Second battery in LR Defender

[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?

[SneakyPete]

Hi Popotla,

Are the sockets for the "household type plugs" 3 -pin 240V sockets? (I hope not).

Cheers

Pete

[popotla]

They're for 110/120-volt, to be used with an inverter. Please elaborate.

[popotla]

P.S. They're 3-pin.

[SneakyPete]

Sorry Popotla, should have elaborated. My concern was whether any 12V accessories would be wired to mains plugs to match mains sockets used in a vehicle and the associated risk that at some stage the accessories could be accidentally plugged into mains power, but it sounds like you've got it under control.

On your other two:

1. All the dual batt installations I've seen haven't used fuses on the main cables (accessories yes). Can be done but needs big fuses to handle charging/equalising currents. I'd take your sparkies advice on that one but others may have a different view.

2. In the application you describe, the cut-off should be fine on either the +'ve or -'ve cables as either way it will effectively isolate the batteries from one another.

Cheers

Pete

[popotla]

Actually, I'm beginning to wonder if the electrician involved in this, at a Landrover dealer, knows as much about the subject as I'm assuming he does.

On the distribution panel, which runs from the second battery, there are two household-type sockets and two lighter sockets. An inverter would, I understand, be plugged into one of the household-type sockets, and electrical equipment then plugged into the inverter. (Am I right?) But why, then, if electrical equipment is to run via an inverter, would I need TWO household-type sockets on the distribution panel? Wouldn't one of these be redundant?

[popotla]

Hi SneakyPete,

Do you have any observations to make on the following? I'd be gtateful for any.

Reasons for putting in fuses:

1. 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.

2. 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 (fail), and it is usually the alternator that blows (fails). With a fuse in the circuit the 25p fuse will blow instead of the £100 alternator,.

3. Another possibility is that if you flattened the second battery overnight, woke up in the morning and needed to run a high-powered load like an inverter for some reason, most people would go and start the engine, knowing that in doing so the second battery will be connected to the starter battery and share the alternators's output, thus re-charging the second battery.

As the alternator is a generator of electricity, it can run electrical loads when the battery can't, so you'd think you can then run the inverter even though the second battery is dead, that the alternator that's now feeding power into the battery will supply the inverter. It might do, but only if you have fitted a large enough cable between the twobatteries to handle the inverter load, which could be as high as 250 amps if it's a really big inverter.

If you have fitted a big enough cable, and if you've also fitted a big enough switch that connects the batteries together when the engine is running, then you're ok, but not many people run 400 amp wire between the two batteries just for the split charge system. (Remember our old enemy, voltage drop, so 200-amp cable won't supple 200 amps safely unless the leisure battery lives right next to the starter battery.)

So if your like most people, and have 20 or 30-amp wire between the two batteries, then with the fuses inline, they will pop and prevent the cable and/or switch from melting when it tries to pass 250 amps or so down it that the inverter demands.

[drivesafe]

Does it matter whether the cut off switch is inserted in the negative cable or in the positive one?
Hi popotla, it sounds like your in Britain ( 25p fuse will blow instead of the £100 alternator ) anyway, 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.


They're for 110/120-volt, to be used with an inverter. Please elaborate.

Next, don’t worry about the cable not being big enough to run the inverter, if when you say he has installed two “household-type sockets” you are talking about domestic mains power type sockets then the maximum current load for these types of sockets is 10 to 15 amps and again thats a nuttso thing to use.

If you have some pictures we could see just what he has done but I really think you need to find a genuine auto electrician as that set up does not sound too safe.

One more point, it’s far safer to use circuit breakers in high current circuits, not fuses.

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.


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.

Again, your electrician ( mechanic more likely ) has no idea of what he is doing if he has not fitted fuses to ALL four power outlets.

You really need to get your set up checked out by someone who DOES know what their doing.

[popotla]

Hi drivesafe.

Thanks for your comments. I'm in Oman, in fact; just that the stuff I got from the internet obviously came from Britain.

The local Land Rover agency is doing this work; it's their electrician.

Next, don’t worry about the cable not being big enough to run the inverter, if when you say he has installed two “household-type sockets” you are talking about domestic mains power type sockets then the maximum current load for these types of sockets is 10 to 15 amps and again thats a nuttso thing to use.

The purpose of installing these "household-type" sockets is to run, via an inverter, household electrical equipment such as a blender and juicer. Could you please clarify what kind of sockets should be installed, if the present ones are wrong. As you will readily gather, car electrics (or any other kind of electrics) is not my specialty. Thus I don't know why the already-installed sockets are unsuitable for my purpose.

[drivesafe]

Sorry popotla, my misunderstanding, I thought you were going to power the inverter from the battery via the household sockets.

The way you are going to use them is fine.

The other problem is that you are going to be using huge amounts of battery power so you must have VERY thick cable from your battery to your inverter.

To get an idea of how much power you will be using, check the wattage of your blender and juicer.

Cheers.

[popotla]

Hi drivesafe.

I have never seen an inverter, and til a couple of weeks ago didn't know that a thing called an inverter existed.

From where, exactly, will I be powering the inverter?

Second point: If the blender and juicer are to plug into the inverter (which, as I said, in other words, above, I don't yet have, what is the point of having the household sockets?

Third point: At present, only two lighter sockets are running off the second battery. As far as I know, there aren't double adaptors (device for running two devices off one socket)for use with lighter sockets, so two of these sockets, I'm now thinking, are likely to be not enough.