2016 Mazda 6.
On about six occasions in the past six years, the battery has been 'flat' in the morning. (Latest occasion was yesterday. The battery voltage was 2.8V) The car had always been used the previous day, and left locked. Each time, the car has been jump started, and has then been ok everafter, so it is not a battery or alternator problem. Checking the switches, nothing was left switched on. e.g. lights, heater fan.
What causes this? Something is draining about 70 AmpereHours in about 12 hours.

 

Youcan not rule out the battery until you first charge to 12.50+ and load test it.

That is the first thing that "MUST" be done before continued electrical problem diagnosing!

ASE

 

Welcome to the forum.

 

Hi Callister. Thank you for your reply.
It is definitely Not the battery or alternator.
I have more information, but did not want to complicate my question.

I am trying to find which bit of the Mazda6 electronics is at fault.

I have had the battery checked most times, and it has always tested OK. The batteries have always gone on to function perfectly for months/years, even when transferred to my 3litre diesel tractor.
It sometimes takes several separate jump start attempts, (Disconnecting and re-connecting the jump-start battery) with no response whatsoever, before it suddenly kicks into life. Other times it kicks into life at the first attempt. This could be because the boosted voltage doesn't get to 9+V, (Unlikely) or perhaps something else.
Breakdown men tell me it is a known fault with Mazda 6's of about this age, but they don't know why. The immobiliser is their main suspect.

 

In your first response the information about the battery voltage points to a bad battery. To drop that low of voltage it would normally take days. Most high voltage and amp draw systems would simply not function after less than 7 volts and low amp draw components would take a longer duration to drop the battery supply voltage.

So, we go back to first you always charge the battery to 12.5+ volts and load test it before assuming or going any further to find an electrical issue on your MAZDA?
The next item reading your responses is a possible continuity issue. So, you first check the battering terminal connections and then the connection at the ground attachment and then the power cable connection at the starter. They must be clean and have a clean and secure connection. Base on some of your information you are surging the voltage which indicates a possible continuity issue.

I might add for anyone reading this, if you use a jumper battery that battery "must" be on good order, (load tested after charged to a stable 12.5 volts) or the battery being jumped will first try to support the battery being used to jump. This is counterproductive and a mistake often done by those using a jumper battery connected to a questionable or low voltage battery. Electrical energy travel in a direction that has the least resistance.