Assume an alternator is charging battery A.

Battery B is connected to battery A via a switch that is open.

If and when the switch is closed is there any way that current can flow out of battery A and over to battery B? Or will any current that flows to battery B be coming from the alternator..

Trying to settle an argument on a boating forum.

Thanks

 

It depends on the relative voltages of the alternator and the two batteries.

ASSUMING that the second battery is at a lower voltage than the alternator and the first battery, then it starts getting charged as soon as it is connected. In general, some of it will come from the alternator and some from the other battery, depending on the specifics. Initially, there will likely be a relatively high current from the first battery to the second in order to equalize their charges (and, while this is happening, all of the alternator current is also going to the second battery), and from that point on both batteries will both get some of the alternator output.

 

If the state of charge and the exact voltages are not identical then certainly current will flow from one to the other. Is that a problem?? Not usually. And both batteries will charge at the same time, but one will charge more that the other, unless the resistance in series with each are identical. That will only be a problem for batteries used in parallel, especially at high discharge currents. So the best choice is to use one and charge the other, or to split the loads and charge them alternately.

 

At the typical charging voltage form the alternator, there will be no current out of either battery.
It's only if the voltage drops below the charged voltage of the battery would there likely be any current flowing out of the battery.

If the generator stops charging then it is possible for current to flow from the battery that has a higher charge to one with a lower charge.

 

At the typical charging voltage form the alternator, there will be no current out of either battery.
It's only if the voltage drops below the charged voltage of the battery would there likely be any current flowing out of the battery.

If the generator stops charging then it is possible for current to flow from the battery that has a higher charge to one with a lower charge.

An alternator can generally only put out a fraction of the current that the battery is capable of. Let's call it 100 A.

If you just connect the two batteries together (no alternator) and if there is much difference in their charge state, charge will flow from one battery to the other at a high rate, limited by the internal resistances of the batteries, which is very low in most cases. Let's call that 300 A.

If you already have the alternator connected to the first battery and then you connect the second battery to the system, does it make any sense that the presence of the alternator will somehow decrease the current flowing into the second battery?

No. What makes more sense is that the new battery will hog the available current from the alternator, all 100 A of it, pulling it down to the voltage of the first battery, which will then make up the difference and supply the remaining 200 A. In actuality, you would expect the newly attached battery in this case to actually pull something more than 300 A, because the fact that the original battery isn't supplying the full 300 A means that it will not be pulled down as far as it was, and so the equilibrium point will be at a somewhat higher point.

 

It is seldom a good practice to connect batteries while high power charging is in process. In fact, there are several very good explanations as to why that is to be avoided. In fact, the list of applications where connecting a second battery during high current draws is very short, given the worst case possibilities.

 

It is seldom a good practice to connect batteries while high power charging is in process. In fact, there are several very good explanations as to why that is to be avoided. In fact, the list of applications where connecting a second battery during high current draws is very short, given the worst case possibilities.

It is also quite common -- how do people often jump start a car with a dead battery? They connect the battery from a running car to the dead battery in the other car and there is a rush of current from the good battery to the dead battery, usually much more than the alternator of the running car can deliver. While it is very arguably preferable to turn off the car with the good battery to make the connections, that risks the possibility that the dead battery drains the good battery to the point that neither car will start -- been there, done that (and out in the boonies with no one else around, so ended having to wait several hours for someone else to come by).

 

Generally boats will have battery selection switches that allow you to select a specific battery or all batteries in parallel. If you wish to drain the two batteries separately but charge them from the same single source, look into a "battery isolator". This way you can run all the loads on the boat (fish finder, lights, etc..) from one battery bank, and dedicate a separate battery bank to the motor(s), BUT all batteries will charge in parallel by the motor. Lots of options depending on your needs.

But yes, if you connect a fully charged battery to a dead battery, some power will flow from the charged battery to the dead one. If there's also an alternator in the equation, the current to charge the dead battery can come from the alternator or the charged battery or any combination of the two, depending on how much current each can supply.

 

When I was in high school (i.e., knew damn little about electronics) I put a second battery in my old Ford Bronco because I was involved in Search and Rescue and wanted to be able to leave my radios on all night but not have to worry about being able to start the truck in the morning. I also had some high-intensity lights that I wanted to be able to use, at least for relatively short periods, when the truck wasn't running. So I put in a second electrical system for the lights and radios (and some other things) but now needed to be able to control which battery was connected to what, and I wanted maximum control and flexibility, including being able to tie both batteries to the main system to have the best chance of starting the truck at -20°F in the Rocky Mountain winter (and it worked nicely on all but one occasion -- and that was in my driveway, so at least I wasn't stranded at 12,000 ft!). So I used four battery isolator (initially I used start solenoids because they seemed to do exactly what I wanted and I had never heard about battery isolators -- but I quickly discovered that starter solenoids don't like being energized for long periods of time, and discovered isolators when I went to an RV shop seeking some advice), one for each battery to each system. They worked beautifully. But I also had some things that needed power continuously (the radio with a clock, as well as my control panel) and I wanted that to come from either battery regardless of how the isolators were configured (including when they were all off). I knew about diodes and was at a Radio Shack when I stumbled upon this part that was called a "full-wave bridge" and had four diodes in it. I only needed two of them, but they were hooked up just like I wanted and had a nice mounting flange, so that was how I learned about those magic words. I actually learned quite a bit about basic electronics designing and implementing that system. One thing that I did that I thought was a horrible, but effective, hack was to control some things by breaking the ground connections. I was to discover a few years later that this exact same approach is used in a number of places on the F-15 fighter, so i didn't feel quite so cheesy for doing it in my truck.