The MPPT controller allows the PV array to operate at its maximum power point
which for example might be 8 amps at 16.4 volts. So the power output is 8*16.4 =
131 watts. The MPPT controller using some sort of power switching topology
transfers almost all of the power to the battery which is for example at 12.5
volts. The current at 131 watts and 12.5 volts is 10.5 amps. So its 8 amp in at
16.4 volts and 10.5 amps out at 12.5 volts - ie,. the power is maintained. If
you simply had connected the PV array to the battery (which is essentially what
most controllers do), the current output of the PV array would have remianed
constant but the voltage would have dropped to the battery voltage so the power
delivered would have been approx 8amps * 12.5 volts = 100 watts - ie,
significantly less than with the MPPT controller.

The one I have draws about 100 ma so it doesnt "break even" until its running
about .5 amps to the battery. MPPT contollers are also a fair amount more
expensive so must compete with simply adding more solar panel area. However, if
you want to keep the PV area as small as possible (like on a boat), it seems
like a good product.

Roger Derby wrote:

Where did the extra 2.5 amps come from?

Roger

http://home.earthlink.net/~derbyrm

"Walt" wrote in message ...
snip
... I have a 160 watt pannel (2x80w Kyocera) and
typically I might have 8 amps coming out of the panel and maybe 10.5 going
into the battery (the
currents will of course vary all over depending on how much sunlight,
angle, ect).