Re: voltage low on car battery

On Feb 5, 2:08 am, cl...@xxxxxxxxxxxx wrote:
On Wed, 4 Feb 2009 15:18:43 -0500, hern...@xxxxxxxxx (Herb and Eneva)
wrote:

 Some chargers do not drop off completly    If your charger does not
have the special circut it will just keep on charging and charging and
charging.

What is the CHARGING voltage at 2.5 amps after several hours?

There is so much misinformation in this thread (about lead acid
storage batteries as used in autos) it's hard to know where to start.

A typical auto battery these days has six cells each, nominally, 2
volts.

Hence such systems are called 12 volts systems. Actually 14 to around
11.8 volts!*

Some boats, heavy trucks and some military vehicles and some aircraft
have 24 volt (and occasionally 32 volt) systems. Back in the 1940s six
volt systems were common. Although European vehicles, generally, have
always been 12 volt. But leave that aside.

But the voltage across each cell, also assuming they are all in good
and equal condition and therefore the whole battery, will vary over
fairly narrow limits.

Measuring the voltage of standing battery without load is not
necessarily a good indication of the state of the battery.

You cannot say something such as; 14 volts = 100%y charged, 13 volts =
75% charged, 12 volts = 50% charged ........... etc. And unlike a
propane tank/cylinder you can't weigh the battery to see how much is
in it! :-)

Also one bad cell in the series of six can spoil the battery. The bad
cell (or cells) either can't pass the electric current through, or is
itself sufficiently deteriorated to not output (i.e. turn chemicals
back into electrical current). Low temperatures can aggravate the
ability of the battery to reproduce the electrcity that has been
stored in it.

High temperatures including overcharging** can also cause damage and
cause lead sulphation. That's when the battery cells start losing
their ability to store electrcity chemically and reconvert it back to
electrcity when demanded!

Given that voltage is not an indication of how much capacity the
battery has remaining nor does it indicate the capability of the
battery to do the heaviest job of all 'Cold Cranking' of a cold engine
in winter.

Measuring voltage with of a good or bad battery outside the vehicle is
not a good indicator. .

Any battery will most likely show on a voltmeter alone (Which draws a
few thousandths of an amp) something over 12 volts immediately after
being taken off the charger or immediately after the auto engine
engine stops!

However with a reasonably accurate voltmeter it is is possible to
measure the battery in operation in the vehicle and get some idea of
a) Is it being charged b) Is it capable of providing small amounts of
electrcity and also regulate the output of the engine alternator. c)
Crank the engine to start it.

a) Maximum voltage should be around 2.3 to 2.4 volts per cell. So 6 x
2.35 = 14.1 volts. So charging voltage should be around 14 volts.
b) A reasonably charged battery with engine idling but not actually
charge will probably be around 6 x 2.0 to 2.15 volts per cell. So that
around 12 to 13 volts. urn on a few lights, with engine idling a good
battery should maintain 12 volts.
c) Cranking; this is not so much a matter of voltage. Not only will
the battery voltage drop as the battery puts out, for very short time,
the up to 100 amperes of current needed to operate the starting motor,
there will also be voltage drops in the wiring, the starter switch and
starter solenoid/relay etc. The voltage might drop to say 8* volts and
then recover immediately the internal combustion engine starts and
starter disengages.

Here are some practical voltages for lead acid cells used in large
installations where the cost is many thousands of dollars per battery
string and they must power critical systems, sometimes for 8 up to 24
hours.

A) Recharging: 2.3 to 2.4 volts per cell. (May depend on slight
differences and whether North America or European administration).

B) Floating: In service but neither charging of discharging, just
waiting with a full reserve, 2.15 to 2.17 volts per cell.

C) Discharging: For very long periods while battery supplies 'all it's
got', 2.0 volts per cell.

D) As the last percentage of the battery capacity is used the voltage
will then tail off rapidly; how quickly depending on the load at that
point. Lights will be dimming etc. voltage will drop to 11.0 volts and
below.

The six cell (auto service) equivalents for these are;
A) 13.8 to 14.4 volts.
B) 12.9 to 13.0 volts.
C) 12 volts
D) Voltage tails off .............. nothing left.

If battery is charged normally for a normal time period (not
overcharged) then allowed to stand for say 10 to 24 hours and then
shows around 11.3 volts it is most likely useless!

However a friend who is a mechanic took an old uncharged car battery
out of a scapped vehicle to his to his cabin and hooked it up to an
old car radio, probably quarter of an amp? And was surprised that it
had enough chemical action to run the radio for most of the summer!

As Herb says above, some chargers will automatically time or adjust
the charging. Others will just grind away (for a week or more) pushing
current into an already fully charged battery. That merely heats up
the battery and if severe can cause battery to sulphate; which may/
will reduce it's life and usefulness.

One can charge a dead battery for ever and it will not come back to
life. Strengthening the acid, unless some has been spilt is not
advisable, it won't make battery any 'stronger'. Water may/will
evaporate, especially in heat and depending on type of battery may
need to be 'topped up', with chemically pure-clean distilled water.
.

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