Introduction: Fitting a Leisure Battery to a Mazda Bongo

If you want to use your Bongo as a camper van then it's a good idea to fit a second battery to run your interior lights, radio and camping junk from. Using a simple split charging system you can insure that you don't run you main starter battery down whilst being able to charge your leisure battery whilst on the go.

If you're fairly competent then the the job should take about 2 hours to complete.

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Step 1: What Exactly Are We Doing Here

First things first. We need to clarify exactly what we are trying to achieve.

I am not going to describe how to wire up your goodies or how to rewire the internal lights etc to run from the leisure battery (that's another howto). I am going to explain how to wire up a split charging circuit so that your leisure battery gets charged up as you drive along.

Your starter battery is connected to the alternator of the van so when you drive along it gets charged. If we connect our leisure battery in parallel to the starter battery it will also get charged as we drive along.

Unfortunately it's not quite that simple. There are a couple instances when we don't want the two batteries connected.

1) Since the starter battery is the most important battery we need to insure that it gets fully charged before we even think about charging the leisure battery.

2) If we are stationary and using the onboard electrics we don't want to risk a flat starter battery so we must ensure it's disconnected from the leisure battery.

To do this we use a special relay that connects / disconnects the two batteries according to the above criteria.

The special relay is a variously called a voltage sensing relay (VSR), smart relay, battery isolator and several other names I can't remember.

It works by only making its switched connection when the input volatge reaches a preset level and unmaking its switched connection below that preset level.

To make sure we only charge the leisure battery when the engine is running and the starter battery is fully charged we need the VSR to make at about 14 volts. When the engine is not running or the starter battery is not fully charged the VSR will see less than 14v and so won't make. This matches the criteria set out earlier.

So our simple parallel circuit now has a smart switch added.

Step 2: What Do You Need


1 x 30 amp voltage sensing relay (VSR)
2m of 27 amp flexible red cable
0.5m flexible black cable (not shown in the photo)
2 x fuse holders
2 x 25 amp fuses
3 x crimp eye connectors
1 x -ve battery strap (not shown in the photo)
Handful of cable ties (not shown in the photo)
Various sizes and lengths of heat shrink sleeving


Side cutters
Crimp tool
Flat bladed screwdriver

There are relays around that can deal with much larger currents, typically they are used for 4x4 and emergency vehicle applications but they start to get very expensive. Unless your leisure battery is very flat a 30A VSR will be suitable. In the worst case your leisure battery will be too flat and draw more the 25 amps. At this point you fuses will blow and the battery won't charge from alternator. You'll have to hook it up to a mains charger to give it an initial boost before putting it back in the van. Under normal use there shouldn't be any problems so it's not worth worrying about.

I got my VSR from Ebay but they are available from other sources.
Try these: or Ebay - Discount Towing Supplies

I got the other bits and pieces from a number of places and at different times for other jobs but all of the parts are available from Halfords super stores.

Step 3: Making Cables to Connect the +ve Battery Posts to the VSR

We need to make two cables up to connect the positive posts of each battery to the VSR.

The VSR is going to be sited just behind the starter battery on the left hand side of the van.

Each cable needs to be fused as close to the battery post as is practical. In the even that too much current is drawn through the circuit the fuses will blow thus protecting all the components from melt down. By placing the fuses as close as possible to the battery posts we only have a very short length of cable that will be subject to the high current and the subsequent potential fire risk. We choose fuse ratings less than the lowest current rating of all the component parts in our circuit.

Our relay is rated at 30A, the cable is 27A so we'll use 25A fuses. There are many types of fuses and fuse holders available and I'd preferred to have used blade fuses. Unfortunately most blade fuse holders come pre-molded with short fly leads. There are two disadvantages to these:

1) You don't know what current rating the fly leads are.
2) You need to make a wire to wire connection which is never very neat.

I chose my fuse holders because I could crimp my cable straight to the fuse holder itself. A much neater job :)

Step 4: Making a Cable to Connect the -ve VSR Input to the Chassis Earth of the Van

You need to make up a cable to connect the -ve input of the VSR to the van chassis.

This cable has nothing to do with completing the charging circuit between the two batteries. It is used to provide power to the VSR to activate the relay and to act as the reference so it can measure the voltage seen across the starter battery.

This means we don't need a piece of high current cable. I don't actually know the rating of this cable, it was a piece left behind after a fitter had installed a new alarm for me.

Step 5: Connecting Up the VSR

At this point we're about to install the charge circuit so before we do anything else we need to disconnect the starter battery to lessen the electric shock possibility.

Always disconnect the battery terminal that is grounded to the chassis first. I our case the van is -ve chassis earth so we disconnect the -ve battery post first.

Now disconnect the +ve battery post.

Now connect the shorter red cable we made to the +ve battery clamp. At this point there are no fuses installed in either of the red cables. As another little safety precaution, don't put them in yet.
Connect the other end of the shorter red cable into input 1 (marked 12v) of the VSR and tighten the screw.

Connect the -ve cable we made in step 4 to the -ve starter battery clamp and then other end into input 6 (marked 0v) of the VSR.

Once you've connected the starter battery to the VSR you need to install the longer red cable we made up. Connect the unterminated end of the cable into input 3 (marked 6) on the VSR and tighten the binding screw.

I chose to site my VSR on the back of the starter battery fuse cover. To allow me to judge correct cable length I taped the VSR to the cover whilst I working. A more permanent fixing will be sorted out a little later.

Step 6: Fit the -ve Leisure Battery Strap

Now you need to install the -ve battery strap to connect the -ve leisure battery post to the van chassis. I bought mine pre-made and simply had to find a suitable place to bolt it to the chassis.

Step 7: Routing the Long Charge Cable

To route the long charge cable I remove the heater air intake scoop and followed suitable bits of the wiring loom and anything else I could cable tie to.

Step 8: Adjusting the VSR and Installing the Batteries

Nearly there now.

Reconnect you starter battery by first connecting the the +ve battery clamp and then the -ve battery clamp.

Install both the 25 amp fuses.

Now check the correct operation of the VSR with a multimeter. You can do this be sticking the +ve probe of you meter onto the leisure battery end of the long red charge cable.and touching the -ve probe to the van chassis. There should no voltage.

Start you engines!

Now measure the voltage at the same point. If the VSR has made you should read about 14v on your meter.

If you don't detect a voltage one of three things has happened.

1) There is a problem with your circuit. Check anything for faults.

2) Your starter battery is very flat and pulling down the input voltage to the VSR below the trigger threshold. This is pretty unlikely since you just started the van which would imply the starter battery is in good shape.

3) The trigger threshold is set too high and needs adjusting to the correct level. Stick a small flat bladed screwdriver in the adjustment hole on the VSR and turn the pot until you hear a click or measure the 14 v with you meter.

Once this is done you should check that the VSR turns off when the engine has stopped. Turn the engine off and measure the voltage again. You will most likely find that the voltage will drop off slowly and so the VSR will not unmake straight away. This is expected. If it doesn't unmake after 30 sec to a minute then the threshold needs tweaking down a bit until you don't see a voltage.

You may have to go round this loop of starting the engine, measuring, adjusting, stopping the engine, measuring and adjusting a couple of times before you get it right. Just don't be too quick to adjust it down, allow the voltage to slope off first.

Now all you need to do is drop in the leisure battery in. For safely remove the fuse from the starter battery end first. Now connect the charge cable to the +ve leisure battery clamp and the clamp to the +ve post of the leisure battery. Finish off by connecting the -ve leisure battery strap to the -ve leisure battery post and reinstalling the starter battery fuse.

Job's a gooden!

Wiring up your electrical goodies is a job for another day.

Put you feet up and crack open a cold one.