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This question already has an answer here:

Is there a specific formula where I can compute how many hours before my 12v car battery becomes fully empty when using 100w light bulb only?

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marked as duplicate by Brian Carlton, uint128_t, Dave Tweed May 17 '17 at 4:30

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    \$\begingroup\$ What is the capacity of your car battery (Wh or Ah)? \$\endgroup\$ – replete May 17 '17 at 1:46
  • \$\begingroup\$ honestly no idea about the capacity. Just a normal 12v battery found on most vehicles. \$\endgroup\$ – anagnam May 17 '17 at 2:02
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    \$\begingroup\$ 4 hours, assuming the actual power consumption of the lamp is 100W. This is a guess based on assumptions. 50 Ah battery is 600 Wh, but you are draining it faster than C/20, so I am subtracting a bit and saying 4 hours. \$\endgroup\$ – mkeith May 17 '17 at 2:36
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    \$\begingroup\$ "honestly no idea about the capacity" - so why no research? You know (or can find by looking at it) the battery manufacturer and model. You have heard of the internet, right? Does Google ring a bell? \$\endgroup\$ – WhatRoughBeast May 17 '17 at 4:10
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You do not want to know. A car battery is a starter. It does not like being fully empty. If it happens more than a dozen times, it quits working.

A starter is not the type of battery you can fully discharge then recharge it like you do your mobile phone or laptop.

There are lead acid EV batteries used to power golf carts. These EV batteries do not have a CCA rating. It is rated in Amp Hours.

Starter batteries are designed with low internal resistance by adding more thin plates for maximum surface area. It is designed to crank an engine for a second or two.

On the other hand the EV battery is Deep Cycle battery, not a starter battery. Deep Cycle batteries have thick lead plates designed to provide continuous power.

Battery life is rated in cycle times with a depth of discharge percentage. This mean if you use your starter battery to watch TV, it will likely die after running the battery all the way down a dozen times.

The following image is from Battery University, How does the Lead Acid Battery Work?

enter image description here

This applies only to Lead Acid Batteries, and not Lithium (e.g. Li-ion) or Nickel (e.g. NiMH)

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  • \$\begingroup\$ Yeah, in my experience, you only get to drain a starting battery deeply a few times. Then you are looking at buying a new battery. \$\endgroup\$ – mkeith May 17 '17 at 3:06

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