En 
SvWhy is the charging performance so poor?
How voltage drop and cable dimensioning influence your battery charging performance
Lead-acid batteries are normally charged at a maximum of 14.4 V to avoid overcharging. Half-discharged lead-acid batteries have an open-circuit voltage of 12.3 V. A larger difference between the open-circuit voltage and the charging voltage allows the battery to be charged faster.
If you want to charge at 14.4 V and your batteries are half discharged, you get a difference of 2.1 V. If you have a battery isolator diode installed, there will be a voltage drop of 0.7 V, reducing the difference to 1.4 V.
If you also have undersized cables and poor connections, you can easily get an additional 0.4 V of voltage drop. That puts you down to a 1.0 V difference, which is less than half of 2.1 V. In other words, you only get half of the charging you could have had.
With better connections and cables, and without a diode-type battery isolator, many users can double their charging.
That’s why Skyllermarks has long been asking: “Who wouldn’t want double charging?”
