Originally Posted by babock
You need an MPPT type controller. You have a PWM controller. You wil lnever get 8A since the battery charges at between 13.6 and 14.5V. 7A is about what you will get if you aim your panel. You usually use 14.0V to determine the current. 100/14 = 7.14
Here's some interesting information (for the OP) found on the 'net that explains in more detail:
Q: How efficient is the MPPT solar charge controller over PWM (in percentage)?
A charging lead-acid battery limits the voltage of the charging source. A silicon solar panel produces a fixed current for a given brightness of sun. The mismatch between a fixed voltage and a fixed current is what an MPPT charge controller is designed to address.
Power is voltage times current. The current is constant, fixed by the brightness of the sun shining on the panels, so the maximum power is obtained at the maximum voltage. During the bulk charge a PWM controller simply connects the panels to the batteries. The power is therefore panel current x battery voltage.
However, the panel will still supply that fixed current at a higher voltage. The MPPT controller converts the voltage, giving the battery the voltage it expects while allowing the solar panel to provide current at a higher voltage. The power is the same (assuming the controller is 100% efficient, which it isn’t quite) but, as the battery voltage is lower, the current is higher and the battery charges more quickly.
To calculate the difference, calculate the loss using PWM as opposed to MPPT. This can be done by calculating the difference between peak panel voltage and battery charge voltage as a percentage.
Typical figures might be 18.5v for a “12v” panel, and anywhere between 13.6 and 14.4v for a “12v” lead-acid battery, depending on how full it is. So those figures might give:-
PWM loss, flat battery: 13.6v / 18.5v = 0.735 = 73.5%
PWM loss, mostly charged battery: 14.4v / 18.5v = 0.778 = 77.8%
Remember these are losses, and should be inverted to get gains.
MPPT gain, flat battery: 18.5v / 13.6v = 1.36 = 36%
MPPT gain, mostly charged battery: 18.5v / 14.4v = 1.28 = 28%
Note how the PWM controller becomes even less efficient as the battery reaches full charge.
The only time you will get max current from a PWM controller is as the above states, during the bulk phase when it's connected directly to the battery. It's also the time the voltage (measured at battery) will be at it's lowest.
Also, if you look at the data tag on the back of your panel there are two voltages and two amperage ratings. One is "Shorted" and the other "Working". To avoid disappointment, use the Working Output numbers.
And as babcock said, you need an MPPT controller. Better yet, add another panel and connect in series, feeding the higher voltage into the MPPT. I have a setup with two panels and you'd be surprised at how much charge current I read even when the sky is overcast. Not a ton but more than you'd expect, especially from a single panel.
When the sun's full I get enough to more than meet my daily "amp hour" needs.