Problem With Flat Battery & VoltGuard Battery Isolator

[Mikefromlondon]

if an LED draws let us say 20mA, then in 24hrs it would have drawn 24x20=480mAh, approx 1/2amp per day, so in 30 days it would draw off your battery 15Amp/hour charge, and so in 4 months= zero capacity!


or if the LED was drawing 10mA using low power, this would be 8 months for battery to drain completely, not forgetting by the time the isolator trips the battery out it must do after the battery has been partially drained away, so the times quoted above would be eve shorter.

 

[chapeau]

Even if it is a phantom voltage that's gonna drop to zero when a load is applied, I can't believe it's showing 12.69V almost a year to the day it was last driven.

Can you explain that?

My understanding of battery chemistry (and it's a rather long time since I studied pure chemistry) is that the voltage will be fixed for a certain type of battery. That's because the voltage is totally dependent on the oxidation/reduction reaction at the anode/cathode and that is fixed by the substances in the battery itself.

What changes as the battery discharges is the internal resistance - it starts lowish and will be a lot higher when significantly discharged. So when you stick your voltmeter across the terminals - when the battery is charged the vast majority of volts are "used" in your meter so that is what gets measured.

When the battery is flat the vast majority of volts are "used" in the battery itself (internal resistance) so your voltmeter sees fewer to measure.

Obviously this is dependent on current draw.

So if you are still reading 12.69v after one year then your voltmeter is a very very good one and hardly draws any current.

It's one of the fundamental rules of fizix. When you measure something, you change what you are measuring. That's why voltmeters have to be very high resistance, and ammeters very low resistance if you want them to show as close as possible to the true amount you are measuring.

 

[securespark]

Didn't know LED's drew that much!

 

[Mikefromlondon]

Quite honestly 10 or 20mA is hardly any drain, but it adds up to a huge amount over a log period of time.

you may have seen some battery powered gadgets like smoke detectors, they do use an LED to indicate it is healthy, but they use a pulse method, whereby the LED flashes very briefly (like for 1/2 second duration) say once per 30 seconds to 60 seconds.

This way a small 9V battery can last years, also the smoke detector circuit which senses the smoke, is also energised periodically (say every every 10 seconds) so as to make the 9v battery last for at least a year, and then the clever circuits have built in detection for low battery voltage. where they emit a short sharp sound every couple of minutes!

On your car it may be other things like radio, which also needs power to keep its security code intact, and ECU is also permanently supplied with live 12v as well as the ignition live, so there is some drain on ECU even when car is switched off., other things like factory fitted car alarm, that energises automatically when you lock your car. so all those minute drains add up a lot over a time period.

 

[Mikefromlondon]

Apollo Gies! Double post