Re: (ET) Fully charged pack voltage

["David Roden" <etpost drmm net>]

It's important to make the distinction between open-circuit voltage 
(resting 
voltage, with nothing connected, 12 to 14 hours after the charge has been 
stopped) and on-charge voltage (the voltage with the charger connected).  

When you read that a battery should be charged to around 2.4 to 2.5 volts 
per cell, that is the on-charge voltage.  

However, that by itself is NOT enough information!  A battery isn't 
charged 
until it reaches the specified voltage AND the charge current has fallen 
to 
about 2% of the battery's 20-hour amp-hour capacity, expressed in amps.

For example, you should charge a 240 amp-hour golf car battery until its 
on-
charge voltage reaches around 7.35 volts (some say as much as 7.5 volts) 
and 
the current falls to 4.8 amps.

Another strategy is to charge the battery until it reaches some 
empirically 
derived threshold voltage, such as 2.3 or 2.4 volts per cell, then hold 
the 
current at about 2% of capacity until the voltage effectively stops rising 
 
(dv/dt charging).  This is what Lestronic golf car chargers do, though 
some 
of them keep the finish current too high (IMO), around 8 amps.

I suggest reading this document :

http://www.evdl.org/pages/hartcharge.html

Open-circuit voltage is an inaccurate indicator of state of charge, but 
it's 
all you have for sealed batteries.  If you're using flooded batteries, 
there's no reason to use voltage, because a hydrometer is much more 
accurate.

http://www.evdl.org/pages/socvolts.html

PS - I say again : in my experience, more batteries die from overcharging 
than from undercharging.


David Roden - Akron, Ohio, USA

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