Thread: Aux Battery Cabling

Stick a flat batteryon the end of an alternator, let me know how much current it dumps into the flat battery.

You can also run the cable under the side trim on the floor, the bits that hold the carpet down and around the trim at the base of the b pillars the up under the carpet alongside the wheel arches and through the trim in the area alongside thebseatbases.

If you only want to run a current limited 10a charge to a battery from memory there is a feed wire for the trailer aux pin you could tap into but you would have to provide a current limiter.

I think you are worrying about things that don't happen.
Every battery has to be able to stand a strong inrush current as the car starts and it is quite normal.
A dual battery is no different to just a starting battery in these circumstances.
In fact a dual battery will probably take a higher inrush as the reasonably full starter battery equalizes after startup.
I have run second batteries with 2GA and my current AGM with 4GA , and they don't blow up, and that is with only a metre run.
It is far more damaging IMHO to run the battery down under say 11.5volts so I always fit low voltage cutouts to my appliances.
Regards Philip A

Originally Posted by Blknight.aus

Stick a flat batteryon the end of an alternator, let me know how much current it dumps into the flat battery.

At least 50 amps

It's not a problem Dave. I have used a clamp meter on the cabling (in D2a) to my D34 Optima and right after startup up the inrush current to the battery can be 40-60A but very quickly it drops away down to between 5-15A depending on the batteries SOC.

Alternator might be 120A but remember it's not JUST charging your battery ... it's also running every other electrical item in the D2a .... cranking battery, ECU's x7, Stereo, lighting, etc .... the alternator will only GIVE the battery what it asks for.

My duals equal 130AHrs. If I run them down to the point the Traxide USI-160 disconnects them (12v), and assuming that is equal 50% SOC (65AHr used), then start the vehicle and go for a drive, it takes maybe 1Hr to fully recharge them .... so 65AHr into 2x batteries is 35Ahr/battery .... been working fine this way for 4yrs. The D2a alternator is a smart battery charger and will reduce the current proportionally as the batteries SOC/Voltage rises.

Get it installed !!

Run wire that is too thin and see how hot it gets. Thin enough will at best melt the cable, or at worst start a fire.


6 B&S has a cross sectional size of 13.56mm. What its diameter is I do not yet know as I have not yet purchased it.

I would expect it to be at least 10mm overall. Multiply this by two(pos and neg) then add the second layer of insulation and it is getting quite large.

Originally Posted by Fluids

It's not a problem Dave. I have used a clamp meter on the cabling (in D2a) to my D34 Optima and right after startup up the inrush current to the battery can be 40-60A but very quickly it drops away down to between 5-15A depending on the batteries SOC.

Alternator might be 120A but remember it's not JUST charging your battery ... it's also running every other electrical item in the D2a .... cranking battery, ECU's x7, Stereo, lighting, etc .... the alternator will only GIVE the battery what it asks for.

My duals equal 130AHrs. If I run them down to the point the Traxide USI-160 disconnects them (12v), and assuming that is equal 50% SOC (65AHr used), then start the vehicle and go for a drive, it takes maybe 1Hr to fully recharge them .... so 65AHr into 2x batteries is 35Ahr/battery .... been working fine this way for 4yrs. The D2a alternator is a smart battery charger and will reduce the current proportionally as the batteries SOC/Voltage rises.

Get it installed !!

Kev. I fully understand all of this. I must have spent an hour and a half on the phone to Tim at Traxide a couple of months ago. I am trying to explain it to *********

Hmm there is a scenario where a 2 BS cable is an advantage and that is the situation where the starting battery say drops a cell.


You can then bridge the dual battery isolator, and some have a switch for this, and disconnect the starting battery and start from the house battery.
Try that with small cable! LOL.

I did have an approximation this situation when I had 2 alternators in a RRC and the main one died. I was able to quickly reorg the wiring to feed from the secondary alternator as I already had 2 B&S in reserve from the house battery to the starting battery but disconnected.
You can do this with a jumper for 2 batteries in the engine bay , but a bit more difficult with one in the back. For me It is worth the extra expense to have the back up, as it is extremely hard to push start an auto in the middle of nowhere.
Regards Philip A

Hi *********, and sorry mate but you are mixing info from battery charger type charging and alternator type charging and they are two TOTALLY DIFFERENT TYPES OF CHARGING.

Battery chargers, solar charger ( regulators ) and DC/DC devices are all constant CURRENT charging devices.

An alternator is a constant VOLTAGE charging device ( also known as TAPERED CHARGING ).

Your reference to 10 and 15 amp maximum charging again, relates to batteries being charged by a battery charger and is for batteries being used in a day cycle situation. Such as batteries used in golf carts or folk lifts or RAPS setups.

As far as Optima Yellowtop batteries go, there is no current limit for charging these batteries other than, as you pointed out, temperature of the battery.

To heat these batteries up while charging, you would need charging currents of 200+ amps, and this is not going to happen in an automotive ( alternator charging ) situation.

I have measured 63 amp being drawn by a low Optima D34 battery, and a guy on the UKD3 site has monitored his Optima D34 drawing 97 amps after he accidentally flattened the D34.

Dave’s 6B&S is idea for charging any type of battery and this size cable is not just for current demands of the system, but to minimise voltage drop while charging low batteries.