2nd battery - some really basic questions

Hi all,
I have a Disco 1 1998 300tdi. Because I often drive it into the desert solo I carry a 2nd battery (70Ah) with me under the hood, though it is not connected. The spare battery is charged manually every 2 weeks using a smart charger. On a few occasions when my main battery (95Ah) went flat I had to move the spare to the main port and that worked just fine. It is, however, a bit time consuming and involves some labor to physically switch the batteries, so I thought about wiring the 2nd one too and using a heavy duty kill switch so that I can connect or disconnect it at will - Not that simple as it turns out.

I find it almost impossible to start the car if the starting battery is not sitting in the main battery port and connected to the main wiring. If I disconnect that battery and try to start the car from the other port using either my pre-installed heavy duty cables or external heavy duty (alligator type) truck cables or even a combination of both the car will not start. I will only get some weak attempts and the charge alarm will sound.

I suspect this has to do with the length of the cables from the second battery and/or the connectors at their ends. I estimate the cables to be approx. 1.2m in length and with a cross section of about 80mm square or more. They have heavy duty flat terminals and cross the hood from side to side to attach to the main battery terminals via standard connectors.

Just to clarify, my only interest in carrying a second battery is as a backup. I don't need extra Amps and I want is to stay disconnected 99% of the time. Just want to skip the part of physically moving the batteries when I need to switch.

What am I doing wrong?

Cheers

Ady

Have you ever jumped it from another vehicle when main battery flat?...are both batteries in good condition...could you make up a system that isolates the flat battery and used your charged second one with a split charge system.

Shippers said:

Have you ever jumped it from another vehicle when main battery flat?...are both batteries in good condition...could you make up a system that isolates the flat battery and used your charged second one with a split charge system.

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Both batteries are good at the moment, and both can start the car when sitting in the main battery hub and attached to the normal wiring. It is just when I try to start the car from a distant battery w/o a battery in the main hub (i.e. adding 1-2 meters of wire) that it won't start.
Yes, I have jump started the car on occasion from another vehicle and that too only worked when my battery was in place and with a good set of truck wires. Jump starting it from a smallish car with regular jumper cables does not work.
There is no problem at the moment, I just want the ability to switch the starter battery without having to move it to the main dock.
Thx.
A

Am I correct in thinking from above you are not attaching second battery to main battery when jump starting?...if so and if lazy cranking sounds like an earth problem...

Shippers said:

Am I correct in thinking from above you are not attaching second battery to main battery when jump starting?...if so and if lazy cranking sounds like an earth problem...

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You are correct, I am trying to be able to use just one battery at a time, either the main or the spare, both wired to the same wires, the only difference being that the spare battery sits 1 meter away so requires extra wires. will try to upload a sketch.

In the attached sketch you will see what I am trying to do. Should be very simple in theory, but doesn't work. I think the extra wires and the switch I am using are all thick enough, but I guess the car thinks otherwise?

I think the wire length is unlikely to be the cause, simply because over the years I've owned/driven cars that had the (only) battery housed in the boot.

Am I right in thinking you've got the second battery wired in to main with an isolater then when you use it you disconect main battery and attach second to disconnected main cables...if so that is not imo how to jump start which is what your doing...leave the main connected and atach second to main clamps...@norseman is correct lead length is not your problem it's a connection issue...

Shippers said:

Am I right in thinking you've got the second battery wired in to main with an isolater then when you use it you disconect main battery and attach second to disconnected main cables...if so that is not imo how to jump start which is what your doing...leave the main connected and atach second to main clamps...@norseman is correct lead length is not your problem it's a connection issue...

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I am not trying to jumpstart but rather create a system in which I can switch batteries. The thing is that when you are far from any assistance and let's say one battery went flat overnight b/c I left something on (or b/c of any other reason) my best bet is to use the spare battery directly to start and drive the car (it is an EDC) rather than have it fill the empty one and possibly be left with two low charge batteries.
The spare battery with its 72Ah is more than enough to function as the primary
I just don't want to have to move it.
Cheers
A

I understand that but the way your doing it is not working...connect second battery to main with leads...start car disconnect second battery and the alternator will provide power to charge flat battery...a booster pack as part of your tool kit would be a usefull addition in your situation...

Shippers said:

I understand that but the way your doing it is not working...connect second battery to main with leads...start car disconnect second battery and the alternator will provide power to charge flat battery...a booster pack as part of your tool kit would be a usefull addition in your situation...

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Yes, it's not working for now but I am sure it can be made to work. After all it's just basic wiring. If it's not working and the length of the cable is not an issue, then there must be a bad contact somewhere. I will find it.
As to boosters, I haven't had a good experience with them (at least not the small handheld ones) and in any case I think an extra battery is the ultimate booster as it can not only boost the main battery but also replace it, plus it can help get you to the nearest road if your alternator fails (something that happened to me a few years ago). I will try to improve the contacts and will report if I succeed.
Cheers
Ady

Exactly how are you connecting second battery?...any pics?...your diagram shows batterys connected together with an isolater...then unconnected to start...is that correct if so why have the 2 batteries connected in first place...pics would help...

Shippers said:

Exactly how are you connecting second battery?...any pics?...your diagram shows batterys connected together with an isolater...then unconnected to start...is that correct if so why have the 2 batteries connected in first place...pics would help...

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will take some pics...
Thx!

The upper image is of the spare battery, the lower of the main (it's negative contact is disconnected). Not the cleanest disco, I know but is spends much of its time in the desert.

Ok run through how you in the past tried to start using second battery...step by step...connections etc...

I have a twin battery in my 101 but they're permanently connected and charging...the reason being I use the same batteries in my smart,fiat and an ape I have so can swop around.

Have you got a DMM?

Ady Gancz said:

View attachment 174260 View attachment 174263
The upper image is of the spare battery, the lower of the main (it's negative contact is disconnected). Not the cleanest disco, I know but is spends much of its time in the desert.

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Are them battery connectors just crimped on? I would be soldering them if they are just crimped.

As @Shippers said, do you have a multimeter (DMM)?

mick 1986 said:

Are them battery connectors just crimped on? I would be soldering them if they are just crimped.

As @Shippers said, do you have a multimeter (DMM)?

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Trying to work out how he's using the second battery...his intro and diagram suggests he is not simply closing the isolater and turning the key which if all the connections are good is what I would do but disconnecting main flat battery and some how connect second...might be wrong but that's how I read it...

Shippers said:

Ok run through how you in the past tried to start using second battery...step by step...connections etc...

I have a twin battery in my 101 but they're permanently connected and charging...the reason being I use the same batteries in my smart,fiat and an ape I have so can swop around.

Click to expand...

As you can see in the pics, the two batteries are connected in parallel using thick (about 12mm diameter) wires with standard heavy duty terminals that attach via screw and bolt to the battery connectors. On the negative pole of the 2nd battery there is a simple isolator (green). The objective is to start and run the car using ONLY battery 2, but w/o moving it.
I tried all of the following:

1) Start it as shown in the pics with the negative pole of battery 1 disconnected (lower pic) and the isolator in the ON position.
2) Remove the isolator and do the same with normal battery connector
3) Add a pair of jumper cables (rated 950amp) to the installed cables using alligator connectors
4) Adding two pairs of jumper cables as in #3, a pair for each pole
5) Connect a pair of jumper cables from the neg. of battery 2 directly to the chassis.

None of these worked. I get a weak sound of starter +- charge alarm.

Both batteries age good (14.3V when charged) and will start the car in about 1 second from the main bay.
I should mention that the car is extremely sensitive to having loose connections. If the negative connector on battery 1 is not fastened really tightly it will not start. Once it's tight it starts easily. This is why I am sure this is just a connection issue, but haven't been able to locate it so far.

A

mick 1986 said:

Are them battery connectors just crimped on? I would be soldering them if they are just crimped.

As @Shippers said, do you have a multimeter (DMM)?

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Yes, just crimped. That's a good point