifications, staying in a campground with hookups is probably a good idea with an occasional 1 night of dry camping mixed in. Hope you enjoyed your trip!