On April 6, 2017 at 14:09, Ernie Gilman said...
I've been thinking about and discussing some details of battery charging and I've realized I've never heard one thing addressed: is there some minimum voltage by which the device supplying the power to charge the battery must exceed the voltage of the battery in order for it to charge?
Looking at this from a slightly different viewpoint, if you connect two batteries plus to plus and minus to minus, with one more charged than the other, will the two batteries come to be of identical voltage?
I'm guessing not. To charge a battery you have to push or pull electrons. Voltage is the electrical analog of a pushing force. That tells me one battery would have to have a slightly higher voltage than the other, and it would quit charging the second battery when the voltage differential falls below the level at which it could push electrons into the other battery.
Just wondering. Ever hear of this detail at all?
As long as a difference in potential exists, the charges will attempt to balance themselves. This isn't the same as neutralizing, which would happen if the opposite poles are connected. In AC, this is usually called 'bucking'. There's no need to push- the difference in charge will cause electrons to move on their own- they want to be in a balanced state.
WRT two batteries in parallel, they'll balance
if their condition is the same. By this, I mean their ability to store voltage and discharge at the same rate with similar loads. This is a good example of how boaters and people with big car stereos often have problems. Boaters (the kind who think they know how to work on their own) tend to be very cheap and they'll take any old battery and try to use it. They often use a cranking battery for their accessories and wonder why it needs to be replaced in a year, but they don't realize that 'deep cycle' doesn't mean kill it and use an alternator to recharge it. They also use deep cycle batteries to crank the engine and wonder why it won't crank as hard & fast soon after. These are made for slower discharge and reserve time and not the demand from a starter which, on a 350 Chevy engine that doesn't have a gear reduction starter, can range from 150A-350A.
Car audio people will chase noise intermittent problems for a long time, even though they did almost everything correctly, except making sure the batteries are in the same condition. When one discharges more than the other, it's easy to have a ground loop when some devices are connected to one battery and some are connected to the other.
If a large difference exists between the voltage source and a battery that's depleted or close to that state, it's best to use some kind of current limiter and if you're going to charge batteries on some kind of non-standard charger, light bulbs work great- just make sure they can handle the load.
This site has a lot of info- maybe it will have what you're looking for. It would help if we knew the specific application.
http://batteryuniversity.com/
