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There it sits, unwilling and unable. All its advanced engineering and high
performance are absolutely and totally defeated by the failure of one of its
most basic components -- the battery. Without this reservoir of electrical
potential, the best car in the world is as immobile as your average tree, as
useful as a piece of metal sculpture. True, it's been proved that bad plugs are
the most common cause of no-starts, but battery problems are a close second.
This automotive prime mover operates on the same electro-chemical principles
it always has, but its design and testing procedures have changed in several
ways over the last couple of decades. One battery maker claims that only half of
its returned units are actually bad, so its important that you understand
current (no pun intended) troubleshooting.
Prelims
Before you jump to the conclusion that the battery itself is the cause of a
no-crank or slow-crank condition, make sure its cables and connections (emergency
clamps are, after all, just that) are in decent shape and that the fusible link
isn't blown. Then there's the starter (is it drawing an inordinate number of
amps?) and the charging system (is the belt okay, and is voltage being produced?),
both of which are covered in other sections. Next, look for cracks in the
battery case and observe the liquid level in all the cells.
Underweight?
Your great-grandfather probably determined the health of a battery by "weighing"
the electrolyte with a hydrometer, then comparing the readings of the individual
cells. Since sulfuric acid is 1.835 times as heavy as water, you can tell how
much is present, hence how strong the charge is, by measuring the specific
gravity of the water/acid mixture. At 80 deg. F., a reading of 1.260-1.270 (subtract
.004 for every 10 deg. below 80) corresponds to the proportions that constitute
a full charge -- 24% acid by volume, 35% by weight. If the float only rises to,
say, a piddling 1.120, enough acid has turned to water to render the battery
deceased for all practical purposes. Charge it (I'll get to that subject shortly)
and try again. If any cell reads .050 less than its mates, and doesn't shape up
after another blast on the charger, replacement is the only remedy. By the way,
hydrometer testing isn't accurate immediately after adding water, if there's
been a recent heavy discharge, and during charging.
This is still a valuable check, but many of today's batteries don't allow you
to do it because they're sealed (I've seen some butchered, even drilled tops,
which were the result of somebody's refusal to accept this situation).
Maybe, maybe not
Checking the voltage across the posts is simple and fast, but just because
you get a reasonable reading doesn't mean the unit has the ability to supply a
useful amount of amperage. To explain, if your meter shows less than the 12.6
volts that indicates full charge (it can even be slightly higher -- say, 12.8 --
just after being rejuvenated), two conditions may be present: All the cells have
a low charge, or five have 2.1 and the last has something less, maybe 1.7, so
you see 12.2 volts. Using a hydrometer would let you find out if there's a weak
link, but how about those sealed units?
Load mode
Well, a careful capacity or load test is the only answer. In fact, even in
cases where you were able to measure specific gravity and found it okay, but are
dubious about the battery's capacity because of its age or suspicious symptoms,
this procedure is necessary. The way it gets right down to the truth makes a
professional-quality VAT (Volt/Amp Tester) attractive, although it's too
expensive for the typical do-it-yourselfer.
Don't even bother with this if you're not going to do it accurately and
completely. First, there's the side-terminal connection problem. Don't just use
a bolt because the thread contact area is too small to carry enough current for
either load testing or charging. Either screw in an adapter or, if one's not
handy, a 3/8 in. coarse bolt with a nut on it. Bottom out the bolt, back it off
a turn, then tighten the nut against the contact and attach the lead to the nut.
Chargification
Then there's the question of charging. Those cells have to be right up there
before doing a capacity test. And charging is actually part of the testing. If
the battery seems not to accept this infusion of electrons, you might jump to
the conclusion that it's bad. You might be wrong. The problem is that the
electrolyte in a really low battery is mostly water, which is a poor conductor,
so initial current flow will be minuscule. It can be a long time before enough
acid is present to allow a decent amount of amperage to move. A typical charger
that provides 16 volts will have to run for four hours before the ammeter will
start to register. A cold battery will be reluctant to take on juice, too.
Volt and amp readings during the charge can give you a rough, preliminary
indication of the battery's condition. The combination of low volts and high
amps suggests an internal short, while the opposite situation should make you
think sulfation of the plates or cold electrolyte (reduce the rate and charge
longer). If both the volts and amps are low, perhaps the unit's totally
discharged, very cold or frozen, or maybe you've made a poor connection.
Another possible impediment is the polarity protection circuit present in
some chargers. A completely dead battery won't have enough voltage to activate
this feature, so it will appear that it's unchargeable when it's really just run
down. You'll have to follow the manufacturer's recommendation on how to override
this circuit and force electrons to start moving.
The Battery Council International says you should never load test a
sealed-top battery if it's temperature is below 60 deg. F. So, if it's cold out,
you'll have to bring the unit inside for several hours.
Step by step by step . . .
Here's the proper procedure for a capacity test (also known as a variable
load or high rate discharge test), for which you can use a VAT, or a separate
voltmeter, ammeter, and variable resistor or carbon pile:
- If you've charged the battery recently, remove the surface charge by
applying a 150 amp load for 15 seconds, or by disabling the ignition and
cranking the engine for that length of time (it'll dissipate by itself, but
that'll take two hours).
- With the load off, connect your tester to the battery. An inductive pickup
must surround all the wires from the negative terminal.
- Turn the amp and volt knobs to the appropriate settings, and adjust the
temperature scale, if present.
- Adjust the carbon pile until the ammeter reads 1/2 the CCA rating of the
battery. If you can't find that number anywhere, use the car maker's
recommendation. Or, you can multiply the amp hour rating by three.
- Apply this load for 15 seconds (some testers have a convenient timer
button for this), then, with the load still on, note the voltmeter reading
and turn off the load.
- At 70 deg. F., the voltage shouldn't have dropped below 9.6 (or, 9.1 at 30
deg., 8.5 at zero). And here's where some judgment comes in. If the reading
exceeded the spec by a volt or more, it's got some safety margin. On the
other hand, if it's right on, it may not have the reserve necessary to do
the job in tough conditions. Keep in mind that this varies with the state of
charge you determined with the hydrometer. If the battery was only at 75%,
yet it met the spec, it's probably in good shape.
- If the voltage was below the temperature-corrected minimum, continue to
watch the voltmeter after removing the load. If it rises above 12.4, the
battery is bad -- it can hold a charge, but has insufficient CCA. Recharge
and do the test again if you want, but there's little hope. In cases where
it doesn't reach 12.4, a low charge is probably the only trouble. Feed it
another good dose of juice, then retest.
Since you probably don't have access to a VAT, you can approximate this test
in a quick and dirty way with just a voltmeter across the battery. Disable the
ignition (traditionally, of course, this was done by removing the coil wire from
the cap and grounding it, but with plenty of modern electronic systems you'll
have to find out which wire must be disconnected to stop the primary voltage
feed, or you might want to kill fuel
injection instead), and crank the engine for 15 seconds. If voltage doesn't
dip below 9.6 (again, corrected for temperature), the battery's probably okay.
A safety note: According to the National Society to Prevent Blindness, 14,238
Americans suffered serious eye damage from wet cell batteries in a recent year.
Wear your goggles or glasses!
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