Interrogate The Integration Expert

Oct. 27, 2008
Power Protection

Matching Batteries

Q: Thank you for your answers last month on power load requirements. I now have a better understanding about how to determine which power supply best suits my installations based on load current. I used the basic formula you gave readers for calculating the power demand of a system (see September Interrogate the Integration Expert on page 48) and it all makes sense. To avoid power problems on long wire runs, I also went online and accessed the easy-to-use calculator that matches load current with wire gauge for determining voltage drop per 100’ increments of wire run. Again, I am much less confused about power requirements thanks to these tools and your assistance. Now I am wondering, how do you determine which battery size is best for an installation?

A: Battery backup time is commonly based on a 20 hour discharge period by the battery manufacturer. Thus a 20 Amp-Hour battery will normally provide 1 Amp over a 20 hour period. However the discharge time is not linearly related to the current. For example, a 20 Amp-Hour battery will supply 1 Amp for 20 hours standby but will only supply 1.8 Amps standby for 10 hours.

Again, the Altronix website (at provides a convenient and easy-to-use reference for determining the correct battery size based on load current and discharge backup times. This takes the complexity out of selecting batteries. Many power supplies also specify a particular battery for various loads and backup times.

Choosing Between Circuit Breaker And Fuse

One last question—when should I use a circuit breaker and when should I use a fuse? Are they application specific as well?

A: There is no hard and fast rule to answer this question as each method has pros and cons. Yes, the specific application will usually determine the choice.
That said, the main difference between the two is that while fuses must be replaced if they blow, they are better suited to protect cameras and other delicate equipment. They will respond much quicker than a PTC (Positive Temperature Coefficient Resistor) to overload conditions.

In contrast, PTCs or circuit breakers are more appropriate for protecting batteries and camera housing heaters and blowers due to their high tolerance to transient over-current conditions.

Jonathan Sohnis is VP of Engineering, Altronix Corp.