DaveS
Well-known member
Voltage Sensitive Relays (VSRs) do not seem to be widely known about, yet they would seem to be a much cheaper alternative to more complex arrangements for keeping domestic and cranking batteries seperate while discharging but correctly charged by a common alternator.
Until this weekend the main DC supply arrangements on my boat were fairly straightforward. One 105Ah "leisure" battery rated for starting and one 110Ah deep cycle battery were each connected via an isolator to a common point which supplied the starter and the distribution board. The 14V 50A machine sensed alternator was also connected to the same common point.
Before starting the engine I would switch on the "start" isolator then switch off the "domestic" isolator. After the engine had run for a few minutes I would switch on "start" and switch off "domestic". This worked well enough and I never actually endangered the alternator by switching off before switching on, but it was something else to worry about. I thought about replacing the isolators with a make-before-break "1,2, both" switch but never got round to it.
What finally prompted me to modernise the electrics was the purchase last year of a Navman 5500 plotter. It's a really great bit of kit, but it does have one annoying "feature": if it sees a supply voltage dip (e.g. when starting the engine) it shuts down. On re-starting, the current route is no longer active. You have to start the route then skip each waypoint in turn until you get to your current leg - and if you overshoot you have to start from the beginning since you can't go back! This is a real pain in the proverbials.
I spent some time thinking about wiring options. Separating the domestic and cranking functions was clearly essential and it would be nice to eliminate manual switching, but then there is the issue of charging both batteries from the alternator. Blocking diodes of course - but the voltage drop would seriously reduce the charge rate. So get into the alternator to try to convert it to remote sensing? Fit a smart charger - and still probably need to get into the alternator? As an alternative approach I thought about a split charge relay driven from the field winding, and then came across VSRs.
One of these is now connected between my battery isolators and the other commoning connections removed. The distribution board is now connected only to the domestic battery, so does not see starting voltage dips. The starter and alternator are connected to the cranking battery. On starting the engine the cranking battery is charged by the alternator: once it reaches 13.7 volts (a few seconds or more depending on how long the engine took to start) the VSR closes, paralleling the batteries which are then charged together. When the engine is stopped the voltage falls: when it reaches 12.8 volts the VSR opens leaving the cranking battery fully charged. Simple!
So both isolators are now left on throughout a cruise. Back at the pontoon, where a smart battery charger keeps all batteries topped up, there is another benefit. With the cranking battery isolator opened the cockpit engine controls are disabled which was not the case before. (The domestic isolator is left on to keep the fridge going and the Navtex receiving.)
We'll see how long it lasts: I hope indefinately, but with anything electronic there's always doubt. I have kept the old cable links so that the system can be readily put back to manual operation.
I bought my VSR from Merlin (usual disclaimer). Its a New Zealand make (can't remember the name - three initials, something like BMR?) It's 100A rated and cost £50 but it's in a nice box with a LED which lights to show when the relay is closed. I've also seen pictures of French made units of lower current rating which are probably cheaper and look like car relays.
Until this weekend the main DC supply arrangements on my boat were fairly straightforward. One 105Ah "leisure" battery rated for starting and one 110Ah deep cycle battery were each connected via an isolator to a common point which supplied the starter and the distribution board. The 14V 50A machine sensed alternator was also connected to the same common point.
Before starting the engine I would switch on the "start" isolator then switch off the "domestic" isolator. After the engine had run for a few minutes I would switch on "start" and switch off "domestic". This worked well enough and I never actually endangered the alternator by switching off before switching on, but it was something else to worry about. I thought about replacing the isolators with a make-before-break "1,2, both" switch but never got round to it.
What finally prompted me to modernise the electrics was the purchase last year of a Navman 5500 plotter. It's a really great bit of kit, but it does have one annoying "feature": if it sees a supply voltage dip (e.g. when starting the engine) it shuts down. On re-starting, the current route is no longer active. You have to start the route then skip each waypoint in turn until you get to your current leg - and if you overshoot you have to start from the beginning since you can't go back! This is a real pain in the proverbials.
I spent some time thinking about wiring options. Separating the domestic and cranking functions was clearly essential and it would be nice to eliminate manual switching, but then there is the issue of charging both batteries from the alternator. Blocking diodes of course - but the voltage drop would seriously reduce the charge rate. So get into the alternator to try to convert it to remote sensing? Fit a smart charger - and still probably need to get into the alternator? As an alternative approach I thought about a split charge relay driven from the field winding, and then came across VSRs.
One of these is now connected between my battery isolators and the other commoning connections removed. The distribution board is now connected only to the domestic battery, so does not see starting voltage dips. The starter and alternator are connected to the cranking battery. On starting the engine the cranking battery is charged by the alternator: once it reaches 13.7 volts (a few seconds or more depending on how long the engine took to start) the VSR closes, paralleling the batteries which are then charged together. When the engine is stopped the voltage falls: when it reaches 12.8 volts the VSR opens leaving the cranking battery fully charged. Simple!
So both isolators are now left on throughout a cruise. Back at the pontoon, where a smart battery charger keeps all batteries topped up, there is another benefit. With the cranking battery isolator opened the cockpit engine controls are disabled which was not the case before. (The domestic isolator is left on to keep the fridge going and the Navtex receiving.)
We'll see how long it lasts: I hope indefinately, but with anything electronic there's always doubt. I have kept the old cable links so that the system can be readily put back to manual operation.
I bought my VSR from Merlin (usual disclaimer). Its a New Zealand make (can't remember the name - three initials, something like BMR?) It's 100A rated and cost £50 but it's in a nice box with a LED which lights to show when the relay is closed. I've also seen pictures of French made units of lower current rating which are probably cheaper and look like car relays.
