On Dec 21, 5:31*pm, "D. Dalton" wrote:
I installed a new battery in my car just two years ago. That seems a bit
recent for a battery to give out...
Is there an easy way to determine which is giving problems: the battery or
the charging system?

Using a DVM on voltage range...
1 - Engine running at 2000rpm. Battery voltage should be 14.3V re
alternator voltage regulator.
2 - Engine off. Battery voltage should be 12.65-12.45V, 12.24V means
50% charge level & nearing end of life.

Many alarm systems & immobilisers fail progressively over years
drawing more current. This is particularly true of factory alarms
which interlink with the door locks, you get a drain of 180-420mA
which is far above the ideal limit of 30mA (0.03A).

Using a DVM on 2A current range (fuse protected)...
1 - Engine off. Remove negative battery terminal. Clamp one DVM lead
to the negative battery terminal. Touch the battery terminal to the
battery negative terminal. Press any keyfob to disable the alarm which
auto-arms on power up. Clamp the other DVM lead to the negative
battery terminal. Remove the battery terminal BUT keep one DVM lead
connected to it AND one DVM lead connected to the battery terminal.
This slightly long winded process is required to avoid the "alarm auto-
arm" and bonnet-open and ECU/Radio powerup initial current surge
blowing the 2A fuse.
2 - You can now read the car battery parasitic drain - which is
typically 30mA or less. If the figure is higher, such as 123mA that
will take most car batteries to 50% capacity within a weak, and in
very low temperatures starting may difficult or even impossible, and
over time the battery aging will result in "weak battery" at about
2yrs.

One unfortunate problem is the drain may be 30mA, but after some time
go higher. A logging DVM ammeter capability can be handy to spot this,
suddenly spiking up to 200mA after a while. Door interlock links are
notorious, it can be necessary to disconnect any link (usually a plug
into a box under the passenger footwell) between the door-lock module
and alarm module. The alarm & powerlock remain operational, but the
link is removed.

Some alarms on battery reconnect auto-powerlock the doors, flash
hazard lights & turn on interior lights so even the 10A DVM current
range will not be sufficient hence the need to do step #1 as above.
Otherwise you can fry a meter quickly. For this reason the OP may want
to pick up a cheap £5-10 DVM from HK-UK off Ebay. They are quite
adequate, just never use them with Mains because the Category rating
is usually low or rubbish (£5-10 meters can and do explode if they
carry a mains fault despite HRC fuses, posh ones just explode your
wallet).

If the battery is an all-black exide unit the 2009 & 2010 winters have
probably stuffed it, it was very brutal on car batteries. You can pick
up Bosch Silver (S3 S4 S5?) units on Ebay or online quite cheaply. The
generic car shop batteries are mean to last the warranty and a day. A
high current drain will kill any battery, as will a defective diode in
the alternator or defective alternator, likewise a courtesy door
switch in the glove box, boot, door, or moisture condensation causing
a low drain in same. Worth giving the door switches a good wiggle
every so often, even remove & electrical silicone grease.

For the most basic battery charger, time (hours) is capacity (Ah)
divided by charge current (Amps). So a 90Ah battery charged at say 4A
needs about a day on charge. You want to get up to about 75%. Of
course, the battery may be too damaged. Ctek XS4003 or MXS4.3 or
whatever the current model designation is a good smart charger with
LEDs to tell you what it is doing and hilariously when it is
blindingly obvious the battery is utterly stuffed (12.08V after a
charge, stinks of hydrogen sulphide and an on car voltage of 0.00V
when asked to light the door lights).

Beware shorting positive to body when working on the battery, things
will get hot & spark. Remove watches, watch spanners.

Oh, another cause - bad battery terminal & car-body ground corrosion.
A corroded battery negative wire & terminal will reduce charging over
time, even creating occasional no-start. The battery terminal clamps
with two pozi drive screws are crap, use a proper stud with crimped
lug and nut tightened down on it. Very good way of killing a
battery... every 1-2yrs and having a variable poor start based on
temperature & vibration.