Hints and Tips



Benefits of FET Battery Isolator upgrade

Peter Sackett explains how to gain a valuable uplift to battery charging


Tom Cunliffe, the UK sailor and marine author, wrote about the first cold beer he enjoyed aboard a sailing boat. This was years ago in the early days of refrigeration and sail boats. Subsequently Tom noted that many sailors managed their sailing around feeding the fridge with power rather than enjoying the sailing.


Refrigeration, ship computer, autohelm, navtext, chartplotter, navigation lights, interior lights, fans, microwave: all add to the electrical load on the Service Batteries. Crew-age can determine the load on the Service Batteries. Younger Crew require chilled water, near continuous charge facilities for their iPhone + iPad + iSomething, frequent hair drying via a hungry inverter. Plus, for crew of any age, it is a shame to waste good champagne by not chilling it properly. Even older sailors deserve better than tinned pies: fresh food equals more refrigeration. Then there is the cold beer needed……and if you have radar running…...


The standard set up alternator on my Jeanneau 45.2 needed to run for 3 to 7 or more hours to get the batteries fully recharged per 24 hours sailing / anchoring. This is expensive on fuel and did not please crew, nearby anchored boats, or the engine.


Alternators and ‘Smart’ Regulators are available delivering 180+ ampere sustained real output, but at a substantial price and substantial engineering work. Low cost fixes improve the situation: for example LED bulbs, but also replacing the Battery Isolator and simple enhancements to some of the wiring.


LED bulbs are the easiest fix. On S/Y Formidable, our Jeanneau 45.2, I changed the Battery Isolator, the unit that prevents the Engine Start batteries subsidising the Service Batteries. A little box, with big cooling fins, 3 or 4 wires connected, near the main battery isolator switches. On The 45.2 it is mounted on the left when looking at the engine through the companion way. If your Jeanneau, like S/Y Formidable, is a few years old this is probably a diode type. In operation mine was getting hot; this is alternator output being wasted. More importantly this type of isolator causes a significant voltage drop (0.5 – 0.9 Volt) in the supply seen by the Service Batteries, meaning the batteries never get properly recharged. I replaced it with the ‘FET’ type available from £40+ (50+ euro). It’s a straight swap. I didn’t scrimp and my top spec FET Isolator, about £100, (120 euro) can handle up to 200 amperes alternator output with a typical voltage drop of a miniscule 0.01 Volts. I fitted a Victron Unit as I have had great service from my Victron Shore Power Charger; ‘other makes are available’.



The Wiring on the Charge Circuit; our boat builder probably sized these wires at a level that meets safety requirements. I duplicated the existing lines: that is the Alternator to the Fuse, Fuse to the Battery Isolator, Battery Isolator to the Battery switch. This is simple and greatly reduces the voltage drop in the wiring so the batteries receive a better charge. If you think you might also latter upgrade the alternator perhaps go up a bit more in wire size. I used wire of 6mm diameter run in parallel with the existing lines and clipped neatly to them. The marine literature can guide you on wire size and advises use tinned copper wire; I purchased a very good quality jump-lead set from a local automotive parts supplier and used that to make up my lines. The same company also supplied me the ring terminals needed. Dependant on the Battery Isolator you have opted for the size of the ring terminals needed to fit might be 6, 8, or 10 mm. Get a few before you start. Clear guidance on fitting these crimped-on terminals is available on YouTube. If your terminals on the existing lines are now too small you can drill them out by one size to avoid having to fit replacements. I also fitted one extra line. It is generally considered better to have a direct return line between the alternator case and the negative side of the Battery Switches. This reduces current flow via the engine block earth. It is a simple task, you have the wire and terminals to hand, and it is worth doing even with the standard Hitachi alternator.



I advise you check and apply the various formal and approved marine electrical guidance data and good practice notes before adopting any of the measures I used.


The Isolator and Wiring changes outlined above gave me an uplift of about 20% or probably more in both battery capacity and charge recovery rate for a few hours work and around £120, (150 euro)--- or in MFUs (Marine Financial Units) almost nothing.


S/Y Formidable - Jeanneau 45.2 - Hull 373 - Peter Sackett