Robbie wrote:

I'm slightly baffled as to why you have not identified the reason for the drain. Or have you had it looked at?

beanie wrote:

Robbie any pointers or a systematic approach to current drain would be appreciated

Hi Beanie,

I keep meaning to write a guide on electrical diagnostics and add it to the wiki. One day, honest! 'Systematic' is the key word and if you think of the power as it starts from the battery before branching out to various systems you will not go wrong.

Robbie's Quiescent / Parasitic Current Drain Testing

1. Always start from a known good point with a charged battery. Bear in mind that batteries can fail and self-discharge internally or even from crud spanning the gap between the terminals. Battery terminal connections should be pushed down the tapered terminal before tightening to ensure a good connection. All aftermarket electrical systems should be disconnected or isolated before testing. Do not convince yourself that the aftermarket kit was 'ok before so therefore it should be ok now'. No exceptions, disconnect it.

a. Good battery voltage:

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2. The vehicle has to be asleep before testing, so open the bonnet, short the bonnet sensor under the RHS latch and either lock the vehicle or mechanically latch the doors open so that the vehicle thinks they are closed. If you are fooling the locks don't set the internal alarm. Even when asleep the alarm LED flashes, which is detectable, and the EAS and potentially the EPB can occasionally draw power.

a. Bonnet latch connector with paperclip:

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b. Depending on vehicle, Model Year and specification it can take a while for the vehicle to sleep:

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3. The preferred method is to use a DC capable current clamp with a millivolt range. This can be clamped on individual circuits or groups of circuits. It is easier to start with large groups (such as the BJB) before delving into individual circuits. It is not possible to remove fuses as part of the diagnostics as this may wake systems or stop others from sleeping which will make nonsense of the diagnostics. Equally, if the drain is caused by an energised relay you may reset it by pulling the fuse and lose the very thing you are looking for.

a. Current clamp for attaching to DMM:

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4. At the +ve terminal the load paths split into 2 directions; one towards the starter/alternator and the other to the battery junction box (BJB). These should be tested individually. Start with the alternator cable; there should be no current running down this lead in this state. Be aware that some alternator failures that can leak current only do so when either cold or hot, so even if you have zero current flow you may have to come back to this if no other faults are found.

5. The main positive lead from the battery to the BJB can be gently manoeuvred upwards to allow for an amp clamp. You should detect a current less than the value shown in the previous table. As an example an older D3 with no TMPS and no tracker fitted should be below 22mA. My own MY13 D4 should be below 30mA, as shown below:

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a. Due to the alarm etc the figure can move around a bit, up to 26mA in my case, but this is still below the 30mA maximum:

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6. If you do have a drain beyond the table limit you have to move systematically through all the various circuits to find out which are drawing more than they should. To save time you can quickly trace down all the fuses with a test light or digital multimeter (DMM) to see which circuits have zero power on them. This allows you to focus on the circuits that have some power on them.

7. You have a few options for testing individual circuits. The workshop method is to release the BJB and clamp the wires from below. This does work but the looms on some later vehicles can be a bit tight. A method I like is to use is via the test points on the back of the blade fuses and effectively treat the fuse as a resister of known value (as that is what they are) and Ohms law to deduce the current flow by measuring the voltage drop in mV and using a calculator or a look-up table to find the actual current flow.

a. The tables are in my gallery and look like this:

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b. The fuse test points if you have never really noticed them before look like this and can be tested thus:

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c. For those that like maths rather than the table the current calculation on this circuit (fuel pump) was:

25 Amp Fuse = 2.36 mOhm or 0.00236 Ohms
Measured @ 0.0235 V or 23.5 mV
= 9.95763 Amps

8. If you do not fancy using a clamp from below or using the voltage drop method you can use a fuse buddy or a home made fly lead to form a wire loop that can be either inserted directly into a DMM (if the circuit is below the fuse limit of the DMM) or to clamp with a DC clamp. There are bespoke kits on the market for those that like to spend money but you can make your own fly lead with spade crimps or get one of these and link it together with a female to female banana plug:

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a. Same circuit again from the example above, producing very similar results to the voltage drop method:

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9. The method above is also useful for very low power circuits as you can wind the wire around the clamp a few times for extra accuracy and divide the number displayed by the number of times you have wound the wire through the jaws of the amp clamp.

10. Current can be measured anywhere on the circuit so in practice you can clamp anywhere with access. This can be helpful when you are down at the component level but a great deal of diagnostics can be done at the BJB or CJB.

11. It is not a complicated thing to do and definitely something you would want to do DIY as it takes time and patience. Even if you have no idea how to fix the problem circuit (although the forum is pretty good) it will save a bundle of cash if you have done the time consuming diagnostics to narrow it down to a particular circuit. Before you know it your workbench or kitchen table looks like this and at least gives the illusion that you know what you are doing:

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A bit of a long post so I hope this helps and I am always happy to help forum members, especially those willing to give it a go themselves. There are many ways to skin this particular cat so I am sure other members will have their own top-tips.

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Battery & Quiescent Current Drain Testing

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