[James-Bob]

I move the charger from one battery to the other. Or if you look at the attached drawings you will see the difference between unbalance and balanced charging looks like.

 

[James-Bob]

Yep reverse-return...I am now a retired plumbing contractor...specialized in Hydronic Radiant Floor Heating and Hydronic Forced Air Systems, well after 45+ years it's hard to stay retired, people never loose my phone number. These two diagrams are part of a very detailed study. I did my own testing and they are correct on the imbalance. I suppose you could make up your own long cables..I would probably just charge each battery on it's own.

 

[James-Bob]

I thought the issue was trying to charge two batteries. I did a simple voltage test on my Toyhauler with 3 - 12 vdc batteries. They were setup from the factory as the unbalanced drawing shows. I checked the voltage on all three and the result was as shown in the drawing. I changed the Toyhauler battery cable wiring to balanced and sure enough all three were exactly the same voltage. It really isn't b.s. I also have a 50' Skyjack lift with 8 very very deep cycle batteries. They are factory wired as balanced. I was actually surprised someone does things right.

 

[James-Bob]

I agree, that is why is said charge each battery one at a time. Putting a charger on one battery and thinking it will charge both the same will not work.

 

[James-Bob]

If you look at the diagram with 4 batteries, and remove two, then you have a vehicle battery layout. The four battery diagram may be confusing. I did the same battery voltage check on my F-350 diesel after running my batteries down with the radio. The batteries were about 2 months old from Ford. I put my charger on one battery, charged until the charger said OK ready to go...I then checked each battery voltages, the battery closest to the charger was charged to a little over 13 volts. The battery on the other side of the truck was just under 12 volts. A note here. I disconnected the battery cables at each battery before the voltage check after charging. There are ways to add voltage regulators to your multiple battery charging setup, to get perfect cell balance, but I think it is easier to just disconnect the battery I am charging, and keep the other battery connected until it's turn.

 

[James-Bob]

Here is a video that may explain better than I have. This video shows two batteries like in our autos and trucks.

 

[James-Bob]

I replied to what I thought was a two dead battery charging issue, or one of two that is not charging properly. It is a complicated issue, I was trying to keep it simple. I am "assuming" the factory thought ahead and has their wiring system designed to charge each battery with the alternator at the same time without overcharging or undercharging either battery. Sorry for causing such a rant. I forgot to mention I have a 4 station Battery Tender charger. I can use it to charge the 1 - 4 batteries at the same time. I have not had to charge my truck batteries with the 4 station charger since the radio incident. I have an OptiMate Pro very nice charger that works great, but just one battery, that is the charger I used on my truck after the radio discharge. But again on my trucks, with two batteries I always disconnect one of the batteries, with any charger just to be safe.
Many years ago I needed to charge two dead batteries in one of my diesel trucks that I loaned to a friend. He ran the batteries down so I had to put my charger on the truck. At that time, I didn't disconnect the batteries put the charger on one battery and let it charge overnight. I was curious if the discharge screwed up the batteries, so I disconnected both batteries, checked the voltage. Sure enough the distant battery was not charged as high as the battery with the charger connected...the truck sat for a while not driven, so I put my charger on the distant battery, left the batteries connected and what I found was the battery that did not charge as high as the other before, now had good charge and the other one that was charged properly did not receive the same charge as before.
I hope this makes some sense. Shoot, I could have washed my truck quicker than trying to explain this...