Batteries Wired in Series & Parallel

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Batteries, or more correctly cells can be wired in series or in parallel for a variety of reasons.

Wiring batteries in series or in parallel enables them to better meet the needs of a situation, electronic circuit design or whatever, that a single cell on its own may not be able to meet.

Commonly a number of cells is used to ensure that the required power can be supplied and batteries and cells may be wired either in parallel or series according to the situation and the needs.

Connecting batteries in series, or connecting batteries in parallel can enable the overall battery to provide the performance that a single cell on its own may not be able to achieve.

Wiring batteries in series has a different result to that or wiring batteries in parallel, so it is necessary to understand the effects of both, as well as some of the pitfalls.

Batteries and cells

The phrases "batteries connected in series" or "batteries connected in parallel" are seen in many places and often used to denote the individual battery cells wired in series or parallel.

Strictly speaking, a single battery unit is called a cell, and a compilation of more than one cell is called a battery.

In this way, several cells wired in series or wired in parallel form a battery.

That said it is very easy to slip into calling an individual cell a battery as the word tends to denote an item that creates electrical power using a chemical reaction. May be using the term "battery cell," would be the easy way to overcome this, although even this is not really correct if it is used as a separate item and not a single cell within a battery.

Single cell applications

Although batteries may be wired in series or parallel using a number of different cells to provide the required output in terms of current, charge or voltage, there are still many applications where a single cell will provide the required output.

Typical examples may be battery powered clocks often using a single 1.5V alkaline cell, mobile phones using a single lithium based rechargeable cell and the like.

When

There is a similar table of voltages for rechargeable cells as well.

There are many different types of cell and battery that can be used, and if there is no need to use series connected batteries or parallel connected batteries to gain additional performance, then it is a lot simpler to use a single cell.

However knowing the different voltages of the different types of cell means that it is possible to judge whether a multiple cell battery wired in series or parallel is needed.

Wiring batteries in series

Connecting cells in series to form a battery is probably the more common configuration, so this is what will be investigated first.

When connecting cells in series to form a battery, the positive terminal of one cell is connected to the negative terminal of the next and so forth for the number of battery cells to be wired in series.

When this is done, the voltages or more correctly the EMFs of each cell build on top of each other, each one adding to the overall voltage seen at the output.

So if each cell is 1.5 volts, then placing three cells in series, stacks the voltages on top of each other to give three times 1.5 volts which is 4.5 volts.

Also six 1.5V cells connected in series give a nine volt battery. This is how the batteries like the 6F22 battery are made. In fact it is possible to see the individual cells stacked in top of each other in series witihin the overall package. It can be seen that the positive terminal of one cell is connected to the negative of the next and so forth.

It is important to note that when battery cells are connected in series, each cell needs to pass the same current, and it is the current flowing though each cell that uses the charge. Thus connecting battery cells in series increases the voltage and not the charge.

This means that if each cell had a charge capacity of 2500 mA hours, then the whole battery would only be able to provide 2.5 amps for 1 hour and so forth.

The normal equation for relating charge capacity, current, and time holds for this. In other words the battery life in hours is equal to the charge capacity in milliamp hours divided by the current in milliamps.

Beware connecting old and new battery cells in series

It is very important to note that if one cell has more charge than another, then the cells with less charge will become exhausted first. Then the cell with more charge will try to force current through the others, effectively reverse charging them

As the internal resistance of cells goes up as they become exhausted, the overall battery will start to fail as the first cells become exhausted and the cel with more charge will not be able to drive current as it otherwise might.

Once the overall battery consisting of cells in series is discarded, then it is most likely that the good cell with charge remaining will be discarded along with the others.

Accordingly it is always best to change all the cells in a battery consisting of several cells in series at the same time.

Having unequal charges in rechargeable cells can be a big issue as rechargeables do not like being reverse charged. If this happens it can reduce the capacity of the cell that becomes reverse charged. This process, if repeated over many charge / discharge cycles can mean that the cell that becomes reverse charged degrades much faster than the other cells in the set. This means that if cells in series are contained in a battery, then the whole battery can more rapidly degrade than it would otherwise.

Wiring batteries in parallel

The other method of wiring cells is to have a parallel wired battery consisting of cells wired in series.

As expected, in this configuration, the connections are made in parallel. As the positive end of all the cells are connected together and the negative ends of all the cells are connected together, this means that the output voltage is that of the individual cells.

Therefore, if several alkaline cells were placed in parallel the overall voltage would be that of the single cell which is 1.5 volts.

The advantage of having a battery wired in parallel from a number of cells is that the amount of charge available is the sum of all the individual cells.

So if each cell had a charge capacity of 2500 mA hours and if three cells were placed in parallel, the overall charge available would be 7500 mA hours. In other words the overall battery would last three times as long for the same level of current.

The point to watch is that all the cells must be the same type having the same output voltage. So you shouldn't place, for example a Nickel metal hydride rechargeable cell with an output voltage of 1.1 volts in parallel with a lithium ion cell having an output of about 3.3 volts.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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