Friday, 8 June 2018

Range Rover P38 - Split charge circuit

The charging circuit.

Grab a cuppa, this is a long blog, mainly as it’s been way more difficult than it should be.

My outline requirements were simple - charge a 2nd battery in the boot which would be running the rear winch (more on the winch fitting in a separate post later) - not hard you'd think....

Well, I had access to an ex-Police '38, and it used a Land Rover voltage sensitive switch (PRC4427) to monitor the cranking battery voltage, and when it was effectively charged, it would switch an Albright contactor to permit charge to the 2nd battery - lovely, copy that then.

Not so - the internet is so full of conflicting and confusing installations - even speaking directly to Albright themselves didn't help much.

The problem lies if you fit a winch, there’s already a front winch on this car, and I’m proposing to fit a rear, so for me, winch electrics have to be taken into consideration for this project.

When the rear winch is at stall point (so under max load) it could potentially draw up to 300Amps - the 2nd battery will deplete, so the winch will then draw from the primary vehicle battery. The same can happen in reverse with the front winch.
The split charge wiring will then have to cope with this draw, and if it’s not man enough, it will promptly melt, catch fire etc. etc.

The common solution seems to be to fit wiring that's an inch in diameter, and a split charge relay capable of coping with 300Amp or so - hardly ideal.

Fortunately, the kindly folk at Gigglepin came to the rescue, and recommended a disconnection relay - basically the relay will disconnect the split charge system when either winch is activated and isolate the 2nd battery from the car battery, and re-connect only when both winches are turned off.
Cabling can be noticeably smaller (but still needs to be generous and capable of around 150Amp) as its now only dealing with the charging circuit, although once the 2nd battery is drained, the rear winch is basically useless as it can’t draw current from anywhere else.

This suits me fine - with a '38 and its BeCM, draining the primary battery is a risky venture, and the rear winch is an occasional "get out of jail free" tool for me, so all is good.

Here’s my wiring schematic – note the two winch switches are interconnected, so both need to be in the OFF position for the split charge system to work.
The winch side of the switches operate a pair of standard 4 pin relays, which switch the control feed to the respective winch solenoids – if either relay is open, the solenoids are “dead” and the winch won’t work. This method enables winch isolation, without the need for massive current isolators – yes the positive from the battery to each winch remains live, but the winches themselves cannot be spooled via the remote or the flying lead controller.
In addition, the split charge circuit is ignition controlled, so the rear battery is isolated once the ignition is off.

There is an issue of surge if the 2nd battery is completely flat and the split charge contactor is closed – the alternator in my car is rated at 120Amp so in theory the system could see this amperage. However, the actual current is unlikely to exceed 70-80amps as the car will be simultaneously drawing current, so I have fitted a 40A MIDI fuse, which will run 80A for short periods, decent cable size (25mm² 170A) and the contactor will cope with 150A quite happily. Despite the current capacity of the system, the cable is only 9.7mm outside diameter, so is still relatively easy to route and pass through the bulkhead (firewall)

The next problem was packaging – the ’38 is a big car, but it is quite full – finding spaces for circuit components involved stripping the interior until a big enough void was found.

On the reference Police car, the Albright contactor was located next to the battery box in the engine bay – ideal for wiring, but when I sent a photo across to Albright, their tech gurus noticed it had melted its casing, and put this down to ambient temperatures being too high. I remain doubtful that was the cause, but the point being made was valid - the contactor needs to be kept cool, and even though they are available IP66 water and dust resistant, the drier the better.

Eventually, I found a perfect contactor shaped void in the passenger foot well, behind the “A” post lower trim – there is a spare threaded mounting here (used for the bonnet release on US and EU spec cars I guess?) ideal for a bracket to mount the contactor. This is a particularly good area as its pretty cold (being behind the front wheel) it’s dry (assuming the pollen filter doesn’t let in water!) and it’s very close to the point where standard wiring passes through the bulkhead. It’s also exactly on my proposed route for the wiring to the rear battery, and is covered by trim, but easy to get to.

There is a small bracket already bolted in here to mount the foot well trim, so the contactor bracket is a purpose built modification of this original.

The vertical slots in the OIS bracket replicate the Land Rover bracket, and pick up the foot well trim mounting clips – I slotted the right hand mounting hole to make fitting easier as the bolt in the foot well is quite hard to get to – I can start the bolt in the threaded hole in the car, slide the bracket on and then tighten it up – far less fiddly!
Here’s the contactor in situ….

The contactor is mounted horizontally, as recommended by Albright (vertical mounting can affect coil operation, especially if the unit is jarred or vibrated) with the connectors towards the passenger cabin – this orientation keeps the cable lugs away from metal work, and permits better wiring routes. The countersunk screw (top right) screws into a rivnut on the bracket so there’s no fiddly nuts and washers to contend with in such a tight space. The screw is M6, passing through an M8 hole in the bodywork – this size disparity permits the countersunk head to sit flush, so it does not interfere with the trim.

Land Rover use the voltage sensitive switch (VSR) to open and close relays in a number of vehicles for a number of applications, most commonly high current draw items like heated screens – here’s an example from a Range Rover Classic….

You can see the VSR in the bottom right (item 72) – when it grounds the supply to earth, the coil in the rear screen relay (item 67) is energised (85-86) and the switch is closed (30-87)
In every application, there is a diode fitted to the input on the coil of the switched relay (arrowed – top right) – therefore, I’ve replicated this circuit protection in my set up – you can see the diode the circuit diagram.
I put the VSR under the passenger seat alongside other supplementary circuits added to the car.

The charging cable route runs like this – from the front battery, through a fuse, down and along the base of the front grill framework, up to the N/S inner wing behind the air box and EAS valve block and down through the bulkhead into the contactor.

From here it drops into the sill, runs under the plastic trim along the length of the car and rises around the rear wheel arch into the void behind the sub-woofer. It then passes into the rear structure of the body shell, to emerge into the spare wheel well. After it has entered the passenger cabin at the bulkhead, the cable remains inside the vehicle for its entire length.

The whole length is covered in heat shrink, and where it passes through the engine bay and where it is inside the cavity of the body shell at the rear, it is also covered in plastic convolute sleeve. There are no joins except at the contactor and at each battery.

At each end there is a fuse to protect the circuit and contactor – here’s the front fuse assembly….

….and here’s the rear, mounted on the frame work of the rear battery - you can just see it between the battery and the rear body at the bottom of the picture.

The 2nd battery.

On a P38, finding space for a 2nd battery is challenging – I’m pretty certain in a 2.5 diesel, there’s room in the engine bay, but not so in the V8.

I have seen people cut a hole in the floor so it sits under the rear seats beside the fuel tank – a drastic but pretty neat solution. However, the spare wheel well on my car was the most obvious choice as it’s vacant – the size of tyres I’m running do not fit the spare wheel well, so it has become a store for ropes and recovery gear.

In addition to the battery, I’m also adding a 2nd air suspension (EAS) pump. The pump, a Viair unit, is needed to re-inflate tyres after off road adventures, and also to assist the original EAS pump in inflating the Arnott Gen III suspension bags, which are much larger than standard ones.
The pump runs off the primary car battery, but nonetheless it still made sense to locate the two items in the same place – there’s more on the 2nd pump in a different blog.

The battery is a normal sealed lead acid, and is as large (in height) as can realistically be fitted into the spare wheel well – the terminal layout matches the front, so it can be transferred if there’s ever an issue with the primary battery.
I made a frame to cradle the battery from 3mm mild steel – this frame sits on rubber bushes directly to the boot floor, and is stepped to follow the floor profile. The battery clamp is a proprietary off the shelf item.

I’ve added a partition to the spare wheel well, and a lid to cover the battery – this is so the battery, pump, cables and air lines are not damaged when hauling ropes out of the remaining space – currently the partition and lid are painted plywood – in due course they will be replaced by laser cut GRP items for more finesse. Despite the addition of the lid and partition, the boot floor cover still hinges and latches as Land Rover intended. When everything is shut down there’s no evidence in the boot compartment of what is going on underneath and the boot space is un-affected, so my carrying capacity remains as standard.

The battery earth goes to the body, and from the body to the chassis, offering a good clean continuous path. The rear winch is earthed directly to the battery terminal.
I’m entirely confident in the wiring routes, installation and schematic design, however time will tell if my split charging proves effective – the voltage sensitive switch will only charge the 2nd battery if it sees 13.8volts at the primary – this may be too high, and I may end up with a 2nd battery that is not being effectively charged. However, it’s not a difficult item to swop out, so we’ll see.

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