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?

The short answer. Two cells connected in parallel will neutralize the voltages between the two. Say we're talking about a 12 volt cell, I know I never heard of them either, cell A is at 12.5 volts. Cell B is at 11.5 volts. Connect them in parallel the voltage output will be between the two starting voltages. I can't give you an exact answer because it will depend on the conditions of the cells. Cell A may not be powerful enough to hold its charge at 12.5 because it's basically charging the cell B.

Look up BMS, Battery Management System. In certain designs, a BMS is required. Like in that lithium ion battery that you use in your cordless tool.

KOT

I know I once had two lithium CR2032 button cell batteries loose in my pocket, and when they made positive to positive contact, they got very hot and I about ripped my pants off to get them out of the pocket.

The ultimate answer depends on the battery chemistry, but you have a similar case with diodes (and transistors) connected in series or parallel. The construct must be managed.

For example, with lead-acid batteries, overcharge will cause hydrolysis (release of hydrogen and oxygen). If the cells (parallel or series) are slightly different, one cell will complete it's charge first, then hydrolysis will become an issue. In serious situations there could be a cell voltage reversal.

Lead acid cells are fairly forgiving. Lithium cells are touchy and bad charge/discharge management results in a story on the evening news.

Never took you for one to wear hot pants

ROFLMAO

SPIT MY TEA OUT.....

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/

I put a 9V battery in my pocket with a bunch of change, thinking that the plastic cap would stay on. I was wrong. As I walked to the back of the shop to meet with the other installers, I suddenly felt a lot of heat and I reached into my pocket to grab everything and get it out. One of the others said he had never seen me move so fast.

My golf cart at the beach is run on 48V. The set up is six 8volt batteries wired in series.

When the charger goes on, all the batteries seem to come back to a individual reading of just over 8.6 volts.

I suppose if one battery has a bad cell, that battery will reach it's limit, but will not hold that limit.

All I know is, the charged shuts off automatically, and all is well once again...

At least for about 5 to 6 years, and then it's time for another $700.00 or so in batteries...

Since then I've always wanted to set up some sort of safe observation method and connect some batteries together and see if they would burst into flames or explode.

Thanks for your post, highfigh.


As I read it, this is batteries in a series short circuit.


I guess I don't understand bucking.


No need to push? Electrons "want to be in a balanced state"? That sounds like a judgment of emotions. I'm saying I think there has to be something that causes them to move, and whatever that is, it must be possible for it to be too small to cause them to move.


I get this. That's great observations about deep cycle versus high current.


I'll be checking it out. Thanks.

The specific application is understanding the details. You can do A LOT without knowing that diodes have forward voltage drop, but more if you know it. I'm guessing something interesting might come from knowing about this.

This happened because of the dead short. Amusing at the time, but if it had been tossed into a box with a bunch of metal, it could have been a problem.

Using a 9V battery and steel wool is a good camper/survivalist method for starting a fire, too.

A short circuit bypasses the rest of a circuit- putting batteries in series is additive, WRT polarity, so one battery's voltage will affect the other battery. This isn't done when the batteries are capable of extremely high current, though.


Bucking is used when AC voltage needs to be reduced without generating a lot of heat via resistors or other small components. If a power transformer in an old piece of equipment was wound for lower primary voltage, sending 120+VAC can cause it to operate outside of its range and some tube equipment has caps that are close to their limit. Rather than use a Variac, a multi-tapped transformer with 120VAC input and 2V, 3V, 5V, 6V or other output can be inserted in series and with reverse polarity to reduce the voltage by the amount of each tap to drop the line voltage to the device's primary to what it wants to see. Connected in series and IN phase, the output from the first transformer would add to the line voltage.

This is used for vintage guitar amps- changing the transformers may not (arguably) let it sound the same as it would with OEM, so some people assemble a box with a switch that allows selecting normal line voltage or another (sometimes several, if they use the amp at places where the voltage is a crap shoot).

http://www.amprx.net/


If you connect a wire across the terminals of a battery, what happens? Electrons flow, but you haven't pushed anything- they moved on their own. Why? Because, they need to. You can add a resistor to reduce the current, but electrons are still going to move. The difference may be minute, but it will still cause electrons (or one electron) to move. You still haven't done anything except make a connection.

What kind of diodes? Silicon, Zener, Shottkey...?

Don't know if you have a need to service anything with tubes, but an incandescent light bulb is a good, cheap current limiter when wired in series with the power transformer.

If two particles are close enough for their charge to affect each other, they'll start to move if nothing is blocking them and that means it will accelerate. We know that scceleration is the change in velocity (or speed) divided by the time required. If the electron wasn't moving at some point in the past, but is moving now, then its velocity changed. That means it had an acceleration.

From Newton's Second Law, we know that things accelerate when there is a net force on them. Therefore, for an electron to accelerate, there must be a net force on it. The question really comes down to "what will exert a force on a stationary electron?".

Basically, an electron moves because it started to move, and it started to move because of the presence of another charged object.

This info was found in several links but they're all based on Coulomb's Law and since motion/acceleration are involved, Newton's Laws of Motion.

Ernie,

In a battery series or parallel arrangement, if the batteries are identical in chemistry, capacity, and condition, things will work out reasonably well. Voltage and current will be shared between the cells. If one or more cells are different, things will go south. For example, the higher voltage cell in a parallel arrangement will tend to discharge into the lower voltage cell until their potentials equalize. This could result in discharging the higher voltage cell and over charging the lower voltage cell. "Overcharge" can result in a permanent mutation of the cell's chemistry or physical structure.

In a series string of batteries voltage reversal of the weak cell is possible. While this is not the worst outcome for a lead-acid cell, other chemistries are not as forgiving. (In a classic lead-acid cell, the freshly fabricated cell has no charge or polarity. "Polarity" is assigned by the direction of charge and could be reversed in the field.)

In a fresh multi-call battery, such as a 12V car battery or a 9V alkaline battery, all of the cells are as identical as possible and for most purposes one can treat them as a single composite cell. In a strapped together contraption the cells may not be identical or external circuit conditions may tinker with the current and voltage sharing.

In highfigh's comment above you can imagine that an electron should be able to propagate through the cells with equal ease. If one cell's chemistry captures an electron or allows an electron to escape in a gas release, the cells will drift from their initial (hopefully identical) condition and the situation will gradually become worse.