**How do I know what size of relay and cabling to install with a SmartBank for any particular installation ?**

A direct question from a boatbuilder.

You must read this entire section before making your decision. The calculations develop as the page progresses.

All you really need to do is work out the maximum possible charge or discharge current passing through the relay at any one time.

For instance if there is one 70 amp alternator and no other charge source then clearly an 80 amp relay will be sufficient. It is obviously not possible for there to be a higher charge rate than this. So the relay and cabling will be chosen to safely pass this current.

Let's say there is a 70 amp alternator and a 50 amp mains powered charger. In this case it is possible that both would be on at the same time. This means there would be a total of 120 amps charge available. Any relay rated higher than this would therefore be sufficient. Naturally the cabling will be chosen to match this current rating.

There is a 55 amp alternator connected to the domestic bank and a 50 amp mains powered charger also connected to the same bank running from the on board generator. This comes to 105 amps so this would indicate a relay of say 120 amps (and appropriate cabling). However if you can be sure that you would never be running the engine and the generator at the same time then obviously the maximum charge rate will be 55 amps in which case a relay (and cabling) of say 70 amps would be sufficient.

Now let's get more clever. There is a 60 amp mains powered charger connected to the domestic bank then a 300 watt wind generator and a 55 amp alternator connected to the engine battery.

Added up this comes to 135 amps available. But wait..... these currents cannot be going through the relay at the same time. Assume the engine battery is fully charged so that all the charge current being provided by the wind generator and the engine alternator are passing through the relay to the domestic bank. This totals 75 amps maximum. The other charger is connected to the domestic bank and it's charge will not pass through the relay so in actual fact an 80 amp relay (and cabling) is sufficient in this case.

The rule is "what is the total amps available at ANY ONE SIDE of the relay AT ANY ONE time?". Add these up, then choose the appropriate sized relay.

There is one other thing to consider here. Assume a 200 amp charger connected to the domestic bank and a 70 amp alternator connected to a single 100Ah engine battery. As these charge sources are on opposite sides of the relay we do not need to add them together. Simply select the biggest one right ?

Well not quite.......... Although 200 amps is available at the domestic bank, even if we assume the domestic bank is fully charged, and the engine battery is completely discharged, there is no way a 100Ah engine start battery is going to draw 200 amps from the charger for any appreciable length of time. A more realistic figure