Charging 24v Battery Bank From 12v Source

[Lostranger]

So.... I've bought a ThermoKing Tripac auxiliary power unit to install in our Gillig. This is a new wrinkle in the conversion plan, and I don't yet have my mind around it. I have some idea of how to proceed with installing the AC and the heat (both backups/supplements for existing systems), but one of the primary uses for this APU will be charging the 490 amp hour battery bank in our solar electric system.

The bus charging/starting system is 24v. So is our house system. The APU comes with a 65 amp, 12v alternator. I need to be able to convert that output to 24v to be able to top our house batteries. What are my interface hardware options?

I want a charger robust enough to handle 65 continuous amps without strain, but I'm also interested in efficiency. I briefly considered replacing the 12v alternator with a 24v model, but I need to charge the 12v battery that will start the APU. Apparently, in most OTR truck installations, APUs are started by the chassis batteries. That's not an option here, even if the bus used 12v. I need a separate starter battery for the APU since it will be operating mostly when the chassis batteries are switched off.

I can think of several ways to approach this, but I'd like opinions. I appreciate your sharing.

Jim

[Utahclaimjumper]

How is your bank made up? 6es 12ves or 24s? If 6 or 12 it can be wired to be charged with the 12 volt system and costs little to do.>>>Dan ( the 12 volt starter battery for the generator can be stand alone)

[Lostranger]

Battery bank consists of 4 12v 8D AGM batteries. How would that wiring work?

[Audiomaker]

Jim,

So you have *no* use for 12vdc in your coach?

Is the APU's alternator the single source of electricity on the unit?

[Audiomaker]

Ok, well I'm sure you've pondered the several ways this can be done.

In my rig, I'm designing a 48v inverter system so I have a similar issue, but my route might be overly complicated or expensive for what you are doing.

My goal was to get the alternator of the truck to charge 48v.   The way I will do this is to use a 12v inverter that outputs 120vac to a 48vdc battery charger.
The inverter will be switched to the ignition key so it doesn't operate when the truck is off.

What I like about this setup is that I get 3 or 4 stage charging of the house batteries vs "dumb" charging from an alternator.

You could do this by using the APU's run signal (there are ways around it if it doesn't have one) to open a relay that switches on the inverter (and subsequently the charger).
There are inverters that have this "switched" provision built in, but it could be done on those that don't using a HD relay directly to the inverters' input.

Option 2 might be to see if there's space to mount a separate 24vdc alternator to charge the house batt's, and have the 12v system only charge the APU battery.

[Audiomaker]

How is your bank made up? 6es 12ves or 24s? If 6 or 12 it can be wired to be charged with the 12 volt system and costs little to do.>>>Dan ( the 12 volt starter battery for the generator can be stand alone)

Yes, please tell us!  Ia this a series/parallel configuration?  I thought that couldn't be done.

[Seangie]

Can you swap out the Alternator?  Whats it powering on the APU?  Are there 24v parts available?  How about a dual powered alternator with 12v and 24v outputs?

Just typing out loud here

-Sean

[Oonrahnjay]

    I'd put a 24V alternator on the APU and use a  24V -> 12V converter to charge the little battery for the APU.  Make the power output of the APU what you really want and need, and do the conversion for the "small stuff".  That is the most efficient way; also, you're only converting a small amount of power, not the largest amount. 

[Lin]

Yes, it seems like the 24v alternator is the answer.  You can wire one of your 12v batteries or center tap the bank to start the APU.  An equalizer can also be used as a 24v to 12v charger.  We do that to charge the house bank OTR.

[bevans6]

There are equalizers around that can charge a 24 volt battery from 12 volts, they exactly double the input voltage and work fine.  They are a little pricey at around $200 and they only handle around 10 amps on the 24v side.  I can't find the link I had to where I ordered mine.  I am using one as a 24v to 12v converter for my trailer lights but when I wired it up to a trailer that had a battery, the brake lights came on on the bus - it was feeding 24 volts back into the bus system.  Here is a bigger brother to the one I have:  http://www.cooperindustries.com/content/public/en/bussmann/transportation/products/power_conversion/battery_equalizers/12040x_converterequalizer.html  It would charge the 24V side at up to 40 amps from a 12V source.  You can only go as high as 32 amps on the 24v side maxing out your 65 amp 12v alternator, so it would have ample headroom.  I don't know how much it costs, probably a bit under $500 from looking at similar sized units.

If you hook up a second alternator to the APU you can float it's ground and use it to charge the 12v to 24v battery of the 24v set, while the regular grounded alternator charges the 0v to 12v battery.  As long as you hard-wired it so you couldn't hook it up wrong that would work fine and probably be as cheap and foolproof as anything else.

Brian

[shelled]

Why are youall making this so complicated ?

Lostranger has 4 12 volt batteries and a 12 volt source on the APU which is (or better be) isolated from his coach's 24 volt system.  Just run positive and negative directly to the 4 batteries - 2 wires in parallel along with voltage regulation and, if needed, isolation.

edward

[Audiomaker]

Why are youall making this so complicated ?

Lostranger has 4 12 volt batteries and a 12 volt source on the APU which is (or better be) isolated from his coach's 24 volt system.  Just run positive and negative directly to the 4 batteries - 2 wires in parallel along with voltage regulation and, if needed, isolation.

edward

This sounds like short to me.  While I understand that it is an array of 12v batt's, and each has it's own voltage of 12vdc, once you have them series wired the voltage potential between any battery's negative and another's positive is different.
It seem like you would need 2 totally isolated alternators to do this (or 4 alternators for 48v).  It would be slick if someone had an isolator for this purpose, but I'm not quite getting how you can tie either the grounds, nor the positives together in a series wired array?

Maybe need a diagram or am missing something?

[bevans6]

I can't see a way to charge a 24v bank from a single 12v source while it's being used to supply 24v loads at the same time, just with wires.  Obviously easy to charge the battery bank if it's temporarily reconfigured to a 12v bank, but that's not the ask here.  I see either make 24v out of the 12v with a converter, add a second 12v source, or add a 24v source.

Brian

[shelled]

First, all the early posts here were about more and more complicated ways of utilizing the APU to charge a 24 volt battery bank and I am downright allergic to complications - that's what the doctor calls it when something goes wrong.

Second, you cannot charge a battery bank without charging the batteries.  The batteries are not 24 volt, they are 4 12 volt batteries wired wired to give 24 volts with 2 batteries in series to make 24 volts and two sets like this wired in parallel to give greater overall capacity.  The individual batteries are still 12 volts and if you add a seperate circuit (both pos and neg) from the 12 volt alternator, it will charge the 12 volt batteries just fine.

Brian, if what you say were fact, you would not be able to run 2 standard batteries in parallel because the 12 volt battery is actually 6 x 2 volt batteries built into one case.

Audiomaker, the wiring to do this is not complicated.  One wire from the alternator goes to all the battery positive terminals and stops.  Another wire goes to all the negative battery terminals and stops.  Voltage regulation and isolating elements if/as needed go in the appropriate wire.

I think it is important to look for simple solutions wherever possible.  That may not be the case here, but it ought to be considered at least as a thought experiment.

edward

[bevans6]

Shelled, what about the fact that in a 24 volt system the 0 - 12v battery has it's positive post connected to the negative post of the 12 - 24v battery?  You've just connected your negative to your positive and created a dead short in your charging system.  I could say trust me, I've been doing this for 40 years, but I won't.  You are missing the point that while the system is charging it is still an operational 24 volt system.  Your approach is to just rewire the batteries as a 12 volt parallel bank, and does not satisfy the requirement.

Brian