Batteries with same voltage but different number of plates

Question

Why do some batteries have the same voltage with a different amount of plates? What is the specific purpose of that?

E.g. 12 V with 11 plates and 12 V with 13 plates.

Answer 1

There seems to be some temptation here on EESE to assume "plates" refer to or equal cells. I believe this is mistaken.

Cutaway images of automotive 12v lead acid batteries I am finding seem to show that each nominal cell consists not of a single pair of plates, but rather of a stack of many interleaved plates, alternately connected to the cell's positive or negative terminal. In effect, each cell is a number of cells in parallel, though sharing the same bath of electrolyte. Six of these cells in series then form the 12v battery.

Likely, the 11 or 13 plate designs are two currently or recently common tradeoffs in the parallelism vs. size or thickness. I'd expect we'd need a battery designer to explain the advantages or disadvantages of each.

Edit: in fact this is the case. Automative-, motorcyle- etc style batteries often have right on their data plate the number of plates per cell, and 11 is a very common number.

Answer 2

In principle, a battery is made up of "cells" (which I think is what you mean by "plates") - and the word battery comes from that (originally a battery is a "a usually large group of things, people, ideas that work together" - so by definition a "battery" ought to be more than one cell although it is now used for a single cell too...)

The cell has a "nominal voltage" that depends on the chemistry (material of the anode and cathode) and temperature. However, as soon as a current starts to flow, the voltage drops due to internal resistance. When you measure the voltage on a "12 V" car battery, you can easily measure 13 V or even 14 V.

I believe the key to your answer is understanding that a battery voltage can be specified "under load" - it is more meaningful to know you can get a battery to deliver 20 A at 12 V than to know it measures 12 V when nothing is connected. In order to achieve voltage under load, one can increase the number of cells (increasing the no-load voltage), or lower the internal resistance (typically, this requires a larger battery).

So if you want a small battery capable of (short term) high peak loads while maintaining the nominal voltage, you can increase the number of cells. When you do so, you may need to put in place a regulator that prevents the low-load voltage from exceeding the capabilities of the circuit under load...

Answer 3

Different types of cells inherently have different voltages due to their different chemistry. It can therefore take different numbers of cells to get to about the same voltage when using different types of cells.

For example, lead-acid cells like in a car battery produce around 2 V, typical AA alkaline cells about 1.5 V, lithium coin cells usually around 3 V, etc. Your 12 V car battery contains 6 lead-acid cells, although it would take 8 AA alkaline cells in series to make the same voltage.

Answer 4

The number of plates is the plates per cell. As others have said, the cell voltage is fixed by chemistry. The number of plates per cell will always be an odd number- a 13-plate battery will have seven negative plates and six positive plates per cell. Ref. Battery Reference Book By Thomas P J Crompton

Available cranking amperes, and life are affected by the shape and number of plates per cell. For the same quantity of lead used in a cell, several thin plates will offer more area in contact with the electrolyte, hence more Amps, hence more cranking. In turn, thick plates will tolerate better repeated deep discharge (up to 80%), that's why forklift, solar or golf cart deep cycle lead acid batteries have less plates than car batteries, but they are thicker. I would think that more plates = higher cranking amperes, but more costly and probably shorter life, all other things being equal. The first part is confirmed by this reference.

Edit: As Olin points out, the references I pointed to and this particular terminology specifically refer to lead-acid batteries. It's been consistent since the 1920s, when battery cases were made from glass jars encased in hardwood, and "battery repairmen" would dismantle batteries and replace the plates (at some risk to themselves).