What Happens When Different Batteries Are Connected in Parallel?

Connecting batteries in parallel means joining positive terminals together and negative terminals together. The arrangement keeps the same nominal voltage as one battery, while potentially increasing the available capacity or current. But if the batteries have different voltages, they do not simply combine harmlessly: current can flow from one battery into the other.

The size of that current depends on the voltage difference and the batteries’ internal resistances. In real batteries, it can be large enough to heat or damage cells, wiring, or protection circuits.

Why different voltages cause current

A useful simple model represents each battery as an ideal electromotive force, $E$, in series with an internal resistance, $r$. When two batteries are connected in parallel, their terminals must have a common voltage. If their emfs differ, the batteries cannot both sit at their preferred open-circuit voltages, so current flows between them.

With no external load, the circulating current is

$$I=\frac{E_1-E_2}{r_1+r_2}.$$

If $E_1>E_2$, this current leaves the positive terminal of battery 1 and enters the positive terminal of battery 2. In other words, the higher-voltage battery is driving current into the lower-voltage battery. The internal resistances limit the current; they do not eliminate it.

This is an approximate model. A real battery’s effective voltage and internal resistance depend on chemistry, state of charge, temperature, age, and current. Protection electronics may also affect what happens.

Worked example: 12.6 V and 12.0 V batteries

Suppose two battery models have open-circuit voltages of $12.6\text{ V}$ and $12.0\text{ V}$, and internal resistances of $0.20\,\Omega$ and $0.30\,\Omega$. The estimated current immediately after directly connecting them, with no load, is

$$I=\frac{12.6-12.0}{0.20+0.30}=1.2\text{ A}.$$

So the 12.6 V battery initially pushes about $1.2\text{ A}$ into the 12.0 V battery. The shared terminal voltage in this simplified model is about $12.36\text{ V}$. The current may change as the batteries’ voltages and temperatures change.

A 1.2 A equalization current might be acceptable for one particular battery system and dangerous for another. The calculation alone cannot establish safety: manufacturers specify allowable charge and discharge currents, and some batteries must not be charged in this way at all.

What if the batteries have the same voltage?

If two batteries have exactly the same emf in the simple model, there is no circulating current between them while unloaded. With a device connected, they can both supply current. Their currents will not necessarily divide equally: the battery with lower internal resistance, shorter wiring, or a higher state of charge may supply more.

For compatible batteries designed to operate in parallel, capacity is often approximately added while voltage stays the same. For example, two suitable 12 V, 5 Ah batteries in parallel provide a nominal 12 V arrangement with about 10 Ah capacity. This is not a universal guarantee; the battery maker’s limits and system design determine what is allowed.

A pair of ideal voltage sources with different voltages cannot be connected directly in parallel in circuit theory. With zero internal resistance, the equation for circulating current has a zero denominator, representing an impossible condition in the ideal model. Real batteries have nonzero impedance, but that does not make every real connection safe.

Safer practice

Before paralleling batteries, check the battery manufacturer’s guidance. In general, a parallel bank should use batteries of the same chemistry and nominal voltage, with compatible capacity and charging requirements. Batteries should be in the permitted state of charge, and connections should use appropriately rated wiring and protection. Some systems require fuses, a battery management system, or a purpose-designed combiner.

Do not directly parallel batteries just because their labels show the same voltage. Different chemistries can have different charging limits and voltage curves; even two batteries of the same type can have substantially different actual terminal voltages. Lithium battery packs, in particular, may include protection electronics and require a manufacturer-approved parallel configuration.

Common mistakes

• Assuming parallel batteries add voltage. Parallel connection keeps the terminal voltage near that of one battery. Series connection is the arrangement that adds voltages.
• Assuming a lower-voltage battery will not draw current. It can draw current from a higher-voltage battery even when there is no external load.
• Treating equal label voltages as equal actual voltages. State of charge, temperature, age, and load can make terminal voltages differ.
• Assuming internal resistance makes a mismatch harmless. Resistance reduces current, but the resulting current may still exceed safe limits.
• Connecting unlike batteries without checking instructions. Voltage alone does not determine whether a battery combination can safely charge or discharge in parallel.

If a device needs more capacity, use a battery bank designed for that purpose rather than testing mismatched batteries by connecting them directly.