Dual Battery

Projects and discussions on ‘how to’ and your Discovery 3 and 4
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clement
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Re: Dual Battery

Post by clement » Fri Apr 03, 2015 8:42 pm

Cheers guys, perfect. :shark:


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Re: Dual Battery

Post by d3_v8 » Sun Apr 01, 2018 10:07 pm

I am also planning the split system, just curious if you can put the cover on with that battery? I don't wamt the second battery to be visible in the engine bay..

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Re: Dual Battery

Post by anglefire » Mon Apr 02, 2018 11:20 am

As long as the battery isn't too tall, then the cover goes on just fine. :thumbright:


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Re: Dual Battery

Post by XDAndy » Mon Apr 02, 2018 6:40 pm

Can I just show my complete and utter ignorance here and ask ....

Why a “dual battery system”?

I’ve got an issue where my battery is settling to about 11.5v after switching off, and consequently I’m getting the HDC fault and programs off errors on starting the car. Also getting the nav system rebooting and about one in 5 the clock-on-dash resets to —:—

Early days, but don’t seem to get very far with charging with a CTEK; perhaps some improvement?

Battery is 2 years old, so guessing this is he source of the issue

But was toying with the idea of a second battery to give more redundancy.

But why don’t people simply just wire up a second battery in parallel?

Guessing there has to be a very good reason I’m ignorant of....


Andy



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anglefire
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Re: Dual Battery

Post by anglefire » Mon Apr 02, 2018 8:19 pm

The second battery is to run auxiliary kit when parked up. So a fridge being typical or to provide extra umph for a winch.

They are therefore connected via a solenoid controlled either via the ignition- which has risks if the aux has discharged a lot. Or via a controller that only connects the two after a few seconds following startup and the voltage is above a threshold of about 13.5v - 14v

Your issue is either incorrect size battery. They need something like 950cca - which is actually more than the original battery spec.

It could also be an inferior alternator fitted. Only the Oram denso seems to work properly.


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Re: Dual Battery

Post by XDAndy » Mon Apr 02, 2018 11:18 pm

Thanks! Will have a look at some point.

But then, what’s the issue with having two batteries in parallel?

Electrically will it not just be taken as a single battery with a greater capacity?



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Re: Dual Battery

Post by PillowSmuggler » Tue Apr 03, 2018 11:42 am

Andy,

If you have two identical batteries, and are using hefty cables to connect them, then probably not too many issues and it will indeed behave as one.

What more often happens though is that the two batteries are not identical, and therefore they have different charge characteristics and internal resistance. I found this out when I was using a YellowTop sealed cell battery as my Aux; an excellent battery, however I discovered that when I was making short trips, the main battery would remain discharged, and it was the Aux that would be fully charged. Eventually I recognised that because the YellowTop had much lower internal resistance, it charged very quickly, and so "stole" most of the higher voltage/current the car provides whilst it's still cold for the first 20 mins or so, which meant that the Main battery remained pretty much discharged at under 11 volts.

The above explanation is contentious as some people have had no such issues with their YellowTop, however when i replaced my Aux with a Calcium battery to match the chemistry of my main battery, all my charging issues went away. I know of at least one other person who found exactly the same as I did.
I do not know for sure, but I suspect that those people who have no such issues may be using a much narrower wire diameter than mine, or just a better split charge controller, and that this helps reduce the Aux charge current for them; Mine is 16mm cable and so has very low resistance, but on the flip side this does mean that I can start the car with a flat main battery from the Aux without smoking the wiring...something you also cannot do if both batteries are full time connected together as they will also go flat together.


Regards, Dom.
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Re: Dual Battery

Post by XDAndy » Tue Apr 03, 2018 12:46 pm

Hi Dom,

Thanks for that. Makes a lot of sense.

Think this means I need to go away and think carefully about what I am trying to achieve, and maybe learn something in the process.


Andy



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