12v power transfer switch/relay

Hi there - Looking for some input on my 12v power/charging setup. Daily Driver 2002 Silverado Truck with dual 130 amp alternators, one battery bank (2-50Ah YellowTops) and isolated accessory/start battery. Batteries and alternators are currently isolated via Charles StartNow relay (which really only isolates batteries in dis-charge state).
Ultimate goal is to be able to run stereo and accessories down to low voltages and have no problems starting vehicle. I am trying to setup two completely separate circuits (battery bank being charged with future smart externally regulated alternator, and start battery/alternator powering all systems, including 150a max. stereo). Upon turning off vehicle, the power source would be switched from starting battery/alternator to aux. battery bank. I love the idea of having a 100% fully charged battery bank being charged in float at my disposal every time I turn of my truck. All the transfer switches/relays I have come across are of the 120v/12v type (marine-shore application). Is there such a thing as a 12v/12v transfer switch? I've thought about higher amperage relays, but this setup would involve dual inputs with common switchable output. I like the idea of my truck running normally, and want to isolate the voltages that are sometimes associated with charging a depleted battery bank via external regulator from the normal operation of my truck. Brands I've been researching are Xantrex, Sterling, Balmar, Charles. Can anyone suggest others? Any and all input-comments welcome.    -NathanNathan Kuehn

Do you have a charge light? (IE - lamp on when NOT charging?) If so you can try the UIBI (Ultimate Intelligence Battery Isolator), but unlike Projectas etc that sell for $270 (here; down under) or others for maybe $70, this will cost about two thousand cents depending on the relay. (This is separate to the fuses or breakers required at EACH battery, and the interconnecting cable, but that is the same irrespective of HOW you isolate.) A standard 30A relay is what - $3? My 60A & 140A relays around around $10. 140A capable MOSFETs are around $3. And I have seen 250A continuously rated relays for $20-$25 somewhere via the12volt.com. Ther operation is simple - if the alternator is NOT charging, then the relay remains off and hence the batteries are isolated. But that assumes a typical single alternator powering multiple batteries. Two alternators cannot be joined together unless specifically designed to do so.    If one alternator is off, then the batteries can be joined. I don't know why you want externally regulated alternators. IMO the best is internal with a Sense (S) line - just take the S to the battery. Hence the common SL alternator is ideal - the L (charge lamp) circuit is used to energise the battery isolator. But single-wire alternators can't remote-Sense, so while their D+ can be used to trigger the isolator, they can only sense what is at their output - not what is at the battery which may be a few volts away.    And forget isolators that claim "priority charging" - I have yet to see one that does. It is usually total bullsh since (obviously) they are ONLY detecting the system voltage - not the state of charge of any battery! Besides, why would you want priority charging? For repeated short trips where the alternator is always at max output trying to charge the batteries. How common is that? In any case, the fastest TOTAL power recharge comes from sharing available current amongst the batteries (assuming there is at least a few Amps for each). Then a "parallel when cranking" can be used which generally increases the life of batteries anyhow (low average "peak" discharge current).     FYI - the UIBI name is in reaction to "Smart" isolators. IMO their operation is not smart, or rather - not smart enough (how can they be?).

Thanks for the reply - it may be my OCD or merely the -25c weather up here right now, but my battery banks' OCV of 12.2v has me feeling that my internal alt. regulators merely taper a constant voltage and cannot efficiently recharge a depleted bank. I only have 50ah right now, but I would like to use one alternator with an external digital regulator to get full charge on the aux. bank and then float the voltage. This would be a completely isolated circuit from the factory alt. and batt. setup. These circuits would never interact, but would be two independent sources to draw upon, one when running and charging, the other when resting and playing. (I like my music at the lake.) The Charles StartNow is somewhat more expensive than the UIBI you speak of, but does accomplish the same thing. I've had one for nearly two years and can say it is stable up to about 150 amps (I know 'cause I blew some 200A fuses and now its acting funny). It is, for the most part, intelligent and isolates when it should. I must now replace it, but after taking my truck to an "auto electrical" shop and being asked what my setup was actually setup to do - I realized this aforementioned scenario would about sum it up. Nathan Kuehn

In the past I have had erratic voltage issues with running dual alternators in parallel, but have changed them to two identical alts and have had none since. Again, with the upgrade I am looking to do, they would not interact at all - one alt. to charge aux. batt. bank, the other to charge starting batt. and run accessories in normal running operation. Has anyone heard of an automatic 12v/12v transfer switch? The trigger could be a relay fed ignition source.Nathan Kuehn

LOL! It's pretty hard for ANY battery isolator NOT to be more expensive than the UIBI. It is just a relay that is connected in a novel or clever way. How mush simpler can you get. (Oh, that's right - you can't - except for a cheaper smaller & quieter MOSFET!) Yes - the Charles StartNow - another "typical" voltage sensor with misleading review info (it may not overcharge your aux batteries, but it does nothing to stop overcharging of the main battery etc! I have a voltmeter instead.).      Great for marine and similar systems that do not use a rotor-controlling regulator and hence may not have a charge lamp.... But otherwise, a smart drain on people's purse. EG - why cross-connect when there is still surface charge? That prevents sulfation doesn't it? And some do not disconnect until 12.5V - that's 10% discharged - fairly significant when cranking batteries generally should NOT be discharged more than 20% (and deep -cycles 50%). Digital regulator... wth? Why? What is better than 14.4V at the battery - or 15.6 whatever that Optima specify? Or do the digital regulators include battery temperature and current sensing? Digital sounds like more hype to me - unless you are talking about 4-stage (3-stage for AGM) chargers. But many auto alternators have that feature - ie, start at 14.4V and later drop their voltage. Mind you, most vehicles sit on 14.2-14.4V even when the battery is fully charged. I'm not aware of any life-longetivity complaints. And then you get the audiophiles with their big power amplifiers that insist on high voltage (ie, 14.4V rather than 13.8V) presumably because of low quality SMPS in their amps - though amp clones like Boss have output powers that vary proportionately to input voltage! Maybe they use a linear SMPS?) With 2x50AH you only require a 10A charger. An alternator should pss that in. (I have a 75A alternator and a 38AH AGM as main battery. Initial recharge is 40A if I crank for 10-15 seconds.)    And at -15C, your OC voltage should only be about 0.1V lower than at 25C - ie, 12.68 instead of 12.78V, and charge voltage up similarly - ie, 0.1V - 0.15V higher at -15C than at 25C. But the above is not Optima specific - it is generic AGM. Alas there is lots of mis-info. Many assert that alternators do not have much headroom for additional loads. This seems to be based on some accountancy theory rather than any reality nor understanding of alternator characteristics. (How many cars can't handle idling too long with headlights, or not handle extra driving lights etc?)    And electricity in general seems to generate confusion... Last night I read a google review for some automotive reference where the author asserted that "electrons" flowed out of battery -ve through the circuit to battery +ve, whereas conventional current flows the other way, and holes "both ways". He than said it was theory only and not fact since "only one can be right". LOL! Electricity ain't always electrons! (Not that they move that fast anyhow!) No doubt he is another that cannot define electricity.... PS....

local343 wrote: Has anyone heard of an automatic 12v/12v transfer switch?

What do you think I have been describing? Else what do you mean by "transfer"? And thence "automatic"?

Appreciate the discussion - you've already got my mind going. I agree that the "smart" isolators ultimately do the same thing as a relatively inexpensive relay (which I originally used, but 'upgraded'). The Charles StartNow will be removed in this new setup, as I will have two completely isolated power sources and no longer need an isolator, but rather want to switch between them. The transfer switch I am looking for would be like a 150amp 1/2/1+2 battery bank selectable switch used to power the main distribution panel (found mainly in solar and marine application). I merely like the idea of this switch being automatic and I do not need the function of being able to parallel my batteries - if setup in the manner I am speaking of, starter/accessory batt. would only be used while vehicle is being started/running, so will always be charged. Selection of which power source (Aux. batts or starter/accessory batt) the factory distribution panel and stereo (ie-all loads) would draw from would be determined by whether or not vehicle is running/charging. More specifically, once ignition turned on, power draw would switch from aux. batts to starter/accessory batt., thus allowing startup, even with a discharged aux. bank. I am not very familiar with circuitry, are mosfets be used in conjunction with the higher current relays? What can they do in this type of setup? My instrument cluster does display a digital flashing battery when lamp wire from alt. is disconnected. What would be the advantage of using this as a trigger over one from the ignition?       You've got me thinking I might be able to make such a switch with one normally open relay and one normally closed relay. This is exactly what I want, but I also enjoy the peace of mind that comes from the experience of professionals and/or manufacturers, which in this field, I am merely an enthusiast. How does this sound? Normally closed (isolated from all loads with ignition on) relay on the aux. batt. bank, and a relay with a normally open (isolated from all loads with vehicle off) relay on the starter batt. This might isolate the two sources from the distribution panel in the way I am envisioning. Can it be that simple? No experience with normally closed (completed circuit) relays - do they draw any current to remain closed? Any issue with both relays sharing the same trigger? Does ignition trigger (turn key to "on" position) seem like the best choice?Nathan Kuehn

As to the external "smart" alternator regulators, I simply want to maximize the AHrs from the YellowTops, not overheat out my alternator and batteries, and prevent excessive engine load, belt slip during heavy recharging. Upgrading my charging system may not be necessary with this new setup, I will have to see once its put together. On the alternators I'm using (Bosch CS144 series), there is a four pin connector. (P/L/I/S) Only L (lamp) and I (ignition) have a wire going to vehicle. I understand 'S' is a 'Sense' connection.   Would there be any advantage to using this connection to run a sense wire directly to the battery? Any ideas as to what 'P' would be for?Nathan Kuehn

Preventing "excessive engine load" or strain on alternators etc - what rot! (Excluding re-wired alternators.) If "unnecessary EXTRA" strain on engines, alternators and batteries was a concern, then there is NO WAY modern vehicles would be incorporating EMS interactive or controlled alternators! The least "strain" and best life and best fuel consumption comes from NOT isolating batteries when a charging source is available, or when being loaded. Not that "strain" is an issue. An alternator will not strain and engine. Sure - its 5kW (500A) output may stress belts etc, but a 50kW or 100kW etc engine - no way! And alternators self-limit with respect to "strain". However, the same applies - the alternator supplies less overall current - and less peak current - the more the batteries are NOT discharged. The exceptions are rewound "standard" alternators for high outputs that cannot house the required diodes or heat dissipation etc. Otherwise alternators are typical synchronous machines that have self-limiting peak, transient and sub-transient currents. And good alternators are designed with that in mind. They should essentially be self-indestructable. Alas Bosch is a manufacturer I avoid. I was not happy with the robustness of their older alternators (1980s etc). Even their common S&L types have a drain of ~12mA on their S terminal (full time!). Compare that to ~50uA on my Hitachi! And blowing Hitachi or Mistubishis after jump starting vehicles? I haven't heard of it. And since charging systems are generally based on required battery voltages, there is an obvious advantage to measuring straight from the (remote?) battery +12V - not from some distant or downsteam ignition-switched source. FYI - the UIBI is far superior to "smart isolators". Although there are some exceptions depending on specific circumstances, by far the best overall is the simple charge-lamp controlled relay. But specific "smart" isolators must be considered in each case - ie, their voltage settings and delay periods. If you study enough "smart isolators", this becomes apparent from the multitude of voltage settings and timing differences, and the inclusion of some extra controls (crank inhibit etc). But do an actual analysis of behaviour under various common circumstances, and the advantage of alternator controlled switching becomes apparent. (Logic - what better source is there to tell you that the alternator is charging?) But threaten a lucrative market an see what happens. Else see the responses from those that do not understand.... Again, this applies to common regulator and charge-light systems - not stator controlled, permanent magnet rotor etc. (But then current should be used - not voltage!) The only improvement is an in-dash voltmeter - but that should be mandatory anyhow (else appropriately monitored). If concerned about heat, I'd be monitoring the yellow tops. Or longetivity - ditto! Do not overcharge them, and limit their discharge as far as possible (50% is the general deep-cycle rule). Ignition-on control is fine if you want paralleling of batteries whilst cranking. That's a BIG advantage for battery life, but requires heavier than normal interconnection (not a problem for big audio systems). Or Acc control for parallel when NOT cranking. But both above suffer from paralleling if left on, or if not charging. Battery independence is thus lost.