How We Picked Generators (or not)

Updated: Sep 6

What to do about a generator?  We prevaricated here a little while, not having a definite opinion on the best way forward.   Sometimes provocation has its uses.  It dawned on us that what we wanted was to understand “energy management” on the vessel. What used energy, what produced it, when, and why.

Hull No 1 – Mobius has a battery-based system. Solar, shore, and the main engine drive power to them; everything else draws from them.   No 2 – Vanguard has two engines, BUT the standard J Deere alternators will not load sufficiently, limiting battery charging to perhaps 3kw/hour each.   FPB’s add additional alternator capacity, but even so, it’s still not going to load the engines much (diesel engines do not like extended low load running). So at anchor, they will experience long periods of charging under a very light load. We also have a Solar or PV array on the cabin roof with a maximum output of 8 kW under ideal conditions. Production at night is zero and reduced under cloudy skies and at high latitudes. Helpful but not continuous.


Charging. Lead Acid Charge Cycle. Lithium-Ion Charge Cycle. Slidepower.com

We have big batteries. This is where it starts to get more interesting.  Lead Acid traction batteries will only accept charge at a slow rate, floating near their full charge voltage for hours.  For longevity, they do need a complete charge cycle.  Fireflies are better, charge faster, and will discharge without damage from only 80% full charge.  Stay with me here.  Lithium, in its various forms, can accept power very quickly (think Electric Vehicles). So if we have a big generator or another charging source, we can run at the full electrical output for a short period and then rely on batteries after that.  Lithium does have its downsides, though (more in a later blog).

We will fit a hybrid diesel-electric main drive.   Initially, we’d selected 20kW electric per engine for manoeuvering.  BUT installing 2x60kWh of battery bank would take min 3 hours to charge. So we upped the hybrid motors to 30kW for a faster charge.  We still only need 40kW for maneuvering on two engines.   However, there is a propeller curve in the mix here.  A drive of 30kW at 300 RPM shaft speed will reverse into circa 90kW electrical output at 2500RPM engine speed when charging and disconnected from the propeller shaft.  An AH-HA moment!  We must select the hybrid drive capacity, not for propulsion but to match the battery rapid charging requirements.   Charge speed also necessitates Lithium battery technologies. We’ve had to develop this idea a little since as it evolved into a single (battery) point of potential failure but its leaves that for another Blog.


Drive train from diesel to gearbox via PHT and E Motor

So back to energy management.  When underway, electrical power capacity is, for all intents and purposes, unlimited.  At the marina, the same applies.  At anchor, we rely on the house and primary hybrid drive batteries and their inverters for power storage, running the main engine for about 30 minutes a day to charge.  Solar tops up the house batteries, and when complete, tops up the hybrid drive batteries running back through the inverters with a small voltage differential.


Praxis Energy Management system control batteries and all energy flows.

So this then begs the question?

Why do we need one, or for redundancy, two independent generators? The answer is, with a large drive battery capacity and hybrid drive on the main engines, we do NOT need the generators at all.  We still have redundant power generation capacity and a near tranquil experience at anchor or in the dock. No matter what is powered up and if we so choose.

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