Dual batteries - Disco 300tdi

[Lindsay]

Gday all
I have just started thinking about a dual battery system for my 98 300 tdi auto-it will eventually run a winch and a fridge. Being somewhat naive about things electrical I had thought a simple solenoid job would be the relaible way to go - but then I found all sorts of worrying informatiion about battery set ups both with solenoids and controller/isolators. What have people found to work well on series one tdi disco's? Is it important to protect the alternator/electrical system from voltage spikes in this car? 8O
many thanks
Frenchy

[LandyAndy]

Hi Frenchy
I wouldnt go the solenoid way,I have one on our Mitsa****ty Trytoo.Over the life of the car(10 years or so) it has consistantly eaten batteries 2 at a time.So much of a pain I have removed the second battery.
For a cheap system a "Cole Hersey" manual switch may be the better option.It would be well worth looking into the "Trakside" electronic management system,they are well priced and support this website.CLICK ON THEIR LINK for more info.
Andrew

[one_iota]

This is an interesting topic and a bit like the subject of springs and shocks seems to be argued more with dogma than science. The flat-earthers tend to win. :wink:

I reckon it boils down to the battery and the need. A winch has a different requirement to a fluorescent light.

In the words of the Hogan's Heroes character Schultz: "I know nothing"

Let the debate begin style_emoticons/<#EMO_DIR#>/biggrin.gif

[drivesafe]

Hi Lindsay, if you can hang off for about two weeks I intend to do a project covering a dual battery set up.

I’ll start on it before doing Projects 2 and 3.

Cheers

[septimus]

If you decide to go for an electronic unit (about $300+ retail) consider buying the kit from Jaycar for about a sixth of that. You (or a friend) will need some basic electronics skills but will end up with a suitable unit.

[drivesafe]

Hi septimus, first off I make controllers so yes I have a vested interest in having people buy my gear. But having said that, the Jaycar unit is next to useless. For a start it works in reverse to what is needed by the battery.

They say that by reducing the current to the battery when the battery is flat, stops the in rush of current damaging the battery.

This is a total load of Bollox. The battery at all times controls the amount of current it will take during charging.

Next, a flat battery actually draws less current than a partly discharged battery anyway so there is absolutely no reason to restrict the current while the battery is flat and you are probably harming the battery by causing it to remain undercharged for longer than is necessary.

When a battery is flat, the liquid is almost pure water and it needs both voltage and current to start the charging process which in turn, as the battery’s charge state rises, increase the acidic strength of the liquid which then increases the the battery’s ability to take and store a charge.

The longer a battery is kept at a lower state of charge the shorter the battery’s life is likely to be.

You are far better of letting a battery charge at the fastest rate possible and as the battery determines it’s own charge rate, let it do so.

This is like the myth about the bigger the charger the quicker the battery will charge. In part this is correct but only up to a point. I saw a posting on another site where a clown after stating he had been an auto electrician for 30 years, reckoned he could fully charge a battery in an hour because he had an 80 amp battery charger in his workshop.

The fact is that as stated above, it is the battery and not the charger that determines the time and the current required to get the battery fully charged. If you have a battery charger can put out 1,000 amps and try to charge a battery that needs 10 amps to charge it, the battery will only accept 10 amps.

There is a lot more to charging a battery than this but this is the fundamentals of it and conversely if you have a charger that is only able to provide 4 amps then the battery Will take longer to charge.

The Jaycar kit was originally published Electronics Australia and both Jaycar and Dick Smiths carry the kit. The problem is that the original project was probably written by some desk bound journo, in need of an article to fill in some blank space in the next edition, and who did little if any research before coming up with the project.

One more point, if you read some of the postings on other sites you will find these kits have a fairly high failure rate and once stuffed, you throw them away. If you buy another one, you will have passed the price of what it would have cost you to buy a ready to fit, tested and warranted unit.

But each unto his own, cheers

[72pug]

.................as per my email to you.......GET y'self a REDARC.............too easy!

[sclarke]

I use a Rotronics Battery Isolater/charger.

Never failed and i had one in the Disco as well..

[septimus]

Thanks for the input Drivesafe. I am sure you are full bottle on this topic and that all forum members appreciate getting a range of opinions.

Just to clarify though, the Jaycar kit does explain why it is designed to limit initial current to a "fully discharged" battery which they for their design purposes define as having a charge of 10V or less (which is a pretty bad state to bring a battery to...)

As they say, they do this because "the new deep-cycle types in particular can be damaged by a large and sudden inrush of charging current" so they limit the current flow that can result from connecting the primary battery in fully charged condition to the second battery in highly discharged state.

The design wont do any harm to the second battery but if, as the article states, there are some deep cycle batteries that can be damaged by a high charging current if they are discharged to 10V or less it would seem worthwhile incorporating.

Of course this is only one aspect of the design and the article inEA from January 1996 gives quite a detailed explanation of their device.

I went for the Jaycar kit based on the recommendation of another guy who had used one for some time to good effect.

So I guess it is up to the forum readers to listen around and decide what suit them best.

Howard