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How long would it take an accessory to kill a motorcycle battery?

Replacement batteries for your bike are 8.6 ah (Amp Hour).

Your cell phone at 1800 mah = 1.8 ah. There are considerations like the efficiency of the charger and so on, but my guess is you have nothing to worry about.

What you don't want to do is leave the moto battery in a slightly discharged state for long. Meaning ride it and charge it back up within a day.

To answer the exact question, the charger will say on it how much it draws at 12v.
Divide that into the capacity of your battery and you will get hours.
In practice the battery will no longer be able to start the moto below
some level (say 20%) charge, and older batteries have lost some of their rated capacity.

If it was me I wouldn't draw more than about 25% before recharge to be safe.
 
Last edited:
OP: Keep in mind that a bike battery at 12.4 volts is 50% discharged. At 12 volts even, it's fully discharged and happily sulfating away.
 
yarghhh

Do this:

Assuming you have a 12V battery
Capacity is in Amp*Hours
Draw is in Watts

time_to_dead (seconds) = Capacity * 12 * 3600 / Draw

Typical Yuasa is 2-3 A*Hr
Typical curling iron is 50W
Time you have to curl your hair: 30 minutes
 
It doesn't take much to discharge a battery. I had a LoJack on my Triumph from a previous owner, and it would kill the battery in a week. They're not terribly strong.
 
All this talk about amps and current and draw and the like is fine and good and educational. But really, just install a relay triggered by turning the ignition on and off and the problem ceases to be a concern. Ever.
 
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