A battery that has just been on a charger or come in from driving may
well read 13.8 volts, but it will settle down over a period of hours. A
brief load helps it settle faster.
The article points out that it's hard to know the best voltage for an
automobile regulator. They all seem to be temperature compensated these
days, providing more voltage for a cooler battery. That helps.
A battery used for for a couple of 10-minute drives a day would probably
last longer with more charging voltage. A battery on the interstate 10
hours a day would probably last longer with less voltage. How can a car
manufacturer predict your driving habits?
I once owned a charger with a switch for conventional or
maintenance-free batteries. The maintenance-free does better with more
On Sunday, May 3, 2015 at 4:44:08 AM UTC-4, J Burns wrote:
Exactly. A fully charged batter is around the 12.6V that you state.
When it's being charged or right after it's been charged, then it will
be temporarily higher, but that higher level is temporary and not
necessary for it to be fully charged. To bleed off that excess, you can
just put on the headlights for a minute, then measure the voltage.
BMW has a solution. Their new cars now have some sophisticated
charging/monitoring system that closely controls the battery. A
real nice feature of this for the consumer is that if you replace
the battery, you have to go to the dealer to have the new battery
registered into the car computer. I'm sure this is really about
making those batteries work better and not making money for the
stealerships, by leading you to them to buy their batteries, right?
I like that! I haven't even been able to find out if my Walmart battery
is low-maintenance or maintenance-free. The cell caps can be removed,
but that doesn't prove it's not maintenance-free.
I'm very curious about the specifics because after charging, it may take
48 hours to come down to a "proper" voltage.
The only "maintenance free" battery is one you cannot maintain. If you
can remove the caps it is possible to maintain the battery - so it
will be a "low maintenance" battery by definition.
It's called a "surface charge". After charging a battery to a full
charge, the voltage will often be higher than the "chemically
dependent" charge - the voltage predicted by the chemical reaction -
but with a light load the "surface charge " will dissappear and the
true battery voltage will be evident.
The true voltage will vary depending on the acid strength used in the
battery - batteried for cold climates generally start with a higher SG
than batteries for a warm climate because to reduce self discharge in
hot climates they start out with a lower SG. This means the battery
sold for use in the Kalahari desert will have a lower voltage than a
battery sold for use in Ottawa, Ankorage, or Nunavit
On 5/3/15 5:58 PM, email@example.com wrote:
Thanks. I have a spreadsheet from batteryfaq.org. It's in fahrenheit.
80F is close to 25C. It says the SG at 80F for sb/sb or sb/ca (low
maintenance) is 1.265. I wonder how widely their figure applies.
One article about stratification says the watery mix at the top causes
corrosion and the concentrated mix at the bottom causes sulphation.
Another article says it's the other way around. It sounds as if a
certain SG is a happy medium between corrosion and sulphation. I wonder
if it varies with temperature. I wonder if it varies with plate composition.
If batteries were really, really simple, maybe some day I could understand!
I've read that the way to tell if a battery is really charged is to see
when the SG stops rising. As it's sampled at the top, that means it's
Darn, I'm going to end up dripping acid on my pants!
Many car makers have this now. There is an article in "QST" magazine about
them and electrically installing ham radios properly. It's not just
connecting a wire to the positive... Both positive and negative sides must
be fused. I wonder how many alarm / radio shops know this?
In the aircraft world they are referred to as a 14 volt system, with
12 cell battery systems referred to as 28 volt.
As for the chargers - a "decent"charger limits the charge current as
well as the voltage, so a 2 amp charger will not put out over 2 amps
into the battery for trickle charging and will limit the voltage to
14.4 so it will never boil a battery dry.. Most chargers out there
fall short of "decent" but there are more and more "microprocessor
controlled" 3 stage chargers on the market for approaching an
Some will test the battery for shorted cells, indicate the failure,
and refuse to charge a defective battery.
On 5/3/15 9:06 AM, firstname.lastname@example.org wrote:
I imagine that's because automobile systems were developed to power a
starter from a battery, and, in the days of generators, you could be
down to 12 volts on the road.
Aircraft electrical systems were designed primarily to power equipment
aloft, without much variation in engine speed.
And the avionics need to be designed to function optimally at the
prevailing operating voltage. Something designed with a 12 volt
maximum voltage wouldn't last long at 14 - and in the aircraft world
they are nothing if not precise when it comes to specs.
Automobile owners, if told their system was a 14 volt system, would be
all bent out of shape to find the resting state voltage to be only
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