Second battery

[popotla]

Thanks to all who replied to my earlier post about this matter.

I have just had a second battery fitted to my Defender, for running auxiliary equipment. Both batteries are 110 AH, and 110-amp cable links the two batteries (I am told), with a manual cut-off switch in the cable between the negative terminals.

Lighter-type sockets and two sockets from which household-type plugs can run have also been installed. The electrician hadn’t fitted fuses for two of the sockets, but I’ve asked him to fit fuses for all four.

I have read that two fuse holders and fuses should be fitted in the main linking cable, one on each side of the cut-off switch, but the electrician says “not needed”. I have also seen that in a recommended set-up, the cut-off switch is in the cable running between the POSITIVE terminals.

1. Am I mistaken about the need for fuses in the main linking cable, or is he?

2. Does it matter whether the cut off switch is inserted in the negative cable or in the positive one?

[LandyManLuke]

There's very little point in putting fuses in the main battery cables - they'd have to be rated to hundreds of amps - and at that rating, what are they going to protect? - you can start a fire with a lot less than that...

You may be getting confused with the bodge-job split charging set up using a standard 30A relay, fuses have to be fitted in that situation as it's not hard to pull 30A through the relay in that application - but as i said, the fuses are there because the job wasn't done properly in the first place.

Positioning the cut off switch comes down to personal choice a lot of the time, without knowing any more info about the setup, there's no particular preference one way or the other. In your situation, I'd have to ask what you expect the cut-off switch to do, and how other things have been wired up.

Why did you choose a switch, rather than something like a VSR or a heavy duty relay?

[popotla]

Hi LandyManLuke

Why did you choose a switch, rather than something like a VSR or a heavy duty relay?

Well, I've let LandRover fit what they recommend and what they have to hand or can get (Salalah, Oman, is not the centre of the universe.

Would you advise me about the following, please?

At this very moment, my Defender is having fitted, at the local Landrover dealer, a second battery and sockets.

1. A cut-off switch has been put in the NEGATIVE lead joining first and second batteries. However, I have wiring diagrams that say "put the switch in the positive cable", and have been told, on this forum, that:

If done correctly, having a switch in the Negative wire can work but why on earth would any auto electrician do it that way. Thats nuts!

To use the negative to isolate the auxiliary ( leisure ) battery, you will have to isolate all the accessory negative wires as well.

I have just communicated the above, politely, to LR, but have been told that "it's fine as it is; all the military vehicles coming in (to the country) (I live in Oman) have it done this way".

2. Fuses or circuit breakers have not been fitted in the cable with the switch in it. There is, in other words, no protection against a possible short circuit. (I should add, perhaps, that the second battery is on the floor between the driver's seat, and it wouldn't take much depth of water for the whole thing to become flooded.)

I understand from an internet source that:

The wire running between the batteries will be live all the time, so if that wire was to ever touch earth for any reason (known as a short circuit), the fuse will blow, and disconnect the power from flowing down the wire. Without the fuse in the circuit, the wire will have to melt before the power can be disconnected, or in rare cases if the wire is thick enough, the battery could explode.

Another reason to put the fuses in the wires, apart from short circuit protection, is overload protection.

A classic example is if a cell in the leisure battery were to die. When they die they short out internally, and thus effectively take 2 volts out of the battery, so the 12 volt battery is now 10 volts. You're pumping 12 volts into the battery to charge it, and the battery can only take 10 volts. Something will have to give, and it is usually the alternator that blows. With a fuse in the circuit the 25p fuse will blow instead of the £100 alternator.

I have also learned, on another forum, that:

As to protecting your set up, if the two batteries and all cabling running between the batteries and the cable running to the battery isolating switch are in the battery compartment under the seat then you really don’t need any fuses or circuit breakers as everything is contained in a safe area, but if any of the cabling comes out of the battery compartment then it MUST be protected.

My second battery is on the floor, with all wires on the floor, too, so must be protected, mustn't it?

I appreciate that fuses can't, I think, go in a 110-amp wire. LR tell me, however, that "circuit breakers are not available here (in this city)".

Please clarify: (1) In which cable (pos or neg) should the cut-off switch go, and why?

(2) Can this set up be run without fuses/circuit breakers in the cable with this cut-off switch in it?

[LandyManLuke]

Yes, if the switch is in the negative wire, the switch will disconnect the second battery from the first, but not necessarily the loads from the second battery. However, the same would be true if the switch was in the positive wire... there's no difference.

Water is not a particular problem, 12v wiring is on the whole not going to short out just because it's under water.

The cable to the starter/alternator isn't fused on standard vehicles, as i said before, the fuse would have to be rather large to allow normal operation, so it will have much less affect in terms of protection. a fuse the size you would require wouldn't cost 25p either!!!

Every load running off the battery should be fused, correctly for the load in question, as near to the battery as possible.

A better approach is to stop shorts in the first place, use heat-shrink on open connectors and run cables through gromets rather than through sharp edged metal holes.

Why is the battery between the seats? you can fit two batteries in the battery box.

[popotla]

Hi LandymanLuke,

Yes, if the switch is in the negative cable, the switch will disconnect the second battery from the first, but not necessarily the loads from the second battery. However, the same would be true if the switch was in the positive wire... there's no difference.

So it doesn't make any diffence whether the switch is in the neg cable or the pos cable?? The switch will not "necessarily" disconnect the loads from the second battery, you say. So it might, or it might not?? (I'm assuming that "disconnecting the loads" from the second battery is the same as "isolating" the battery.)

However, switch in neg cable/switch in pos cable is, according to another poster on this thread, not a matter of indifference:

If done correctly, having a switch in the Negative wire can work but why on earth would any auto electrician do it that way. Thats nuts! To use the negative to isolate the auxiliary ( leisure ) battery, you will have to isolate all the accessory negative wires as well.

(Every diagram I've looked at, all involving negative earthing, show the cut-off switch in the pos cable. None of the accompanying instructions say that switch in neg cable/in pos cable is a matter of personal choice.

and

As to protecting your set up, if the two batteries and all cabling running between the batteries and the cable running to the battery isolating switch are in the battery compartment under the seat then you really don’t need any fuses or circuit breakers as everything is contained in a safe area, but if any of the cabling comes out of the battery compartment then it MUST be protected.

You would say, on the other hand, that the answer is to use heat-shrink on open connectors and run cables through gromets rather than through sharp edged metal holes.

I can see, though, the difficulty with fusing the 110-amp cable that's been fitted in my set-up.

[mmgemini]

popotla

Frankly I don't understand why this is being done.

Why are you using such heavy cable ? Winch ?

My second battery runs the fridge, the GPS. The computer if carried and camping lights.

There is no cut off/battery master switch fitted. It runs through a Lucas Split Charge Relay.

The only battery master switch that must be fitted o the positive terminal, assuming a negative earth system, is an FIA battery master switch. The reason being that the FIA switch switches the engine off when the switch is switched off.

Again like Luke I cannot understand why the second battery is not being fitted in the battery box.

I fit ALL electrical cables through grommets when passing through and metal surface. Likewise I use Lucar wiring connectors covered with the apropiate sleeve to insure no unwanted shorts.

mike

[LandyManLuke]

(I'm assuming that "disconnecting the loads" from the second battery is the same as "isolating" the battery.)

Not necessarily, look at this picture

Two switches are needed, one to isolate the loads from the battery, and one to isolate the first battery from the second. The switch on the left, isolating the batteries from each other, could be in the live or the earth, Personally i'd put it in the live, but it doesn't matter, there's no hard and fast rule.

[Graham Thomson]

Not necessarily, look at this picture

Two switches are needed, one to isolate the loads from the battery, and one to isolate the first battery from the second. The switch on the left, isolating the batteries from each other, could be in the live or the earth, Personally i'd put it in the live, but it doesn't matter, there's no hard and fast rule.

Ok, I almost get this.....so if you were running say an electric coolbox while u travelled say 200 miles to destination, you would have all the switches open? and then on arrival at destination you would isolate using the left hand switch?

And what is to stop chaining a third battery, given an additional isolation switch etc.....

[LandyManLuke]

Yup, though (possibly confusingly) a 'closed' electrical switch is one that allows current to flow, and an 'open' one does not. so the inverse of what you have said is true.

Of course, using a manual switch is not the ideal way, hence my question in my first reply, using a relay or a VSR is a much better solution in my opinion.

this post might help explain a few options.

[Graham Thomson]

That is a really useful post Luke - so many things now on the list for the Landy - need a decent lottery win I think!

I wonder what the draw is on my coolbox.........

[popotla]

Hi mmgemini, you asked me:

Frankly I don't understand why this is being done.

Why are you using such heavy cable ? Winch ?

Idon't know why it's such heavy cable. I left it with Landrover to do, assuming that their full-time electrician would be fully conversant with how to do it, and fully competent.

I never thought of fitting the second battery in the battery box under the driver's seat (LHD), and Landrover never suggested it.

[bleyseng]

They are using heavy cable because, in the event of a flat primary battery, the secondary battery will be wired sufficiently to be able to handle the current, required to crank the engine over.

If a 100amp VSR was between the primary and secondary battery, then it would be so much better. It would remove the possibility of human error, ie forgetting to isolate the batteries manually.

[Frax]

Can some one please clarify some points for me if I fit the X-Charge unit.

1. Can I fit batteries with different amp hours.

2. Does the X-Charge relay work but cutting the voltage from the main battery and only uses the aux battery for say a winch.

3. Is the relay voltage sensitive.

I have two good batteries and just wondered if it is possible to use them because they have different ah.

I have just ordered an X-Charge system from Si.

[Adam001]

Can some one please clarify some points for me if I fit the X-Charge unit.

1. Can I fit batteries with different amp hours.

2. Does the X-Charge relay work but cutting the voltage from the main battery and only uses the aux battery for say a winch.

3. Is the relay voltage sensitive.

I have two good batteries and just wondered if it is possible to use them because they have different ah.

I have just ordered an X-Charge system from Si.

The X-charge requires a feed from the alternator no charge light wire (field exciter) to the coil of the split charge relay. So when you start the car the relay links the 2 batteries and automatically disconnects the batteries when turned off. It is not voltage sensitive, if you give it 12V on it's coil it will connect them the same as any other relay.

You can have 2 different batteries using the system with differing amp hours, so long as you install it as per the instructions and do not leave them connected without the engine running.

The kit is ideal for running auxiliary equipment but a word of caution, I am not trying to go against x-eng here (I have their X-cap and X-brake) but this split charge is NOT suitable for winching or high loads. The 100amp relays are strong enough and if the kit still uses 16mm^2 wire, it is not heavy enough. Still better than a small relay, but if you want to do high loads, get another product that uses a device with a greater contactor area and a minimum of 25mm^2 wire (35-50 is ideal, 50 essential for prolonged winch use). Average 16mm^2 wire has a safe current handling of 100amp...a basic winch pulls ~400.

[Frax]

Thank you for that Adam

I only occasionally use the winch but have 6 extra lights in total and a night heater which I would like to add to the aux battery. Don't suppose it matters really if the batteries are joint when in use, Night heater is one for the Aux though as I do turn it on with the Land Rover stopped.

Nice to see I can run two batteries with different ah though as this saves me buying another battery.

[Adam001]

If you want to be completely thorougher just put a switch inline with the relays coil circuit for when you use the winch. You won't have any issues then. The X-eng will be perfect for the other things you mention.