Battary charge capacity in series and parallel - Page 1
Forum section: electronics / beginners

author: dinushka95
author_rank: Newbie
timestamp: February 11, 2021, 12:59:09 pm
content: Can someone explain what is charge capacity? in depthWhy is charge capacity not doubling when you connect 2 batteries in series?Thanks

author: tunk
author_rank: Super Contributor
timestamp: February 11, 2021, 01:22:25 pm
content: Short version: If you measure in Ah it does not double, if you measure in Wh it does.

author: tooki
author_rank: Super Contributor
timestamp: February 11, 2021, 07:35:12 pm
content: Quote from: tunk on February 11, 2021, 01:22:25 pmExactly.Long version:Quote from: dinushka95 on February 11, 2021, 12:59:09 pmThecapacitydoesdouble. The problem is that the commonly-stated mAh ratingis not a capacity rating. It’s only half of it: you need to know the voltage, too, which multiplied together gives you mWh, which is capacity.When you take two batteries and put them in parallel, the voltage stays unchanged but the current (the mAh) doubles. Your total capacity in mWh doubles.When you take two batteries and put them in series, the voltage doubles, and the current remains unchanged. Your total capacity in mWh doubles, exactly as above.For example, suppose you’re using 3.6v, 2000mAh batteries. The capacity of a single cell is 3.6V x 2000mAh = 7200mWh.Two in parallel gives you 3.6V x (2x 2000mAh) = 3.6V x 4000mAh = 14400mWh.Two in series gives you (2x 3.6V) x 2000mAh = 7.2V x 2000mAh = 14400mWh.Which approach is right depends on the application, there’s no one right answer.

author: msat
author_rank: Regular Contributor
timestamp: February 12, 2021, 02:04:32 am
content: The answers given above also elucidate the reason why automakers of electric/hybrid cars give their battery specs in terms of kWh; because it's a better unit of measure to not only compare against but as a single measurement tells you more than either pack voltage or pack current alone (though the techie in me would like to know all those specs!)Fun thought experiment: why are the voltages additive when cells placed in series in a battery? That took me a while to wrap my head around and figure out.

author: golden_labels
author_rank: Super Contributor
timestamp: February 12, 2021, 01:21:16 pm
content: dinushka95:Amperhour is a non-SI unit ofcapacitancecharge(thanksSiwastajafor spotting the mistake). It has the same dimension as coulomb (C). While the characteristics of batteries are not well modelled this way, a very rough, fist-order approximation of a battery is an ideal capacitor. Knowing how capacitors act in parallel and in series, apply that knowledge to batteries.Another way to see it is looking at Ah as a piece of information that tells you how long a battery will last under given current. With 2A current draw, a 4Ah battery lasts 2h, 10Ah battery lasts 5h and so on. If you connect batteres in series, both batteries see the same current, right? So both are discharged at the same rate as if they were separated. A 10Ah battery lasts 5h and it doesn’t matter to it, if the charge actually goes through another battery.Ah is useful while comparing batteries of the same type. In practice they are often seen as a constant voltage source. Since they all have the same voltage, it can be removed from the equation. If your device draws average of 30mA and you power it from 2× AA cells, the calculations are then boiled down to dividing capacity by current. Of course that is a simplified model and there are situations in which it is insufficient… like all models. If batteries of different type are compared, the Wh (or Joule) is a more convenient unit.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 12, 2021, 01:56:19 pm
content: Amperhour is not an unit of capacitance. The unit of capacitance is Farad.Amperhour is unit ofcharge, like Coulomb.Charge relates to the number of electrons.Simplified, charge (Ah, or Coulomb) is the number of snowballs you throw, while voltage (V) is the strength you throw them with. Current (A) is how many snowballs you throw every second.