Nita- excuse my butting in, I'm an old mechanic, so I'll try to help... it may get technical, feel free to ask questions.
It will take longer than 15 minutes with a car's charging system- how long depends on how big the battery is, and how much current ( Amperes ) the charging system puts into it.
However, about 15 minutes at fast idle should tell you if the battery will take a charge, and give you some idea of whether to keep charging it or try the next one.
A battery is like a bucket of water- you can't take out more than you put in, and it takes some time to do that with a car.
If you have any choice in vehicles to use, pick the one with the heaviest-duty electrical system, like a truck. It will charge faster.
How old are those marine batteries? Three to five years is about the extent of any storage battery's life- they should have a date tag with dates punched. In any event, pick the newest one.
Check the electrolyte level-- there should be two rectangular blocks atop the battery ( if there aren't you have a "sealed" battery and can't check the levels in it )-- wipe around them with a cloth you don't mind losing ( acid will eat it ) and using a flat screwdriver, pry under the edges, working around the edge, until it comes off.
There will be holes about an inch in diameter- those are the cell tops where you check & top up electrolyte levels.
Remember, this stuff is sulphuric acid, so be careful-- if you get any on skin ( can't mistake it, it is oily and will start burning & itching ) rinse with water, a bad case, neutralize with baking soda ( Arm & Hammer, not Calumet ).
Using a flashlite ( no flames! hydrogen gas is present ) sight straight down-- you should see fluid. In fact there is a little split ring in the hole to indicate proper levels in each cell, and ideally, the fluid level will be up to it. If it is below that, or you see the plate tops ( grey metal lines ) you must add distilled water until the plate tops are covered and the split ring reached. One source of distilled water is the runoff from an air conditioner, but it may need to be passed thru a coffee filter to get junk out of it. In an emergency, you can use tap water, but it will eventually kill the battery. However, it is vital the plate tops be covered- if not done, the battery will not hold a charge, even if it normally can.
Optional- proceed with caution, this does not always work-- an old mechanic's trick to revive an old dead battery is to mix a few tablespoons of Epsom Salts with the distilled water, and top up all cells with that. Let it sit a while, and it may reduce sufation enough to revive the battery. If your batteries are over three years old, I would recommend tyring this first. If they are newer, just try topping up the cells with distilled water and a test charge.
To summarize:
Pick newest battery
Clean top, pry off cell covers, and cover the plate tops with distilled water or a mix of that & Epsom Salts
Charge for 15 minutes and try it. If it runs your gear a while, try charging longer. If not, repeat with next battery.
Best of luck- I wish I could be there, it's easy to do, but not easy to describe.
Nita,
As usual backhoe has the answer!
I know less than he does but I will FReemail my phone number to you if you need additional help.
Tom Eaker
In this case, don't you think it would eaiser to just connect the inverter to the vehicle battery in her car and run an extension cord inside?