Saw on a youtube video, that an alternator will only charge a leisure
battery to 85%. What is 85% of 13.6 volts ? 11.56 ? I've not allowed my batteries to go that low.
Would this mean, there is nothing wrong with my vehicle charging, my batteries are already above the level the alternator would take over? All this is making my brain hurt.
that is not how
battery capacity is measured. and also that video has by the sound of it oversimplied things when you are talking leisure batteries and split-charge systems.
Ok. To fully charge a
battery, the voltage of the charger need to be typically around 14.4V. Many Alternators - and many standard Motorhome Mains Chargers for that matter - will not sustain or even reach 14.4V. (A Sargent integrated charger will typically put out a steady 13.8V). That is why an Alternator is unlikely to charge a
Battery fully - and why it is good practice to put your Starter
Battery on a charger on a fairly regular basis to give it a full charge and lengthen its life.
Here is a graph showing the voltage and current levels to charge. The End of ABS (Absorbtion) is when the
battery is deemed Full
Victron MPPT Charging Scheme by
David, on Flickr (top line V; bottom line A)
for just about any
battery people are likely to have (but some ARE different, so you must check for yourself), that voltage during ABS is 14.4V
So Alternator charging a Leisure
Battery.... Depends on the charger.
If using a basic relay system that just connects the Starter and Leisure Batteries together when the voltage hits a certain value or the D+ signal is live, then nope, that won't fully charge the
battery. Just the same situation as it charging a Starter
battery.
If using a decent B2B charger then YES, it will fully charge a Leisure
Battery if run long enough (if
battery is at 80% say and a 100Ah, and you have a 5 hour drive and a decent B2B charger, then you will be fully charged well before the journey is up). This is becuase the B2B will boost the voltage output to the required level - doesn't matter if the Alternator is putting out 13V - the B2V will boost the chargers output (NOT the alternator output) to 14.4V or whatever is required for the charging algorithm.
This is why a B2B at say 30A will provide a better charging solution than a 100A Relay