Battery idea!

[RedSledTeam]

Zurc,
Cool idea, however you need to compare battery capacities for an accurate comparison. And when it comes to capacities, there is a matter of milliamp-hr versus amp-hr:

Milliamps Hour (mAh) is important because it's the easiest way to distinguish the strength or capacity of a battery. The higher the mAh, the longer the battery will last. Batteries with different mAh ratings are interchangeable. If your battery is rechargeable then the mAh rating is how long the battery will last per charge.

Milliamps Hour is 1/1000th of a Amp Hour, so a 1000mAh = 1.0Ah Think of your truck's gas tank. Voltage is how much gas is being used, and mAh is the size of the gas tank. The bigger the gas tank (mAh) rating the longer the device will run. If your battery is rechargeable, then think of the gas tank as refillable (rechargeable).

Connecting 2 - 7.4 batteries in a series (+ to - ) will give you about 14.8 volts, however capacity ratings on the side of your batteries are about 1200 mah or 1.2 ahs. Safety dude mentions a 7 ah battery, your system is about 2.4 ah, 34% of Safety dude's. So in this case, capacity size matters!!