Battery drains and how to test

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Battery drains and how to check. Part 1


Battery drains fall into two groups, depending on when the model was first introduced. All cars consume slightly less current when locked.

Part 2 will cover and describe the instruments required to check and find the faults, also how to use them.
Part 3 will cover common ECU faults
Part 4 will cover wake up systems

Early Cars

The earlier cars like the 124, 129, 140, 201, 202, 208 that use relays do not suffer from these faults, the worst is a leaking diode pack on the alternator or a boot lamp switch so will not be included here.

Normal faults. This includes some later 129, 140, 202, 210 cars often from mid 1997 on

All cars with ECU’s controlling the seats, door windows and central locking. Shorts or leaks in these cases will continue to drain the battery till it is flat

These are the most common ECU’s that can leak and drain the car over several days when the leak starts to draw currents of 150 m/a or more in some cases.
On these cars a drain of under 50m/s is normal.

How to check on these cars will be in part 3

Cars with a wake up system

Car models brought out after 2000 have a wake up system namely the 203, 209, 211, 215, 219, 221, 230 and part of the 220.

On these cars it is possible that they can fail to go to sleep as the CAN B remains active, this will drain the battery in a short period, or to a point where the consumers are turned off when the battery falls below 11.75 volt.

All of the above cars should have a drain of no more than around 50ma, though in some cases it is impossible to get some cars below 65m/a

How to check on these cars will be in Part 4

 

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Part 2 Meters and their uses

It is said that when checking for leaks and drains in electronic circuits that the power source should never be broken as this can temporary stop the path of the leak, but the only way of checking to see if there is a leak is with the most expensive instrument of all the clamp meter. Until last 2010 they cost anything around several hundred pounds for one that measured milliamps in DC. There is one now available from RS components that resolve down to 1m/a for circa £46 but this is still very expensive for a one off fault. The advantage this meter has is that you just put the clamp on one of the battery terminal and you get an instant read out of any current flowing, without breaking any circuit to join it in. It comes with leads and a case; this meter can be used as a multi range meter.

This is it in action on my 230, the reading includes the auto boot lamp that consumes 15m/a for the auto boot light, and you may just see that it is reading 68m/a. It does help if you know a little about the car on test like the auto off boot lamp

To be honest there a very few faults where breaking the circuit causes the leak to go away for a time, so we can deal with normal ammeters both digital and analogue and the special very cheap to buy ammeters with a fuse blade plug.

If you are going to use a normal multirange meter, then this must be connected between the battery post and the large cable clamp that goes to the battery, it does not matter if you use the positive or negative terminal, but the negative is safer if you should touch a test lead to ground (chassis). In all they are not very convenient as if you pull a fuse from the front fuse box and the battery is in the rear, then you have to run round to check the current reading with every fuse checked.

Yes it is possible to take a fuse, cut the fuse part and solder leads to the 2 spade pins and use this on a normal multirange meter, but the fuse blades are very difficult to get any solder to take to the spade pins, but hardly worth the effort as the fuse blade ammeters are so cheap and much easier to use.

Multirange meters digital meters

The meter on the left cost circa £4 and will do the job. The center one is what I use most at £18. The one on the left is auto ranging, they are OK but far to slow to use for me at £27. Fine for those not sure of what they are doing, but take care that you read it correctly.

The fuse blade ammeter

The best value of all, and the most useful is this little fuse blade meter that read out current only, there is a 20 and 40 amp version costing from £5 to £10 from places like Maplins

Old Avo’s and movement meters

One can use the old type Avo’s, they are good for 10 amp and resolve very low but they are polarity conscious so the leads have to be the correct way round, also too bulky to place anywhere. As can be seen mine does not get used much but it is 60 years old but still has some uses.

 

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