Battery draw

Is there a way to tell if an aftermarket piece of equipment is hooked up ,how long would the battery last and still start the car if .....12v, 5w, 0.41a ...applies. Is there some math I could do to know if I don't start the car for say 4,5,7 days I'd still be o.k.?

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The Interlock GUY

Too hard to tell. You could do all the math you want and will still be wrong. There are far too many variables to be able to accurately estimate battery life.

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Certified Security Specialist
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As Auex stated there are many variables which affect how long a battery will last, and still be able to start your vehicle.

The following are some of the things you need to take into consideration.

1. Age of the battery.

2. Your driving habbits.

3. Ambient temperature.

3. Batteries RC (Reserve Capacity) rating

4. Type of vehicle and its start up load.

5. The total parasitic current draw from all systems.

6. The length / duration the vehicle will be stationary.

7. Batteries state of charge.

8. Vehicle maintenance (ignition, oil viscousity, etc)


Typcially this is the general method of doing the math as to how long (estimated) it will take for the vehicle to drain, and still be able to start the vehicle.

Every battery has a RC rating, the following is the definition of it:


A battery's Reserve Capacity represents the length of time the battery can maintain the vehicle's electrical needs in the event the alternator fails.

Battery Council International defines Reserve Capacity as a measure of the time (in minutes) a lead-acid battery can deliver 25 amps at 80 degrees F and maintain terminal voltage of at least 1.75 volts / cell

For example my Optima Yellow Top battery has a RC of 120 minutes. Meaning that it will be able to supply 25 amperes at 80'F for two hours (120 minutes).

Now, what you need to take into consideration is how much starting current the vehicle requires to turn over the engine.

If the weather is extremely cold, then the internal resistance of the battery will go up, and the battery will have reduced cranking power as temperatures drop.

Most Honda's start over very slowly, but conversely they do not draw down alot of current during start up. Whereas, GM, Fords, start up very quickly, and also draw down almost double the current as a Honda.

The next factor is what the overall parasitic current draw is from the vehicles electrical system.

The general accepted threshold is 45 mA per hours maximum. If you take the above Yellow Top battery as an example then the duration before the battery was unable to start the vehicle would be 23.148 days.

In the information you have give, assuming it is correct. A 410 mA current draw would kill any battery within 2-4 days.

Obviously the batteries current state of charge is a very important factor as is the rest.

If in doubt, either remove the fuse to the device, or install a master battery disconnect while you are away, or install a trickle charger to maintain the batteries state of charge.





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Thank you very much for the help

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The Interlock GUY