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ChrisCM



Member Since: 10 Jun 2013
Location: Cornwall uk
Posts: 572

United Kingdom 2010 Defender 110 Puma 2.4 SW Santorini Black
Voltage sensing split charge relay current
Hi Guys,

Perhaps one of our electrical/electronics gurus may have an answer to this.

I want to fit an inexpensive Voltage Sensing Relay split charge system in SWBOS 90 to charge a 100Ah removable power pack we intend using to power my ARB fridge and other electronic goodies whilst camping.

One thing I cannot get my head around is the rating of the relay, they seem to range from 30A upwards. What I do not understand is how the charge current is regulated. If my leisure battery in the power pack is completely depleted then I assume that it would take a high charge current, as much as the alternator can supply, certainly more than the 30A of the smaller VSRs. Does this mean that the VSRs 30A fuse would blow? If so is it necessary to match the current rating of the VSR to the alternator? Which, as this is a low spec vehicle I assume to be 85A.
Which then begs the question as to why anyone would ever fit one of the low amperage VSRs as most vehicles alternators will put out >30A and be continually blowing fuses or melting wiring. I suspect I'm missing something? can anyone enlighten me?

Chris 2010 Defender 110 Station Wagon, Santorini Black.
Post #636456 8th Jul 2017 10:23pm
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dorsetsmith



Member Since: 30 Oct 2011
Location: South West
Posts: 4554

Post #636472 9th Jul 2017 6:39am
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ChrisCM



Member Since: 10 Jun 2013
Location: Cornwall uk
Posts: 572

United Kingdom 2010 Defender 110 Puma 2.4 SW Santorini Black
Thanks Dorsetsmith,

I've already perused their site and, as you can see, theirs range from 20-500A! But no indication as to how they regulate the current so that using, say, the 20A VSR the liesure battery draws no more than 20A?

Still confused Confused

Chris 2010 Defender 110 Station Wagon, Santorini Black.
Post #636500 9th Jul 2017 9:42am
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DuncanS



Member Since: 06 Dec 2013
Location: Cumbria
Posts: 295

United Kingdom 2008 Defender 110 Puma 2.4 USW Santorini Black
The majority of the VSRs (Voltage Sensitive Relays) are purely just as they are named, voltage sensitive. This means no regulation etc etc, bar the voltage based open/shut.

They are ideal if you have auxiliary 12v sockets in the back, that you only want running if the engine is on, but you are wiring them from the permanent live. (Just one example)

If you are wanting something that charges an auxiliary battery "properly" rather than just floating the charge up to the same level, you'll want a DC-DC Battery Charger. These devices where they show their current draw are based on maximum limited output rating. The reason they are so much more expensive is they will do the same bottom-up charging profiles and others that proper mains chargers will do, and they also aren't affected by a "smart" alternator in modern vehicles.


Back to the use at hand, your best bet is to use something like this: https://www.durite.co.uk/itm/55875/Voltage...40A/072733

That will be more than happy linking up the two battery banks, and they are reasonably cheap and readily available at any remotely decent Auto Electrician supplier. Fuse at 100A with a mega fuse, and fit to an Anderson connector and bobs your uncle. The alternator will put out whatever its happy with, so having a 140a relay makes no odds and gives you plenty of headroom for future adaptation.

The reason most people still use VSR's rather than going to the DC-DC chargers is twofold 1) They are too friggin expensive and 2) Most people aren't discharging their auxiliary batteries down to below 50% before restarting their vehicle. However I do always recommend that people fit the ability to plug a charger in to their aux battery so once every few months (usage dependent) they can do a proper "bottom-up" charge with a mains charger to keep it in good condition.

Hope this helps

Any further questions, fire away. North Cumbria Search and Rescue - Team Leader
Post #636504 9th Jul 2017 9:55am
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ChrisCM



Member Since: 10 Jun 2013
Location: Cornwall uk
Posts: 572

United Kingdom 2010 Defender 110 Puma 2.4 SW Santorini Black
Thanks Duncans,

Yes, aware of the dc-dc chargers, I have a Ctek S250 in my 110 and it works really well. It also has a split charge relay on the 13pin towing socket which also works well, but the same question applies to that as well!

I'm happy that the cheaper VSRs will work and as this may well be a one off trip I don't want to spend too much!

It's more seeking knowledge Very Happy, I like to think that I have a good basic grasp of electrical theory but this is eluding me. I am assuming that as the battery charge state lowers then the internal resistance of the battery lowers thus allowing more current to flow through the battery?, perhaps more than 20A so why doesn't it blow the fuse on a 20A VSR?

Perhaps I should just fit and forget and just rely on this electrickery to do it's job 😳 ?

Chris 2010 Defender 110 Station Wagon, Santorini Black.
Post #636509 9th Jul 2017 10:13am
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DuncanS



Member Since: 06 Dec 2013
Location: Cumbria
Posts: 295

United Kingdom 2008 Defender 110 Puma 2.4 USW Santorini Black
Treat the VSR's just like automated gates. You'll get the same results as if you whacked jump leads across two batteries generally. In this instance, go for a 140a durite or similar for about £35 and you can't go far wrong. When you're running a CTEK unit, the transferred current is regulated by the charger, based on its point in the cycle. (Usually 3 stages, initial charge, main charge and float) This is to optimise life expectancy and charging time.

The amount of imbalance in the batteries, and what parasitic load is hanging off the end of it will massively affect the current draw into and through the cable, and (I'm also assuming here as I'm not the battery expert in my sales office) that there will be a theoretical chemical maximum it will take anyway. Generally (from my limited knowledge on the subject) for Good ole lead acid batteries its C/5 (a charge rate in amps of one-fifth the overall battery capacity in amp-hours) but I'm not entirely sure how that rationalises over to Glass Mat. North Cumbria Search and Rescue - Team Leader
Post #636511 9th Jul 2017 10:24am
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ChrisCM



Member Since: 10 Jun 2013
Location: Cornwall uk
Posts: 572

United Kingdom 2010 Defender 110 Puma 2.4 SW Santorini Black
Hi Duncans,

I'm going to go with a Durite VSR as you suggest, which I shall mount in my power pack box with an Anderson SB50 panel mounted, then use an Anderson SB50 connector mounted on a fly lead via mega fuse to the vehicle battery. This will give me interoperability with my 110 which has a fly lead mounted SB50 for powering my portable compressor which can then also charge the power pack which I will also use for Ham Radio.

Thanks for all the info.

Chris 2010 Defender 110 Station Wagon, Santorini Black.
Post #636542 9th Jul 2017 2:22pm
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