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      My Bav 32 (2003) is fitted with a “smart” battery charger and two batteries, one domestic and one engine. (Sorry don’t have the tech specs to hand). I believe the charger is original factory fitted equipment.

      On charter boats I have been on in the past, the domestic and engine batteries had separate isolator switches, so that when you are under sail or at anchor you can turn off the engine battery and be sure that the engine is going to start next time. I was surprised to find that my boat has only one isolator – under the chart table – which means (I assume) you can have both batteries on or off.

      In the user manual for the charger it claims to preferentially charge up the engine battery over the domestic one – which is good – but is the circuitry smart when you are discharging the battery?

      So if I go sailing and forget to turn the fridge off for example, or lying at anchor overnight with lots of lights on, does the domestic battery discharge first, followed by the engine battery? Or perhaps they are on separate circuits with no crossover? Or perhaps I will be stuck with no way to start the engine!

      It would be good to know the answer to this one, as otherwise the tendency when lying at anchor is to annoy the rest of the crew by obsessively turning off lights, and perhaps worse, causing them to drink warm beer when it’s not strictly necessary!


        Warm beer = short plank!

        On a serious note though, very good question.
        My 32 does have seperate isolation switches… if the batteries are wired simply in parralel I guess that they will discharge in unison….not a good situation.
        But then I ASSUME (dangerous ground) that if the banks are seperated somehow there is hopefully some charge diode the will prevent the starter battery draining into the house bank?


          Forgot to ask, what is the make / model of the charging unit?
          Maybe this will give us an insight into the tech spec.


            All the bavarias I have been on have a single battery on/off switch and like yours mine is under the chart table, however the engine battery is only used for engine starting. This is achieved by a factory fitted split diode which is in the engine bay. Mine is on the panel behind the engine and to port of the engine. You can therfore relax about using the fridge.
            We have a smart charger for use from shore power and an Adverc control unit fitted to the alternator circuit which allows the batteries to make better use of the charging available from the alternator. We have an engine start battery plus 2x140amp hour domestics and we monitor battery use so we can see how many amps/hour every piece of kit on the boat is using.
            Our fridge uses 2.4 amps/hour when pulling down to temperature and 1 amp/hour therafter. By comparison if you switch the cabin lights on they use 0.9 amps/hour each and you could have 12 on. This is the best argument i can give for replacing the halogen lights with LED’s.
            If you do fit a device that allows the alternator to charge beyond the normal voltage controls then you need to fit an additional switch to take the engine battery out of the charging circuit when motoring for extended periods or it will get overcharged and shorten its life.
            The original owners manual has all the electrical circuits included and I think it is available on this website.


              Thanks for your advice Saloma and CaSam.

              I will check the battery and charger spec when I am next on the boat and post them here, but it sounds like I can relax about running the engine battery down.

              I think I will conduct an experiment while the boat is in the marina. I’ll disconnect shore power then run the fridge and some lights and check battery levels from time to time just to be sure the engine battery is not discharging.


                The single battery isolator on all Bavarias is in the negative (earth or ground) line and removes both batteries from the common ground bus.

                The positives are isolated from each other by a split charge diode arrangement as described by Saloma so there is no draw on the engine battery from the domestic stuff (fridge, lights etc.).

                So, nothing to worry about!



                  I have replaced the split charge diode on my 37 with a VSR which is more efficient than a diode, but does the same job of charging the start battery first then the house battery. Typically a start battery needs minimal charging if the engine is a quick starter. The battery charger is also split to charge the two banks separately.

                  To cater for the unlikely situation where the start battery fails I also have a switch cluster that isolates the +ve of each bank independently plus a parallel switch so that the house bank can start the engine. The standard -ve isolator is still there, but normally left on.

                  The switches and VSR are made by BEP Marine and supplied by Merlin Power. A big improvement in functionality over the standard system.


                    After a couple of weeks cruise up and down the NE coast I can now confirm from experience that the engine start battery is indeed isolated from the domestic stuff. After several nights at anchor, the engine start battery never dropped below about 13V whereas, after anchoring overnight with the anchor light and the Chartplotter on to provide the anchor alarm, the domestic battery was occasionally down as low as 10V.

                    Many thanks for all the good advice on this topic.


                      The chart plotter uses a lot of current. The depth alarm on the tri-data uses a lot less and depending on the anchorage can be as useful. An led on an extension cable, plugged into a 12v socket and tied to the backstay makes a good anchor light. Lots of things stop working when battery voltage is at 10v, especially autopilots.
                      Does anyone make a chart plotter that can have the screen switched off yet keep the alarms working so that less current is used?


                        Bavaria 36 2002
                        Because we have changed our engine and it has a much bigger alternator (115amps) we had to increase the size of the wiring to the batteries. We have therefore fitted a VSR(Voltage sensitive Relay between the engine and Domestic batteries. This has the advantage that you can get rid of the diode (which would have had to be bigger as well). You can use the thick starter cable to charge the starter battery and the only thick wires you need are the ones between the starter battery, VSR and Domestic battery. We tried a normal VSR to begin with and when it was engaged it got very very hot which must have been using current to keep itself engaged. We have since bought a Latch VSR from Stirling and this is working great. It only uses power when it switches on or switches off and doesn’t get hot.

                        The secondary advantage of the latch VSR is that if the solar panels get the domestic batteries up to 13.2 volts it joins the domestic batteries up to the starter battery and keeps that topped up too.

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