Why do people use mechanical relays, switches, and solenoids as battery isolators?
Very simply, they're cheap and easy to understand, but in no way should they be considered a battery isolator for high current DC applications.
A very common, old-school way of creating battery isolation is to install a starter solenoid (relay) between two batteries, and have that solenoid close the connection between the two batteries when the vehicle is running. When the vehicle is shut off, the solenoid will open creating "isolated" batteries. This works great on paper, but if one of the batteries is deeply discharged, when the solenoid is closed, the current transfer from the "full" battery to the "low" battery can exhibit very high inrush current creating a possible fire danger if the wiring, plugs, and/or the solenoid overheat. It's not uncommon for solenoids, relays, and/or switches to become stuck when mechanical contacts effectively weld closed under high current, high arc'ing conditions. This can lead to a situation where the user thinks that the mechanical disconnect is open when in fact it is stuck closed and therefore, no disconnect is occurring.
Another serious problem is when the engine is stopped shortly after starting. The two batteries have been connected during the cranking, starting, and running process just long enough to cause the cranking battery to discharge into the auxiliary battery and now the vehicle will not restart. If the auxiliary battery is of a low voltage potential, the main battery WILL
discharge into the auxiliary and the engine may have to be operated for a considerable period of time before it may be safely turned off. Many an engine has failed to restart because of this problem.
The constant opening and closing of mechanical disconnects also lead to high resistance, high heat connections.
When contacts are new and flat, they yield low resistance transfer of current which means little heat dissipated. However, each time the contacts open and close, an arc is generated. The greater the current and/or difference in voltage, the larger the arc. In short order, the contacts are no longer perfectly flat and are instead pitted and deformed. What was an efficient transfer of current has become poor conductor of current and the additional resistance yields heat which will be the ultimate demise of the mechanical connection. It's not a matter of if it will occur, it's a matter of absolute deterioration over time guaranteed to end in thermal failure.
MOSFET-based battery isolators, specifically the patented POWER-GATE single rectifiers and dual rectifiers have no moving or mechanical parts. There is no arc'ing and no deterioration in performance over time. Unlike typical silicon and Schottky diode isolators or rectifiers, POWER-GATE devices achieve ultra-low resistance....we invented and patented the technology to achieve the lowest resistance devices in the marketplace. There is no such thing as a "no resistance" isolator and until someone figures out how to defy the laws of physics, when it comes to advanced, solid state battery isolator technology, POWER-GATE is the ultimate choice for high reliability systems.
