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Which Battery Charging Link Connector Information

ProConnect I: Ignition feed/signal feed relay

This range of low cost low signal activated relays is the simplest in the range, it offers the ability to link as many battery banks as you wish to the starter battery to link and therefore charge different battery banks on a boat/vehicle when an engine is running.

Ideal applications: Boats with outboard motors and simple cabin batteries with no heavy loads on the cabin batteries, small vehicles/boats with auxiliary battery systems with no loads on the auxiliary battery bank in excess of the rating of the relay so as not to overload the relay circuit.

Its limitations: There is no current limit with this type of device, so avoid using it on a system which has a high secondary battery load, such as a big inverter. anchor winch or bow thruster, as the surge currents associated with this type of equipment can destroy the unit (see current limiting relays for this application).
Also there is no low voltage shut off, ie in the event that the secondary battery load is higher than the alternator's ability to cover it (for example at low R.P.M the At would not product full power) then not only will the axillary battery/domestic battery be drained but so would the starter battery. A common example of this would be a high power stereo system on a vehicle where, at tick over in a town, the alternator cannot cover the power used, so when the vehicle is switched off the starter battery could be to low to restart the engine.

Advantage over a standard relay. A standard relay has a activation feed via a cable, this feed takes about 1 amp to run, this adds the 1 amp to where you are taking the feed from , either the d+ on the alternator or key switch or oil pressure switch . This can place enough extra load on these circuits to cause a problem. there is also the secondary problem of voltage drop in the ignition feed cable affecting the relay. This product, even though it also needs an ignition feed cable it simply uses the feed as a signal and takes no power from the ignition feed wire, the signal then activates a internal fet which in turn activates the relay from it own power source internal. This also has the advantage of not suffering from voltage drops in the feed line. The time delay prevents the high load on the starter battery on start up from destroying the relay.

This unit is also water resistant and in a much safer package form. The package also contains anti-spark protection to help reduce the effect of back EMF from open circuiting the relay under heavy loads, without this relays will weld close if the unit is switched off under high current pass applications. The connection terminals are also high quality brass nuts and bolts and not poor quality crimp connectors which are not suitable for high sustained current flow.

ProConnect VS: Voltage Sensitive Relays

This product would be regarded as the next level up from the signal feed relay (as above), the main difference being that it is totally automatic, this relay does not require a feed to operate it, it works on monitoring the input voltage to the device (usually the starter battery or battery bank with other charging source such as a battery charger or wind gen), when this voltage exceeds 13.3 volts the processor makes the assumption that the alternator/battery charger or other power device is active and as such it will automatically engage the relay to connect the main battery bank to the auxiliary battery bank and so charge the aux battery Conversely, when the voltage drops below 12.9 volts the processor in the relay assumes that either the engine  has been stopped or the load on the aux battery is pulling the starter battery down too much and for safety reasons it shoudl be isolated. In a nut shell this is easier to install    the above and is smarter, however, it suffers from many of the same limitation as the standard signal relay.

Ideal applications: See the above signal relay applications, also ideal for charging a secondary battery bank from a battery charger that has only one output such as a combined inverter charger. The starter motor interlock wnnection prevents the starter motor current being pulled through the relay and damaging the relay.

Its limitations: See the above limitations for the signal relay. Another major problem with standard relays is their ability to switch off under high load. The main job of a voltage sensitive relay is to sense any major current being taken out of the starter battery (primary) and to stop it. For example, if we take a typical 12V boat system with an 80 amp alternator and install a standard 80 amp VSR on the engine starter battery bank to charge the secondary battery (domestic battery bank, anchor winch or bow thruster battery bank). On engine start up the alternator will raise the voltage on the starter battery and this will engage the relay to charge the secondary battery bank (so far so good). However, for the sake of argument say the secondary battery bank is discharged (i.e. first thing in the morning) and then someone switches on a 2000 watt inverter, or a bow thruster or the anchor winch, these items will attempt to draw 200 amps + from the secondary battery bank. However, because the secondary battery is nearly empty, the load (the 200 amps) will automatically attempt to draw this current from the highest voltage source which, in this case, would be the starter battery bank, obviously we do not want this to happen as we do not want to drain the starter battery or burn out the 80 amp relay or even worse set fire to the cables as the cables would be rated to take 80+ amps and you are now trying to pa 200 amps plus down these cables (a fire in the making). In theory the large current flow through the VSR will drop the starter battery voltage and so trigger the relay to open circuit and so switch off this circuit stopping the drain and saving the day. However, a relay has 3 main ratings, for example an 80 amp relay can take 80 amps all day no problem, (hence its continuous rating) and has a short term over load of about 400 amps for about 1/10th sec. It has a third rating, however, that is much more important; that is the maximum current it can open circuit at. The assumption by the public is that an 80 amp relay can open circuit at 80 amps, but that is not so, it has an open circuit current rating of about 20-30 amps. And so the problem becomes apparent, the high load on the secondary battery system (in the above example) causes an instant load of 200 amps + on the relay (that's ok, it can deal with this for a split second) then the control circuit attempts to open the relay to stop this discharge, then BANG you open the relay with 200 amps going through it (with am open circuit rating of about 25 amps) and so you cause a large back E.M.F. that, if you're lucky, wi11 vaporise the contacts blowing them in to small pieces and open circuit the relay destroying the product. To reduce this effect, Sterling puts anti back E.M.F. spark reducers on Al relay products.

ProConnect CLR: current limiting voltage sensitive relays

These relays have built in current limiting, in a nutshell you can do whatever you want with them, if you overload them they simply switch off safely. The trick with a relay is not to open circuit it when it is overloaded, but to remove the load, then open circuit the relay within its rating. With a current limiting VSR in an overload situation such as the example in the section above (Voltage Sensitive Relays) the 200 amps will surge through the relay for a split second (within the relay's capability), then the built in current limiting device will see this overload and reduce the current from the dangerous 200+ amps to a very safe 6 amps, the relay will then safely open circuit with a 6 amp load and not a 200 amp load, and so protect the relay and your installation. Once the high load demand has been removed then the relay is safe to re-engage and continue doing its job.

Current limiting Voltage Sensitive Relays can be used for different things other than charging batteries, depending on what the setup is in the relay program. The Sterling unit can be used for the following depending on which of the preset programs you chose to use. The program can be altered using a magnet and run it across the unit to activate different performance requirements.

The Sterling unit has 3 programable settings , you select which function you require by passing a magnet over the unit and selecting the program you require.


Function 1: Charging activated by starter battery voltage increase (engine on). This is the most common setting and the factory default setting. This setting also does not have an ignition feed requirement. This connects the primary and secondary based on a voltage in excess of 13.3V being sensed on the primary battery only (the engine starter battery). The 2nd battery bank, once connected would only disconnect when both battery banks drop below 13.0 volts on both sides. The 13V safety threshold is still activated to protect and seperate the batteries if, or when, the drain that invoked this function (voltage drops below 13 volts) once the unit has tripped, will not re-engage until the voltage on the primary battery bank (the engine start) has exceeded 13.3 volts again. No ignition feed required on this setting.

Function 2:. Charging either way, based on a voltage rise on either side of the unit. This function will close the relay at 13.3 volts sensed on either side of the relay, a good example of this would be the use of a combi inverter charger on the domestic battery bank, because most combi s only have a single output charging line (to the domestic battery bank) then with this function the unit will charge from the combi to the starter battery bank (in effect giving you 2 outputs from the battery charger) when the combi is on, but also charge from the alternator to the domestic battery bank when the combi is off. In either mode the unit still has the 13.3V on and 13V off mode and the current limit function. No ignition feed required on this setting.

Function 3: Charging batteries one way only. This function allows a charge to flow from the alternator, or battery to battery charger, or any other charging source etc to a secondary battery bank , but only when the engine is running. In effect this mode only allows current flow from the primary source to the secondary, and will continue to flow until either there is an overload threat or a low voltage threat to the primary battery or the engine is shut down. This function requires an extra wire to the unit, an ignition feed , that informs the unit if the engine is running or not. None of the other functions require an ignition feed.

Other advantages of a current limiting VSR

  • volt drop across the device ensuring a more even charge.
  • cables can be thinner as the overload is contained and reduced automatically not only protecting the relay but also the cables.
  • No fuse required to protect the product , the product can even deal with a dead short and switch itself off with no damage.
  • ignition protected SAE J1171.
  • IP66 water proof.
  • Can be used in different charging modes
    In the unlikely event that the pre programed voltage settings are not suitable, then the voltages are adjustable using the magnetic reed switch.

What size of relay do I need?

There is no point purchasing a relay which is too small for the job as it will simply protect itself and shut down leaving the system ineffective. To size the correct unit, ensure that the unit is equal to if not a little large than the main charging source. This is very important if charging a main domestic battery bank (in conjunction with an advanced regulator) where the full alternator current flow will be very common, and so if you have a 110 amp alternator you should purchase the 120 amp Current Limiting Relay unit. However if you are only charging, for example, an engine start battery for a generator where high current flow is extremely unlikely then a lower relay rating would be fine.

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