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

would be 50 amps. It may draw well in excess of this for a few minutes at maximum but quality relays have a huge overmargin for short term currents.

So now we can modify this rule. It now becomes 2 questions.

A) "what is the maximum total amps available at ANY ONE SIDE of the relay AT ANY ONE time ?"

B) "What is half the battery capacity (in amp hours) on the other side of the relay ?"

Take whichever is the smallest of these 2 and then select the next highest available relay size and select your cabling to match.

NOTE that you cannot use this "get out" to reduce the required size of relay if there is a continuous load on the battery that can draw excessive current. If that is the case then you will have to rely entirely on rule A) above.

There is one other thing to consider however. That is, would it be possible, in your installation, for a heavy load (say a bow thruster, engine starter, windlass, large inverter etc) to attempt to draw it's current through the SmartBank relay and cabling in the event of certain batteries being flat?

An example of a scenario where this might happen would be as follows:-

A windlass or bowthruster is connected to it's own battery which is charged via the SmartBank, from the domestic charging system. The windlass or bowthruster battery is flat, due to running it last night after the engine was shut down. This morning the engine is started and (due to the house bank alternator) SmartBank connects in the windlass or bowthruster battery (which is flat remember). If the windlass or bowthruster is now used then almost it's entire current (perhaps hundreds of amps) will be drawn through the SmartBank relay. The relay and it's cabling must be sized to handle this current safely.

This may sound like a very odd scenario and highly unlikely but it is a possibility and we have seen it happen.

And finally, is the emergency engine start facility of SmartBank to be used?

This is a manually triggered paralleling of the engine start and auxiliary bank to enable the engine to be started in the event the engine battery is flat (or faulty).

If this is the case then the relay and cabling must be capable of handling the full starter current.

With all these variables it can be somewhat difficult to decide on the required relay and cabling sizes. For this reason we now recommend the following for all standard installations in narrowboats, small to medium sized ocean going vessels, yachts and vehicles. This also has other benifits in that charging losses are greatly reduced. The decision is made for you and is very simple.

We recommend the use of the SW180-2 relay as the battery paralleling relay. This relay is rated at 200 amps continuous and will surge to 1000 amps for engine starting. All cabling should be 50mm as a minimum. We use 70mm on our own installations.

This may sound excessive but remember that SmartBank is a fit and forget solution to your split charging needs. It will take care of all future expansion (such as wind turbines, solar panels, combi inverter/charger units, extra alternators etc) of the system without any changes being required. This cannot be said of any other split charge system that we are aware of. So you can rest assured that once the system is installed using this high powered relay and very heavy duty cable it will never have to be altered. So do it once, do it properly, and do it for good.

But don't these heavy duty relays use a lot of power to keep them energised comparaed to a much smaller "cube" type relay

Well, yes and no. The SW180-2 relay will use about 14 watts of power to keep it energised if used as a standard relay in the normal manner. However due to the unique switch-mode power saving feature of SmartBank this is reduced to around 4 watts. This is comparable to a "cube" relay of around 30 amps capability and substantially less than the typical loss of 100 watts or more using a split charge diode.

The system is complicated and I can't decide what size of relay and cabling to use.

Simple. Tell us what the system includes (or better still send us a schematic) and we will work out what size of relay will be required and what size cable to use. Or just take our advice and use the SW180-2 relay and 50mm (or bigger) cabling.

Can I add another relay for another battery bank ?

Yes. The drive circuitry will easily handle another relay to enable say a bow thruster or windlass battery to be charged. But bear in mind that SmartBank will not monitor that battery bank for an available charge source. Only the first 2 banks will be monitored.

Despite the above, do you have a recommended relay type that you use in your own installations?

Yes. As stated above we always use the Albright SW180-2 relay. This is rated at 200 amps continuous, 1000 amps surge. These are true ratings, unlike those on some cheap foreign imports. These relays are practically indestructible, which is why we use them.

This relay will happily live with starter currents, bow thrusters, windlasses etc. In fact this part number is a continuously rated version of the typical relay seen on bow thrusters. More like a heavy duty contactor rather than the typical small cube relay some installers seem to think will do the job.

For smaller installations with much smaller equipment we occasonally use the Albright SW80 which is much smaller and rated at 100 amps continuous. Even this relay will still handle starter currents for small to medium sized diesel engines, say up to around 5 litres.

Be very careful with relay ratings. We know of several where the rating is er.... shall we say "optimistic"? One very popular type (obviously we can't name names) rated at 180 amps by the importer actually self destructed at less than half of this in our tests. Be careful what you buy. See here for further details.

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Page last updated 02/04/2008.
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