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Single/Twin Alternator Concepts and Misconceptions
With ever increasing electrical loads now being common place on marine craft and automotive vehicles, larger alternators have appeared (100 and 120amp outputs are common, these days) and, in an increasing number of cases, twin alternators.
Large alternators can be justified, obviously, when large battery banks and high electrical loads appear to warrant the move, although there is a tendency to be indiscriminate in the choice of alternator output when, arguably, more attention should be paid to making the existing charging system work better, which is where ADVERC comes in.
The higher the alternator output the higher the cut-in speed, so the perceived advantages are often negated on low revving engines and those ticking-over for long periods.
Question: So the next step is to increase the alternator r.p.m. by reducing the alternator pulley diameter?
Not so because, in so doing, the ‘wrap round’ capability of the driving belt i.e. the amount of belt in contact with the alternator pulley, is reduced. This is not helped by the fact that, in many instances, the same belt drives the engine water pump
Question: ‘Got it’, you will say, so increase the crank-shaft pulley diameter?!
You’ll have a job! Engine manufacturers, especially marine ones, seem pre-occupied with a 125mm (5") crank-shaft pulley. You need at least a 2:1 crank-shaft:alternator pulley ratio, otherwise belts squeal and premature belt wear can result. A 125mm crank-shaft pulley equates to an alternator pulley diameter of 62.5mm (2.5"), therefore.
This is just about viable with a multi-groove or poly-vee drive-belt, common on automotive engines but, where single-vee pulleys are concerned, common on marine engines, our experience has shown that a 70mm (2¾") diameter pulley is necessary to achieve a satisfactory ‘wrap-round’.
So, clearly, a problem can exist, which is directly related to the output of the alternator
Question: What about two smaller alternators?
Good idea. You have lower cut-in speeds, accompanied by a kind of fail-safe advantage, should either alternator happen to fail.
Use of twin alternators does involve more complex mounting bracketry and driveability considerations because there is still the matter of the 125mm diameter crank-shaft pulley, although it is possible to bolt a second pulley onto the original. In the case of certain marine engine manufacturers, who shall be nameless, having grasped this approach, what do they do? Answer: they bolt on another 125mm diameter pulley, when a 150mm or 200mm diameter version would have been better. In fairness, there is always the matter of the water pump speed and bearings to consider, as well.
Automotive manufacturers have got the alternator driveability equation spot-on, using poly-vee belts and some thought. With the Mercedes Sprinter, used on A&E ambulance applications, for example, where there are huge electrical loads, a 115amp alternator, working with ADVERC, can deliver 80amps at tick-over. A similar situation exists with the Ford Transit and V.W. in similar applications.
Marine engine manufacturers, generally speaking, are in a different world, with the exception of Thorneycroft and Sabre Engines, who are wisely addressing the issue of effective battery charging as an important part of engine design and marketing. Another notion widely practised e.g. by Oyster Yachts, Lister, Shire and Beta Marine engines and the Ford Motor Company, is to use a twin alternator configuration for a dedicated purpose, the most common of which is to have an alternator charging the engine start battery and the other, the auxiliary or domestic battery bank.
Whilst this approach reduces the risk of an engine non-starter it does not optimize the full charging potential of two alternators
Two examples: The first involved a Ford Transit A & E Ambulance belonging to Mersey Regional Ambulance NHS Trust. An experiment employed twin Lucas A127 70amp alternators, one for vehicle starting, the other to charge the auxiliary battery and support electrical loads in excess of100amps.
The vehicle start battery was reasonably well charged requiring a mere 8amps, from a 70ampalternator. The second 70amp alternator had to support a 100amp load and charge the batteries as well. The total output from both alternators was 72amps.
The solution was to parallel both alternators, controlled by a single ADVERC Battery Management System, with split-charging of the two battery banks via two 95amp blocking-diodes. The combined output was 136amps, 5amps to the vehicle start battery, 100amps to meet the electrical load and 31amps towards charging the auxiliary battery bank.
Using the same technique on a Jeanneau Lagoon 472 catamaran, in Nantes, with two Yanmar Engines and a single Hitachi 80amp alternator on each, a 145% improvement in charging was achieved, with electrical loads applied.
This twin alternator technique has been extensively developed by ourselves, over the past three years, and it is now possible to parallel alternators of different makes, outputs and speeds, controlled by a single ADVERC System. Often, there is still a case to be made for a suitable single alternator, however.
Two general configurations are illustrated, though it must be stressed they are only general configurations and we do not advise the uninitiated to embark on such an ambitious project without technical help.
Notwithstanding, the benefits of this approach, if properly thought out, are immense.