----- Original Message -----
From: <M35products@aol.com>
To: "Military Vehicles Mailing List" <mil-veh@mil-veh.org>
Sent: Monday, November 19, 2001 9:15 PM
Subject: Re: [MV] Wanted: Battery Guru's
> Some random rules about charging lead-acid batteries:
>
> 3. All diodes, regardless of their voltage or current rating, have an
> inherent voltage drop of 0.7 (zero-point-seven) volts. Whatever voltage
> setting that is available at the input (anode) is reduced by 0.7 volts at
the
> output leg (cathode) of these isolators.
>
We usually regard the forward diode voltage drop of 0.6V for ordinary
silicon devices, germanium devices show a lot lower drop but are virtually
extinct and do not come in high current form. Shottky and other modern
constructions may show a lot more. It is important to note that the forward
volt drop of a diode P-N junction is totally independent of current flowing
through it.
> 4. A maintenance charge of 1 to 3 amps should be sufficient to keep good
> batteries up to par, in temperate weather. 25 amps is way too much
current.
> You will cook your batteries. (You will never see a truck's ammeter at
that
> reading for more than a minute or two, if that long.) This current
> recommendation assumes that all parasitic loads are removed. These are
loads
> that are caused by moisture and insulation breakdown in the vehicle, along
> with devices such as alarms, clocks, etc. (Disconnect the positive cable,
in
> other words.)
>
We are missing the point here, the charge current is dependant on the
voltage of the source supplying it.
Both dynamos (DC generators) and alternators are controlled by voltage
regulators, usually externally for dynamos and internally for alternators
except very large ones purely for convenience.
Either type senses the battery voltage and adjusts it field winding current
(hence output voltage) to achieve its reference set, battery full charge
voltage, for a dynamo this is electro-mechanical (a very voltage sensitive
relay) and for the alternator an accurate electronic reference (zener diode
or band-gap reference).
For either type when it senses its terminal voltage (the battery) to be
below the reference setting, it increases its excitation (field current) to
compensate and this leads to charge current, the dynamo needs additional
current sensing to restrict its power output (watts or V x I) to safe design
limits. The alternator is current (hence power) limited by design, at full
output the internal magnetic field is saturated and you get what you get.
Increases in electrical load, turning on headlights for example, drop the
battery voltage which is immediately sensed by either charging machine type
which jacks its self up to compensate, thus supplying, or balancing, the
extra load and maintaining whatever charge current the battery wanted
previously.
> 5. Commercial chargers have a circuit that will limit, and ultimately
reduce
> to almost zero, the current presented to the battery. The internal
> resistance of a lead-acid battery increases with its charge condition,
which
> is a self-limiting feature. This is another reason not to try to use a
25-amp
> supply.
>
This is achieved by designing the charger to inherently supply only a few
hundred millivolts more than the maximum terminal volts of a float charging
battery. Its a voltage thing again, no matter what the current capability
of the "charger" at nearly full charge it will only deliver a few amps
because its voltage and that of the battery are very close.
To grossly overcharge, or supply 25A as is inferred above to a fully charged
battery, the supply would have to be set very high indeed. The danger with
heavy current capable chargers comes when using them on small batteries that
are very discharged and cannot stand this level of current.
An earlier posting suggested modern (low or no maintenance) batteries in
domestic cars being taken to 15V, this is way too high and I suspect the
meter used was seeing the ripple of the alternator or just reading high.
Modern gas recombination batteries usually are labelled that 14.5V is the
absolute maximum, at this level the battery is gassing to the maximum
capability of the recombination arrangement.
> 6. Automotive batteries lose half their potency at 32 degrees F. They
lose
> 3/4'ths their potency at 0 degrees F. While this fact has nothing to do
with
> charging, it is recommended to take the batteries indoors during extreme
low
> temps, to aid the charging process.
>
Important point.
> 9. Put on your safety glasses, when connecting, disconnecting, or messing
> around with, batteries, even if they do make you look like a nerd. Better
to
> be a seeing nerd than a blind tough guy.
>
Everyone should very aware that conventional lead acid batteries out gas at
around 13.8V (for a 12V battery or 2.3V/cell), it is independent of size
being a function of chemistry.
The bubbles given off are in a perfect ratio down to the molecular level of
2 hydrogen atoms and one oxygen atom. Any form of ignition near the cells
will have this combine to form a drop of water but with great violence, it
goes bang with surprising force.
I did this in ignorance in 1961 with a car size 6V battery whilst
disconnecting the still powered _domestic_ charger causing a small spark.
It blew out all four sides (luckily) like a card house immediately followed
by a lot of sulphuric acid which (also luckily) I was not in the way of.
I would not like to repeat this experience ever again and suggest its more
excitement than anyone would wish to handle.
Richard
Southampton - England
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