Battery Bank Sizing

[Frank B]

Thanks Frank. By the way, where are you guys?
Kev

We:re back home in Calgary where the snow has finally disappeared. It was a long and brutal winter up here.

 

[solarman]

Thanks Frank. By the way, where are you guys? Solarman.. I agree to respectfully disagree on a few points, and offer this as a handshake over the Forum.

Kev

likewise..

 

[Isaac-1]

you connect it to the coach starting battery,  that's how I would do it. that way if you have an issue with the house side, you can
still start and use the genny.

Personally I like the idea of having the generator started by the house bank, so it can be used to recharge the starting battery should something be left on and the starting battery be fully drained.  A big enough solar arrays would allow the same thing, if not for cloudy / rainy weather, ...

 

[HueyPilotVN]

Solarman that DC-DC converter may be fine for keeping the control electronics working on the water heater, fridge, etc, but how do you hand cranking the generator from the house bank if it is running at 48VDC?

There is another option for getting 12 volts from a higher voltage battery bank.

Your 48 volt battery bank is probably made up from multiple batteries wired in series.

An example is a golf cart where 6 volt batteries are wired in series to add the voltage up to either 36 or 48 volts.

If you start measuring the voltage from the negative ground you will measure 6 volts at the first battery.  Each additional battery in the string will have an increase of 6 volts.  You will go up in steps to 6, 12, 18, 24 and so on.

The way my and most golf carts get the 12 volt supply for the radio and other 12 volt accessories is by tapping the second battery in the string at the positive post.  You can actually use any two of the 6 volt batteries in series but then you would get a floating ground.

I am sure that someone will mention that this might deplete the first two batteries more than the others but from a practical standpoint it works fine.

 

[Frank B]

Personally I like the idea of having the generator started by the house bank, so it can be used to recharge the starting battery should something be left on and the starting battery be fully drained.  A big enough solar arrays would allow the same thing, if not for cloudy / rainy weather, ...

With a big enough solar array, you will never need the generator at all!  ;D


Admittedly, the house battery is typically far larger than the starting battery in the engine compartment. One time I mistakenly left a cell booster running in our truck overnight in near zero weather. Truck battery was too weak to start the truck the next day. I boosted the truck from the house batteries using three sets of booster cables clipped together.    Good thing that the truck was not parked further away!


Now I make sure to always have a small battery charger with me in our travel trailer. If this happens again, I can always recharge the truck battery off the inverter.

Yes, I guess the jury is still out on 12 volt house batteries versus higher voltage house batteries. I'm looking forward to having Kevin post back what he discovered with regards to wiring 8 x 12 volt batteries together in his coach in a more efficient manner.


Frank.

 

[jje1960]

First, enjoy seeing all the experience and members spending their time posting to help others, thank you.  As long as these posts are done in good faith, everyone enjoys hearing others perspectives and experience to enjoy this great RV sport of ours!  Some of us take things slow with limited budgets, simply trying to make our experience meet our expectations.  We love stopping on our travels and having power for certain needs, while not connected to shore power, that's how I got started with expanded battery bank, inverter and solar mods.  I love learning along the way to understand what can (and can't)  be possible with converting the 12V systems to 120V requirements.  With our experience, there is always gives n' takes with how we meet our requirements, that's for sure!  Along the way moving from a single 12V battery in our 5th wheel, we knew we wanted more time available with the system and to use 120V needs for travel.  What we ended up with was 4 12V AGM batteries (because I don't wish to service water requirements) a 2000 watt inverter, battery cutoff switch and a monitor that we could use with our smart phone or ipad near the trailer (in the tow vehicle or anywhere around the trailer).  What we ended up with.... Well, we are super happy!  For what we have into the system, it serves our needs and does not put undue burdens (in our opinion and needs) to deal with.  What we do with our setup....  Well, with 220Ah of batteries, we don't worry about the furnace running out if we need it in a cold night.  We charge all of our electronic 'stuff', phones, ipads yada yada.  We have an extra fridge in the toyhauler bay running off 120V, a maxxair fan that keeps the 5th wheel really cool while going down the road.  We use the keurig for coffee, watch directv with our dome, have the lights on, use the water pump... You get the idea.  We have dipped into the solar arena with a 100W suitcase panel.  Most of the time while on the dry and not needing aircon, we use a 1000W Mitsubishi generator, or our Powerhouse 4K if we need the aircon.  Not science, if we need to charge the batteries we do but not as we did with one 12V battery that's for sure.  With our setup, yea, definitely need to remember to turn off the converter in the 5th Wheel while using the inverter, no auto switch. Funny thing, Gary from this forum once advised "you will forget"  yep!  We did one time, was a chuckle when I realized I forgot!  Point of all this is?  For reasonable cost, we were able to do things we were not able to do before!  We are at 3 years with our batteries, they are doing fine, we don't draw them down past half on a regular basis and wow, we are happy.  Now, advice from our experience.... Take the plunge in the battery bank and solar experience at the pace you are comfortable or can afford, you will learn and enjoy your major or minor investment if you understand clearly your expectations and limitations.  Do your research carefully, however don't be discouraged by advise that may come from extreme use limitations.  Hope this helps those that may feel intimidated with specifications that are outside of your means or expectations.

 

[Ernie n Tara]

That's the longest paragraph I've ever seen

Ernie

 

[Kevin Means]

Sorry I haven't posted the wiring diagram I mentioned. Been slammed with things around here. When Solarman mentioned balanced vs. unbalanced batteries, I remembered that someone else had mentioned that to me some time ago. I meant to look into it but never did.

The image below shows two wiring options for 12 volt batteries that are wired in parallel. The bank on the left is how ours are wired now, and I'm pretty sure that all RV manufacturers wire theirs the same way. When wired in this manner, there is (apparently) an unequal draw from the batteries. Basically, the batteries on the end of the bank get most of the draw - hence, an unbalanced system. The bank on the right is wired so that the draw is equal from all four batteries. Some important requirements of wiring batteries in this manner, is that you have to have an equal number of batteries, and the cable lengths have to be equal, or close to it.

The logic of wiring the batteries in this manner makes sense to me, but I wonder how significant the imbalance actually is when wired as ours are now. Maybe Solarman or someone else knows. I suppose it would have a lot to do with how many batteries you have, and how much current is being drawn. I can't help but think that the impact of wiring them as ours are now can't be too severe, because our battery bank (8 AGMs) lasted seven years, and during the first three of those years, there were only six batteries. When there were six, their SOC was significantly lower in the morning than they are now. (55-60% vs 75-80%) When I replaced the batteries in January, they were still taking a full charge per our Trimetric, but it was taking about an hour longer to do so (on solar.)

So is it worth the cost, time and effort of adding more cables? Would I get noticeably more life out of our battery bank if I wired it like in the image on the right? I don't know. I'd love to hear from anyone who's wired theirs in this manner.

Kev

 

[jje1960]

That's the longest paragraph I've ever seen

Ernie

Sorry bout that....

 

[Frank B]

Kevin:


OK, I think I get the idea. Evens out the wiring runs, and therefore the resistances.


Might be tougher to approximate that with 6 GC2's, but I already connect to opposite sides of my battery bank for + and -.


That balanced wiring probably still wouldn't prevent a heavy discharge through a shorted cell, which was one of the issues of multiple parallel strings.


Anyway, with the cost of GC2's, not sure how much real gain would be achieved. I typically get 7 years out of mine, as well.


Frank.

 

[butchiiii]

It's been my experience that most RVers don't monitor their battery's SOC very well, usually because they have no way of doing so. They just want their solar panels to recharge their batteries, and they don't want to have to think about it. Getting them to measure their actual usage, for the purpose of determining how much solar and AH capacity they'll need, can be a lesson in futility.

Kev

Kevin then the tools I will need to determine actual usage are the 2 you mentioned above? I have already setup a spread sheet to do a
theoretical usage so I want to see how close I will be to it.
Thanks

 

[Kevin Means]

Butch, a good battery monitor will not only tell you your battery's SOC, in percentage, it'll also help you determine your RV's actual consumption, for the purpose of sizing your battery-bank and a solar array.

Start with fully charged batteries (let's say it's a 200 AH battery-bank) then go boondocking, or just camp in your driveway. Do the things you would normally do... watch the same amount of TV, charge your phones and laptops, use lights as you normally would, etc. Do that for 24 hours, then check your battery's SOC. For our purposes, let's say your batteries are at 50%. That means you've consumed 100 amps in a 24 hour period.

Once you've determined your actual consumption, then you can decide if you've got enough battery capacity to meet your needs, or if you need more. Remember, lead/acid batteries can take occasional discharges to 50%, as long as they get fully recharged soon thereafter, but if you anticipate frequent deep discharges, I wouldn't recommend going that low. Personally, I don't let ours get below 75% and our last set of batteries lasted seven years.

The nice thing about using a battery monitor vs. a chart to measure consumption, is that charts tend to use the consumption of generic devices, and you have to approximate how much you think you use them. That'll get you close, maybe even close enough, but measuring your actual usage is more accurate. A battery monitor measures the actual consumption of YOUR devices as you normaly use them. We're all different. It also factors in your RV's actual parasitic loads. For even more accuracy, you could run that exercise several times, then average them.

Once you've got your battery bank sized for your needs, then you can start working on how much solar you'll need to meet your needs... whatever that is.

Kev

 

[butchiiii]

Kevin thanks for the info. Would that be the trimetric 2030 rv battery monitor?

 

[Back2PA]

Would I get noticeably more life out of our new battery bank if I wired it like in the image on the right?  I don't know. I'd love to hear from anyone who's wired theirs in this manner.

Kevin, can I assume from this you are running 12V batteries?

 

[Kevin Means]

Butch, yes. I'm using a Trimetric 2030 RV and I love it, but lots of folks have a Victron battery monitor, and they love that one too. I'm sure there are others out there as well.

Scott, yeah, I considered making the leap to 48 volts a few years ago, and Frank's earlier question renewed my interest in it. I think 48 volts would be a more efficient voltage from which to work, for a variety of reasons, but I just can't justify the additional work and expense - just to get back to the end-user's 12 volt requirement. I'm very pleased with the performance of current setup.

Kev

 

[Back2PA]

I'm very pleased with the performance of current setup.

So if you were to replace your batteries today, and were planning on sticking with a 12V system, you'd do 12V batteries again? If yes, I'm interested in your reasoning as I thought 6V series/parallel was the go-to setup. Hopefully I'm a few years away yet but when battery replacement time comes I'd assumed it would be 6V, even if I add a second bank.

 

[Kevin Means]

Yes, I would stick with our 12 volt system. I just can't justify doing all the mods that would be necessary to switch to either a 24 or 48 volt system. Is a 12 volt system as efficient... no, but it does work very well.

Ideally, I'd switch to lithium batteries though, which I almost did in January. Ultimately, howevever, I decided to stick with 12 volt AGMs for the following reasons (which may not apply to everyone.) Lithium batteries would have enabled me to go from our 840 AH battery-bank to about a 600 AH battery-bank, because you can repeatedly draw Li batteries down to 20% capacity vs my self-imposed minimum of 75% on our AGMs without damaging them. Unfortunately, it was going to cost a minimum of $6000.00 to make the switch to lithium, and while it still cost me about $2700.00 to replace my AGMs (after core returns) that $3300.00 difference is significant. 

The Winnebago Tour's battery bay is located between the frame rails, beneath the basement floor and is completely exposed to ambient air conditions. We occasionally boondock in sub-freezing temps, and often in 90+ degree temps, both of which can cause problems for Li batteries. Combine that with other technical limitations (no more AGS, the need for a different type of battery monitor) and it became clear that Li wasn't for us at this time.

The reason I didn't switch to high capacity 6 volt GC batteries was simple - they won't fit. Our Group 31s barely fit, with less than an inch of clearance above the terminals. I'd love to have those 220 AH GC batteries, and if I could make them fit without having to sacrifice a huge amount of storage space, I'd do it, and I'd definitely stick with the AGM versions.

AGMs are truly maintenance free, I've never had to equalize them, you can mount them in any position you'd like, the battery compartment doesn't have to be vented and they (seem to) charge faster than standard lead/acid batteries. Yes, they cost about twice as much, but in my opinion, they're worth it.

Kev

 

[solarman]

AGMs are truly maintenance free, I've never had to equalize them

Kev

How old are they and what is the make, I will let you know the equalize interval.

 

[Back2PA]

The reason I didn't switch to high capacity 6 volt GC batteries was simple - they won't fit. Our Group 31s barely fit, with less than an inch of clearance above the terminals. I'd love to have those 220 AH GC batteries, and if I could make them fit without having to sacrifice a huge amount of storage space, I'd do it, and I'd definitely stick with the AGM versions.

Interesting. I thought 6Vs were the standard install, clearly not

 

[solarman]

Interesting. I thought 6Vs were the standard install, clearly not

standards ? in the RV world... ???