Interesting, i have now tried to charge with this method and unsure of my results (i have no meter to read volts etc...)
I knew my main battery was low as i struggle to start the engine. I put the trickle charger on and after a few minutes the light on the split relay came on?? I was presuming that it would take a few days before it charged the main battery then switch over. However what i found strange was the light on the relay was a bit dull, not as bright as it would be while charging through the alternator. I think the trickle charger i used only buts in 4A so maybe this is why.???