My brother-in-law is buying an underwater camera for use with his marine biology studies.

Here are the electrical specs for the camera. Input voltage 12 VDC Power Consumption: 1.4 W Cable: 30, 50, 100m (basically a really long cable with an RCA connection) 21 LED lights Image Sensor: Color CCD

He's trying to understand the options that he has available for the Battery Kit. He can choose from:

  • 4.5 A Battery + charger
  • 7 A Battery + charger

Why would he get one over the other? Is it actually paired up with the length of cable that is connected to the camera?

Doing some quick Math: P = VI => I = P/V I = 1.4 W / 12 V = .12 A.

So the camera doesn't really require that much amperage. So why would he choose the 7A Battery over the 4.5 A Battery b/c that seems more than adequate.

  • \$\begingroup\$ Will this battery be used for all components on the platform? \$\endgroup\$
    – sptrks
    May 2, 2012 at 6:03
  • \$\begingroup\$ I'm in no way endorsing this product...but here's the link. theimportsworld.com/nauncasytide.html This bundle comes with the camera and a USB connection Kit...assuming that you will connect your computer. Perhaps @sptrks is hitting the nail on the head that the Battery Kit is also used for other devices. \$\endgroup\$
    – milesmeow
    May 2, 2012 at 6:10

1 Answer 1


The misunderstanding is about the unts: the battery capacity is expressed in Ah, which means the number of Ampères that it can supply for an hour, or the number of hours fot which it can supply 1 A of current.

The cable hasn't a great influence on this, other than dissipate power due to its length. But at that low current, it shouldn't be an issue if it's big (and I bet it is!).

So, given that your current is .15 A (150 mA, rounded to the top ti be safe), the smaller battery should ideally be able to supply the camera for 4.5/.15 = 30 hours, if the values are correct. But probably the capacity will be a bit less and decrease with time, and the consumption may be higher, and you will have a slow leakage depleting the battery.

But for this scenario, 4.5 Ah seems to be enough, I guess (depending on how long he needs the camera on).

With the 7 Ah battery, the duration will be 7/.15 = 47 hours.

  • \$\begingroup\$ I had a feeling that it was also a mislabeled option. That would make a lot more sense. Thanks @clabacchio \$\endgroup\$
    – milesmeow
    May 2, 2012 at 6:16

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