My 12.5 kw Onan is hooked up to a 200 amp manual transfer switch. I want to use the genset to charge batteries and then run lights only off an inverter and batteries. How do I hook up an inverter and battery bank to this existing setup? Help!

-- James (b@b.com), December 07, 1999

Answers

Is the inverter a straight inverter ( DC input, AC output, period. No relays, no built in charger, no gen start, etc.)? Or is it a Trace with all the bells and whistles?

-- Ken Seger (kenseger@earthlink.net), December 07, 1999.

What kind of Inverter are you using or planning to use?

Does it have an integral battery charger?

-- plonk! (realaddress@hotmail.com), December 07, 1999.

First I need to clarify what you have/want. It sounds like you have an AC generator (ok, alternator), 120/240 volt, ~ 50 amps. connected to your house. Right? You want to have it charge a bunch of DC batteries (what total voltage?) and use the DC to operate some lights. Correct?

If you are going to use a twelve volt DC system just hook the generator output to a charger. You sure aren't likely to need the whole capacity of the generator, but that's fine.

How many lights do you need to run? What is the DC amperage load? Anyway, when the lights get dim, run the generator to power the charger. If the charger's got enough capacity it can power the lights while it is charging the batteries.

Does this help?

-- Gary S. (garys_2k@yahoo.com), December 07, 1999.

Sorry for the delay in responding. Been out of town. I want to run 300 watts worth of lights for about 4 hours per day. I have not bought the inverter or batteries yet. Need advice. Thanks much.

-- James (b@b.com), December 10, 1999.

Well..... 300W x 4 = 1,200W/day right? Your problem is that if you buy say a 200AH battery, you can only charge it at a 20-40 amp rate depending on its type. If you are talking a 12VDC batt and inverter that = 40A x 12V = 480W. Batteries prefer the electricity to come in slow and go out slow, otherwise they age quicker, a lot quicker. So this would be just under three hours charging time. If your Onan is a diesel I'ld say just run the diesel, it will probably only use about a quart or two an hour.

Meanwhile back at the math. If you've got more money you can up the amps or the volts. If you go with more batteries, like 4 at 200AH each in parallel at 12VDC that's 800AH you can charge at 160 amps x 12V = 1,920 W/hr. That's only about 2/3 of an hour BUT that's also a scary amount of amperage. 4 in series running a 48V inverter and charging at 40A would give you the same results at 1/4th the amperage.

Does running a diesel for those 4 hours keep looking better?

How about 16 batteries in series-parallel at 48V for 1/4th the time? Nice idea? Well.... You are not going to find an inverter with a builtin charger that is going to put out 160A at 48V. The big $4,000 Trace (street price $2,600-3,400) only puts out 75A at 48V (3,600W at max. which would be 20 minutes/day + more for in/out/convert loss). You don't want to know what a 160A at 48V battery charger costs, trust me.

To complicate matters you are going to be losing some power going in and some power going out, plus a slight loss running the inverter.

See both http://www.windsun.com/Batteries/Battery_FAQ.htm and http://www.dcbattery.com/faq.html for information that will help you a lot.

I hope this helps you. P.S. I'll assume that the lights you want to run are flourescent? If not, by switching to flourescent you can cut your power requirements by about 2/3rds.

While your are on the internet check out http://206.67.59.5/wwwboard11/ for a lively discussion of off grid systems.

-- Ken Seger (kenseger@earthlink.net), December 11, 1999.