To check for a bad cell in the battery, you need, a voltmeter, a headlight,  two nails about 2 inches long.
Take off all the caps of the cells.
Clip the nails into the alligator clips of the volt meter.
Hook up the head light so that there is a modest load on the battery.
(with no load this test may not reveal anything)
Start at one end of the battery by touching one battrey terminal with the positive lead of the volt meter and  insert the nail gently into the second cell away from the terminal. This tests the first cell only.  It should read about 1.2 volts.
Move the nail to the third cell and put the other nail into the first cell. This tests the second cell, it should read about 1.2 volts also. Move each nail one cell further each time to test each cell. The cell being straddled by the nails is the one being tested. If any cell is significantly lower than the others, ( more than one half volt or more different), the battery must be replaced.  Since you cannot replace one cell of a battery, the whole thing is junk, get a new one.

BTW  I have had a solenoid fail and replacing it is PITA.  After you finally get it out, cut a notch in the lower bolt hole of the new one.
This is so you can have the lower bolt in place but loose.  Put the bolt through the upper hole and screw it part way in. Now rotate the new solenoid until the notch slips over the lower bolt and tighten both.
This method requires fewer four letter words than any other.
Having the stealership do it also requires fourletter words when you pay the bill then find out they didn't do it right.

Phelonis