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Why did my battery go flat overnight?

Posted: Sun May 22, 2011 1:25 am
by warpc0il
The OEM battery was rated at 60Ah (AmpHours)

The Ah of a battery defines its overall capacity, e.g. 60Ah means 10Amps for 6 hours, or 60Amps for 1 hour, before it is flat.

However, "flat" in our case is anything less than it takes to start the engine. So it may unlock the doors and light-up the panel but when you turn the key you get anything between a chug-chug turn over and the dreaded machine gun rattle of the solenoid. The rotary engine needs to be spinning at least 180rpm for the centrifugal forces to maintain a seal for the combustion chambers and start.

So let's assume that the battery needs at least 2/3rds of its capacity to start the engine - without getting into the negative impact of cold on producing high currents - which means we have 20Ah to play with.

Mazda were obviously aware of the potential issues of a partially flat battery as they ensured that the radio can't be left on with the ignition key out and similarly the cigarette light socket is also left dead, in case anything is left plugged-in that could steal precious power.

The quoted Back-up current is 20mA, this being the constant flow of current present (for the audio unit, clock, PCM, alarm, central locking, etc.) when the ignition switch is off and with the key removed.

20mA is 0.02Amps so is would take 20÷0.02=1000 hours or 40+ days, before we should have issues starting.
Certainly something to think about if leaving the car unused for more than a month.

This can be extended by disabling the internal alarm, using the button on the key fob, to reduce the Back-up current.

So how can the battery be drained in less than 24 hours, at least to the point when it won't start the car? :scratch:


1: The available battery capacity is way less than 60Ah because it is insufficiently charged or just past it.

If there's a good reason why it's not fully charged then give it an external charge or jump-start and take the car for a 10 mile run to charge it, otherwise it's time for a new battery. :(

2: Something else is taking power even though the ignition is switched off.

The interior light is 10W, which is about 840mA, giving a total drain of 860mA.
Leave that on and we have 20÷0.86=23.3 hours.

The boot light is only 5W, about 420mA, giving a total drain of 440mA.
20÷0.44=45.5 hours.

So the interior light will drain a fully charged, good battery within a day and it takes less than two days for the boot light to stop the engine starting. :shock:

If you have the old-style remote fob then there's no built-in delay on the boot-release button.
The smallest knock on that button while within range of the car, the boot will release and the light will come on. #-o
Even in the dark this isn't noticeable since the lid is still shut, and the clock is ticking on not being able to start the engine... :bom:

The simplest and cheapest fix is to replace the boot light 501 bulb with an LED.
This doesn't need to be a fancy multi-faceted job as we just need the same amount of light as the original but a much lower energy consumption.

A typical 501 single LED takes 20mA (where have we seen that number before?) so by simply replacing the bulb we get 20÷0.04=500 hours and now have over 20 days before risking a flooded engine. :thumleft:

501 LED bulbs can be bought from eBay for as little as £1.18 for a pack of two, delivered from a UK seller.

What are you waiting for?

Re: Why did my battery go flat overnight?

Posted: Sun Jul 29, 2012 2:35 pm
by warpc0il
If your battery is totally drained (flattened) then you may find that your charger ignores it when you try and charge it back up.

Most "Smart chargers" won't start their sequence unless they detect a battery, so connecting another 12V battery in parallel, using a pair of jump leads, is the way to go.

For older trickle chargers you just need a load of some sort across the leads, so you can either use another battery as above or just connect any old 12V bulb, something around 5W would be ideal, across the battery terminals.

Eventually the dead battery should recover enough to be detected in it's own right and the bulb can be removed.

Just make sure that the bulb isn't resting on anything that could burn or melt, as it will get very hot.

Re: Why did my battery go flat overnight?

Posted: Sun Mar 10, 2019 11:40 am
by warpc0il
If all else fails then

Re: Why did my battery go flat overnight?

Posted: Tue Oct 20, 2020 7:00 pm
by warpc0il
If your battery is losing voltage when parked-up, faster than expected, then you need to know why.

A good battery can take a relatively high drain current with minimal drop in voltage, while an old/tired/poor battery will drop a significant voltage with only a tiny drain.

Ambient temperature is also a factor, dropping the voltage much faster, particularly if the battery is old.

To know if it's the car or the battery that has the issue, you need to measure the drain current.

For this you need a ammeter with a capacity of at least 2 Amps DC.

You disconnect the battery negative terminal and then connect the meter between the battery and the cable - in series, so all the current goes through the meter.

The steady drain current should be less than 50mA but it could spike at over 1 Amp (1000mA) as you make the connection, due to components in the audio system, this it why you need a 2 Amp capacity meter.

If the drain current is within spec then you know that it's the battery that's the problem.

If the drain current is higher than expected then you can pull fuses, starting with the passenger footwell fusebox, to determine which circuit(s) are taking the current.

If the current remains high with any/all fuses removed then the problem is with something that's not fuse protected, like the alternator.

After reconnecting the battery, you need to reset some systems, see viewtopic.php?p=595730#p595730

Note that switching the ignition On, with the meter in circuit, can damage the meter, unless it's rated at 20Amps or more
Attempting to start the engine with the meter in circuit would blow it into tiny pieces, you have been warned.

If you want to check the starting current then a DC clamp-on ammeter is the tool to use, but these aren't sensitive enough to check drain current.