We frequently get asked the question, "How am I supposed to connect my battery if I want to double the capacity but not the voltage?", or similar questions. It can be confusing if you've never done it, but hopefully this'll make it simpler. Be sure to read the important notes at the bottom to protect yourself from damaging any equipment!
Connecting in Series
When connecting your batteries in Series you are doubling the voltage while maintaining the same capacity rating (amp hours). This might be used in a scooter, Power Wheels kids vehicle, or other applications. Just use a jumper wire between the negative of the first battery and the positive of the second battery. Run your negative wire off of the open connector from the first battery and your positive off of the open connector on your second battery.
Connecting in Parallel
When connecting in Parallel you are doubling the capacity (amp hours) of the battery while maintaining the voltage of one of the individual batteries. This would be used in applications such as laptop batteries, some scooters, some ups backups, etc. Use a jumper wire between the positives of both batteries and another jumper wire between the negatives of both batteries. Connect your positive and negative wires to the same battery to run to your application.
Important notes: When connecting batteries in a pack there are some important things to keep in mind - - Find out the requirements of your application. For example: Don't double the capacity on your Power Wheels vehicle if you're not supposed to...you could burn up the engine. Follow the recommended guidelines for your application. - Don't use two different chemistries when connecting a pack. Usually the voltages will be different, but more importantly the charge rates will be different and the capacities may be different, thus resulting in a shortened life span. - Try to match capacities as much as possible. When connecting batteries in a pack you should try to match the capacities as much as possible to avoid discharging one battery quicker than another. A pack operates at a combined voltage so your one cell that discharges quicker will likely discharge deeper than it may be able to recover from.