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10forcash
Member Since: 09 Jun 2005
Location: Ubique
Posts: 16534
  
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Maybe something to do with which side of the equator you're on 'cos my clamp ammeters, voltmeters and 25+ years of working with DC electrickery in the northern hemisphere show that (provided the terminal voltage is at 'charge' levels) a fully charged battery will discharge into a partially discharged battery - as demonstrated here
http://www.disco3.co.uk/forum/post201879.html#201879 of course the raised voltage has something to do with it...
The reason I mentioned the float valve in the analogy was to illustrate that there needs to be some form of replenishment, to use the 'pipe' analogy is probably too simplistic but without getting too deep into the electrochemical properties of storage batteries I felt it was a reasonable comparison, as stated before - I don't advocate connecting two batteries together with different states of charge unless there is some form of charging input (although I use solenoids with a 14VDC operating voltage, they will actually trigger at 12.8VDC) purely for the reasons you expanded upon 
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 29th Nov 2007 1:27 am |
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drivesafe
Member Since: 23 Feb 2006
Location: Gold Coast, Australia
Posts: 867
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Yep 10forcash, I read that thread when it was first posted.
Amazing but I don’t think I'm game to experiment with it.
At the end of the day, it comes down to just what you want to get out of you dual battery set up.
If the auxiliary is just there as an emergency back up in case the main fails then any isolator will probably work.
If on the other hand your out to get the longest operating time between charges then there is only one choice.
My Traxide isolators will give you far longer operating time, they literally convert your 55 A/H auxiliary capacity into at least 80 A/H with out installing a bigger battery.
Cheers. 2008 TDV8 RR Lux + 2009 D4 2.7
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| 30th Nov 2007 8:28 pm |
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xtc
Member Since: 27 Mar 2007
Location: Hampshire
Posts: 1072
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Quote...
My Traxide isolators will give you far longer operating time, they literally convert your 55 A/H auxiliary capacity into at least 80 A/H with out installing a bigger battery.
Only by 'pinching' 25ah out of the main battery.... Nothings for free. 25% of the vehicles capacity and as you should never run a vehicle battery below 50% it only leaves 25% for starting and any other vehicle related usage.
Also, if two batteries are hard wired together there terminal voltages must be the same, albeit a current may be flowing one way or the other. Neither individual battery will dictate the cut in voltage... Sorry to butt in, but it is a forum! Cheers, Richard.
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| 30th Nov 2007 9:45 pm |
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10forcash
Member Since: 09 Jun 2005
Location: Ubique
Posts: 16534
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drivesafe wrote:...will give you far longer operating time....
2 years with the current configuration and battery, 20+ years (albeit across several vehicles) prior to that - how long do you want???? 
There's no pee'ing match from me - I just don't like the 'smoke & mirrors' approach to a simple problem, I'll install any system the client wants providing they know what they're getting - other than that, what XTC said
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| 30th Nov 2007 10:15 pm |
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drivesafe
Member Since: 23 Feb 2006
Location: Gold Coast, Australia
Posts: 867
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xtc wrote:Sorry to butt in, but it is a forum! Cheers, Richard.
Hi xtc, I don’t consider it “Butting Iâ€, more like some honest opinions.
First off, not taking a vehicle battery below 50% is a recommendation of the battery manufacturers and is only stated as such to protect the manufacturer not the battery.
Further to this point, if you had a look around our web site you would have seen that my normal SC40s and SC80s actually cut out at 12v which means the cranking battery is continually taken to 50% and as we have been manufacturing these devices for around 19 years and never had a problem ( unless a fault already existed that would cause problems whether there was an isolator or not ).
On top of all this, the first units we supplied for D3s ( before LRA asked us to make a special version for the D3s ) were actually set at 12v cut out and these D3s are still running around with the same set ups and none have had a problem.
Last but not leased, as the batteries will initially be connected only after the motor is running, therefore the alternator’s voltage will rule that the only current that is going to either battery is from the alternator itself, not current from one battery to the other and once the motor is stopped, both batteries will be at the same voltage anyway.
One more point, after more than 30 years of working in the auto electrical field, I am yet to see a single problem caused by connecting two batteries together, even when they have different SoC, with the exception of when one of the batteries is stuffed and in such cases, common sense SHOULD prevail.
Cheers 2008 TDV8 RR Lux + 2009 D4 2.7
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| 1st Dec 2007 12:52 am |
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xtc
Member Since: 27 Mar 2007
Location: Hampshire
Posts: 1072
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I Have to say, all I ever do is connect a 300a solenoid linking the batt's to the ignition... The few seconds the second battery is connected to the charged one before the engine starts is immaterial. Also has the advantage that if the cranking battery has become low the secondary will kick in!
Think a lot of the sensing relays etc were invented to make wiring easy, i.e. so a ignition feed etc was not needed for the DIY installer.... I'll duck now!  Cheers!
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| 1st Dec 2007 1:24 am |
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AM510657
Member Since: 01 Dec 2006
Location: nr Paris
Posts: 25
 
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Great thanks « drivesafe » and «10forcash ». Your comments are very instructive. Discovery 3 (2007) HSE TDV6 Java Black/Ebony Black, Privacy glass, Manual gear box
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| 2nd Dec 2007 12:37 pm |
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