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| MadTom Member Since: 10 Sep 2013 Location: Olomouc Posts: 616   
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DC DC charger can limit maximum charge current - necessary, when you use some "camping" battery, as they are not designed for high currents.
And it can overcome thin wires to the AUX battery - like when the battery is in a tin tent. And for sure - more complicated, more expensive, better for those, who sell it.  "Drobek" = The Small One - Discovery 2, "Blufínek" = The Blue Thing - Defender 130, and for me at least Ford Mondeo  |
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9th Aug 2017 10:52am |
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| miker Member Since: 13 Sep 2015 Location: Surrey Posts: 1763  
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It does a "proper" charge cycle rather than just the constant voltage charge off the alternator.
Better for the battery long term, especially in high performance large battery banks |
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9th Aug 2017 9:04pm |
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| MadTom Member Since: 10 Sep 2013 Location: Olomouc Posts: 616
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"Proper charge cycle" is good for camping battery used for cycle use = full charge and nearly full discharge. But starting battery is not used this way. Normally it is not charged for 100%, because that results in heavy gas generation from battery, and it is discharged on a bit, for short time but with high current, during normal starting.
I have 2 batteries setup in Discovery 2 - starting/main/winch is Banner AGM 95Ah, camping was some Banner 100Ah camping battery. It was good for fridge, but not good for vibrations of car when offroad. I made simple charging adapter to limit the inrush charge current with 2 relays and one big limiting resistor. Later I have changed this battery to another Banner AGM, but only 75Ah - it is the same size. And use only one big relay to disconnect the batteries when engine is not running. Simple and working. I have a plan to add 2 separate voltmeters for monitoring, but no time. "Drobek" = The Small One - Discovery 2, "Blufínek" = The Blue Thing - Defender 130, and for me at least Ford Mondeo |
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10th Aug 2017 6:10am |
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| Bluest Member Since: 23 Apr 2016 Location: Lancashire Posts: 4209 
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I found this video quite informative. Didn't realise modern cars charged at s lower alternator voltage, and thus won't full charge a second battery without a dc to dc charger.
2007 110 TDCi Station Wagon XS |
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10th Aug 2017 10:09am |
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| MadTom Member Since: 10 Sep 2013 Location: Olomouc Posts: 616
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Standard alternator output voltage is 14,0V (13,8 to 14,2V) - it is to protect the battery from extreme gas production, so you can use sealed batteries with no need to fill in the water. For cycle use (charge/discharge/...) wit full capacity, you have to charge to higher voltage, or it will took too long time to charge the battery. This is not suitable for sealed batteries.
You have always check datasheet of your battery for charging recommendations. And last but not least - there is big thermal change of the battery voltages - higher temperature means lower charging voltage. Also check the battery datasheet. And most important - sealed batteries have no way how to add water in the battery. So once they are "overcharged" with high voltage and loss of internal water, they are permanently damaged. "Drobek" = The Small One - Discovery 2, "Blufínek" = The Blue Thing - Defender 130, and for me at least Ford Mondeo |
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10th Aug 2017 11:37am |
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| ChrisCM Member Since: 10 Jun 2013 Location: Cornwall uk Posts: 572 
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This is interesting! We have 2 Defenders, my 2010 110 SW and SWMBO's 2010 90 Soft top.
We have been on a camping trip to Spain in the 90 and before we went I made up a battery box holding a rather old, but good quality, carbon fibre 100Ah deep cycle battery which has been on a cetek maintainance charge for a couple of years. In the battery box I installed a durite VSR, this was then connected to the main vehicle battery via an Anderson Plug. The battery box has a voltmeter on it. When I connected up the batteries and started the engine the relay switched and I was rather surprised to see a reading of 14.5v on the voltmeter. I checked the voltmeter against another, same result 14.5v, even on tick over. I then disconnected the battery box and checked the voltage on the vehicle battery with the engine running, still 14.5v. The battery box voltage slowly dropped to 12.8v where it stabilised, so fully charged. A bit alarmed now I thought I must check my 110 to see what it was doing. I have a cetek dc/dc charger in the 110 powering a 50Ah gel battery under the drivers seat. This has worked with no problem for a couple of years now. I also have a scan gauge permanently connected, this normally gives voltage readings from 13.8 - 14.3v perfectly normal, but I decided to check the voltage at the starter battery, this was also giving a reading of 14.5v with the engine running. As both vehicles battery voltages were the same I reasoned that it must be ok. I used the battery box to power my ARB fridge, this it did successfully for 5 days or so then would not hold a charge and within minutes of switching off the engine the battery voltage dropped below the fridge cut off voltage and continued to drop to just over 3 volts! When I disconnected the fridge from the battery box, on engine turn off the battery box dropped to about 11.8v and then stabilised. So, is 14.5v normal? Seems a it high to me but for the dead leisure battery, (which may just be old age and not liking to work hard in 40 degree heat) I wouldn't have worried, it's also strange that both defenders, with different size alternators, are showing 14.5v at the battery whereas the scan gauge is showing a more "normal" system voltage of 13.8-14.3 on the 110. Out of interest, Land Rover specify an alternator voltage of 13.6 to 14.4v at 3000 rpm using "suitable test equipment", whatever that is! and where is it connected? 2010 Defender 110 Station Wagon, Santorini Black. |
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10th Aug 2017 3:18pm |
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