Power & Energy
Different-voltage batteries in parallel: what happens?
Wire two batteries of different voltages in parallel and the higher one discharges into the lower one. Nothing limits that current except the two batteries' own internal resistance, so it can be amps, with or without a load attached. It wastes energy, heats both batteries, and with lithium cells it can be dangerous. Batteries in parallel must match.
Matched: the load is shared
Two fresh 9 V batteries side by side, + to + and − to −, on a 45 Ω lamp. The lamp sees 9 V and draws 200 mA, and each battery supplies 100 mA. Neither works as hard, so each lasts twice as long: two 1000 mAh batteries run the lamp for about 10 hours instead of 5. The voltage doesn't go up; only the hours do.
Mismatched: one charges the other
Now replace one with a tired battery reading 7 V. The two are joined at both ends, so they form a loop of their own, with only their internal resistances in it:
Two amps flows out of the fresh battery and backwards into the tired one, round and round, whether or not the lamp is connected. The lamp sees a little under 8 V, somewhere between the two. The fresh battery drains in a fraction of the time it should last, and the tired one, being pushed backwards, warms up. Alkaline cells driven backwards can leak.
Live experiment
Share the lamp, then swap in a tired one
9 V batteries with half an ohm inside, + to + and − to −, run a 45 Ω lamp that wants 200 mA. One lasts 5 hours, two share the current and last 10, three about 15. Swap in a tired 7 V one and the fresh battery pours about 2 A into it, lamp or no lamp.
“Cells side by side” is lesson 3 of 6 in Watt Harbour, Power & Energy World 1. Free with an account. Lessons unlock in order, so you’ll do the 2 before it first.
Internal resistance sets the size
The smaller the internal resistance, the bigger the current for the same mismatch. A 9 V battery's half ohm keeps it to amps. Lithium-ion cells have just tens of milliohms, so a few tenths of a volt between two of them can drive several amps, enough to heat and stress both cells. That's why battery builders bring parallel cells to the same voltage before connecting them.
Parallel or chain?
Check which you've built with a meter. Side by side, the pair reads one battery's voltage. Joined + to −, it's a chain, and two 9 V batteries read 18 V. On the 45 Ω lamp that chain pushes 400 mA, twice what the lamp was built for.
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 side by side” is lesson 3 of 6 in Watt Harbour, Power & Energy World 1. Free with an account. Lessons unlock in order, so you’ll do the 2 before it first.
Or start from the very first lesson, no account neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
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 output does sit between the two, but current running from one into the other wastes energy, so the pair drains faster, not slower.
- Why not “Work fine”? Parallel cells must match. Here the 9 V one pushes current backwards into the 6 V one, heating both.
- Why not “Give 15 V”? Adding voltages needs a chain, + to −. Side by side the two are joined at both ends, so their voltages can't add.
Show the answer
Answer: 18 V and 400 mA. A chain adds the volts: 18 V across 45 Ω is 400 mA, from both batteries, double what the load was built for.
- Why not “Nothing”? A chain across the load is still a complete loop, so current flows, twice what the load was built for.
- Why not “9 V and 200 mA”? That's what side by side gives. A chain adds the two voltages.
- Why not “4.5 V and 100 mA”? Nothing here halves the voltage. Chained + to −, the batteries stack to 18 V.
Show the answer
Answer: + to + and − to −. Like with like. + to − makes a chain. Joining only the + pins leaves one battery dangling and doing nothing.
- Why not “+ to − and − to +”? That joins the two head to tail in a closed loop: a short circuit that drives a huge current round both batteries.
- Why not “Only the + pins”? With only the + pins joined, the second battery has no return path and does nothing.
- Why not “Any two”? Which pins you join decides what you build: like with like for side by side, + to − for a chain.
“Cells side by side” is lesson 3 of 6 in Watt Harbour, Power & Energy World 1. Free with an account. Lessons unlock in order, so you’ll do the 2 before it first.
Questions people ask
Can I connect a 12 V and a 9 V battery in parallel?
No. The 12 V one drives current backwards into the 9 V one through their internal resistances, which can be amps with no load at all. It wastes energy, heats both, and can make them leak, swell or worse. Parallel batteries must be the same voltage, type and state of charge.
What voltage does a load see from two mismatched batteries in parallel?
Something in between, pulled toward the battery with the lower internal resistance. With equal internal resistances it's close to the average: a 9 V and a 7 V battery give a little under 8 V.
How do I wire lithium cells in parallel safely?
Use cells of the same type, capacity and age, and bring them to the same voltage before joining them, by charging each fully or measuring them to within a few hundredths of a volt. Lithium cells have very little internal resistance, so even a small difference drives a large current.
Related topics
- Batteries in series vs parallelChain cells for more volts, put them side by side for more hours, and never mix a tired one with a fresh one.
- How to calculate battery life from mAhRuntime = capacity ÷ current: mAh, watt-hours, C-rate, and why real batteries give a little less.
- Kirchhoff's laws explained: KCL and KVLWhat flows into a junction flows out, and the drops round a loop add up to the rise.
- How to charge a lithium-ion batteryConstant current, then constant voltage at 4.2 V: the two-stage curve every lithium charger follows.