My trailer batteries are no longer good, they don't hold any charge.  Doing a little looking this morning it appears that even a "cheap" deep cycle battery is $125, so about $250 to replace 2.  We go to lots of open and 4H shows where we show off of the trailer, during the summer we run the generator for the AC but in the Spring and Fall we don't always need AC, usually just the radio and the water pump, both of which work when the trailer is connected to the pickup.

Would it be practicle to just allow the radio and water pump to run off of the pickup battery at the shows when we aren't running the generator (usually about 10 hours), what about just replacing one of the batteries?

Running the trailer off the truck batteries can run the risk of a no start, just when you need it the most. 8 -10 hours of a water pump, radio and lights can effect a significant draw, and may drop the battery voltage lower than is what is needed for starting.

Having two batteries in parallel that are bad, and changing one, will quickly degrade the life span of the newly installed one. You would be better off using just the new one and disconnecting the old.

If you have always a generator at your disposal, you would lessen the chances of your truck being dead in the water, and you not getting home. You have to determine if the cost of two batteries is worth messing with your tow vehicle.

My truck will not allow any B+ voltage to the trailer, unless it is physically running. This is done just so, to alleviate any starting problems caused by drained batteries. Various truck brands and construction dates, have different priorities. You may or may not be able to "trick" the circuitry, to allow a hot B+ all the time.

I would spend the money for two new batteries, enjoy a great summer and not worry about whether or not I'd be able to leave at the end of the day.

On several occassion, I have run into the Interstate Battery Center here locally and bought used reconditioned batteries. They sell them for $29.   The salesman says they are batteries that various reason have come back to them and after testing they have determined that they are still good.  They have no warranty.  They may last 3 months, they may last 3 years.  But for the $29 gamble, I've bought a few and they have worked fine.

You may want to check the local battery suppliers in your area. ( don't go to a local Wal-mart or Auto-Zone asking for this type of used battery. But go directly to the battery company or distribution center).  You might also check into wrecking yards, They pull the batteries from wrecked cars, trailers etc. And while a Deep Cycle would be the prefered choice, you may get away with a normal car battery to just run the radio in your trailer. I see my contrsuction workers with their truck radios blasting all day on construction job sites and their trucks still seem to start at the end of the day.

I'm assuming you have checked your exisiting batteries and made sure they are full of water, the terminals are clean etc. That is usually the reason people are trading in good usable battery as their core when they buy a new battery. The distribution center, cleans, fills and test and then resells those batteries.

Thanks Paul, I was thinking about discarding the two batteries that I have an replacing with one....would that be enough?