Mahe 36: Batteries And 12vdc Systems

[rabbi]

This is my battery charge enhancement project. Maybe it will help some fellow like all the stuff on this forum helped me get to know my Mahe.

The setup of my Mahe is that the port engine is connected to the house batteries and starboard engine has it's own battery.
So port charges the house battery and starboard only takes care of its own battery. Motor sailing on starboard engine does not charge the house battery. Unless the emergency start crossover switch is engaged to bridge both batteries, which I don't like as I know for sure I will forget to disengage it and be left with no emergency start capabilities some day.

Both alternators are rated at 115A with a fixed 14,4 voltage with no temperature compensation and no compensation for voltage drops in the wires. With a relatively deep discharged battery it would begin to charge at 80-90A, way too much for the 315Ah battery capacity. On the other side a relatively full battery would not accept many Amps at 14.4v (minus losses) since the Mahe comes with Ca batteries that like higher voltages.


I did not like this setup so I changed it:
I moved the wires for the port engine to the terminals for the starboard engine so both start off and charge the single starter battery that was intended for starboard only. Moving these wires is a nasty job, as the terminals are located on the right side of the box with the battery switches. At least the older models with all batteries on port have little space there. No worries, emergency starting is still functional.
Then I added a battery-to-battery or B2B charger with full galvanic isolation between starter and house battery .

IF at least one of the engines is running this B2B charger simply uses whatever amps it can get from the alternators and converts it into a temperature compensated and adjustable three stage charge scheme for the house batteries.

The alternators are not touched, both still operate at their fixed voltage. As the charger kicks in they sense the load and increase the output, theoretically up to the maximum they can deliver.

The B2B charger starts charging only if the alternators deliver more juice than required to charge the starter battery and uses only surplus energy. It adjusts its charge current so that there is no load on the starter battery even if the alternators produce little current while iddling. Beside the battery wires it uses separate sense wires for both the starter and house batteries to know the real battery voltage, as well as a connection to the alternators D+ to switch it on only if an engine is running.

This setup works well and produces a fully charged battery in roughly the same time as the main battery charger.

The B2B charger I installed is made by Votronic in germany. It is rated at 45A so just OK with the 315Ah capacity. It really delivers these 45A without any thermal issues. Cost was 250 euro, installation took two hours to run the wires.

Bigger B2B chargers are available from Sterling.

This was done mostly for night passages and late season cruising when solar does not produce enough juice to keep the batteries and watermaker happy. This should not happen to often as I now have 560W of solar (240w on davits, 320w on bimini).

[captainjay]

Just an FYI every modern FP cat that I have worked on including three different Mahe's have a battery combiner that let all the batteries charge on either or both alternators. They use a voltage sensing relay to trip a solenoid that combine them for charging purposes. If yours wasn't doing that then there was something wrong with the combiner. It is usually mounted on the bulk head in one engine room and is labeled in French. Typically mounted in a grey water tight box.


Jay

[rabbi]

Thanks for sharing. I know the charge distributors from my previous boats.

However my Mahe did not have that installed.
And from reading the other posts I got the impression that others don't have that as well. See post #100


There is a grey box in the port engine room, which contained an unlabeled VDO circuit. The way it was wired it doesn't really do anything, e.g. there was no connection to the starboard alternator, no solenoid, not even a + connection to the battery (so the VDO circuit was living on the D+ which it should not).

I had to take the VDO circuit apart to understand what it does (voltage sensing and switching, but in this setup actually nothing). All this was obviously original wiring from the factory. I suppose it was the preparation for a combiner solenoid but it wasn't connected that way, several wires and the solenoid were missing. Maybe the combiner solenoid was an additional option back in 2007 or maybe they just decided to leave it out for some other reason. But then I don't understand why there was no connection to the starboard alternator. To me this was just a french mystery.


I thought about installing this solenoid but decided against it. Cost would have been a bit less than the b2b charger but the charging regime of a three stage charger leads to a much better charge than a simple solenoid with a flat uncompensated 14,4v alternator voltage.

I replaced the VDO circuit with two standard 12v relais, driven by the two alternator D+ to activate the windlass and the B2B starter on any of the two alternators.


Bootom line: My writeup makes only sense for those who don't have a combiner solenoid in place.

[rabbi]

Originally Posted by rabbi

Bootom line: My writeup makes only sense for those who don't have a combiner solenoid in place.

From re-reading the thread it seems that FP changed the setup to include an automatic battery combiner solenoid sometime between 2009 and 2010.

So my writeup is useful only for those who have a Mahe older than that.

[1rkghost]

I just replaced my house battery bank from lead acid to 4 225amp hour AGM 6 volt Batteries. Does anybody have AGMS a what charge rate do you have your charger set at. With the lead acid we had 14.4 but Im not sure if it needs different for agm

[Dod42]

Originally Posted by 1rkghost

I just replaced my house battery bank from lead acid to 4 225amp hour AGM 6 volt Batteries. Does anybody have AGMS a what charge rate do you have your charger set at. With the lead acid we had 14.4 but Im not sure if it needs different for agm

Our AGM's where set for 14.8 Bulk and 13.8 Float

[ErikFinn]

Originally Posted by 1rkghost

I just replaced my house battery bank from lead acid to 4 225amp hour AGM 6 volt Batteries. Does anybody have AGMS a what charge rate do you have your charger set at. With the lead acid we had 14.4 but Im not sure if it needs different for agm

You should consult your battery manufacturer.

[Dod42]

Originally Posted by Dod42

Our AGM's where set for 14.8 Bulk and 13.8 Float

Sorry I also meant to say ours are the Varta AGM's fitted by the factory and the settings are what the Victron Multi plus charger inverter fitted at the factory was set to. I have set my solar controllers to the same. The exception is the Volvo alternators that I think are hard set to 14.4 anyway. It isn't going to be a big issue but AGM do need an occasional "equalization" charge to (in our case) 14.8 to avoid sulfation.

good source of info

Absorbent Glass Mat (AGM) Battery Information - Battery University

[Dod42]

On our lipari (2013) it has the factory battery combiner looks like this
but next to it is another box that has 2 small relays that look like this

Any idea what they are for before I start trying to follow wires?


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[DotDun]

Originally Posted by Dod42

Any idea what they are for before I start trying to follow wires?

The windlass interlock? (assuming the windlass does not operate without an engine running - a normal feature from FP)

[rabbi]

Looks like the windlass interlock that disconnects the windlass if no engine is running.
In our 2007 Mahe the purple wire would run to the junction box where the windlass solenoid is connected.

We changed that so the controls directly at the windlass work with engines off. The remote control works only if engine is running just in case some kids play with it. Our kids know the danger but visiting kids are a different story.

[captainjay]

Originally Posted by Dod42

On our lipari (2013) it has the factory battery combiner looks like this
Attachment 103267 but next to it is another box that has 2 small relays that look like this
Attachment 103268
Any idea what they are for before I start trying to follow wires?


Sent from my iPhone using Cruisers Sailing Forum

The one on top is a resettable ATC fuse/breaker the other is the relay tied to the alternator output that makes the windless work. It actually powers the low voltage switches on the windless relay.

Jay

[Dod42]

Thanks guys. That's great. Makes sense


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[Rufduck]

I am a new owner of a Mahe 36 Duo and am overjoyed to find this thread.. I've just finished reading the whole thing and, overwhelmed is an appropriate descriptive term.
I managed to run the house bank down so that neither motor would start...neither one.
Remember reading about an isolated emergency battery but it should start the starboard engine..
In fact, running the house bank down should not effect the starboard battery at all if the transfer switch (don't know what else to call it) is in the open or off position. The previous owner said both motors charge everything. So, who knows.
I expected to have to wait until the solar brought the bank up to a level to start a motor but decided to have a look in the starboard engine bay.
Sure enough, there is a battery box with a battery in it but it was disconnected. The positive battery connection is just sitting there pushed away from the battery terminal.
Hooked it up and vroom engine started...vroom port engine started. The house bank charging.

Question: do I presume the battery was intentionally left unhooked? Or do I presume the battery was accidentally left unhooked? Either is possible.
I hope this is the proper place to ask my questions.
Thanks,
Rufduck

[Dod42]

I cannot think of any reason for intentionally leaving it off. On out Lipari there are separate battery isolation switches in each engine bay for disconnecting the battery from its associated engine and a battery combiner switch that can be used to join both banks together for emergency starting. I would think that leaving it off unless the combiner switch was set, would 1. not allow you to start the engine and 2. the Volvo engine battery alarm would go off, and 3. stop it being recharged by anything.