inverter and starter batteries

[ttomas]

Can I use an inverter hooked to the starter batteries (while running) to run one high efficiency AC unit without hurting my alternator? thanks, Tomas

[Sean]

That depends on the size of your alternator.  You don't even mention what engine you have.  Give us a few more details and we can try to answer your question.

FWIW, I can run two air conditioners from the power supplied by my main engine alternator while driving.  That being said, few inverters are up to the task, and you really need a pure sine wave inverter to properly run air conditioners.

-Sean
http://ourodyssey.blogspot.com

[ttomas]

 Hi sean,  Sorry for the lack of information. I am converting a mci9 1983 model with a 6V92ta. That's about all I know abouit it. It was a N.J. transit bus.    thanks

[chris4905]

Tomas,

Do not get in a habit of using your starter batteries for anything, other than starting.  The start batteries are not the same as deep cycle batteries (which is what your inverter should run off) and the start batteries are not made to handle the constant deep cycling that an AC and other appliances would put on it.

But.........the main concern is your start batteries need to remain dedicated to starting, not used by anything else, to lower the possibility of having dead batteries when it's time to start.

Get a set off deep cycle batteries, separate from your start batteries, then hook that new fancy inverter to the deep cycle batts.  After starting the coach you can ties the batteries together and charge them with that hefty 280amp (approx) alternator on the coach engine.

I can also email you a wiring diagram of how to separate the battery systems until started and then charge them together.  Has worked for me for 7 years.  Can recharge both sets of batteries while going down the road and run the front AC unit.

When you ready for the diagram email me and I"ll send it to you as an attachment.

Chris Christensen
1974 GMC 4905  with 8V71 & Spicer 4speed
currently have the 8V71 and 4 speed removed and getting ready to put in the 6V92TA & V730 w/ air shifter
Eagle, Idaho

[ttomas]

thanks Cris, i will email you.    Question: will the starter batteries overcharge while the alternator is charging the inverter batteries?

[ttomas]

Hey Cris, I couldn't find your email address. mine is: jesusrulesbus@yahoo.com  Thanks  Tomas

[chris4905]

Tomas,

The response to your questions and the documents & pictures are enroute via email.

Good luck,
Chris

[TomC]

You can run both the starter and deep cycles off the same alternator without overcharging.  If you think of batteries as a big glass of water, and that level of water being the voltage the alternator is set at.  Once the batteries reach their voltage saturation point, the current overflows into the least restriction, which in this cast would be the battery that is still not charged up.  Once voltage is met, no more current will flow into the battery (as long as the voltage is correct-like my 14.1 volts for my AGM).  The best is to have a three stage regulator, then it will reduce to float voltage-like about 13.2 volts.  But have yet to have problems with my big alternator over charging my batteries (2-size 31's for starting and 2-8D AGM's for deep cycle).  Good Luck, TomC

[jjrbus]

You do not need a pure sinewave inverter to run AC's. I and many others run AC's with modified sinewave.  HTH Jim

[Jerry32]

I tie all the batteries together while going down the road whith a constant duty relay and that keeps all the batteries charged. The inverter is powering the fridge which is household type but don't have AC yet other than bus ac . Jerry

[Sean]

I am converting a mci9 1983 model with a 6V92ta. That's about all I know abouit it. It was a N.J. transit bus.

Most likely, this will have a 50DN alternator, capable of around 250 amps (270 is the max rating) at 24 volts.  More than enough to run an air conditioner.  But you should double-check that that is what you have.

The advice about not running anything off the "start batteries", while normally good advice, is not relevant if your intent is to only do this when the alternator is charging.  That's because you will not actually be running the inverter from the batteries, but, rather, directly from the alternator output.

Conventional wisdom in the conversion business, though, is to achieve this, as has already been mentioned, by connecting your inverter to house (rather than start) batteries, and then having a mechanism to provide the alternator current to the house side while running, either with a battery-bridging solenoid (my preference) or with a battery isolator.

That said, you can have a separate inverter on the chassis side if that's the way you want to do it.  (One reason to do this might be to use an electric, rather than engine-driven, air conditioner to service the driver area, as we do.)  In this case, I recommend a disconnect solenoid, so that the inverter or its loads can not possibly be connected unless the alternator is running.  The solenoid can be run from the old blower circuit, or from the relay terminal on the alternator.

HTH.

-Sean
http://ourodyssey.blogspot.com

[Sean]

You do not need a pure sinewave inverter to run AC's. I and many others run AC's with modified sinewave.

Yes, one does not need a sine wave to run air conditioners.  BUT:

Running AC motors on other than a good sine wave will have two unwanted effects:

(1) The motor will use more current, and draw more energy from the batteries (or alternator).  Now, if you never run on batteries, then this is likely not going to cost you much, unless you're pushing up against the current limit of the alternator.  Depending on just how "square" your MSW output is (and there is a whole spectrum, from nearly square to, say, 50 steps per cycle or so), this will cost you anywhere from 5% to 15% efficiency.  If you run from batteries at all (as we do) you will pay for the less expensive MSW inverter by either having to have more battery to begin with, or having shorter battery life.  And

(2) The motor will heat up more.  This will not only further reduce the efficiency of the AC unit, but it will also shorten its life.  The 5%-15% efficiency loss I mentioned in (1) goes mostly into heating of the AC motor (although some of it goes into additional heating in the inverter, also shortening its life).

So, yes, you can do it, but, IMO, it makes little economic sense to do it.  Combine that with the additional fire risk, and I would not chance it.  No MSW inverter manufacturer recommends running high-load, continuous-duty induction motors on their product.

FWIW.

-Sean
http://ourodyssey.blogspot.com