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Author Topic: Ford Galaxy - Testing Parasitic Battery Drain / Battery looses charge quickly  (Read 7086 times)

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Offline Mirez

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Ford Galaxy - Testing Parasitic Battery Drain / Battery looses charge quickly
« on: July 22, 2013, 02:07:51 PM »
« Last Rated on: October 24, 2014, 08:50:38 PM »
This is a test that can be conducted with most multi-meters and measures how much current the car is drawing when in an idle state, or "asleep". A vehicle battery in good health that discharges over the course of a couple of days will generally have what is considered a "Parasitic Drain", ie, an electronic circuit draws more current that would be expected.

Sequence of power-states:
  • With the engine running the alternator should provide enough power to supply all the vehicles electronics and charge the battery.
  • With the ignition on but engine off, all the electronics are powered purely by the battery.
  • With the ignition switched off then some circuits will remain powered (Most Control Modules, Interior light circuits, 12V Sockets)
  • After around 10 minutes the BCM will cut power to all but critical circuits (Radio memory, ECU memory, Alarm etc)

At the last stage, the car is considered to be "Asleep" and it's at this stage that we NEED the current draw from the battery to be minimal.
So why's it that important? Well if you have a 90 amp hour battery (90Ah) as fitted to most diesel Galaxy's, then your battery can supply one amp for 90 hours (or 90Amps for 1hr). Let's say we have a parasitic draw of 500mA, the battery would be dead in around 7.5 Days - But, that's to a theoretical 0 value - realistically you wont' start the car once the available power has dropped only a fraction of this and at 500mA it could potentially be "dead" within just a few days. Our target range would be between 30 and 50 mA during sleep.

So how do we measure? Well you'll need a meter capable of measuring current - nearly all do - but you'll need to move the meter probe from the "volts" input to the "amps" input (or similarly marked) so we can measure current. A word of caution here, most meters are rated to 10amps - any more and you'll blow the fuse within the meter (or worst the meter itself if its not fused) so you need to ensure you don't have or turn the ignition on during the test, you don't lock or unlock the car and you don't operate the global opening/closing. If your meter has two inputs for current, one for Amps and one for mA (MilliAmps) then use the Amps socket. Since the battery will be briefly disconnected ensure you have the radio code and the keys are out of the car!

So, start by moving the red probe from the Volts (Far right in this image) to the Amps (Far left):
1370-0

Next, we need to connect the meter in series to the car. This means the meter makes up the circuit, and isn't just applied "on top" of it, like you would normally measure volts. Our test here centres on the supply to the cabin so we need to disconnect the supply to the first fusebox (10mm Socket) and then connect our meter from the battery + terminal to the end of this cable:



Note, as above, the circuit is now going through the meter. Ensure you have good, stable connections at both ends and that the lead can't "bounce" back towards the battery post.

Turn the meter to it's "current" position, it'll look similar to this:

1374-2

A quick note here : Don't forget that you are limited by the maximum current your meter can take - in my case 10 Amps. This means you can't operate things like the headlights, main beam, ignition, horns etc - you'll blow the meters fuse if you do!!

So now we are now ready to observe some readings, Some controllers will fire up as soon as the meter is reconnected and on some car's the interior lights will also power up. The interior light circuit will draw close to 7.5 Amps so you might see this reading before the value drops. Here, we see the meter just after this has happened and it's now measuring a current of 137 mA.

1376-3

We need to leave the car to stand now for between 10 and 20 minutes (depending on controller's fitted) and hopefully we'll see the current drop to that target range of 30 to 50 mA (0.030  > 0.050 on this meter).

If your meter supports mA measuring and the interior lights didn't come on when you first connected, you can switch the probes to measure mA and watch the readings with a little more accuracy. Here we see it drop to 84 mA after 2 minutes and finally down to 38 mA after 12 minutes as the car enters it's sleep state:

1380-5

So, in this situation the vehicles electronics are drawing a perfectly acceptable current during sleep and we can look more towards a battery or alternator fault. It's worth noting that anything still connected to the battery + post isn't being measured during this test so if you have auxiliary circuits such as amplifiers or chargers then these should be measured individually.

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14 VW Tiguan 2.0 TDI 177 R-Line in Deep Black Pearl
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08 Ford Transit 2.2 TDI 115 in Frozen White
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