Use of One Battery in a Battery Bank

JRM

The solar part of my small off-grid system is finally working - even better than I expected.  It provides AC electricity to my trailer day and night, and the three 300-watt panels charge the four 12-vots batteries in my 48-volt battery bank to 100% before noon on a sunny day.  The only use problem (so far) is that my 3500-watt high-frequency inverter won't start the trailer A/C, but this is my fault for not realizing soon enough that more power is needed to start the compressor, and the solar system wasn't expected to run the A/C long enough to make a difference anyway.

Now I am moving on to setting up a generator to charge batteries on cloudy days or, once it is working, run the A/C.  And this leads to my question below.  For background, I have a Westinghouse iGen4500DF generator that has some nice features - it will run on propane or gasoline and has an automatic choke, it has push button automatic start built-in, and comes with remote start and stop.  My problem is that this generator also has a run-switch that must be turned on for the generator to operate, and (when on) the run-switch has a draw of about 0.3 amps that runs down the very small 12 volt starting battery unless the generator is running to charge it.  I can get a voltage control unit to automatically start and stop the generator for battery bank charging, but the run-switch needs to be left on.  The only solution I can think of to prevent a dead starting battery is to use one of the much larger and solar charged 12-volt batteries in the battery bank to start the generator, which would subject that battery to draw down by the run-switch and starting the generator, and this same battery will be charged while the generator is running.  My hope is that someone can tell me how the draw down and the charging of one battery would affect the overall battery bank?

JR Munn

Graham Parkinson

It's not a good idea to use power from one battery in a multi-battery bank.  This will throw the batteries out of balance and result in either under or over charging of each of the batteries in your bank.  

What does work is to use a 48V to 12V buck converter (available in a range of amperages from Amazon etc.) run off of your 48 V bank.

However a better solution might be to figure out the wiring of your generator so that you could use a relay to power the run-switch when the control unit requests generator run.

Marc Kurth

Install a "soft start" kit on your A/C unit to drop the inrush starting current to 30% to 50% of what it is now. They work wonders on systems running on inverters and/or generators.

JRM

Thank you Graham and Mark for suggestions.  I have purchased a "soft start" for my trailer A/C and hope to get it installed in early July.  I think I can figure out using the "buck converter", which also solves the same problem of how to power 12v shed lights if the inverter shuts down at night.  I don't understand relays well enough to know how this might work, but am going to ask the more technically proficient people where I am getting the control unit.  Thanks again!  JRM

Graham Parkinson

Your generators "run switch" can be replaced by a relay which is just a remote actuated switch powered by a small current.  The current from the autogenerator start signal could also operate the relay to put your generator into "run mode" when generator run is requested.  Otherwise the relay could leave your generator in the "off mode" to save the start battery.

The only complication might be that the "run switch" might be multi-pole (i.e. have more than two wires) but have no fear - there are multi-pole relays too.

JRM

Graham, thank you for the follow up about relays.  I will keep this in mind for the future or if I have problems with the set-up.   Speaking of possible problems, does anyone know what happens to a buck converter if my generator sends a charging current back towards it?  My plan is to reduce the battery bank voltage from 48 to 12 volts with a buck converter, and then use the 12 volts from the converter to start the generator.  However, the generator will try to charge its battery through the same wires that lead back to the buck converter.  I suppose this can be blocked by diodes, but this is another complication.   JRM

Dave Angelini

Impossible to answer with 100% certainty. Circuits have various designs.

 The good news is the converters are cheap so buy 2 and if you fry one, (it is OC protected right?)
use the 2nd one with diodes or a switch.

JRHill

I have a 12vdc regulated power supply (120vac) that was used for a 2 way radio base station for years when I lived on-grid.. I thought to use it for eliminating several 12vdc wall warts plus for some other 12vdc items. But it was very inefficient. So I thought to try a 48vdc to 12vdc converter. I can't recall what the no/low load draw was and its efficiency was also terrible. So I just stayed with the 120vac to 12vdc wall warts that I thought to eliminate. Bottom line is I didn't think too much of the Buck converter.... Maybe there are some that are better than the unit I bought.

Graham Parkinson

Perhaps a more robust strategy to power the 12 V to start the generator from a 48 V main bank is to use a 48V to 12 V charger module (similar to buck converter but designed to maintain a battery) to charge a small 12 V start battery for the generator.   

Doing without a generator start battery would require a very large buck converter to handle the generator starting current spike.

By the way, there is a wide variety of buck convertors available, some with good specs and some not so good.  Anything that is going to be left on all the time better have good efficiency or it will eat your lunch.

Dave Angelini

That is what I thought he was doing Graham.   Getting rid of the battery would be a stress on circuitry unless it was robust.

 Might have read it correctly.....Cheers!  Float it with a 20 watt solar panel is what I have done in the past.

Graham Parkinson

Good point Dave - Even up here in the cloudy PNW only a small panel and a cheap little PWM controller is all that's required to keep up a generator start battery - simple and effective is always the best solution.

Dave Angelini

For you Graham, it might be an upsize from 20 to 40 watts😂😉

NANOcontrol

Why not get the voltage relay to turn on the run switch. 

Voltage relay turns on the run switch
A short delay timer closes the start switch

What does the run switch do, operate a fuel solenoid or field coil?
Any solenoid or relay will hold in with 30% of current. There are ways to accomplish that.
I think turning it on only when you need it is the way to go.

JRHill

I'm curious what JRM ended up doing. The 12vdc solar panel seems the best so far.