Re: (ET) charger voltage

["David Roden (Akron OH USA)" <etpost drmm net>]

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One pretty good one is to monitor amp-hours withdrawn, and replace exactly 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?