Charging a 24v house bank with 12v alt/gen

[AndyyCoulic]

So ive been racking my brain for the past couple months, trying to figure out a way to charge my 24v house battery bank without changing my 12v 300amp generator. Since i'm running next to no load on the bus with the otr ac gone i was debating on swapping the gen to a 24v and adding a 12v belt driven alternator.

An idea i came up with today. Was what if i ran a 12v power inverter from my bus 12v with a power isolator. (Inverter would only work when bus was running to prevent battery drain) Say a 2000 watt inverter to put 120v into a magnum inverter charger to charge my 24v house battery bank.
I know its not a 300amp charge. But between my 2.6kw solar rig and that 100 amp charge. I should be able to run my minisplit a/c plus everything else over the road with no worries.

Thoughts?!


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[Iceni John]

Why can't you charge your house batteries from 2.6kW of solar?   That's more PV power than any RV or bus conversion I know of  -  I thought my 2kW was a lot!   How many aH of batteries do you have?   What depth of discharge do they normally get down to?   Do you not get enough useful insolation where you are?

If you convert DC to AC merely to produce more DC, that's inefficient.   There are 12V-to-24V converters, but they may be equally inefficient.   I also don't know if any 12V inverter will work straight off an alternator  -  some inverters' high voltage disconnects may be too low for a typical alternator?  

[bevans6]

I see this as an over-the-road issue, therefore efficiency is moot - to some extent.  You can draw X% more power from your 12 volt 50DN (I assume) to run stuff.  One thing I don't understand is using the Magnum 4024 to charge the house bank to run your air conditioner.  I would have thought you wanted to charge the house bank so the Magnum can draw 24v power from it to run the 120 AC loads.  Anyway, assuming that is indeed what you want to do, here is how I break it down...

You don't say how much power your AC loads draw, both starting and running.  You will use the Magnum 4024 drawing power from the 24 volt battery bank to run the AC loads, so starting loads are OK (power will just be drawn from the batteries for the instant that the load is starting) so you need to replenish the power drawn on running load only.  I will use 15 amps at 120 volts as the draw, so  that is 1800 watts.  You need to figure a way to put 1800 watts of power into the 24 volt battery bank from the 12 volt source.  That is nominally 75 amps at 24 volts.   The correct way to do that is to find a 75 amp 12V to 24V converter, which I could not find easily and which would cost roughly $5K if you did find one.   The other way to do it is to build one, starting with a 2500 watt 12 volt inverter (over size a bit to accommodate losses and so you aren't trying to run it at full power all the time), and a 100 amp or so 28 volt power supply to provide power to the batteries and the Magnum 4024 inverter.  You would run the 12 volt inverter from the bus start batteries and the bus alternator, no issues with that as people do that all the time.  The issue is the 28 volt power supply.

A battery charger is not a power supply per se.  It's similar to a power supply but it's designed to charge batteries in steps, not supply high current on an ongoing basis so you can't use a battery charger to run the Magnum 4024 while you are driving.  Thankfully, aviation requirements make such power supplies common, since airplane guys need to do what you want to do fairly often so they make high power supplies to run airplanes on the ground.  A typical power supply is https://www.aircraftspruce.ca/catalog/elpages/bycanpowersupply.php, at $1350.  

Assuming 15% efficiency loss at each stage, you will need around 180 amps at 13.5 volts nominal out of your bus alternator.  Should be no problem, check your cable sizes, etc.  I estimate around $2500 to $3,000 for the hardware (inverter and power supply).  But it should work.  Not sure that it's a good idea, though.

Brian

[scott332]

If your 24 volt inverter sees 120 volts coming in from the 12 volt inverter, the 24 volt inverter will just pass that 120 volts through to your circuits. It should charge the batteries, but it will not invert unless it is one of those hybrid inverters. If this happens your 12 volt inverter will need to be sized large enough to handle your 120 volt electrical needs while driving.

[AndyyCoulic]

with the magnum 4024 ill be running 220v in the bus. (running two 4024 linked into a 1600ah 24v bank) so the 120v into the magnums would possibly power the ac and charge the battery bank with the help of the solar.

the ac is a 3 zone 30000btu split pulling 15 amps at 220v max. (running a 220v mini split has a higher seer rating than a 120v split). 2.6kw of solar sure is a lot for a bus! i'm surprised i figured out a way to get that much on my roof. but being up in canada it makes it a little bit less efficient with only having 3 months of summer and even less sun in the winter months. (don't need ac in the winter, but do plan to use the heat pumps until they won't work in too cold of temps).

the plan is to still be able to float my system with the a/c on in the daytime during summer. but dawn/dusk/night time driving or cloudy days may hinder that with other appliances. so if i can just push some extra power like faking 120v to the magnum from the bus' power that would be great. and it would help with my voltage regulator from cycling constantly, from not having much of a load on it.

regarding insulation i have 3 inches of closed cell spray foam on the walls and ceiling. and 1 1/2 inches of foam board for the floor.

i'm not really worried about the efficiency loss as i have most of the parts laying around to do it and it won't really cost me anything to do it. (cheaper than finding a 24v gen and a 12v alternator and rewiring the bus). i was thinking if the inverter that id use for putting 120v into the magnum be connected via relay so it will only turn on when the bus is running, and have that inverter connected to the bus system without an isolator so it would draw comfortably and pull power from the gen/start batteries with no issues.

i want to make sure this all makes sense, and tho it may not be the most efficient process, i am more worried about damaging something else from trying to find a work around.
i don't want to have to be frugal with power, but also don't want to connect to a pole if i don't have to

kind of all makes sense ya?

[bevans6]

Are the Magnum's Hybrids?  They are the only type that could work in this application.  Your AC will draw around 4,000 DC watts after inverter losses (the magnums put out 120 VAC each so you will have 240 VAC, not 220 VAC), and your 2,600 watts of solar will probably put out no more than 1,000 watts on an normal day (capacity factor of around 25%, double that for daytime use only, discount it for sub-optimum installation angle and orientation), so I'm not sure where you are going with this.  It will be a balance of cycle time on the AC vs power generation and who knows, it might work.  You have a nice big battery bank so you do have a lot of leeway on discharge, but the charge time of that bank is also very long and running it at partial discharge for long periods of time won't be good for it.  You'll have to arrange to fully charge and soak the batteries regularly

If your Magnum's are the Hybrid model and you have a handy 12V inverter, I'd sure as heck try it, the theory is good.

Brian