ECN Forum Forums Technical Discussion Electrical Theory and Applications Amp Hours of Emerg. Lt. Battery

A particular emergency lighting unit (battery pack) is rated for 100 watts. Remote lighting heads will also be fed. The voltage is 6VDC. I'm trying to understand "amp hours" which I see listed on replacement batteries for emerg. lighting units. For this 100 watt unit, how many amp hours should the battery (batteries) be rated for?

100 watts @ 6 V for 1 hour is 16.67 A·hr.

If your actual wattage is 50, a 16.67 A·hr battery will last two hours.

What is your actual wattage and how long do you want them to remain fully lit?

eg. 75 watts for 90 minutes.

75 W <= 100 W

75 W @ 6 V = 12.5 A

12.5 A * 1.5 hr = 18.75 A·hr

Thank you. I now understand.

One other thought -- I remember reading that the amp-hour relationship is non-linear.
A 10 amp hour battery will deliver 1 amp for 10 hours, but will not be capable of delivering 10 amps for a full hour.

Correct. That's why it is important to note that his 75 W load is less than or equal to the 100 W rating of the battery. If he had a 200 W load, it would severely underperform. If he had a 10 W load, it would significantly overperform.

I'd like to give one more example. A 36 watt, 6VDC emergency lighting unit has it's battery missing for one reason or another. I would order a 9 amp hour battery based on the actual wattage (36) fully lit for 90 min. Am I correct?