At 8:02 AM -0400 7/13/05, Dennis Haynes wrote:
>The capacity of the alternator has little effect on the rate of charge
>to a battery. The charge rate is determined by voltage. Since the
>alternator is regulated at a fixed voltage and does not compensate for
>battery state of charge or temperature, it is also a poor choice for
>really charging deep cycle batteries. Typically, at the 13.8 volts, a
>battery will charge at ~ 20-30 amp hours up to ~ 75% state of charge,
>then the rate will drop even lower. You really need to plan on 4 hours
>drive time to get batteries even close to fully charged.
>
>Dennis

I stand corrected.

I had forgotten that these alternators are usually only running at
13.8V. And it's true that at such a low voltage it's very hard to get
a fast charge.

As I understand it, this is where a AGM type battery will shine. The
do have very low internal resistance and so will charge much faster
at low voltages.

It is possible to turn up the voltage, and even implement real good
charge algorithms, but that get's pretty complicated (or expensive).
I remember now that I had turned my alternator up to 14.5V when I was
doing a lot of charging.  But that could damage a battery when it's
hot, so that's where it gets complicated.
--
  -Otmar-
82/86 Stretch Vanagon Westfalia GTI. "Power of two" Soon to be TDI.
http://www.evcl.com/vw
1 914 EV, 1 Insight Hybrid