Running batteries in series or parallel?

Below are pictures and an explanation of what wiring in series or parallel accomplishes. It shows older NiMh packs, but the same holds true for wiring lipo packs.

There are several advantages to running two separate battery packs in a particular boat. One reason is if you need 12 cells but have other hulls that use only 6 cells, you will still be able to run these packs in the other hull at a different time. Another reason is weight distribution. Some hulls need weight moved around to help balance things off. It's easier to do this with two small packs than one large one. The only downfall is the added resistance from longer wires. 

The bottom two graphs assume we are running the average 7.2volt 2400mAh(milli amp hour) NiCad batteries. The end result of what the motor gets for exact voltage and mAh (Milliamp Hour) will vary depending on many factors. Including but not limited to, wire & electronic speed control resistance.

It is very important that in either of the two bottom scenarios, the two packs being used are both from the same manufacturer and have the same mAh rating. Both packs should also be peaked individually within a few minutes of connecting them for use. If not serious problems could occur including fire and possible explosion.            

Series Graph
 

This is what we run in most Fast Electric setups. The output to the motor and ESC will see, will be the combined voltage of each pack. So in the below example, we have 2 x 7.2 volts packs totaling 14.2 volts and the mAh will be 2400. You could also combine packs of different voltages, but it's important to use similar packs from the same manufacturer with the same mAh rating. Both packs must be fully peaked prior to running them. Also don't run the packs until they totally dump, once the boat starts to slow down bring it right in, just incase one pack has a bit higher mAh capacity than the other. Also, make sure the ratings of both the motor and the ESC will handle the higher voltage, you don't want any premature equipment loss.

   Parallel Graph

Lots of scale or enduro boats will run this type of setup. It's very important that both packs have the same voltage. If one pack is stronger the the other, it will try to charge the weaker pack when they are connected. The motor will see the same voltage as a single pack but close to double the mAh. Because of this, you will see close to double the runtime but slower speeds than the single pack due to the added weight of the second pack. Take care to make sure all your electronics is up to the task of double your normal runtime. Extra heat will build up in the motor & ESC. These should be water-cooled.