Second battery

Hmmmm
It's flat?
Yr jumper leads are crap?
Starter motor is the problem, not the battery?

How long is a piece of string?

What is used to separate the batteries and when last did you test it.

Have a look here: Isolator-Test

Regards Derek from ABR

Gday Grundle
Dont worry about the earlier comment- I seriously wonder why they make any comment.

Are both batts the same? I'm sure those with more tech knowledge will explain but I know some of the deep cycle batts dont have a great deal of punch when it comes to cranking and have faced the same issue years ago.
Hope you get it resolved.
Cheers

Hi Grundle

No technical jargon - just thinking out loud for ya!

Door ajar probably accounts for the crank battery going flat - my 100 series avoids this by turning of all interior lights after about 5 minutes - you use to be able to get a "thingy" that you cut into the circuit that did the same thing. Don't know if they are still available (think I got it from Marlows).

Is the secondary battery getting enough/any charge? Earth connection good? Alternator doing its job? How have you got your charging system set up? Is it working? I had a Matson set up and it died with the same result as yours.

What have you got connected to your secondary battery - is there something that has a light, timer or memory circuit in it. These could be slowly draining the secondary batt.

Like I said - just thinking out loud for ya!

Cheers

Anthony

As future insurance and a battery saving measure.

Fit an LED array as the light in the roof so it takes longer to Batten the Flattery.

You will also notice the brighter interior light easier.

Had a similar problem years ago. Both batteries flat. Finished up being a faulty isolator. Changed over to a Redarc isolator. No problems in five years. Redarc the only way to go. See details here

Cheers

Terry

Are both batteries AGM (or other deep cycle) by chance ?
These are being used as crank batteries a lot nowadays, as well as their usual purpose as the 2nd battery.

If so, you usually need to leave the leads connected for a few minutes before cranking, sort of puts a bit of charge across to the crank battery before going for the start.

Good idea for the interior LED globe, but spend a bit, the real cheapies aren't terrible good.

It is possible that the charge from the second (charged) battery is going straight in to the primary (flat) battery rather than going to the starter motor to crank the engine as current takes the path of least resistance. Before you replace any other parts try disconnecting both batteries and connect your jumper leads from the charged battery to the leads which were connected to the flat battery (which should go straight to the starter motor and earth) and the car should start. BTW, do not disconnect the jumper leads with the engine running until the leads have been reconnected to the batteries. In other words, do not run the alternator on 'open circuit'

A couple of thaughts

How long did you leave this door ajar?
Should have taken the best part of a full day to flatten a battery even if there where multiple fairly bright interiour lights.

Now if indeed you did properly flatten the main battery and the second battery still had a pretty healthy charge.......you may not be able to jump start the vehicle with the flatt battery in place.

A properly flat battery is nothing but a mill stone arround the electrical systems kneck.

Even in theory..(the reality is far worse than the simple theory)..if you have one properly flat battery and one fully charged battery....if they are connected together...even after there has been time for them to equalise you will be left with two half flat batteries....as I say the reality is worse than the simple theory...you may end up with two pretty close to flat batteries.

During the equalising process the flat battery will be sucking down everything the healthy battery can give......there may be hundreds of amps flowing.

Another posibility is that...if the main battery has been properly flattened...it may be irreparably damaged.
best case it will takes quite some charging effort to bring it back up to a stable full stste of charge.

To the matter of the jumper leads.......most are just rubbish...I can probably count on one hand the number of actual realy good quality jumper leads I have seen other people using.

I have been involved in many casses where a fully charge battery wont even come close to jump start a car with a properly flat battery......most cases.... I have found required the engine on the jump starting vehicle to be running for a while before a sucessfull jump start can be achieved.....what is happening these is the jump starting car is charging the flat battery ( chucking in 50 to 100 amps) to a point where the battery in the jump starting car can help.

In some cases I have found that with only a full battery on hand...the only option is to replace the flat battery with the full one...to avoid this millstone effect

hope this helps.
cheers