Use 24v Batteries to run 12v System... Possible?

[furyous68]

Sorry for the long post....

Hi everyone. I am new here. I am an RV greenhorn... don't even own one yet. But, we're in the market & the biggest thing I want out of it is to be as self-sufficient as possible. The very first thing I will be doing (besides checking over the engine, trans, rearend, and general health of the coach) is updating the charging system with a solar system. I do not want to run the generator unless absolutely necessary. I HATE when other campers run their generators for 12 hours out of the day, & I refuse to be one of them.

I'm looking at a 12v/24v solar system w/ a 60A MPPT controller. I know I could use 1 or 2 12v AGM batteries. BUT... I'm really interested in seeing if it's possible to run the RV's 12v system off of 1 or 2 24v batteries. If so, I could use 1 or 2 smaller batteries & due to you only needing to pull 1/2 the amps @ 24v's. I assume you would need a step-down converter between the 24v batteries & the 12v converter in the coach... right?

Somebody school me!

[Almot]

Those who have ever been to Turkey or Middle East, must've seen on of these:



Put coffee grounds in cold water, make a LOW heat, remove it from the stove as soon as it's just beginning foaming. If you pour slowly, most grounds will stay in the pot. Still have one somewhere.

[MrWizard]

normal drip maker
coleman stove top drip coffee maker
old fashion stove perculator
french press

forget the 12v gizmos, they gimmicks, that seldom work satisfactorily

[Almot]

Now you just wait, people with 1,000W Keurigs will stone you for using presses like Melita 🙂

I am so happy that switched to black tea long time ago. Still has caffeine, just not much.

It's not that you can't use 120V appliances with solar, it's that you need more battery (and a good inverter).

[jplante4]

furyous68 wrote:
The only person who drinks coffe is my wife, and I see a lot of single serve 12v coffee pots available.

I have a Melita coffee maker. Easy peasy - just boil water. No electricity is harmed in the making of this coffee.

[pianotuna]

Hi,

Save your money. The 12 volt pots simply don't cut it unless you get one that can draw 40 to 50 amps (with #4 wire).

furyous68 wrote:
The only person who drinks coffe is my wife, and I see a lot of single serve 12v coffee pots available. Nobody uses a hair dryer in this house... which is weird since I'm the only male.

[Almot]

Not much of a coffee drinker, but it appears that French press is more popular than 12V gizmos, in this scenario.

250-300 AH bank - total bank, not "usable 50%" - might work for your needs. Especially if there is none or very little use of microwave and no other high-power 120V items.

2 pairs of 6V batts for a total 500 AH would've been better, if you can carry that much, but with 250 AH and a big solar to charge it quickly it might just work for you. The beauty of a big solar is that you always have a few amps trickling in, even when it rains.

[furyous68]

Thanks Almot... and everyone else. I know I need a rig before I buy anything, but I'm trying to figure out a budget for solar & batteries... especially the batteries. Solar systems are relatively cheap, but not decent batteries! I think I have an idea of what I need to budget for now.

I really hope to not run a lot of 120v appliances unless absolutely necessary. I think I can minimize a lot of it. We don't use a microwave very often @ home, I prefer using an oven to warm up food. Takes longer, but I think it tastes better. The only person who drinks coffe is my wife, and I see a lot of single serve 12v coffee pots available. Nobody uses a hair dryer in this house... which is weird since I'm the only male.

[Almot]

You are overthinking this not because you need to hire a pro, but because solar is not something to be planned in details before purchasing an RV.

When you get your bus, go on the roof with a few 40"x65" sheets of cardboard - a typical size of 230-260W panel - and see how many you can fit safely. You need to be able to walk on the roof around the panels, and there are tall structures like A/C, and clear lids of "sunroof" that can't be covered.

330AH/10A = 33 hours discharge to 50%, but this isn't everything. 10A @12V is basically pumps, lights, TV and laptop. When you run 120V appliances, you draw 100-150A @12V and battery hates this, responding with a reduced energy available, i.e. at 100A it will last not 3.3 hours but only 1.5-2 hours.

It's hard to tell whether you will be able to recharge battery with 800W solar. If you don't run any powerful 120V loads, you should be alright with as little as 500W. Now, try telling DW that has just washed her hair because it's been raining and nothing else to do - try telling her not to use 800W hair drier because battery is low 🙂

[pianotuna]

Six 6 volt jars make it a requirement to have a buss bar and equal length wiring to each 6 volt pair.

Four are easy, and twin banks of four are wonderful.

[MrWizard]

Your idea is correct, many people run multiple 6v batteries in, a 12v bank
But 335 amphrs at 10 amps is 33 hours
So you could camp for the week end , (2) nights, then recharge the batteries at home, on shore power,

can't put that back with two hours of generator run time
Solar of course will return part of your nightly use

[furyous68]

I live in CA, so unless I'm up in the Sierras during winter, I shouldn't have any issues running lithiums. But I can't afford them, so it doesn't matter! Lol.

pianotuna wrote:
2 six volt, 4 six volt, or 8 six volt, but not 6 six volt.

So you can't create 2 or 3 packs of 2- 6v batteries in series (making a 12v pack), and then run those packs in parallel to increase the amps? There are a few 225Ah 6v batteries made. 3- battery packs (2- 6v in series making 12v) connected in parallel would give me 675 Ah, right? Since I can't (or shouldn't) let them drop to less than 50% charge, that would be 335Ah usable until I'd need to recharge... right?

Theoretically, that would let me run a 10A load for at least 15hrs before needing to be charged back up to 100%... right? 10A seems like a lot for evening & overnight use, is it? Should (& I know there are several factors to account for) an 800W solar system typically be enough to be able to recharge the batteries during the day?

OR... am I overthinking this and should I just hire a pro? LOL

[Almot]

MrWizard wrote:
if you want to talk ampHrs cycled, and discharge the lithium bank to 20% SOC

you 'can' have a smaller size lithium bank with the same 'safely cyclible' number of ampHrs as a larger lead acid bank

In other words - smaller physical size for the same amount of usable energy. Could we agree on this wording? 🙂

Physical size and shape of 12V 100AH Li is often the same as 12V 100AH flooded. They do it on purpose so that people could just "drop in" Li in place of flooded. This is why they call it "drop-in replacement".

Weight is important for RV users. Li units weigh 3-4 times less, in usable AH per pound.

[MrWizard]

Almot wrote:


The only way to have a physically smaller bank with the same amount of energy, is to use Li batteries.

common misconception

Lithium will weigh less, but for the same total AmpHrs, it will be very close in space used as lead acid

now if you want to talk ampHrs cycled, and discharge the lithium bank to 20% SOC

you 'can' have a smaller size lithium bank with the same 'safely cyclible' number of ampHrs as a larger lead acid bank

but it won't be the same 'total capacity' from full to zero SOC

example (2) grp27 lead acid 105 amphrs batteries 210 amphr total
105amphr to 50%
are the same physical size as (2) lithium 100ampHr batteries 200amphr total
160 amphr to 20% SOC and the lithium weigh a lot less

for the same 'total capacity' Li has more usable(aka safely cyclable) power for the same space, less weight,
BUT not physically smaller for the same 'total amphr capacity'

(3) 105 amp hr lead acid give 150Amphrs to 50%
take more space, weigh more, but have 50% more total power capacity in a run dead use it all emg situation ( its cold the heaters on and there is no gas for the genny, or it won't start for some reason}

[Almot]

TV set itself shouldn't be a problem.

Other 120V appliances, OTH, drain battery very fast. Toaster, coffee maker, microwave, hair drier. Some of this (or most?) can be eliminated and/or replaced with propane. It boils down to habits and preferences.

Solar should be planned for worst case scenario, estimate your needed wattage of the array and then add 30-50% to this number. Panels are cheap these days, especially big 230-260W ones, no need to skimp on it.

[pianotuna]

60 watts per 100 amp-hours @ 12 volts. Then do an energy audit for 72 hours to determine battery bank size in watt hours. Use a Kill-a-watt meter and keep the peak load below 1800 watts.

Right now I'm using 1600 watts to keep my RV at 20 C (68 f). The ambient temperature is 1 C (33.8 f). Even with a huge lithium bank it would be impossible to have enough panels to cover the load and still have the system portable.

I will limit the heated area of the RV to keep the bedroom nice and warm. The rest of the RV won't freeze up as the projected low is -5 C (23 f).