[alfa] Batteries, charging and current draw

["John Fielding" <johnf@xxxxxxxxxxxxxxx>]

Hi All,

It seems the same discussion repeats itself every so often.

I am not familiar with the Optima brand of battery, as they are not sold here,
but my guess is that they are a "quasi-leisure" type of battery and may not be
very suitable for automotive applications.

Some years ago the company I worked for had a contract to supply battery
backup supplies to the military to power two-way radios.  We designed a float
charger using a well known battery manufacturers battery.  This turned out to
be a disaster as the battery was unsuitable for this application.  The battery
was claimed to be a type suitable for jump starting cars and could be float
charged indefinitely.  After many hours of consultation with the manufacturer
we were talked into using this particular battery in our design.  Shortly
afterwards the manufacturer pulled the battery off the market as it had such a
poor reputation in the automotive trade.  It was reintroduced a year or so
later under a different name as a conventional car battery!  We ended up
replacing over 2,000 batteries in the field with a revised battery type, and
expensive task.

What most readers do not seem to understand is that there are many different
types of lead-acid batteries in common use, some are applicable to automotive
duties and some are most definitely not intended for this duty.

The "Deep-Discharge" and Deep-Cycle" types belong in a family intended for the
leisure market.  These, although employing a lead-acid type of composition,
are mostly unsuitable for automotive duty.  These batteries were designed for
the caravan and allied uses market where a battery is required to supply a
limited amount of current for a limited period with long periods of not being
in service.  For example, a battery used to power 12V fluorescent lamps or a
portable television for a few hours and only recharged intermittently.  This
type of battery can be left in various states of charge or discharge and will
maintain a reasonable level of charge over long periods.  They are not
intended for high current discharge applications, but will accept a long
trickle charge without ill effect.

All batteries, regardless of the chemical construction, have an inherent self
discharge - some worse than others.  If the Nickel-Cadmium battery (used in
many power tools etc) is considered the self discharge rate can vary between
1% and as much as 3% per day.  So if the battery was placed in storage after a
full charge it can be regarded as completely discharged in as little as a
month or two.  Manufacturers of Ni-Cad batteries generally recommend storing
in a fully discharged state, although any state is acceptable in practice as
with a month or two it reach the discharged state!

The early types of lead-acid battery used in automotive service were
"open-construction" types with filler caps to top up the individual cells.
These types when charged showed "gassing" which is the generation of hydrogen
and oxygen, an inflammable mixture!  Later types of sealed construction have a
different charging regimen because of the potential loss of electrolyte if
over charged.  These sealed cell types use a different chemical composition
for the plates, often with Lead-Calcium being the major constituents.  These
subtle differences cause the cut-off voltage to change from the accepted norm
for automotive batteries.

GM/Delco some time ago issued a warning about the change over to sealed
"maintenance-free" batteries in which it advised that owners of vehicles when
fitted with the later battery type needed to change the alternator regulator
to deliver a higher cut-off voltage to allow an acceptable level of stored
charge.  The voltage difference isn't very much but it is quite significant to
the charge retention in the newer battery types.  Generally the cut-off
voltage needs to be raised about 0.4V by the adjustment of the regulator
parameters.  If the original alternator regulator is used the charge retention
will only be about 85% because of the lower cut-off voltage.  Similarly, if
the sealed type was subsequently replaced with the previous type the gassing
problem became severe and the regulator needed to be changed to the lower
cut-off voltage.  As long as the regulator is matched to the battery type all
should be OK.

All lead-acid batteries, even the "Deep-Cycle" types show an inherent self
discharge.  Older designs with vibration or excessive gassing could shake
loose some of the plate material and this fell to the bottom of the cell and
caused a "soft-short" across the plates which caused excessive internal
discharge, and left the owner with a flat battery.  The early Land Rover
series 1 when fitted with the diesel engine had a bad reputation for
destroying batteries due the excessive vibration.  This was cured by fitting a
thick rubber pad under each battery.  (The LR in the early days had 2 x 6V
batteries connected in series fitted under the front seats).

In general, the quiescent current in a stored vehicle should be less than
50mA, this is normally due to ancillaries such as clocks, alarm systems and
memory back-ups in the onboard computers.  The Bosch Motronic ECU fitted to
the Alfetta & Spider series draws as little as 2mA when the ignition is
switched off.  This is the back-up memory retention.  The main 12V to the ECU
is switched through the ignition switch, the "Keep-Alive-Power" is powered
directly off the battery.  50mA is about the expected inherent self discharge
level within the battery and should not cause any long term effects.
Disconnecting one pole of the battery will only isolate the ancillary current
draw and not the internal battery self discharge.

All rechargeable batteries are inherently inefficient.  To replace the lost
charge requires more energy to be put back into the battery than was
originally drawn out.  Typically a factor of 120 to 140% is considered the
norm.  If 10A is drawn for 1 hour then the total capacity depleted is 10AH, to
replace this requires the charging current to be equivalent to 14AH.  So a
current of 14A for 1 hour or a current of 1.4A for 10 hours would be needed to
replace the lost energy.  Generally, most rechargeable batteries are better at
accepting a charge of a low current over a long time than a high current over
a short time.  The chemical conversion needs time to fully reconvert the
positive and negative ions in the electrolyte and plate material.  This is a
very complex equation and things such as the specific gravity of the
electrolyte, cell temperature and the charger's source impedance and current
waveform all have a bearing on the rate of charge retention.

In the lead-acid battery now used in automotive applications the maximum
charge state is not normally achieved because of the gassing problem.  If the
battery is allowed to reach the gassing state, and maintained at the required
level for the final chemical conversion to occur, which requires a "top-off"
voltage of about 15.6V, then the possibility of 100% charge state can be
reached.  However, this is a big problem with the sealed type of construction.
If the gassing state is achieved the internal cell pressure rises and it could
cause the internal protective vent to open and vent some electrolyte.  This
depletes the electrolyte and releases the inflammable gasses, which are both
undesirable.  Hence, the best result we can achieve is about 95% charge level
and this is why today manufacturers fit slightly bigger capacity batteries to
offset the lower charge capability of the sealed types and to allow for
diminished capacity due to aging of the battery.

my 2c worth.

John
Durban
South Africa
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