If you want to dramatically see the affect of wire size on voltage drop between you converter/charger and your battery(ies) during recharge, use a four-place digital voltmeter to measure the voltage at your converter's/charger's terminals while it's charging your battery(ies), and then use the same voltmeter to measure voltage on your battery's(ies's) terminals.
The voltages won't be the same ... but ideally they should be the same for fastest charging for any type of charger/converter you might be using. The larger the wire size the less difference there will be in the voltage at the two locations while the battery(ies) is(are) being charged. Increasing the wire size will seem almost "magical" in how it speeds up the charging process.