Battery charging question

[Chris142]

My old Peterbilt sits alot. It has 3 12v batteries. They are wired so that it's 12v but about 3000 cca combined. I dont know what you call it...+-+-+ and - - -.

I put it on the charger once a month or so. Say 1 battery was run down. If I hook my charger to all 3 at the same time will it put more power into the run down battery or will it overcharge 2 while bringing the 3rd back up?

How does this work?

 

[mechanicx]

I think the battery with the least charge (lowest voltage) will draw the most current from the charger and the most charged battery will draw less.

 

[JHZR2]

So the electrical system is still 12V right? If so, then they are in parallel.

If so, Id think they would balance each other. The charger should recharge in the path of lowest resistance, which would be the battery that is in the best shape/newest.

There may be data for cell resistance versus SOC out there that could be of help.

 

[Stu_Rock]

It sounds like the batteries are wired in parallel. As long as they stay connected, all three of them will always be at the same state of charge. You could just think of it as one big battery.

 

[Bob Woods]

Three batteries in parallel will allow all three batteries to remain at the same voltage, if one is weak it will drag the other two down with it. Charging all three will charge to the same voltage equally but not the same stored energy. To verify this you can use a hydrometer to check the specific gravity of each cell. RV's have this type of hook up and when one battery goes bad we replace all of them. You shouldn't have the problem of one battery run down as they will all discharge to the same potential at the same time. Don't worry about under/over charging as the three voltages will equalize.

 

[Chris142]

Originally Posted By: JHZR2
So the electrical system is still 12V right? If so, then they are in parallel.

Ya 12v just 3000cca combined. I wonder if it would be better to charge each seperatly?

 

[pcoxe]

What you describe is a parallel connection - all the +'s connected together and all the -'s connected together. This should create no problems unless one battery is really bad. If they were series connected +-+-+- , a weak battery would certainly cause the situation you mention. Batteries have an internal resistance that increases as it ages. In a series string, all would receive the same current flow, but the weak battery would receive the higher voltage. Series batteries should be evenly matched. Parallel batteries not as critical.

 

[Chris142]

Yes they are a matched set. I know about only replacing 1 at a time...........Don't work.

 

[meep]

woah hold on.

the weak battery will have a lower state of charge (SOC) and WILL rest a a slightly lower voltage. the stronger batteries with higher SOC will have slightly higher voltage and will spend their time trying to bring the weak battery to the same level. they will basically trickle charge the weakling, losing their charge in the process. this will happen until they are all three at same chemical SOC, meaning your stronger 2 will not be able to hold more charge than the weakling. this will shorten their lifespan.

M

 

[The_Jeff]

When a battery is more discharged its internal (charging) resistance is less. You'll see this on the ammeter on a "stupid" manual charger, it starts off pegged then goes down near zero when it's fully charged.

With a lower internal resistance, it's going to take most of the amps your charger puts out.

I'd do a bulk charge with all the interconnects hooked up then a 6-10 amp overnight charge on each isolated battery, and take note of how many amps each is taking after the "full charge".

If you find one of the three batteries to be a dud you could move it to the end of the chain furthest from the starter leads and put a knife switch on it. Cut power when not in use so the other batteries aren't always trying to top it off.

 

[mechtech2]

Chris - Continue putting the charger on it every month.
It will be just fine with your system. I'd make sure your cables are clean and shiny at BOTH ends of each + and -.
[Although for other parts, it is best to drive it here and there]

 

[3311]

If you charge them in parallel (like you have them connected) you must have a charger of sufficient size. Batteries need to reach there gassing voltage, about 2.37 volts per cell, in order to properly mix the electrolyte otherwise you get what is called stratification. This is when the acid separte form the water will actually settle to the bottom of the jar/case. On the other hand to much current will cause rapid active material loss and abnormal heating, try to keep them below 115f. Over heating will result in thermal runaway, which is when a battery gets to hot and the on charge voltage actually starts to drop causing an increase in current and actually destroying the batteries if not caught in time.
If you using a typical automotive charger I doubt you would ever adequatley charge them.

Sorry to ramble on but I've been in the industrial battery business for a long time and see some misinfo some times(none here ). I guess what I'm trying to say IMO what would work best for you and your rig/batts would probably, as others have said, to run it for an hour or so each month. Your alt/gen is much better (tighter voltage regulation) than your run of the mill car charger.