I'm not entirely sure what this is?

[mikeiso1192]

I have a B38 2004 and behind the starboard salon seat is a blue electrical box to do with charging the batteries.
It has both a voltmeter and an ammeter.
The photo that I have could be better as the flare from the flash means that it is not possible to read the details on the box front.
My boat is in Greece so it means that can't just pop by to have a quick look.
Can any of you wise guys give me more information as to what exactly this box does?

Mike

[tiger79]

It's some sort of DC-DC dual battery charge device, intended to control charge of the main battery plus an emergency battery. 

Later version shown here - http://www.shelectro.gr/en/dbc.html

[Yngmar]

It means you bought an ex-charter boat, which have these to charge an emergency battery for the VHF due to coding requirements.

[Nigel]

Personally I would replace it with a multi-stage charger.

[mikeiso1192]

Thanks guys; I now understand better, I think.
Yngmar - you are quite correct she was originally a charter boat.
Nigel - Your suggestion is duly noted. It is because my charger became faulty (charging at in excess of 17volts) that prompted me to investigate this side of things.
Ironically, my faulty charger is a (3 way) multi-stage charger, but only the primary circuit was connected.
So, as I am not needing to have my boat coded; should I do away with the back-up battery and switch the VHF to the house bank which should cope at 240a/h and now has the support of 200w of solar charge through an MPPT controller.
Just one thought though, should I get a multi-stage charger again but this time use the extra charge circuit to charge the engine battery which I think is only charged from the alternator.

Mike

[Yngmar]

So, as I am not needing to have my boat coded; should I do away with the back-up battery and switch the VHF to the house bank which should cope at 240a/h and now has the support of 200w of solar charge through an MPPT controller.

Yes. Depending on what the emergency battery looks like, you can either join it to the house bank (often it originally was part of it and simply separated to satisfy the need for a backup), or remove it (if it's a smaller/different type from the house bank ones). Although if your faulty charger cooked the house bank with 17V for any length of time, you're probably looking at a new set of batteries anyways. The DC/DC charger can definitely go.

Just one thought though, should I get a multi-stage charger again but this time use the extra charge circuit to charge the engine battery which I think is only charged from the alternator.

Yes. Other things worthwhile doing while you're fiddling with the DC system are a bridge/selector switch, so you can start from the house batteries if the starter battery is dead, and an electronic alternator regulator, which is nicer on the batteries than the alternator integrated one. Oh, and a VSR so the alternator output charges the starter battery first and then the house bank.

[Lyra]

Had also in my boat a DC to DC charger, but in my case the charter company already removed the extra battery, so i simply removed it and connected the VHF and GPS, which were on this circuit, to one of the spare circuits in the main panel.

Unless we use different terminology, a multiple stage charger does not mean how many separate battery banks it can charge but rather what is the "algorithm" of charging (i.e - bulk, absorption, float and boost charging).
Whether a charger can charge separately more than one bank would probably be defined and "multiple output charger" (I think Sterling has one such charger).
Personally I do not think you need to have the shore power charger charge the starting battery, and I have it (and also my solar charger) connected only to the  house batteries. The starting battery, just like in a car, should never get discharged and routine engine operation (in and out of the marina) should keep it full.
I also do not like those 1,2,both,none switches because someday someone will leave it at BOTH and drain the starter battery, or will switch to NONE while the engine is running and fry the alternator diodes. I simply keep a set of jumper wires for emergency starting from the house battery.

However if the boat is left unused for many months then indeed something needs to be done to maintain the batteries to overcome their self discharge. Assuming the house bank is left connected to shore power (personally I do not like this either) then you could leave the DC to DC charger installed, connect the house battery to its input and the starting battery to its output (the starting battery takes the role of the previous emergency battery).
Anther option, which I used several year ago, is take a small (5W is more than enough) 12V solar panel, and connect it to the batteries. take a separate line to each battery bank with a diode on each line (current will flow from panel to batteries but cannot flow from battery to battery). Since it is a very limited current panel there is no need for a solar charge controller, and it will keep the batteries alive. If the boat is kept ashore then you can also totally disconnect the batteries (when in the water you would like at least the bilge pump connected)

[mikeiso1192]

Right Oh!
So on the basis that my backup battery is small (can't read the Ah) but my guess is 35/40Ah and is down to 6.2v - Then I consider it Kna....red and will dispose of it, remove the dc to dc unit and reconnect VHF and Chartplotter via panel to the house batteries.
This leaves me with 2 x 140 Ah house batteries - new last autumn, strongly suspect the faulty charger cooked the old ones last summer. Currently maintained by my 2 x 120Amp solar panels, alternator (need to check what split charge system is in use) and a replacement charger.
With the above in mind - what size charger do you think I should go for.
N.B. Boat is ashore Nov - Apl inclusive with the batteries disconnected.
Mike

[Symphony]

25 amp charger would be more than adequate for 280AH bank, plus your engine start, and the Cristec charger that Bavaria now fit is very quiet and well priced.