Powering second battery from wiper wire

Hi gone bush,

It wasn't a thread as such, I think it was a follow up. I thought I read that the power is cut to wipers whilst cranking. This is why I want to read it, but can't find it!

Not sure this is quite correct....(?)

In most vehicles, the wipers will stop mid-way through a stroke whilst the starter motor is being cranked. As such, the act of using the wipers as a trigger for the dual battery solenoid, serves to actually "separate" the 2 batteries during the act of cranking the motor.

The reason for this (AFAIK) is so that you minimise the risk of a depleted 2nd battery "dragging down" the main battery when cranking. If the 2nd battery has been running accessories all night and is down to (say) 10.5 volts, then if it were automatically linked to the starter during the whole starting process, the 2 batteries may end up not being able to crank the donk over.

For this reason, I have always preferred a manually-switched solenoid with a voltmeter so I can see what the state of either of the batteries is. The big benefit of a manual system is that, assuming the 2nd battery is showing to be in good condition (say 12.8 volts at rest), then it is perfectly okay to flick the switch "ON" and enjoy the benefit of cranking with both batteries at once.

YMMV

Roachie

But why, why, why, would you want to join both batteries together???
Isn't the starting battery big enough to turn the donk over?

The purpose of a "smart" isolator, is to keep the primary and auxiliary batteries electrically separated from each other, at all times.
It gives preference for charging the primary battery first from the alternator, then allows the charging circuit to be swapped over to the auxiliary battery. At no time is the two batteries connected to each other.

The primary battery is for starting and basic low current devices.
The auxiliary battery is for running higher current draining devices and is generally therefore a deep cycle battery.

Bill.

Hi All,

Firstly G'ay Roachie (Bill) your old storage box is great!
Let me point one thing out, I have the silver contact relay ready to fit to a vehicle and don't want to use second battery to assist primary in starting.
In my earlier follow up I said I thought that when using wiper wire power is cut to relay when starting. Roachie mentioned the same, and Sand Man agrees. I only wish I could find the thread that has the follow up about it. I would be right.
Thanks all. P&J

G'day P&J and thanks for the comment about my sorry-old box....I'm sure the one you made would be much better looking and professional.... (As an aside, I haven't been able to see the photos on the Ultimate forum, cos Annette put some sort of block on the computer so the kids couldn't look at .....hmmmm... inappropriate piccs, and I'm not smart enough to work out how to over-ride her security thingo.... So I can't get photobucket photos....).

To answer Sandman about the question: Why, why, why etc...

In my case (and anyone else who has a larger-displacement motor), the 2nd battery is a god-send on those really cold mornings (especially if you're in the high country). For the big Chev, I actually have 2 x 760CCA batteries permanently coupled just for this reason (and Kadina isn't all that cold). I also have a 3rd battery (Fullriver 120a/h) which is charged via a Arrid Twin Charge and is used for accessories only, in the same way most people use a 2nd battery).

I would also point out that a number of blokes in our club (including Pesty who runs a 110 litre Waeco fridge 24/7) have their dual batteries permanently coupled together. These blokes have both batteries powering their fridges and other accessories all the time and because they start and drive their rigs virtually every day, they never have any issues surrounding batteries (apart from the fact that batteries DO expire eventually etc of course).

Roachie

Hey Bill your old storage box is same, a good clean up and some new carpet in draw, looks the same.

Thanks to all above I have found the thread I wanted. Number is 60717.
Cheers P&J

Ah Bill,

In your example, joining two batteries to give enhanced starting capabilities is not the issue. Joining them in parallel, you basically have one bigger battery with double the CCA's. No problem with that.

The issue is when auxiliary batteries, used for running fridges, etc. are joined together with the starting battery or batteries.

IMO this is inviting big trouble. In this sort of case, you run the risk of draining all the batteries, if you forget to manually switch over to separate them. This doesn't make for good sense to me.

It's one thing to drain the auxiliary battery bank down and lose the ability to run a fridge.

But it's an entirely different matter to drain down the starting battery bank to the point where you can't start the car. Then you are completely reliant on not being alone.

I have inadvertently buggered two auxiliary batteries on separate occasions because I didn't have a low voltage cutout in place. (I now do)

But, at no time did this effect the starting battery because the automatic isolator did it's job and kept the two circuits separated.

Each to their own I guess, but I'm happier with the "smarter" setup.

Bill.