Hi, new to the forum. I have converted an 86 Chev G30 mini school bus into a camper, and i have mounted a Danby 5.1 "window shaker" air conditioning unit into a plenum, which creates the equivalent of a half indoor / half outdoor installation just like a normal wall or window installation. 'Til now, I have mostly run it when hooked up to grid power, but I would like to be able to run it off the bus's engine/starter battery while driving... seeing as the school bus does not have factory a/c.I'm considering connecting a 1000w (continuous) pure sine wave inverter to the starter battery, and then running the a/c unit off it. The unit draws ~ 430 watts continuous, so I know the inverter is sufficiently sized, but I'm concerned with drawing down the battery while driving. My question is, would the vehicle's charging system be able to keep up??? I'm not sure of the alternators output, but I've seen replacements rated at 100+ amps, and it's a standard 12v battery.Thanks so much for any help you can offer.

What is the locked amp draw of the AC unit?

Doubtful that your 1000w inverter will be enough to power the AC through the locked rotor draw of the compressor. If it could, the 430w would drain a single battery very fast, that is a 40 amp draw. Assuming you have a 80AH starting battery, you would be dead in less than an hour or two. It's also doubtful that the alternator would work in this situation since it would be maxed out and won't last when asked to do that.

donn0128 wrote:What is the locked amp draw of the AC unit?sorry, not sure what you mean by "locked" amp draw. i do know it draws 430 watts once running, so about double that on start-up... that works out to about 36 amps continuous.

westend wrote:Doubtful that your 1000w inverter will be enough to power the AC through the locked rotor draw of the compressor. If it could, the 430w would drain a single battery very fast, that is a 40 amp draw. Assuming you have a 80AH starting battery, you would be dead in less than an hour or two. It's also doubtful that the alternator would work in this situation since it would be maxed out and won't last when asked to do that."locked rotor draw of the compressor"??? i'm not sure what you are saying? i have run the a/c unit off my 1000w xantrex inverter tied to four 6v golf cart batteries with no problem.yes, it would drain the starter battery very quickly, which is why i would only run it while driving.indeed,the alternator is my concern. but why wouldn't a 100 amp charging system be able to handle the ~40 amp draw, especially if i wasn't running headlights?

The truth is that once they warm up, in just a few minutesMost automotive alternators can only sustain approximately 70percent of rated outputIf your bus alternator can sustain the 100 amp output you should be okay, but I would add another engine battery,What is the, MFG and model of the alternatorSince that is the G30 chassis I'm assuming you have a Delco automotive style alternator

Running a 120v a/c unit off starter battery | Good Sam Community

62 Replies

MrWizard
Moderator
Dec 24, 2013
Pt
That would be great, , but it's unlikely
The mini bus is built on a GMC G30 chassis used for box trucks and class C motorhomes
pianotuna
Nomad III
Dec 24, 2013
Hi,

If you get lucky the bus starter battery will be 24 volts, as will be the alternator. That makes what you are attempting a lot easier.
jrnymn7
Explorer
Dec 24, 2013
Salvo wrote:
If you already have all the components, then give it a try. Using short leads, connect inverter to chassis battery. Monitor chassis battery voltage while you're driving with A/C on. If voltage drops below 13.0V then alternator is not big enough to power A/C.

Sal

i do not have the components as of yet. i was hoping to know for sure if the idea was even feasible before i buy another inverter. canadian tire has a 1000w pure sine wave on sale this week for $190.

i am willing to spend the money on the components; including a higher output alternator and better battery, if necessary; if it means not significantly reducing the lifespan of my house batteries. it would appear the idea is feasible, but now i need to decide on the best way of doing it. house batteries vs. starter battery?

i might be best to buy the other inverter, and keep it as a back up if nothing else? but i like your idea of doing a trial run... at least i'll know where i stand with the existing equipment. my hope is the alternator is already a high output, seeing as it's a bus. thanks!
jrnymn7
Explorer
Dec 24, 2013
Peg Leg wrote:
With 4 golf cart batteries already installed you should be able to run a inverter for the AC unit. Use the golf cart (GC) batteries to power the inverter not the starter battery.

The GC batteries should already have a charge wire with a isolator. The isolator looks like an old Ford starter solenoid that mounted on the fender well but is rated for continuous duty.

The isolator allows the GC batteries to charge from the alternator when the vehicle is running. Then it shuts off when the engine is off to keep the house from pulling down the starting battery.

i do not already have an isolator in my wiring. this is not a manufactured rv, it is my own conversion. presently i charge the bank with a 3 stage charger, and am concerned about charging with the alternator. the big question is, will charging with the alternator significantly reduce the life of the house batteries?
mena661
Explorer
Dec 21, 2013
pianotuna wrote:
Hi Sal,

Thanks for the "go, no go" for the voltage of the chassis battery. I've often wondered about that.
X2
Salvo
Explorer
Dec 21, 2013
We're talking about two different approaches. I'm talking about connecting the inverter to the engine battery, not to the house battery. A 6awg cable from alternator to house battery is still too small if OP wants to connect to that battery.

The first step is to see if his alternator is up to the task. He should do that with minimum amount of modifications. Place/secure inverter in engine compartment. Run extension cord from inverter to A/C. Monitor chassis battery voltage.

Sal

jauguston wrote:
One thing I would add would be if you already have the circuit with the relay going to your house battery consider upgrading the wire size as that circuit will be asked to carry more amperage than it was intended to carry. I wound think something like 6ga would be a good upgrade.
jauguston
Explorer
Dec 21, 2013
One thing I would add would be if you already have the circuit with the relay going to your house battery consider upgrading the wire size as that circuit will be asked to carry more amperage than it was intended to carry. I wound think something like 6ga would be a good upgrade.
pianotuna
Nomad III
Dec 21, 2013
Hi Sal,

Thanks for the "go, no go" for the voltage of the chassis battery. I've often wondered about that.
Salvo
Explorer
Dec 20, 2013
If you already have all the components, then give it a try. Using short leads, connect inverter to chassis battery. Monitor chassis battery voltage while you're driving with A/C on. If voltage drops below 13.0V then alternator is not big enough to power A/C.

Sal
Peg_Leg
Explorer
Dec 20, 2013
With 4 golf cart batteries already installed you should be able to run a inverter for the AC unit. Use the golf cart (GC) batteries to power the inverter not the starter battery.

The GC batteries should already have a charge wire with a isolator. The isolator looks like an old Ford starter solenoid that mounted on the fender well but is rated for continuous duty.

The isolator allows the GC batteries to charge from the alternator when the vehicle is running. Then it shuts off when the engine is off to keep the house from pulling down the starting battery.