split charge, voltage sensing

I am wondering if it is possible to use a straightforward system to achieve what a relatively expensive bit of kit does,that is,charging a second battery in your vehicle once the intitial battery reaches its peak capacity. there are plenty of heavy duty relays out there that you can trigger from the alternator so the second battery starts to charge once the vehicle is running,and there are quite expensive "intelligent" relays that do the job mentioned above,but the highr the capacity,the (much!) higher the cost.    Since I will be doing this on Land Rovers the secondary battery will be used for high power lighting and possibly 12v winches,so the carrying capacity of the switching relay needs to be in the 100amp range,I am assured by those who do these things.     I can see a few ways round this,like using a cheap low capacity voltage sensing relay to trigger a higher capacity one,or building/sourcing the voltage sensing equipment myself. But I dont know how to go about it.Other than "intelligent split charge relay" I dont know what to look for,what other applications they might have,also I would like to be able to set the peak voltage of the first battery myself.Or at least KNOW it as most suppliers wont tell me.    Anyone got experience here? Im in the UK,not that it makes a lot of difference......

Split charging systems - as in priority charging? Ha ha ... No such thing... well, not by those systems that are voltage sensing only (as per the video at projecta.com.au - that is complete and utter bullsh designed to sucker in dumb aussies and justify paying 2x to 3x as much for a typical voltage sensing battery isolator. They have already taken that "priority charging" crap off their Australian packaging - I'm surprised the video site survives! If your alternator or vehicle has a charge lamp, then yes, there is a simple, cheap and robust system. It is called the UIBI which is the "Ultimate Intelligence Battery Isolator" - so called to counter the Intelligent or "Smart Battery Isolator" name given to the above system for morons (unless you have a stator system as in boats & some RVs & bikes etc; else prefer something off the shelf and don't mind spending the money). You buy a relay of sufficient capacity to handle the battery & load current you require and have that controlled by the alternator's charge lamp (aka the L or D+ terminal/circuit). Most alternators/regulators will control a typical automotive 30A - 140A relay - ie, up to ~250mA coil current. But if in doubt, supply more info - what current etc, and what alternator type. Otherwise yes - just have whatever capacity "smart" or "intelligent" (LOL!) isolator switch a suitably rated relay instead of the battery(s). Simple. And don't forget fuses at EACH END of the interconnection as near each battery as possible (unless perhaps they are co-located and you don't care to protect the relay). FYI - I prefer self resetting circuit breakers if economical and no fuse or 2nd battery monitoring is available. And forgive my wit, but the "simple relay" solution has been around for quite a while.... And my sarcasm is not directed at you, but others that waste my time trying to defend "intelligent" isolators (unless they have capacity sensing AND CAN ALSO explain why the heck you'd want to limit the total charge rate?)!

OK,cool,thanks.I had searched and HAD read and taken in your previous on this subject,so I was hoping it was you who would reply,mate. My current caravan/trailer charge supply is switched by the alternator light and that how I would usually do a bigger system,but with all these "intelligent" relays on the market,I liked the sound of the switch over taking place once my No1 battery was full. I have a 100amp alternator on my L-Rovers which is uprated from the standard 60amp. So,the product I need is an intelligent battery isolator?Where would I find one?Marine supplies?Electronics? Anyone else?Like the circuitry inside one of these smartboxes so I can build it myself?

Off the top of my head the alternators are all basically a Lucas 127 type,but of other brands,like denso and marelli.But the same thing,essentially.I will have to check that though.Got to go fence in a pig,now.

shirker wrote: I liked the sound of the switch over taking place once my No1 battery was full...

But they don't. So if "priority charging" is your only reason for wanting an "intelligent" relay, then forget it. The only thing "intelligent" about them is their suitability for systems without charge lights assuming a current battery interface isn't available, and their misleading marketing... In efect, all those "intelligent" isolators are trying to do (with their voltage sensing) is determine whether the system is charging or not. What does a charge light tell you? Meanwhile, the voltage sensing systems have two major complications: (1) their voltage switching thresholds & (2) the need for various switching delays depending on what has caused the voltage change. I could also ask WHY do you want priority charging? Is your alternator not big enough to charge both? You prefer less total recharge overall (because you consistently only run your vehicle for a few minutes which is not enough to recharge your main battery so it is going flat and the alternator has no spare capacity to charge the 2nd battery as well?)? Apart from the fact that the voltage sensing relays/isolators CANNOT priority charge(Note**), there is no logic as to WHY do so - unless there is no spare alternator capacity during run-time duration. (Note** - excluding current sensing, capacity can be determined via signal injection. When you find a system that uses such frequency injection and capacity determination, let me know.)

PS - I see the Lucas A127 is a single-wire alternator - aka a D+ (which is the charge lamp circuit). They are usually NOT preferred for multiple or remote battery installations because the alternator only senses the voltage at its output (B+) not at the battery as do the SL types (Sense & charge-Lamp) or SIL types (...Ignition...). (The older IL types can have their Ignition power come via an IGN controlled relay that connected the alternator's Ig to the battery +12V.)     The S-type alternators can have their sensing from remote batteries. Whilst this may boost system voltage (that's another issue, and detectable via your dash voltmeter), it's the usual method use to ensure proper charging of all batteries concerned. But it is not used for hi-power audio systems or anything that could suffer prolonged remote voltage sags. Excess voltage can be reduced with a PWM circuit (a $3 MOSFET controlled by a PWM voltage regulator) which is easier than boosting the remote battery via a dc-dc converter. Of course, it is better minimising or eliminating the voltage drops involved, and if the remote battery's loads are small, the remote battery will eventually recharge fully (based on alternator/system voltage) as the voltage drop gradually reduces.

Well,you sure know more about it than me! I was told what I needed to do was use a second battery with a deep cycle to power my extras,including camping gear like fridge/coolbox and aforementioned lights etc,while the vehicle is not running.Im just going on what I have been told on my local landrover forum,thats all. But,since you put it like that,it sounds like I would be best doing it the way I have done before and use a simple high current relay triggered by the alternator charge light.Its a system I know and trust and is relatively cheap to build with the relays costing around £12.In fact I have some on my old lorries tail lift system that would do the job. Thank you for de-bunking all of this for me,as I said in my OP I was unimpressed by the fact that I couldnt adjust the threshold of switch.Now,I am unimpressed by the whole "intelligent" thing.So....back to the tried and trusted. Thank you for your help.

Dear Shirker, If I have managed to de-bunk disinformation, then this has been a welcomed pleasure! But Maybe you should change your name from Shirker to Enhancer or something....? If I am incorrect in any of my comments, then I look forward to correction. Part of what you have been told is quite correct - ie, a 2nd battery, deep cycle preferred, and isolated when NOT being charged (or loaded/in-use). (And recommendations are not to discharge a deep cycle below 50% capacity.) But it's how the batteries are isolated that has been prone to misunderstanding or money making ventures & misleading marketing (sometimes innocently - or is that my stupid naivety?). And if paralleled (interconnected) ONLY when being charged - or being used (eg, cranking or reasonable loading) - then battery matching is NOT required. [As opposed to permanent interconnection where - not only is connection (charge & discharge) symmetry required - but also battery type, batch, history, temperature etc matching for maximum life, which will still be subject to the first to failing which in turn takes out the remaining battery(s).] Just be careful of alternators that cannot power the relay solenoids (coils). There is a simple solution to that (somewhere) - namely the addition of a MOSFET which means no loading of the alterator's charge-Lamp circuit (no more than a few micro-Amps). In fact. the relay is often not needed - MOSFETs that switch over 100A can be bout for a few dollars.... (I'll update this or PM if/when I get such details... it's currently an issue of interest for me...)

'Spark,if you can tell me how to go about setting up a MOSFET system I would be eternally grateful.Its a real change conversing with someone who knows what they are talking about and isnt trying to sell me something....so thanks,mate much appreciated. Another thought,if my alternators are ignition switched by relay,and if its the relay I THINK it is,its rated at 70amps.Would I need to be uprating this too? I cant think why I would,but the understanding I thought I had of all this is fast evaporating ! Im glad that this is an interest of yours cos then we BOTH may get something out of this. Oh,I currently have,on both the discovery 300tdi's that I use,alternator charge light switching of of two relays,one of 20 amps and one of 40....not that they handle anything like that,they are for the fridge and battery charge supplies to my caravan/trailer hook up.From what you say,it may be that adding another,heavier coil relay to switch the secondary battery may push the charge light system out of its comfort zone.And yes,I do realise that its the coil current that is relevent in the relays not the switching contact capacity in this case! My only experience of MOSFETS has been as bits of power amps that blow up all the time.I used to be a sound engineer on big live rigs si I have hauled a lot of MOSFETS around over the years.....its about time they started repaying the favour.

So the Durite 12v 140a split charge relay at the bottom of this page is a waste of time,then?This was my idea of priority charging,prior to all this...... http://www.vehicle-wiring-products.eu/VWP-onlinestore/accessories/towing.php#scr I had planned to use one of these,or a lower powered (so called)sensing relay to trigger one of these 12v 120a relays,the R120R..... http://www.vehicle-wiring-products.eu/VWP- onlinestore/relays/relays.php#high to switch the load to the secondary battery.But from what you say,there is no point.....is there?