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Split Charging Systems
VSR’S v BLOCKING-DIODES – ANYTHING FOR A SALE!
A few articles extolling the merits of VSR’s (voltage sensitive relays) have appeared in marine publications, amply endorsed by one or two companies keen to make a sale, claiming there is now no need for an alternator controller and that, if an AGM battery is used as well, batteries recharge twice as fast, so you only need half the capacity!
One company has repeatedly made the above claim, and has since had the temerity to promote its new alternator controller. Anything for a sale!
Another marine publication has, by implication, suggested Adverc have diametrically opposed views on VSR’s. Not true. Over the years, Adverc have looked at everything there is – knowledge and know-how are king in our book – and we are certainly not diametrically opposed to relays, whether they are standard or voltage sensitive (VSR) types. We actually use them ourselves, though in doing so, it is very much a case of horses for courses.
If we have any criticisms concerning relays, firstly, we have never been totally happy with what is inside them. Open one up and see for yourself. The components don’t generally inspire confidence, but this is strictly a personal point of view, albeit based on experience. Secondly, as stated, we feel claims made for VSR’s can be somewhat romanticised with a flavour of the month appeal. Like the flowery terms ‘leisure’, ‘deep-cycle’ or ‘spiral technology’ applied to batteries, their promotion (and claims) have been enhanced by the marketing men to boost sales.
Whatever the claims, VSR’s basically are simply relays, at the end of the day, but the readership has been wooed into thinking they give ‘prioritisation’ to the engine-start battery, for example. Since time immemorial, or at least the last twenty years, ‘prioritisation’ of charge to the engine-start battery has been the major pre-requisite in the eye of the marine customer and accessory suppliers have gone out of their way to provide an electro-mechanical means to achieve this to give the customer peace of mind.
With a properly thought out and balanced charging system, an engine-start battery does not require prioritisation. In a dedicated twin battery system, it doesn’t do anything but start the engine!
The VSR exponents will argue that an alternator, for example, charges the engine-start battery first and then, at a pre-set voltage, brings the auxiliary battery into parallel via the relay.
What happens, in most cases, is that, as soon as the alternator fires up, the subsequent output voltage is sufficient to energise the relay and pull both batteries into parallel, immediately, so where is the prioritisation?
However, what can also happen is that a high electrical loads on the auxiliary battery, say from an inverter, for example, can cause the relay to momentarily disengage, so the battery then continues to discharge, or the relay starts to ‘chatter’ as it pulls in and out due to the voltage changes.
One or two suppliers of VSR’s have dismissively described this phenomenon simply as ‘buzzing’, when what is happening is that arcing is taking place between the relay contacts, dramatically affecting the life expectancy of the relay. Arcing could well impinge on radio reception quality and there is also mounting evidence to suggest possible emc implications (RFI) between some VSR's and radio transmissions.
So, how does the often maligned and seemingly antiquated blocking-diode differ in performance?
Firstly, all battery banks are totally isolated from each other. Nothing affecting the auxiliary battery can affect the engine-start battery and vice versa.
Secondly, the charging process is automatic. Whichever battery bank wants a charge gets it. If both (or more) battery banks are down, all receive a charge commensurate with their needs.
Yes, there is a significant voltage drop across the diode-splitter and, yes, some form of compensation is required, which is where an Adverc system or other battery sensed device comes into the equation. We would never recommend the diode-splitter route without this compensation. However, a relay based split-charging system does not compensate for other voltage losses or ambient temperature changes which, on narrowboats and yachts, in particular, is an important consideration, nor does the low-loss splitter which, although good, is at least three times more costly than the common diode-splitter or blocking-diode.
An interesting example may perhaps illustrate the above points, quoting A & E Ambulances which have high electrical loads ‘on a shout’, in excess of 100amps.
About five years ago, a particular ambulance service had fourteen brand new Ford Transit A & E ambulances, all of which ended up off the road with flat batteries. Apart from the seriousness of the situation, it was not without its funny side, as the wailers began to change in frequency and pitch, sounding like a wounded cow, creating much mirth amongst the local populace!
The problem was attributed to VSR’s used in a French ‘battery management’ system and was solved by the installation of an Adverc system, split-charge diodes, as well as the implementation of other recommendations. These fourteen vehicles have not missed a beat since.
As stated earlier, nothing beats a well thought out, balanced or even planned charging system (our advice is free), which out-performs the bolt-on goodies approach every time. This is what Adverc are good at. They should be after twenty-three years of practicing! Anything for a sale has never been part of our business philosophy.
If an Adverc system with a split-charge diodes approach is good enough for Ellen MacArthur, Asda / Scania trucks, A & E ambulances and Bomb Disposal vehicles, they should serve the narrowboat and yachting fraternity equally well!