elec schematic for 24 volt RV battery system

my daughter and hubby have chosen to turn a Promaster van into an RV.

They want a water heater which will be operated off 2 100AH deep cycle batteries and 3000w. inverter for shower.

Need schematic for wiring and charger to charge 12v. batteries while maintaining 24v. supply to inverter.

Advice is welcome.  I am an elec engineer and can insure wiring is adequate for application.

thx. in advance for help

mark in MN

I can't help with the schematic, but I will note that those two batteries won't provide hot water for all that long, and will require long recharge times between uses. 2 * 100 = 200 amp hours to dead (figure half that, or 50% discharge if you want the batteries to last more than a handful of uses), or 100 amp hours * 12V == 1200 watt hours to feed a 3000 watt inverter. I don't know the draw from your electric heater, but I think a LOT more batteries might be in order.

There are others here who can provide much more and better information.

Larry, thx for response.

water heater is 1650 watts.  Watts = Volts X Amps means 13.75 amps at 110v or about 65 amps at 24v.  that's a big load but assuming a shower lasts 5 minutes the draw should be less than 10% of battery capacity when fully charged even allowing for losses in the inverter.

If I've got this wrong somehow I'm very open to being corrected.

mark s.

are they going to set up some propane for other uses?
if so that is way to heat water.
I have both elect and propane in my toy hauler.
the propane is faster and hotter for little gas.

oldryder said:

Larry, thx for response.

water heater is 1650 watts.  Watts = Volts X Amps means 13.75 amps at 110v or about 65 amps at 24v.  that's a big load but assuming a shower lasts 5 minutes the draw should be less than 10% of battery capacity when fully charged even allowing for losses in the inverter.

If I've got this wrong somehow I'm very open to being corrected.

mark s.

Click to expand...

Calculations may be a bit more complex than that. A couple of things to consider is what is the desired temperature of the water when heated, and what is the temperature of the incoming water. Then figure the rate of consumption of the hot water. Then get the BTUs needed to heat the incoming water to the desired temperature at the rate of consumption. Figure out how many watts are needed to provide the required BTUs.

oldryder said:

Larry, thx for response.

water heater is 1650 watts.  Watts = Volts X Amps means 13.75 amps at 110v or about 65 amps at 24v.  that's a big load but assuming a shower lasts 5 minutes the draw should be less than 10% of battery capacity when fully charged even allowing for losses in the inverter.

If I've got this wrong somehow I'm very open to being corrected.

mark s.

Click to expand...

Mark, you have to figure the amps consumed at 12V, since that's what the batteries have, and there should be some allowance for the inverter overhead (it's not 100% efficient at conversion), so 1650W/12V == 137 amps. So that means less than an hour to 50% depletion. I guess that means you could do a shower or two before recharging, but is that ALL you're going to use the 110V AC for?

Hopefully someone else can help you more.

A 5 minute shower? Hmmm...

Everyone, thx for responses.  At the moment they are not assuming any propane although at this stage it's still a possibility.  They are concerned with the necessity of properly venting a propane heater. (of course, the battery storage area will also have to be vented.)

re: current load on batteries; 65amps is at 24 volts.  However, I just realized I was assuming 200AH which is true for 12v. but not 24. so a couple more batteries are probably in order to have a decent reserve capacity given the electrical load of the water heater.

(4) 100AH batteries would provide the current for a couple of 5 minute showers with ample reserve.  KDBgoat is correct about the other variables that would need to be included to accurately predict the total draw on reserve capacity.  I'm not going there but rather trying to determine what would constitute an ample reserve for a long weekend of camping.

The kids are assuming about 400w. of solar charging but even a couple days of that won't matter much against the draw of one 5 minute shower.

1.  I would strongly consider having the Van engine alternator be set up to also charge these batteries.  Keep in mind that is a 12vdc system not 24vdc.  Is there a possibility to pay extra 10-25$ and get a 12v inverter?  Would make the wiring much easier.

2.  I would also try to run the van engine while taking one of these 5 min showers. 

Bobtop46 is on to something. Consider a heat exchanger type water heater that will make hot water off engine coolant heat. My dad did that home brew in their 68 Travco with a 5 gallon electric water heater. (That's a story for another thread...)

And, a 5 minute shower can be about a 1-2 minute shower if you wet down, turn off the water, lather up, and then rinse. AKA navy shower...

SLOweather said:

And, a 5 minute shower can be about a 1-2 minute shower if you wet down, turn off the water, lather up, and then rinse. AKA navy shower...

Click to expand...

His first words "My daughter" ;D

kdbgoat said:

His first words "My daughter" ;D

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LOL.. a five minute shower is more like 30 minutes or until the water runs out..

Good stuff all.  Regarding 5  minute showers and "my daughter"; she is a recently discharged marine so a 5 minute shower is something she can deal with.  Ditto for her hubby.

Running the engine during a shower is a great idea assuming a HO alternator and some fat cables.

I actually thought of a water tank heated by engine coolant but didn't like the risk associated with hot water that might be 150 degrees or hotter given typical engine coolant operating temps.

kdbgoat said:

Calculations may be a bit more complex than that. A couple of things to consider is what is the desired temperature of the water when heated, and what is the temperature of the incoming water. Then figure the rate of consumption of the hot water. Then get the BTUs needed to heat the incoming water to the desired temperature at the rate of consumption. Figure out how many watts are needed to provide the required BTUs.

Click to expand...

here are some quick calcs to show you why you need to use propane not batteries..


I'll do this for the typical six gallon dsi that is in a lot of rv's

for this we need the water temperature, amount and final temperature.

so:

1. water in is 60 F
2. Amount is 6 gallons
3. Final temp is 125 F

so we need to raise 6 gallons by 65 degrees

energy required:

to heat one pound of water 1 degree F takes 1 BTU ( British thermal unit )

1 gallon is approx 8.3 pounds

we get 8.3 pounds x 65 degrees * 6 gallons = 3237 Btu assuming no losses

now it just so happens that 1 kW/hr = 3413 Btu so we will use 1 kW/hr as it's close
( that's 1000 Watts applied for 1 hour )

your heater is stated to be 1650 Watts.

so if we apply that energy to the water it will reach the desired temperature in 1000/1650 * 60 minutes = 36 minutes..

you are using an inverter and they are typically 90 % efficient.
and you have a 24 Volt source, so you need to draw ( 1650 * 1.1 ) / 24 Volts = 75 amps from the battery.

great you say.. well there is another factor here and it's the Peukert factor.

Peukert's law, presented by the German scientist Wilhelm Peukert in 1897, expresses approximately the change in capacity of rechargeable lead?acid batteries at different rates of discharge. As the rate of discharge increases, the battery's available capacity decreases.

so your 100 A/h battery is not 100 A/hr when you discharge it at a very high current..

i did the calculation and a 100 A/hr battery with a 75 amp load has an effective capacity of  64 A/hr
you need to draw 75 amps for 36 minutes.. so if you could do this without the massive voltage drop tripping your inverter then
your battery will be seriously mad at you !

next, deep cycle batteries will only tolerate a discharge of C/4 at best.. so for 100 a/h your looking at 25 amps.. !
not enough.. so I did a reverse calculation to establish the capacity you would actually need to achieve the goal..

basically you need 75 amps * 4  plus a Peukert correction and you get: 465 amp/hrs at 24 V..

with this size battery you will stand a chance of some success..

so you see it's a tall order to heat water this way.

now, propane has an energy of 91500 btu's per gallon.. much better.. !
this is approximately equivalent to 23 kW/hrs of electricity..

so i would strongly suggest you heat with propane.. !

Solarman- thanks for doing all the hard work! I now recall the you had posted about the Peukert factor before, but completely forgot about it.

Oldryder, please notice the grin emoji, my statement was meant as humor. Please pass my "Thank You" to your daughter and son in law, I do appreciate them serving our country,  and defending our freedom.

Why 24V inverter?  Everything else, including charging system, van power and other accessory power is all 12V  Use a 12V inverter and simplify the wiring.

Based on Solarman's calculations, I would consider (4) 6v T105's (or equivalent brand) for the battery bank in 12v configuration.  each is 225 amp hours.  2 pairs connected in series doubling the voltage to 12v, but not the amp hours, then the pairs connected in parallel doubling the amp hours. This will give you a of 450 amp hours.  The inverter to battery cables should be close to 2/0 and as short as possible, no more then 6 feet.

Try something like this:

https://www.westmarine.com/buy/west-marine--6-gallon-front-rear-mount-water-heater-with-heat-exchanger-120v--13915103?recordNum=1

Could be used off inverter or shore power electrically, and engine heat when not connected.  With proper planning they could recharge the batteries (with inverter switched of) while running the engine and take showers at the same time. 

These have water temp protection.

I am not familiar with the vans in question, but this doesn't take into account water tank or pump.

oldryder said:

Need schematic for wiring and charger to charge 12v. batteries while maintaining 24v. supply to inverter.

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Can't be done unless you have a 24 volt charger.  You can't charge at one voltage while simultaneously drawing out current at a higher voltage.  This also precludes using the 12 volt engine alternator to supplement the batteries while using the water heater.  The batteries have to be wired in parallel to charge from a 12 volt source, then connected in series to provide 24 volts.

You can automate the changeover using a couple of relays, but the operation will still be one or the other.  Charge or use, not both at the same time.

It would be much simpler to design the system for 12 volts and use a 12 volt inverter.  If you can't find a 12 volt inverter large enough to feed the water heater, it may be that the idea of running it off of batteries is not practical.

Lou Schneider said:

it may be that the idea of running it off of batteries is not practical.

Click to expand...

exactly why I posted my calcs..

propane is the correct and most economical solution, period.