On 30 Jul 2006 at 9:21, Michael S Briggs wrote:
just trying to
learn more about what voltage a fully charged pack should maintain
The answer is - "it depends."
But for one example, let's take US Battery's golf car batteries. USBMC
tells
us that their float voltage for standby use is 2.17 volts per cell at 80
deg F,
with a temperature adjustment of -0.028vpc per 10 deg F (the sign means
that the voltage is lower as temperature rises). This is the voltage at
which
(theoretically) no current will flow into a fully charged cell or battery.
So that
might be considered the fully charged voltage for USBMC's products - 6.51
volts per 6v module, or 13.02 volts per 12v module. This is approximate.
Other products will be different, and age will also be a factor.
The problem is that fully-charged voltage depends on several factors,
including the design of the batteries, which varies from manufacturer to
manufacturer and even within lots; their age and condition; and the
batteries'
internal temperature.
This is one of the challenges in charging batteries for good service and
long
life, and the reason that absolute voltage isn't an ideal indicator for
use in
charge control.
The best, most reliable determinant of state of charge is a hydrometer.
Obviously this only works for flooded batteries. I've read that there are
optical
methods of reading SG and it makes me wonder whether anyone has yet
designed a charge control to use this method. (Anyone know?)
A lot of methods besides that and absolute voltage have been used for
charge control.
One pretty good one is to monitor amp-hours withdrawn, and replace exactly
that amount plus an amount for charging inefficiency. The drawback is
that
charge efficiency declines with battery age, so some means to compensate
for this has to be provided. Typically chargers which use this method also
use a second one, such as absolute voltage or DV/DT. By looking at the
difference between the methods' results, they try to estimate drift in CEF.
Another intriguing method was tried many years ago. It used closed cells
and monitored the gas pressure in the cells to determine when and how
much gassing was occuring, and thus regulate the charge. This is actually
where we get the term "valve regulated."
One of the easiest, most direct, and reliable methods is DV/DT - monitoring
the slope of the rise in on-charge voltage. When the *on charge* voltage
stops rising and/or the current stops falling (or the slope falls to some
very
small value), the battery is full, for all practical purposes. This
method works
with lead batteries of any design, any age, and any temperature. The only
concern is that, with VRR batteries, when they exceed the gassing voltage
the charger must limit the current so they don't produce more gas than they
can recombine. Most DV/DT chargers also monitor absolute voltage as a
backup method, and to make sure they don't sense a flat DV/DT too early,
during the bulk phase.
Amp-hour monitoring is uncommon (which is too bad). Valve regulation was
tried many years ago but AFAIK today it's almost never used. The only one
o these in significant use is DV/DT. It's implemented in Lester's
Lestronic
range of chargers. I believe Lester have a patent on it, though it may
have
expired by now.
But by and large absolute voltage alone, usually without temperature
compensation, is the most common means of charge control.
No, actually, that's not quite right; it's the most common where *any*
charge
control is used. The most common charge control is none at all. Often
you're lucky to get even a voltage regulated (taper) charger, and/or a
timer
like low-end golf car chargers and the GE. Charge control costs money and
raises the price of the consumer products. Although some high-end products
have decent charging, in most cases it's considered better business to
price
rechargeable appliances and such cheaply by throwing a low-bid Chinese
wall wart into the box. The only charge control is current limiting from
the
inherent resistance of its transformer and wiring. They let the user deal
with
the more frequent battery replacements later.
The good news is that flooded golf car batteries are pretty forgiving, and
the
ET doesn't stress them much because it's seldom run more than once or
twice a week. That's why the GE charger can abuse the batteries rather
greviously and they'll still last 8-10 years.
Does this help?
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