The most common issue is that many of the split relays on the market are designed to trickle charge the system battery.
Now with any charger that is on a 'trickle charge' setting, its going to take a while to charge a 110ah battery, especially if your work is very local to you!
Now, the split relay kicks into life once it reads a certain voltage from the vehicle battery, thus slowly filtering charge through to the system. Of course, the less you drive, the less the trickle of charge to the battery!
On top of this, many new vehicles have smart ECU's which will essentially turn the alternator off when its not needed. This clearly gives an issue when it comes to a split relay - as this means the split relay will hardly ever kick into life, as it will never read enough voltage.
Thats where the SmartRelay split relays come into play - as they are not set at one particular voltage reading to charge, and work with the ECU to ensure your system gets charged when required. It always puts the vehicle battery first, so you don't have to worry about a vehicle battery failure due to the system drawing all the juice!
The other thing to mention is these systems are rapid charge - we have not yet had a SmartRelay customer who has had to charge his system battery!