Battery, Charging and Starting diagnosis

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Battery, Charging and Starting diagnosis

Post by PaulW » Sun Mar 15, 2009 1:50 am

From a practical point of view it is useful to know that the battery terminal voltage under various conditions can tell us almost everything we need to know about the state of the battery and charging system so accurately measuring the terminal voltage is key to fault finding in this area.

A cheap digital multimeter the like of which is available in Maplins or at most motor factors is the perfect tool for the job. Set it to a voltage range of around 20 volts DC if it has manual ranging, ensure that the test leads are inserted in the connectors marked "common" and "V" and connect them across the battery terminals, ideally with crocodile clips so you don't have to hold them conencted while you perform the tests. DO NOT connect a multimeter on the Amps range across a car battery, and make sure the test leads are not left in the "common" and "A" connectors.

Battery condition and charging circuit

Obviously the circumstances in which you find yourself with a starting or charging problem can vary, but in an ideal situation, when faced with a problem, I like to take the following sequence of actions to diagnose the fault:

1) Try to charge the battery, ideally using a mains powered battery charger for a couple of hours, to ensure it is likely to be fully charged.
2) Allow the battery to settle for perhaps 15 minutes after charging so the terminal voltage reflecs its' true condition.
3) Measure the terminal voltage of the battery.
4) Apply some load (e.g. car headlights) and watch the terminal voltage for a minute or two.
5) Switch off all loads, start the engine, and, ideally, have someone watching the battery voltage while cranking the engine.
6) Measure the terminal voltage with engine at a normal idling speed.
7) Increase engine speed to a fast idle (2,000 RPM) and measure the voltage.
8) Add some electrical load (headlights, blower fan, heated rear window) and measure the voltage at idle speed.
9) Increase engine speed to a fast idle (2,000 RPM) and measure the voltage.
10) Check electrical connections to battery, starter and alternator and also all chassis ground connections in the engine bay for heat.

I interpret the readings taken above as follows:

3) Normal reading would be 12.5 - 12.8 volts indicating a healthy battery fully charged. Suspect battery if it has failed to reach and stabilise at this voltage after a charge.

4) Expect the voltage to drop to perhaps 12.2 - 12.5 volts due to internal resistance but to remain stable for a minute or two. If voltage continues to gradually decline below this level, and certainly if it reduces to 12.0 volts or below within a minute or two, and was previously fully charged, suspect battery.

5) While cranking, battery voltage will fall significantly. If it falls below 10 volts or so when battery was previously fully charged, I'd suspect the battery. If it's also cranking the engine rather slower than usual, it adds more weight to this diagnosis.

6) Expect 13.5 - 14.5 volts. Greater than 14.5 volts indicates battery is being overcharged (voltage regulator failed within alternator). If it's significantly greater than 15 volts do not drive the car until resolved as the battery could be damaged or even explode and other electrical accessories in the car may be destroyed.

7) If the voltage didn't make it to at least 14 volts when idling it should do now otherwise alternator suspect, or perhaps wiring from alternator to battery or chassis and engine block to battery negative terminal connections. Should remain under 14.5 volts, as before.

8) Expect perhaps 13.0 - 14.5 volts. voltage may well have dropped due to load on the alternator.

9) Expect 13.5 - 14.5 volts. Most of the drop with load in the previous step should recover at normal cruising RPM. If not, suspect alternator or wiring fault.

10) Electrical conductors may become slightly warm to touch with electrical load, but any conductors, or particularly crimped connectons, that become hot to the touch have a high resistance (loose crimp perhaps?) and are reducing the effectiveness of the charging system. It doesn't take much voltage drop before the battery is not being adequately charged so rectify poor connections before they leave you stranded.

It may not be practical to perform the whole sequence above if you are stranded in a rainy layby. However, the battery terminal voltage will still give you some vital clues as to what's going on:

Engine stopped
No load: < 12.5 volts : Battery not fully charged or defective
Dropping to < 12 volts under load : Battery flat or, most likely, defective

Engine Running:

< 13.8 volts: Battery not being charged adequately. Ideally expect to see 14.0-14.5 volts at cruising RPM with normal levels of electrical load

Quescent (standing) Current Draw:

There is one other failure mode to consider if the above tests confirm that the starting and charging systems are healthy but there is still a starting problem: drainage of the battery while parked through excessive Quiescent (standing) current consumption of the car's electrical system.

Measure the terminal voltage after the car has been parked for long enough to cause poor starting following steps 3 and 4 above. If the measurements indicate a poor state of charge, yet the above tests have confirmed the charging system to be performing normally and the battery to be in good condition, it is possible that a load is draining the battery while parked. This can be measured by carefully measuring the current draw of the vehicle when stopped.

Before carrying out this measurement ensure that you have codes for audio equipment in the car and take steps to disable the power sounder as disconnection of the battery is required.

Ensure all electrical loads in the car are switched off and that the doors are unlocked but closed. Disconnect the battery negative terminal. Set the multimeter to the highest DC Amps range (10 or 20 amps if possible), connect the test leads to the appropriate sockets on the meter for current measurement and connect the positive lead to the battery clamp that you removed from the negative terminal and the negative lead to the negative terminal of the battery. If the multimeter indicates an overload or over-range condition immediately disconnect it and check that everything is switched off. Ensure that the loose terminal doesn't short to the battery terminals or vehicle components while testing.

The current should settle quickly to a low value of less than 100mA (0.1 A) and you might need to switch the meter to a lower current range to get an accurate reading. You may need to wait 5 minutes or so to allow the accessory timer relay to switch off before you achieve a final reading. When the current has settled it should typically be around 70 mA (0.07A) or less. The actual figure will depend on the specification level of the car and the alarm and audio equipment fitted. DO NOT attempt to switch on any electrical loads while the meter is connected as the car battery is able to deliver enough current to destroy the multimeter, or at least to blow its' internal fuse. When the measurement is finished, switch the meter back to a "Volts" range and remove the test leads so you cannot accidentally connect it across the battery on an "Amps" range.

If you find that you have a problem with this so-called "quiescent" current reading, you will need to go through a process of determining which circuits are drawing the excess current and eventually narrow it down to a faulty component. The obvious things are boot lights that don't extinguish when you close the door but other known culprits are sticking accessory timer relays and defective power sounders where the internal batteries have leaked. Repeat the above measurement with a suspect device disconnected. Alternatively, devices are available that allow a fuse to be removed and the current through its' circuit measured and this can help to narrow down a quiescent current draw problem to a single circuit in the vehicle. Failing that, you can always remove fuses in turn until you find one that is passing the extra current. Bear in mind that some systems on a car will take a few minutes for their consumption to settle down after the battery is connected, and if you have opened the doors you will have extra current drain from the interior light and accessory timer.
All the above comments are deemed an opinion, and in most cases :spam: