Re: Battery Charge Time?

[Dave1massey@cs.com]

In a message dated 9/22/2005 2:16:36 PM Central Standard Time, 
tr3driver@comcast.net writes: 
> The exact "correct" value depends on battery temperature and construction.
> A quick search turned up this paper with a nice curve for sealed lead-acid
> ... flooded (wet) lead-acid will be similar but not exactly the same.
> http://www.actec.dk/SLA-charge.pdf
> 
There are chargers that use a three step charge cycle more or less covered in 
this text.  The first step is a constant current charge which produces the 
fastest recharge within the capabilities of the charger.  Once the battery 
voltage reaches 2.45 v/cell it reverts to the absorption mode or constant 
voltage 
mode where it remains until the current falls off to a threshold at which point 
the charger will reduce the voltage regulation point to 2.25 V/cell.  This 
type of charger is quite common in applications where batteries are the prime 
source of power such as golf carts and wheel chairs.  (see 
http://www.chargetek.com/ct500.html for just one example)  Most car hobbyists 
are unwilling to 
spend this kind of money for a charger.  What they get for their $29.95 is a 
transformer and a rectifier.  These are fine for injecting a partial charge 
sufficient to start a car after a few hours of charging but are no replacement 
for a 
proper battery charge management system.  And there is one on the car and it 
is called an alternator with a voltage regulator.

The battery tender I bought from Harbour Freight regulates at 13.5 volts 
(2.25V/cell).  The other tender I bought from Mouser regulates at 14.25 volts 
(2.38V/cell)  Either one of these can be left on the battery indefinitely but 
they 
have a max charge rate of 0.5A.  I use one of these on my TR6 when it goes 
idle for more than a few weeks at a time.

A good compromise is to get one of each (a battery charger and one of these 
tenders) and use the charger to partially recharge a dead battery and then 
switch over to the tender for topping up.

Dave