Internally, I mean. I know that externally, nothing happens, ta-da.
IIRC from elementary physics class, when you complete the circuit, the current flows. So if you complete the circuit by attaching one 9-volt to another, does the current flow between them? What if one of the 9-volts was completely dead, and the other charged fully? Would connecting them effectively charge the dead one and deplete the full one to half capacity each? How long would that take? Am I making any sense?
If you mean hooking them together so the connectors of each battery snapped to the other one you’d have have a mess on your hands. Current flow would be limited only by the internal resistance of the cells and ability to deliver it. You’d have two very hot batteries in a few seconds and probably a rupture or explosion soon after that.
Yes, you might partially charge a dead cell from a charged one depending on chemistry but high current flow levels would probably cause overheating as in the first example.
Huh. Weird… I’ve done this before, numerous times, with 9-volts and have never noticed over-heating or explosions. Wouldn’t they be designed to avoid such things, given that their ends do snap together so nicely?
How much current are we talking about?
True, the individual cells in a 9v battery are pretty small so peak current flow won’t be that high with alkalines. I wouldn’t do it with a NiMH or NiCad 9v but you hardly see those to begin with.
You probably wouldn’t be able to charge a battery in this way. To charge a battery, you want to hook positive to positive and negative to negative, whereas here you’re hooking positive to negative and negative to positive. The full battery would try to charge the drained one in reverse, but I don’t think that the internals of a 9v battery are such that this is possible.
:smack: One dopeslap for me please. If I had thought about that for a couple of seconds I might not have given such a wrong answer. I should know better considering I’ve charged and jumped a few batteries in my lifetime.
Padeye is correct.
To a first-order approximation, a 9V battery can be modeled as a 9 VDC voltage source in series with a small resistor. The resistor represents the internal resistance (a.k.a. source resistance) of the 9V battery.
Now let’s say you have two 9V batteries. We’ll call them Battery A and Battery B. Each battery is modeled as a 9 VDC voltage source in series with a small resistor. Therefore you have the following four circuit elements:
V[sub]a[/sub] = 9 VDC voltage source of Battery A
V[sub]b[/sub] = 9 VDC voltage source of Battery B
R[sub]a[/sub] = Source resistance of Battery A
R[sub]b[/sub] = Source resistance of Battery B
Note that, numerically speaking, R[sub]a[/sub] = R[sub]b[/sub] = R.
When you snap two 9V batteries together, all four of the above circuit elements will be in series. V[sub]a[/sub] and V[sub]b[/sub] can then be added together to form an 18 VDC voltage, and R[sub]a[/sub] and R[sub]b[/sub] can be added together. So after you snap the two batteries together, the circuit can be modeled as an 18 VDC voltage source connected to a resistor that has a resistance of 2*R.
So when you snap two 9V batteries together, maximum current will flow through the batteries (18/(2R)) and maximum power will be expended by the batteries (324/(2R)). This means the batteries will get hot and discharge quickly.
But as mentioned above, this is a first order approximation. In a real experiment the voltage sources will not remain at 9 VDC; they will decrease somewhat. Also, the source resistances are fairly nonlinear, and are a function of temperature.
How do you connect 2-9v batteries together internally?
Nine volt batteries are not normally rechargable. You would only run down the good battery and have little effect on the other.
If you connect the + terminal of A to the - terminal of B, then the voltage from the + terminal of A to the - terminal of B will be 2 x 9 = 18 volts. IF you connect the two terminals producing 18 volts together you have shorted the batteries and they will produce considerable heat, even to the point of exploding due to the short circuit!
OTOH If you connect the two + terminals together and the two - terminals together you will get 9 volts but twice the apmpeage capacity and hence twice the life at the same load.
How do you connect 2-9v batteries together internally?
Nine volt batteries are not normally rechargable. You would only run down the good battery and have little effect on the other.
If you connect the + terminal of A to the - terminal of B, then the voltage from the + terminal of A to the - terminal of B will be 2 x 9 = 18 volts.
IF you connect the two terminals producing 18 volts together you have shorted the batteries and they will produce considerable heat, even to the point of exploding due to the short circuit!
OTOH If you connect the two + terminals together and the two - terminals together you will get 9 volts but twice the apmpeage capacity and hence twice the life at the same load.
Seems to me this would be the same as connecting the two in series (making an 18V battery) and then connecting the two not-yet-connected terminals with a wire – only if you snap them together their own terminals take the place of the wire. As described, mucho current, mucho heat, something’s gonna melt or break.
How do you connect 2-9v batteries together internally?
Nine volt batteries are not normally rechargable. You would only run down the good battery and have little effect on the other.
If you connect the + terminal of A to the - terminal of B, then the voltage from the + terminal of A to the - terminal of B will be 2 x 9 = 18 volts.
IF you connect the two terminals producing 18 volts together you have shorted the batteries and they will produce considerable heat, even to the point of exploding due to the short circuit!
OTOH If you connect the two + terminals together and the two - terminals together you will get 9 volts but twice the apmpeage capacity and hence twice the life at the same load.
My 4yr old son came across two 9 volt batteries and connected them together, then said “Mommy, feel these batteries, they’re hot!” I touched them and to my alarm they were extremely hot, at which point I quickly pulled them apart. Just wondering what would have happened if he hadn’t told me about it and left them connected, so I started searching the web and found this thread. Scares me to death that there could have been a potential explosion/fire had he not brought the connected batteries to my attention.
No explosion or fire, but they can get uncomfortably hot.
(Even zombie batteries can get hot).
Well, to be fair, while the batteries can explode, it’s not a grenade type of explosion. The battery case pops open and hot battery goo spurts out. Your 4 year old might have taken a trip to the doctor for minor cuts and burns, but he wouldn’t have lost a hand or anything like that.
Because the terminals on a 9 volt battery are both physically on the same side of the battery and are close to each other, putting one in your pocket can easily cause it to short out on anything conductive, like a coin or a key. It can give a new meaning to the word “hot pocket”. 
The real question is, if the hot batteries were placed on a big block of clear ice, how deep would they melt themselves into the ice before they were exhausted and the water re-froze again behind them?
PS
Shorted battery current is above 10amps for new alkaline 9v batt.
ha ha I’ve done that before!:eek:
Which would mean 180 watts. Which is a lot of power, for something that small, and not designed to dissipate heat.
a 9V disposable battery connected to another would get hot enough to burn skin or burst. a wire across the terminals might get hot enough to start a fire.
a 9V rechargeable battery (really 7.2 to 9.6V depending on the battery) will definitely get hot enough to burn or start a fire if a wire is connected across the terminals or two connected together.
rechargeable batteries of any sort should be stored so that the terminals can’t accidentally be connected through any metal path.
Heck, that can happen with a AA–I had a NiMH in my pocket with my keys, and at some point during the day the keys had somehow arranged themselves to form a conductive path. Got pretty warm!
any rechargeable battery if shorted out can get hot enough to start a fire or cause burns.