There are 3 important rules
There are 3 important rules that will help you understand any electrical circuit:
• Ohm’s Law V = I×R Relates the voltage ‘consumed’ by a component to the current flowing through it and the component’s resistance (only works for regular ‘ohmic’ resistance components, mostly used for standard resistor calculations)
• Kirchoff’s Voltage Law Basically states that in any circuit ‘loop’, all the voltage added by batteries, etc must be completely consumed by the other components in the circuit.
• Kirchoff’s Current Law At any junction in a circuit, the current flowing into that point must be equal to the current flowing out of that point. Just be careful using this near batteries/inductors/capacitors/etc, as these components can store or release current.
If you know how to use the 3 laws above, you can work out the currents and voltages throughout practically any circuit. Just remember that voltage is produced or consumed (increased or decreased) across a component, measured from the input to the output terminals. Current on the other hand flows through a component, and the current along any single circuit loop will always be the same.
When you add multiple cells/batteries in series, you add the voltages together to get the output voltage. Assuming you’re adding several of the same type of battery together (which you really should, mixing batteries is a bad idea) then the maximum current (in A or mA) and the capacity (in Ah or mAh) will be the same. Kirchoff’s Current Law tells us that the same current must flow through the chain of batteries.
If you add multiple cells/batteries in parallel the opposite happens, you add the maximum currents and capacities together, but the output voltage will be the same as a single battery. When all the cells/batteries are identical, the same current flows through each cell and all the output currents are then brought together to form the output current.
If you need more voltage and more current/capacity you can add cells/batteries in parallel to get the current/capacity you need, and then add multiple blocks of parallel cells/batteries in series to boost the output voltage.
Some manufacturers will give you specifications (particularly for rechargeable batteries) on what the nominal capacity of the battery is, and sometimes they’ll also give you a maximum current too. As rik mentioned you’ll get one hour from a 2000mAh battery if you draw 2000mA from it, or 2 hours if you draw 1000mAh from it. Most rechargeables can supply at least the ‘1-hour’ current (also known as 1C or just C) without straining too hard, but the more current you pull from the cells the less run time you’ll get compared to your predicted value.
As far as the terms negative and ground go, they’re normally somewhat interchangeable. A fresh AA cell produces ~1.5V between the positive and negative terminals, so if you connect the negative terminal to ground (which is just a reference ‘zero’ volt point), then you’ll get +1.5V from the positive terminal. If you connect the positive terminal to ground instead, you’ll get -1.5V from the negative terminal. Ground is just considered the ‘resting’ voltage of the circuit, it allows us to have a convenient reference point that we can define all the other circuit voltages in relation to.
Think of it this way: you have two fresh AA cells, measured to have 1.5V across the terminals. You know that means that between the positive and negative of cell 1 you have 1.5V, and you also have the same condition for cell 2. Now, what is the voltage on the positive terminal of cell 1? What about cell 2? Are they the same?
The problem is you really can’t say; the only thing we know is what the positive terminal voltage is in relation to the negative terminal of the same cell. We don’t know anything about the 2 seperate cells.
If we connect the negative terminals together however, they must now be sitting at the same voltage which we’ll call ground (zero volts) for convenience. Because we know that the positive terminal of cell 1 is 1.5V above ground, and the positive terminal of cell 2 is also 1.5V above ground, we can now say with certainty that both positive terminals are the same =D
It may seem like nothing has changed just by connecting the negative terminals, but what if cell 2 was carrying a static charge from sitting in your pocket? That would cause a difference in the ‘absolute’ voltages of cell 1 and cell 2, but we’d never know about it. ‘Grounding’ the two cells allows us to compare them under the same conditions.