Weinstall a lot of the B&C standby alternators. The advantage is that you can adjust the output voltage as it has an external regulator that is adjustable. The disadvantage is that you have to mount the regulator.
To me it is well worth it, especially for the lithium-ion and odyssey batteries as you can adjust the output to better match the battery.
Most of the standby alternators are set to come on around 13 volts, so you really don't have to install a switch. They will begin to carry the load after the main alternator fails and the bus voltage drops below 13 volts. Initially you will think the standby is not working, as amps will read zero for a quite a while after the main alternator fails (or you turn it off for a test). Also, the standby alternators will not output anything until you are around 1500 RPMS, so again there is a chance to be fooled if you don't wait for the bus voltage to drop.
My preference is to set the standby voltage around 13.8 and the primary alternator around 14.2-14.4.
Vic