I'm adding a leisure battery to my van so I can run some stuff with out fear of draining the starter battery.

My leisure battery is wired to my starter battery so it can charge from the alternator when driving but is separated by a relay to avoid draw while stationary. I have split a wire from my dashboard which becomes live with the ignition and reads 14.5 volts with the engine running to use to activate the relay, and grounded the other side.

My Problem

When I connect this wire to the relays trigger circuit (pin 86 I believe), it's voltage drops to 0.6 which doesn't seem to be enough to close the circuit.

Any one know what I'm doing wrong?


I'm using this relay

I have wired it up like this;

the switch

  • pin 30 positive on starter battery
  • pin 87 positive out of relay


  • pin 86 live from dash
  • pin 85 ground.

Van is mk6 ford transit, wire is white and green one from display behind the steering wheel which I thought might be alternator warning light...

hope that helps.

  • 1
    \$\begingroup\$ Schematic? Relay datasheet (or at least the part number)? \$\endgroup\$
    – Null
    Jul 10, 2015 at 15:12
  • \$\begingroup\$ The control wire for the relay must come directly from the ignition or auxiliary circuit - there may be a spare terminal in the fuse block for such applications. The wire you have tapped must have something (lights or ?) between the ignition or aux switch and the place you connected. \$\endgroup\$ Jul 10, 2015 at 15:41
  • \$\begingroup\$ It is generally possible to find harness diagrams on the internet, they are definitely available in service manuals. Find out what you are tapping and see what else is in line with that circuit. There are generally free aux lines in the harness you can use and last resort is to pull a line through the firewall directly from the battery. I would not rely on voltages appearing at different key positions without understanding what you are tapping into. E.G. A CANH line will steady state high if nothing is talking on the bus giving you a positive voltage, but is not a line that can be tapped into. \$\endgroup\$
    – crasic
    Jul 11, 2015 at 4:41

3 Answers 3


You should be looking at installing a battery splitter instead of this relay. The splitter is a pair of high current diodes mounted on a heatsink and equipped with nice stud terminals for connecting up the wires and a case cover. The alternator output goes to the common anode terminal of the splitter. The two cathode terminals connect off to each battery.

No special control wiring is required!! Also no mechanical part to wear out.


simulate this circuit – Schematic created using CircuitLab

The diodes block the leisure load from discharging the running battery. Such splitters can be found at common RV supply outlets.

  • \$\begingroup\$ This sounds lovely but the alternator is a nightmare to get at as I'd need something to lift the van up with and the original battery lives under the driver seat which means I shouldn't need to run any wires externally this way. \$\endgroup\$ Jul 10, 2015 at 19:30
  • \$\begingroup\$ @aaaaargZombies I deleted the part in my answer which related to Michael's answer as I put what I was trying to say badly. Main point was that the voltage of any battery being charged MUST be directly monitored by the vehicle regulator IF the voltage is properly controlled for float and boost operation. Running a battery behind a diode with boost voltage based on the before diode voltage leads to early battery death. I saw batteries charged via a diode with main battery direct from alternator in a taxi fleet. The auxiliary batteries all died young due to too low voltages. \$\endgroup\$
    – Russell McMahon
    Jul 11, 2015 at 1:58
  • \$\begingroup\$ If one does use a battery splitter (a.k.a. battery isolator) the better way to connect them is to have the voltage sense at the running/starting battery + terminal instead of at the alternator output. This can be a challenge to achieve with some alternators that have built in voltage regulators without a separate sense lead back to the battery. This at least allows both batteries to be charged from more similar voltages. \$\endgroup\$ Jul 11, 2015 at 3:18

What you are doing SOUNDS correct.

Many examples here

enter image description here

It sounds as if the pin 86 feed from the dash is not full battery voltage but has something else in series with it. Try connecting 86 via a wire directly to battery. This should operate the relay without any drop in voltage. If that works then you need some other dashboard feed.

  • \$\begingroup\$ The battery splitter device does NOT need the engine running be able to utilize the "leisure" battery to drive loads that are attached to it. Then engine running is ONLY needed for charging such battery. \$\endgroup\$ Jul 11, 2015 at 1:06
  • \$\begingroup\$ @MichaelKaras Whoops - yes. What I meant (and put wrongly) is wrt the current input to a remote load that is relay enabled. You CAN charge a "leisure battery" via a diode from the main battery but the diode voltage drop causes the LB to suffer an early death (based on installations I have seen). \$\endgroup\$
    – Russell McMahon
    Jul 11, 2015 at 1:54

So after some investigation it seemed the voltage drop was caused by a lack of current(amps) which was needed to power the coil. Soooo I added a new wire straight from the battery to pin 86, put a diode between 86 and 85. 85 went to ground but was controlled by a transistor. The original low powered cable from my dash was enough the operate the transistor which opened the ground for the battery wire. It works though its a bit convoluted.

Probably should have just brought a voltage sensitive relay to begin with...

yo dawg


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