In our field, especially the heavy duty and agricultural side of things, we are often explaining the difference between batteries in series and batteries in parallel. How can you tell what voltage a system is at by looking at the batteries? This may seem obvious, but if there are multiple batteries it may get confusing.
When referring to batteries in parallel, all of the positive posts are tied together and all the negative posts tied together, (either by a cable or by frame for grounds). In parallel, the voltage stays the same and the amperage of each battery is added together.
When referring to batteries in series, the positive post of one battery is tied to the negative post of the next. This will leave open one positive post on one battery, and one negative post on the other. In series the voltage is added together, but the amperage remains the same.
It is also possible to have a total of 4 batteries, 2 in parallel tied in series to the other 2 also in parallel. This will double voltage and double amperage.
*This is a basic wiring guide and will not cover every application or scenario. Always use caution when modifying a system. While Smith Co Electric deems this information accurate, we are not liable for problems arising from use of this information.