Power & Energy
Batteries in series vs parallel
Wire batteries in series, each one's − to the next one's +, and their voltages add: four 1.5 V cells make 6 V. Wire them in parallel, + to + and − to −, and the voltage stays the same while their capacity adds: two batteries share the current and last twice as long. Either way, the batteries must match.
Series: volts add up
Each cell is a step up. Stack four 1.5 V steps and the pack is 6 V; a 9 V battery is six tiny cells chained inside one case.
A chain is one loop, so the same current flows through every cell, and each one drains at the same rate. Four cells in series give four times the volts but run flat in the same time as one. The pack's capacity in mAh is one cell's; its stored energy, volts times amp-hours, is four times as much.
Live experiment
Climb the cell staircase
Each 1.5 V cell is a step up, and Pip's 60 Ω lamp is the slide back down. Add cells, one to six, and the top climbs 1.5 V a step. Tap a cell to turn it round and it becomes a step down. It's one loop, so one current flows through every cell.
“Cells in a row” is lesson 2 of 6 in Watt Harbour, Power & Energy World 1. Free with an account. Lessons unlock in order, so you’ll do the one before it first.
Parallel: hours add up
Side by side, the load still sees one battery's voltage, but its current is shared. A lamp drawing 200 mA from two batteries takes 100 mA from each:
Two side by side are a bigger battery, not a stronger one.
They have to match
In parallel, a fresh 9 V battery next to a tired 7 V one pushes current straight into it, round and round, with or without a load. Through their small internal resistances that can be amps, wasting energy and heating both; with lithium cells it can be dangerous. In series, a weak cell runs flat first and the others then drive current backwards through it.
So use the same type, the same age and the same charge, and never mix chemistries.
Both at once
Real packs do both: rows in series for the volts, groups in parallel for the capacity. A 3S2P lithium-ion pack is three series groups of two parallel cells, about 11.1 V nominal with twice one cell's capacity. Laptop batteries and power banks use parallel groups for capacity; an electric car chains around a hundred groups in series to reach several hundred volts.
Big series packs need a battery management system that watches every cell, because one cell running low or high would otherwise go unnoticed.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Cells in a row” is lesson 2 of 6 in Watt Harbour, Power & Energy World 1. Free with an account. Lessons unlock in order, so you’ll do the one before it first.
Or start from the very first lesson, no account neededWork it out
Practice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: 9 V. Side by side, the voltage stays at one battery's worth. Only a chain adds volts.
- Why not “0 V”? Both push the same way across the load, so it sees their shared 9 V.
- Why not “4.5 V”? Parallel batteries don't split the voltage. Each holds the full 9 V across the load.
- Why not “18 V”? Volts only add when batteries are chained + to −. Side by side they share the same two points.
Show the answer
Answer: 100 mA. One loop, one current. Every cell in the chain carries the same 100 mA, so each drains at the same rate.
- Why not “400 mA”? No cell can carry more than the loop's current. 400 mA would mean the cells were somehow adding currents.
- Why not “25 mA”? A series pack is one loop, so the current doesn't split between cells. Each carries all 100 mA.
- Why not “Zero”? Every cell in the chain is part of the one loop, so they all carry the current.
Show the answer
Answer: Fight: the 9 V one pushes current backwards into the 6 V one. Parallel cells must match. The stronger one drains into the weaker, wasting energy and heating both.
- Why not “Give 7.5 V and last longer”? The pair may settle between the two voltages, but it won't last longer: the 9 V battery drains into the 6 V one as heat.
- Why not “Work fine”? Mismatched batteries in parallel never work fine: current flows from one into the other even with no load.
- Why not “Give 15 V”? Volts only add in series. Side by side, both batteries are forced to the same voltage, and they fight.
“Cells in a row” is lesson 2 of 6 in Watt Harbour, Power & Energy World 1. Free with an account. Lessons unlock in order, so you’ll do the one before it first.
Questions people ask
Do batteries in series add voltage or current?
Voltage. Each cell's − joined to the next one's + stacks their volts: four 1.5 V cells make 6 V. The same current flows through every cell, so the capacity in mAh stays that of one cell, while the stored energy goes up with the voltage.
What happens when you connect batteries in parallel?
The voltage stays at one battery's worth, and the load's current is shared between them, so the pack lasts longer. Two 1000 mAh batteries side by side act like one 2000 mAh battery at the same voltage.
Can I mix old and new batteries?
Avoid it. In parallel, the fresh one pushes current into the tired one even with no load, wasting energy and heating both. In series, the weak cell runs flat first and then gets driven backwards. Use cells of the same type, age and charge.
What do 3S and 2P mean on a battery pack?
S is cells in series, P is cells in parallel. A 3S2P lithium pack has three groups in series, each of two cells side by side: about 3 × 3.7 = 11.1 V nominal, with twice one cell's capacity.
Related topics
- What is voltage?The push that makes charge flow: what a volt is, and why stacked cells add up.
- How to calculate battery life from mAhRuntime = capacity ÷ current: mAh, watt-hours, C-rate, and why real batteries give a little less.
- Series vs parallel circuitsOne path or many: what stays the same, what adds up, and how to combine resistors.
- Electrical power explained: P = V × IP = V × I: how fast energy turns into heat, and why a resistor has a power rating.