More Volts or More Hours?
Batteries in series & parallel — and which wire is which · By Onion Madder · Mess O' Pedals
Two batteries, two wires, two completely different outcomes. Wire them one way and you get more volts. Wire them the other way and you get more hours. That's genuinely the whole idea — everything else on this page is detail, cautions, and how to prove it with a multimeter.
This is the companion to my vape battery pack build, where I wire two salvaged lithium cells in parallel. That guide shows you how; this one explains why, and what to do when your project wants two different voltages at once.
SERIES — positive to negative, in a chain. Voltage adds. Capacity stays the same. Want 9V instead of 1.5V? Series.
PARALLEL — all positives together, all negatives together. Voltage stays the same. Capacity adds. Want it to run twice as long? Parallel.
Memory hook: series → rise (the voltage rises). Parallel keeps things level.
⚠ Read this before you join anything tap to expand / collapse
Never short a pack. Positive straight to negative with no load is a dead short. Batteries dump enormous current into a short — that's heat, burns, melted wire, and with lithium, fire. Keep loose terminals taped and never let your snips bridge both ends.
Only parallel matched cells. Same chemistry, same voltage, same state of charge. Joining a full cell to a flat one dumps a huge current from one into the other the instant they touch. Check both with a meter first and get them within ~0.05V of each other.
Only series matched capacities. In a chain, the same current flows through every cell, so the weakest one empties first — and then the others try to push current backwards through it. Reverse-charging a cell is how you get a hot, venting, angry cell. Don't mix old and new, and don't mix brands or sizes in one string.
Lithium deserves extra respect. Salvaged Li-ion has no safety margin for carelessness — don't puncture, crush, or short it, work on a non-flammable surface, and only charge with a properly rated charger (~4.2V/cell max). Full cautions live in the Vapocalypse guide.
Know your cells at a glance
Before you can add voltages up, you need to know what one cell brings to the table. Here are the usual suspects — plus the one that trips everybody up: rechargeable AAs are 1.2V, not 1.5V, so four of them make 4.8V where you expected 6.
| Cell | Voltage | Spot it |
|---|---|---|
| AA / AAA alkaline | 1.5V | The everyday cylinder; “alkaline” printed on the wrapper |
| AA / AAA rechargeable (NiMH) | 1.2V | Same size, but the label says NiMH / rechargeable / 1.2V — the sneaky one |
| C / D alkaline | 1.5V | Fat cylinders — same voltage as AA, just more hours |
| 9V “block” | 9V | Rectangular with snap terminals on top (it's six little 1.5V cells in series inside) |
| Coin cell (CR2032) | 3V | Flat silver disc; the number's printed right on the face |
| Li-ion (18650 / vape cell) | 3.7V (4.2 full) | Rechargeable cylinder; the salvage star of the Vapocalypse guide |
Series — volts add up
Chain them nose-to-tail: + of one to − of the next. The leftover − at one end and leftover + at the other are your pack's terminals. Each cell's voltage stacks on the one before it, like standing on each other's shoulders. What doesn't change is capacity — the same current flows through all of them, so the pack lasts as long as one cell does.
Parallel — hours add up
Now line them up side by side and tie all the positives together and all the negatives together. Every cell sits at the same voltage — which is exactly why they must all be the same voltage to begin with. What adds up now is capacity: three cells share the load, so the pack runs about three times as long (and can supply more current without sagging).
Series-parallel — have both
Nothing says you have to pick one. Build two series strings, then parallel the strings together. You get the voltage of one string and the capacity of all of them added up. This is how basically every real battery pack in the world is built — your drill, your e-bike, your laptop.
Two voltages, one project (9V and 6V)
Here's the one that trips people up. Say a build wants 9V for one chip and 6V for something else. You have three options, and they are not equally good.
Option A — tap the series string
Six AA cells in series make 9V. But you don't only get to use the ends — you can solder a third wire partway along the chain. Tap after four cells and that wire sits at 6V. All three wires share the same negative, which becomes your common ground.
Option B — two separate packs, grounds tied
Build a 9V pack and a 6V pack, and wire their negatives together. No imbalance, because each pack only feeds its own load. Costs you an extra pack and more space, but it's honest and it works. The shared ground is mandatory — without it the two halves have no common reference and your signals will misbehave.
Option C — one pack + a regulator (usually the best)
Run a single 9V pack, then make the 6V with a regulator — an LM7806 is the classic (in, out, ground, plus a cap on each side). The 6V stays rock steady as the battery sags, nothing goes lopsided, and you only maintain one pack. The trade: a linear regulator burns the difference as heat, so it's a little less efficient. For small noise boxes, who cares.
The rule that outranks all three: whatever you build, every section must share a common ground. “9V” and “6V” are meaningless on their own — they only mean something relative to a reference point. That reference is ground, and everything has to agree on it.
Using a multimeter to find out which wire is which
This is the part that turns guessing into knowing. A multimeter measures the difference between its two probes — that's the whole concept. Voltage is never a property of one wire; it's always “this point, compared to that point.” Once that clicks, multi-voltage packs stop being mysterious.
Setting up
- Turn the dial to DC volts — marked V⋮, V‒, or DCV. (Not AC. AC is the squiggly one, and it's for wall power, not batteries.)
- Manual-ranging meter? Pick a range above what you expect — the 20V range for anything up to 20V. Auto-ranging meters sort it out themselves.
- Black probe → the COM jack. Always. It never moves.
- Red probe → the V jack (usually marked VΩmA). Not the 10A jack — that one is for measuring current and will short your battery if you probe voltage with it.
The move: park the black probe on ground
Here's the technique that answers “which wire is the 6V one?” — clip the black probe onto the pack's common negative and leave it there. Now touch the red probe to each mystery wire in turn. Whatever the screen says is that wire's voltage, because everything is now being measured against the same reference. No more guessing from wire colors.
| Red probe on… | Healthy reading | What it means |
|---|---|---|
| The common negative | 0V | That's ground — it's the reference, so it reads zero against itself |
| The 6V tap | ~6.0V | Four cells' worth. Fresh alkalines may read a little high (~6.3V) |
| The 9V rail | ~9.0V | The full chain. Fresh may read ~9.5V |
| A single vape cell | 3.7–4.2V | 4.2V = full, 3.7V = nominal, under ~3.0V = over-discharged, treat with care |
Reading the screen like a pro
- Negative number? (e.g. −6.02) Your probes are backwards — you're measuring ground relative to the 6V wire. Harmless. Swap the probes, or just read it as 6V and move on.
- 0V where you expected volts? Broken connection, a cold joint, a dead cell, or your black probe isn't actually on ground.
- “OL” / “1 ”? Over-range — the reading is bigger than the range you picked. Turn the dial up.
- A number that drifts around? You're probably not making solid contact. Press harder or use clips.
- Measure under load, too. A tired battery reads perfectly fine sitting there doing nothing, then collapses the moment something draws from it. If a pack tests at 9V but your circuit acts starved, measure it again while the circuit is running. Big sag = the pack's done, or a joint is too resistive.
Before you solder anything
Two measurements save the most grief. One: check every cell individually before you parallel them — get them within about 0.05V of each other. Two: after you build the pack but before you connect your project, probe every output wire against ground and confirm each one reads what you think it reads. Two minutes with a meter beats a fried chip and an evening of confusion.
Ready to actually build one? Go wire two dead vapes into a rechargeable pack: Do You Want to Survive the Vapocalypse?
The concepts here are plain electronics — series adds volts, parallel adds hours — but I want to credit what made it finally stick for me: DigiKey's Overview of Batteries in Series or Parallel, and specifically its habit of showing the physical terminals and the schematic side by side. I've borrowed that teaching approach and drawn my own diagrams; the words, the artwork, the matching cautions, and the multimeter section are mine. Go read theirs too — it's good.