Connecting Batteries: Serial/Parallel/Serial and Parallel—DC Voltages, Current, and Wattage on Each
Introduction: DC vs AC
Batteries are everywhere around us; our cars, our MP3 players, our cellphones and laptops. Every portable device needs some source of energy, and that comes from batteries installed inside them.
Batteries are sources of Direct Current electricity (DC). That means, if the output is connected to an oscilloscope so the graph of the voltage is shown, it will be a flat line located at the output volts amount. DC current is way different from the electricity sockets we have in our houses, which provide Alternating Current electricity (AC); in an AC system, the output is constantly switching from positive to negative through a sinusoidal graph with frequency same as the line frequency (60 Hz in the US, 50 Hz in most of Europe, etc.) and magnitude same as the line's voltage (120V for US, 220 or 230V for European countries).
Batteries come in many different sizes, capacities and types; however, technically all batteries share some characteristics which derive from their nature as a DC voltage source. Like any DC source, batteries have a contact which is marked with + and is the receptacle for positive voltage and a - contact where 0 V is applied. Don't let the - tag confuse you, batteries do not have negative voltage; the 0 V receptacle is almost always considered the ground and is connected as ground in DC circuits too. The voltage difference between the + and the - receptacles is what is called the DC Voltage of the battery.
Aside voltage, another crucial characteristic of a battery is its capacity, or, put simply, for how long the battery can keep a device operating. Battery capacity is typically measured with Ah, mAh or Wh. Let's show some examples so the units are understood:
A stands for Ampere; one ampere is 1000 mA - ampere is a unit for electrical current.
h stands for hour
The unit Ah indicates for how many hours the battery can supply 1 ampere before it empties. An example:
A 52 Ah battery (if totally full) can provide 52 A for 1 hour, or 26 A for 2 hours or 13 A for 4 hours, etc.
W stands for Watt and is a power unit; power can be calculated when the Volts are multiplied with the Amperes, W= V*I.
As a result, a battery which is rated at say 100 Wh can supply, if full, 100 W for one hour, or 50 W for 2 hours, etc.
Batteries can be connected with each other in multiple ways, to provide different voltages, to have higher capacity or both.
In a series connection, the + contact of a battery is connected with the - contact of another battery, thus forming one "new" battery. In the two ends of this battery (from now on called battery bank) there are one + and one - contact unconnected. These two contacts are the positive and negative pole of the bank. A battery bank which has been formed through series connection has the same capacity (Ah) as the batteries it consists from but its voltage is the sum of the voltages batteries. As you understand, series connection is used when our circuit or appliance needs more voltage than the voltage one battery can supply; supposing you need 48 Volts, you would connect 4 batteries of 12V in series.
In a parallel connection, the positive poles of the batteries are connected together and the negative poles are connected together too. The receptacles for the battery bank that is formed are any + contact and any - contact of the batteries. One would choose to connect his batteries in parallel when he needs higher capacity; the battery bank has same voltage as the batteries its consists from, but its capacity is the sum of the batteries capacity. Supposing you need 12 V but 104 Ah, you could connect two 12 V 52 Ah batteries in parallel.
This is a combination of the previous connection methods. You can achieve increased voltage and increased capacity, depending on the batteries you connect.
Series and Parallel Connection
Advice and Tips on Batteries and Connections
1. Regardless of the connection method, you must avoid the following:
- connecting batteries of different age together (shelf age before you bought them also counts)
- connecting different capacity batteries
- connecting batteries with different nominal voltage
- connecting batteries which at the moment of connection have different charge status
All of the above are typical mistakes made by people who feel in urge to get the advantages of a battery bank; most of them will not cause a problem at once, but eventually the capacity of the batteries will decrease.
For example, if you connect a full battery with an empty battery in parallel, the full will attempt to charge the empty one - a large current will be formed instantly, causing temperature increase in both batteries, sparks and possible insulation breakdowns. You could instantly end up, in the worst scenario, with two batteries which are damaged.
If a recent battery is connected with an older one, eventually the fresh battery will degrade faster because it will be constantly "supporting" the older battery whose capacity has surely dropped over time.
2. Connections between large batteries of many Ah, for example car batteries, should be accomplished with the proper gauge wire for the current. Car batteries can provide huge amounts of instant current and if the wire is thinner than what it should, it could break or melt and cause further problems in the circuit. Proper contacts should also be used, sufficient for the power distribution. If solder is used, the joints should be stress-tested and held firmly by additional hardware. Fuses of the appropriate rating are also a must; should a short be caused in any part of the circuit, the fuse will melt and break the circuit, possibly protecting other devices.
3. Selecting the proper way of connecting batteries to form a battery bank has to do with our application and devices. We cannot have a 12V battery bank if our devices need 24V and we cannot push a small capacity battery to the limit by applying constant high loads which empty it in a matter of minutes.
4. The power rating, in Watts, of the battery bank is always the sum of power ratings of the batteries it consists from, regardless the connection method.