How to get a rough approximation of how long it would take to discharge battery almost fully when keep ignition on?

Question

I want to do a simple 'test' to see if my motorcycle battery is draining too fast. I've heard that if you keep motorcycle ignition on over night, it will drain. But I could not find any single source where I can get an idea how many hours it will take to drain.

Before asking this question I read this answer from electronics SE.

It means that the battery has a capacity of 25 Ah when discharged in 10 hours. 25 Ah is 25 Amps for 1 Hour which is equivalent to 2.5 Amps in 10 Hours.

So if you load the battery with 2.5 Amps it will last 10 Hours.

If loaded with a higher current usually battery capacity decreases so that is why the 10 Hours is mentioned, it results in a higher battery capacity making the battery look "better".

How many watts: simple 2.5 Amps x 12 V = 30 Watts and that for 10 Hours.

My motorcycle is a small one with 110 cc engine and has following battery specifications:

ETZ5, ATZ5L 12 V-4 Ah (10 HR)

Your vehicle has a maintenance-free type battery. You do not have to check the battery electrolyte level or add distilled water. Clean the battery terminals if they become dirty or corroded.

When I turn the ignition on (not the engine), I think the main things that consume battery are: Headlight (default is low beam), tail light and instrument panel which also have a digital fuel meter and a clock as shown below:

And the information about headlight and tail light:

My bike specifications don't mention the thing I am interested in. So I think I have to roughly calculate it but I'm not sure how to do it.

So is there any way to get rough approximation of how long it would take to discharge battery almost fully when keep ignition on?

I don't need something accurate. If I can figure out something like it should last 2 hours OR 6 hours OR 12 hours OR couple of days (in other words the total time bike ignition on is kept on before it's unable to start the bike), that would help me a lot.

It would give me idea to know that something is not right. For example, the theoretical time is 6 hours and my battery is practically discharging significantly in less than 1 or 2 hours, I would obviously know something is wrong.

PS: If you think there are some other non expensive ways to test health of this kind of battery, feel free to recommend.

Answer 1

I want to do a simple 'test' to see if my motorcycle battery is draining too fast. I've heard that if you keep motorcycle ignition on over night, it will drain. But I could not find any single source where I can get an idea how many hours it will take to drain.

Let's assume you're doing this out of curiosity and not to diagnose any specific problem with the bike. If you do, however, suspect the battery is bad, just remove it and take it to a shop that has a battery tester. They can load test it and determine its health.

But YES, if you leave the ignition on it will eventually drain the battery. How long depends on two main factors:

1. How much energy is stored in the battery.
2. How much power the bike draws when in this state.

ETZ5, ATZ5L 12 V-4 Ah (10 HR)

This gives you an idea of how much energy is stored in the battery. The spec 12V - 4Ah means that it will supply 4 Amps at 12 Volts for 1 hour. It would also supply 2 Amps for 2 hours, etc. So if your bike draws only 1A it should stay on for about 4 hours.

When I turn the ignition on (not the engine), I think the main things that consume battery are: Headlight (default is low beam), tail light and instrument panel which also have a digital fuel meter and a clock as shown below

There are likely some other power drawing components that you are failing to account for. What about the engine electronics, the fuel pump (if there is one), the ignition system, etc. It's going to be difficult to calculate the total amount because you don't know exactly what is turned on and you also don't know the power consumption of each of those things. Also, going by the fuse specs is not realistic since those circuits are not likely going to draw that amount of current except in a fault condition.

A far better approach would be to actually measure the current draw with an ammeter, this would give you an actual figure to use.

The other problem is that you don't really know how much battery capacity is needed to start the engine. You'd have to also do this experimentally by, again, using an ammeter, and measuring the engine start loads.

Using that you should be able to get a rough idea of how long you could leave the bike on and still be able to start the engine.

A far better idea, however, would be to avoid leaving the ignition on for long periods of time. Most motorcycles and automobiles are not designed to be left on without the engine running for more than a few minutes at a time.