Economy mode warning is NOT a fault condition. It is to protect the car from flat battery; if the battery voltage drops to a certain level, Economy Mode kicks in to help preserve power by progressively shutting off non vital functions, such as radio, heater, wipers. It should clear immediately the engine restarts.
although I have sophisticated test gear, I use a simple 12v battery voltmeter in the cigarette lighter socket like this for a quick check.
It should indicate around 12.3 volts with engine off - rising to around 14.3 volts with the engine running.
The battery condition is critical to the operation of the car as many functions - ignition, steering, sliding doors, air conditioning all rely on the battery being up to scratch. The negative is connected to earth.
Note also that a faulty alternator can flash up fault messages on the multiscreen - and be aware that modern batteries can fail without warning
A battery can go flat in a short time if it has gone low capacity or there is a discharge through a fault on the car.
On all three of my 1007's (2 x 1.4 Dolce and 1.6 Sport) I found that initially there was a discharge of 1.6 amps after switching everything off - this dropped to 15-20mA after 30 seconds or so - enough to drive the essential circuits. The battery should last for several weeks without use at this discharge rate.
Putting some figures to it: the battery discharge when the car is not in use should be around 20mA = 0.02A. This is just enough to keep the security module operational.
At this rate, a 60amp-hour battery should last for 60/0.02 = 3,000 hours to run flat. But we don't ever want that so after 1,500 hours it will be only half discharged; which is acceptable.
1,500 hours is 62 days or 2 months. So a good battery should safely last 2+ months without any need to charge it - and indeed mine often stand for over a month without being used or charged.
So if your battery "goes flat over Christmas" then it has lost capacity and is well past its scrap-by date and should be changed for several reasons -
Out of interest I checked on the battery volts on my 1.6 petrol auto with my max/min voltage meter. This measures the lowest instantaneous voltage across the battery terminals which can't be seen on a conventional digital or analogue meter. The car battery is a top quality silver calcium two years old.
Having stood a couple of days -
Before turning anything on, battery volts were measured at 12.13 volts
On starting, this dropped momentarily to 9.62 volts - not far from the voltage for the electronics to hiccup.
Immediately the engine was running voltage came up to 14.41 volts.
On second start-up; the battery voltage dropped to 10.21 volts - not surprising as the engine had had a short run to remove the initial stiction.
So if your battery is a little tired; could be (more so with a diesel) that the battery voltage drops momentarily below that needed to keep the electronics happy - and so a no-start situation.
. . . . but not everyone will own one of these. This is a simple one Gunson Tools G4184 although it will only test up to 100 amps. I have an ex-garage one that loads a battery up at 250 amps (see below)
If the voltages are OK (12.3 and 14.3 approx using a voltmeter or the tester suggested above) then you could also try leaving the headlights on for a while (engine off). That would take about 10 amps out of the battery which any good battery should do easily for two hours (monitor the volts and if they drop below 11 volts discontinue the test). Have a battery charger or jump leads handy though!
To carry out a CCA pass/fail test, you should load a fully charged starter battery with half the rated CCA value for 15 seconds. To pass, the voltage must stay above 9.6 volts at 10 °C (50 °F) and higher. Colder temperatures will cause a larger voltage drop.
* On the 1.6 Sport I found the battery would deliver 125 amps and drop to 10.3 volts. At 250 amps this dropped to 10.0 volts so passed the test. * 2 years later this battery was changed as it failed a drop test.
** However the battery on the 1.4 Dolce, (which appeared to be an original 2004/5 Varta battery so 7/8 years old at the time) dropped to 9.6 volts at 125 amps but went right down to 7.0 volts at 250 amps which is well below acceptable. * 2½ years later this battery was changed again under warranty as the drop test showed just 7 volts.