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Sargent ec155 fitting


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Pretty much any auto relay with a rating of about 70amps will do.

However, These contacts (depending on how you are wiring it and the Battery bank size) can switch a lot of current so the contacts are prone to minor deterioration.


Just a little bit of burning of the contacts can drop the voltage enough to affect the charging of the habitation batteries significantly so suggest you get a quality relay?


Don't be tempted to get a relay that has a very high rating, like 150amps, as these also tend to draw more current and most Alternator lamp (D+ ) circuits have a max rating of just 2 amps which may be used for switching other things like the Step alarm, Satellite retraction, etc



If it was my van, I would create a D+ switching system using a good quality 50a relay, then use this D+ amplifier circuit to then drive individual Big relays for the other D+ functions.


So for example use a 50A relay just to 'amplify' the D+ signal then that would supply the power to switch 2 other relays, a big 150 amp for battery charging and a separate circuit for the Fridge (Power drawn directly from the Starter battery), satellite, etc.


It might seem over the top, but just a little bit of deterioration of the contacts can drop the voltage.

Just a 0.25v drop is the difference between a healthy 14.2v and 13.95v, increasing the charging time.







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You can use Voltage sensing relays, which are easier and quicker to wire, but there are several disadvantages.


The first is that any time the Starter battery voltage is raised, say above 13.2v, the relay will trigger and connect in the Habitation battery, which in your case may also bring in the Fridge? So simple operations like charging up the Starter battery via a Car battery charger will hook in the Habitation battery resulting in the Car battery charger taking longer to charge the starter battery.

If it does also hook in the fridge, which can draw 13amps, when the Car battery charger is only 4amps, the batteries will slowly discharge, not charge up. This last scenario is not likely, just showing possibilities.



You also need to be very careful about 'Jumpstarts'. The rule when 'boost starting' a Motorhome battery is to put on the jump leads then run the Donor engine for 10 minutes to put some 14.4v charge into the dead battery. If a voltage sensing relay is in operation, it will connect in the Habitation battery to the Starter battery, so sapping some of the charge and reducing some of the effect.

However, worse than this is that when you then operate the Starter Motor it has three batteries to draw power from : a half 'dead' Starter battery, a potentially fully charged habitation battery (or two) and the Donor vehicles Starter battery which may or may not have a perfect connection through those jump leads?

Guess what might happen if the Habitation battery tries to supply 200amps through it's thin cabling?

If that power is routed via a Power unit like the EC155, guess what might happen to that? Pretty sure you can kiss good bye to the VSR the current goes through.



However, the biggest disadvantage, in my view, is the continuous power draw that the voltage sensing relay takes while it is 'watching' for the voltage to rise. It is only small but adds up over weeks to a significant drain.


A 'dumb' relay approach, as per Sargent has none of those draw backs. It only triggers when the Alternator is spinning, no other time. Which is why it is used by all the big Motorhome electronics manufacturers.


You also have to be careful if using Solar Power charging of the Starter battery as this will obviously raise the starter battery above the VSR trigger voltage, which may or may not be a good thing depending on whether the VSR brings in the Fridge on 12v?


A voltage sensing relay is the lazy way to wire up, not the best.

A quality VSR relay will also cost a lot more than a 'dumb' unit.







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