Confusing starting problem

make sure to buy all 4--and dont cheap out--might be cheaper in the long run

 

Your batteries are connected in parallel, otherwise you would have a 48 volt system.

My advice is this:

A battery bank should be equalized charged about once a quarter (every 90 days or so). An alternator charges the batteries to about 13.8 volts. Over time batteries develope a memory of how much charge / discharge is expected of them. An equalization charge uses a pulse current to raise the voltage to about 15.4 volts. This knocks out the sulfurization that takes place. It doesn't completely eliminate it, but it does reduce the effect of it.

It really is ok to just replace the bad battery in a bank. You might be replacing another battery in a year or so, but the overall cost is less, especially if you are aware of the possibility. Replacing all four batteries maximizes reliability, but not cost. I know this is opposite to what generally is said, but I don't have a dog in this hunt. (I'm not selling anything)

Consider running a line from the alternater to the solenoid for the starter. This will reduce the power loss to the starter a little bit, giving a quicker start when the batteries are good, meaning less wear on the starter, and a more dependable start in cold weather or when the batteries are weak.

I've been running a set of Optima's for 5 years now, for 4 years I've mechanics with their battery testers tell me I have a bad battery. They really don't understand AGM batteries, and half the time they have the battery tester poorly hooked up.

 

Battery #3 would be bad as it was trying to support #4. It is understandable that the outside battery would fail first as it is the one getting the most draw of amps.

Unless those are premium batteries plan on replacing them on 12-13.

Since they are hooked up in series, seems to me like it wouldnt hurt to have grounds and power at both ends, then the batteries are all working and not just one end of the battery bank.

 

i guess you could replace just the bad 2--not ideal--but the world wont end--it might shorten the life of the new pair --but sometimes we cant do everything we want
hell somepeple have to replace their truck after 3 years--and i am sure most people realize it sure isnt worn out
you do what you can do--

 

Here's what's going to happen if you replace just one:

Think of a battery as a container of water. Now think of four containers of water with different levels in each. Hook them all together with a pipe on the bottom of each container and they will seek their own level--equalizing the level in all four containers. Think of this as the state of charge in a battery bank.

Now if one of those containers has a hole in it (the bad battery), the others will try to seek a level with the leaky one, draining themselves in the process. So you turn on the faucet to replenish them (the alternator) and the three containers that do not have holes become overfilled (overcharged) and the one with a hole in it never fills.

When you replace one battery in a bank, there is going to be a mismatch in capacity. One battery will always be lower than the others and will cause the better ones to cook eventually. In addition, the newer battery will be constantly draining toward the older ones.

Replace the set and sell the good batteries to a battery shop.

Exactly

 

If the batteries are wired so the load comes off of the end of the set, instead of the middle, then the battery closest to the load gets worked a little harder, and the one furthest away gets worked a little less. But that's more of theory issue than a real life one.