First of all I know Voltage goes Up in Series and Capacity/Total Current in Parallel.
But I have confusions,
For example let’s say we have 2 Li-ion cells,
Both 3.7 volts and 2000 mAh or say 2 AH.
When their volts go up to 7.4 volts why doesn’t it go to 4000 mAh as well?
Where does the other 2000 mAh go?
I know because Battery is now delivering current at faster pressure (volts) but it still doesn’t explain why battery is not called 4000 mAh after It’s put in series.
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In: Physics
Because it’s not just a higher voltage, it increases power too, that has to come from somewhere. Watts = ampere * voltage
You’re discharging both batteries when you draw current at 7.4V. Let’s say you attach a 3.7 ohm resistor, that’ll draw 2A of current, and “drain” your battery in an hour. If you had just the single 3.7V cell, that would have drawn 1A of current, and “drained” it in 2 hours. Because the increased voltage will “push” more current, you’ll drain the capacity sooner, but the total Ah you get is the same. Both cells are involved in “pushing” the extra current.
Look at it from the perspective of the load – adding cells in parallel doesn’t change the pressure, as you say, but it linearly extends how long you can receive the pressure for. Adding cells in series doesn’t change the duration, but for the same load, the higher voltage linearly increases the current you can draw for the same duration.
A hallway has two doors at either end. Each door can handle 100 people per minute. How rapidly can people go through the hallway?
a) Even though each door can handle 100 people per minute, everybody who goes through the hallway has to go through both doors. Therefore, the hallway can only handle 100 people per minute
b) In normal circumstances, the doors and the hallway are not pushed to their limit; how many people go through the hallway usually depends on what is on either side of the hallway more than the architecture of the hallway itself
Applying the analogy:
a) The current rating of a battery is a maximum of how much current can go through it. When two batteries are connected in series, all the current that goes through one also has to go through the other
b) The current rating is again the maximum current that can go through the battery. How much current actually goes through the battery usually depends on what the battery is connected to.
Milliamp hours are not a measure of energy, but rather, electrical charge – they tell you how long a cell can supply a given current not how much energy it stores. To get the latter (in mWh), you need to multiply by the voltage.
If a battery has a capacity of 2000mAH, that means it can deliver 2A of current for one hour or 1A for two hours. When you put two cells in series, the same current must flow through both of them. If you put two 2000mAH cells in series, and your circuit draws 1A of current, then 1A of current flows through both cells, and therefore, both cells will be drained after 2 hours.
But the voltages add, so with 2 3.7V cells you deliver 7.4V to your circuit. Therefore, the two cells in series are delivering twice as much power at the same current because the voltage is doubled. You have twice as much energy available, but you are draining it twice as fast. If instead you wire the batteries in parallel, then the opposite happens – now, the voltage is the same for both batteries, but the current is shared between them. If your circuit is drawing 1A, then with the cells wired in parallel, you would draw 0.5A from each cell, so they would last 4 hours.
> I know because Battery is now delivering current at faster pressure (volts) but it still doesn’t explain why battery is not called 4000 mAh after It’s put in series.
This is it though.
Power = Current x Voltage
Work (energy used) = Power x Time
So to get the energy you multiply the mAh by the voltage.
This means two batteries separately delivering 2000mAh at 3.7V is the same energy as one delivering 2000mAh at 7.4V.
If you were getting 4000mAh at 7.4V you’d need to somehow double the energy in the batteries.
/mAh is a bit of an odd unit for expressing the energy batteries contain as it’s not a value for energy.
Most batteries are 1.5V
We have 0V at the negative end and 1.5V at the positive end.
If we put two in parallel, we have two 1.5V regions connected with a 0Ω wire and two 0V regions connected with a 0Ω wire. That’s fine, it just means we have a battery with twice the capacity (more mAh) and each battery is supplying half the current the circuit needs at 1.5V.
Let’s assume it is a 15kΩ circuit, that means if we apply Ohm’s law, R=V/I, the circuit will draw .1mA from the batteries, or .05mA from each battery
If each battery has 10mAh of charge, then a single battery can power the circuit for 100 hours, or two batteries in parallel can power it for 200 hours.
If we instead put the batteries in series, we have on battery at 1.5V that’s now putting the “floor” for the other battery (which was 0V) up to 1.5V, and the positive terminal is still 1.5V above that, to we have 3V at the positive end, 1.5V in the middle, and 0V at the negative end. It’s like we have one big battery with twice the voltage.
Let’s put it in the same circuit. Since it’d 3V on a 15kΩ circuit, it’s now drawing .2mA of current. Since each battery still has 10mAh, and we have 2 of them, that’s 20mAh, so we can only power the circuit for 100 hours
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