external power supply on conversion van

I am considering adding some additional power supplies to my 1993 GMC Vandura 2500 Conversion Van by Explorer. I occasionally use the van for camping on longer road trips. I would like to be able to power a few things without having to run the engine to keep power up for the accessories while camping. My intended install is to use a through hull marine 20 amp plug to bring the 110 ac 20amp power in. Attached to this plug will be a standard power strip with the gfi breaker built in. This power strip will operate the heater, digital converter tv box, tablet pc charger, and the power supply. The 12v power supply will be a 350w computer power supply rewired to use the clean 12.6v dc outputs to power the vans accessories without needing to run the engine; ie the stereo, tv, 6-pack cooler, phone charger etc. I've run a small camper with one of these in the past using relays to disconnect the 12v battery when the 110 cord was powered up. The question I have is how should I tie the power supply into the van's 12v system. I am wondering if I can just tie the 12v from the power supply into the vehicles battery and use a standard 40a auto spdt relay to disconnect the power supply from the vans system by the ignition circuit, and will the steady 12.6v output from the power supply hurt the van's battery tying in like this? Do I need to use blocking diodes between the two? Thanks for any advice. :)

You can't use a PC PSU to connect to a battery - you must use a battery charger (a 12V car battery system is not 12V whereas a PC PSU is; and chargers are typically up to 14.4V). Disconnect the battery if using the PC PSU, not diodes.

I appreciate the quick answer... and just another quick question based on your answer. Is there a relay that would be of a high enough amperage that I could use to disconnect the battery when I power up the power supply or would I have to use one of those manual auto parts store disconnects? I would like to put in as many fail safes as possible in case someone other than myself is using it. When I set up my small camper, I just had normal 14 gauge wiring to deal with as it was only running 3 1156 bulbs and a car stereo so I was able to use relays to disco the battery when the power supply turned on. I'm also going to see if I can separate the conversion accessories bus so that it would be simpler to use a relay to separate the systems when on shore power. I appreciate any suggestions you may have. :)

Relay size is not a problem, it's more a case of how? A relay that disconnects the battery (NC contacts) which - if it fails - reconnects the battey. Or a relay of any type between the battery and vehicle which - if it bounces or fails - could be a hazard. Or a change-over relay selecting battery or PC PSU which ditto - may compromise driving safety. Best is an interlocked mechanical switch, or simply mechanical. You could leave the starter motor (heavy) connected to the battery (that saves a few hundred Amps of switch/relay rating). Many people use battery chargers which suits vehicle electrics (13.8-14.4V) and recharges or floats the main battery. Why not boost the PSU output to 13.8V, that might solve all the problems? It may be able to be left connected when not being used, else a (~400W/12V 33.3A=) ~30A or larger relay that connects it when being used. Many convert PC PSUs to other voltages. But boosting from 12V to ~13.8V may be as simple as adding 3 series diodes to its sense/feedback from the 12V output. (IE - 3x 0.6 - 0.7 diode drops = 1.8 to 2.1V.) That voltage subtracted thru the 12V output sense loop means the PSU will boost output by 1.8 - 2.1V => 13.8 - 14.1V. Purrrrfect! (Depending on battery, but normal range is 13.8-14.4V.) That also means you have your battery to supply the voltage sags when your audio hits those big thumps... oops sorry, wrong OP.... But that rectifier (aka charger) and battery is a UPS - an Uninterruptible Power Supply/System (not all UPS are AC output!). If your PC PSU blows, you can get to it without tripping over things in the dark.... Mind you, you might want an alarm followed by a timed relay else battery protector to disconnect the battery is the PSU fails, or a dual-battery, etc etc... Are you game for some PSU hacking? Why not - they usually only cost ~$10 or less. Just ensure they are disconnected from the AC supply (as in cable pulled out - don't trust switches), and that the internal capacitors are discharged (they can be over 200A DC - more than enough to maim and kill!!).

I am all about messing with the PSU's... I used to be a 12v installer though installing and modifications are 2 diff things. The camper I install the 300w supply to was as simple as throwing the green sense wire to ground to get the supply to power up. In that case I dedicated a couple of the 12.6v wires to diff lights and bulked the rest together to a bus for future needs. I think I had estimated around 20 amps from that which is more than enough for my needs. The only reason I wanted to use a 350w is because I already have 3 of them in my garage. :) I had not heard of boosting the voltage and I will be rereading your posts until I understand it fully. I appreciate your help.

Yeah - sorry, the PSU boosting was a lat minute thought... (Obvious in retrospect.) As to PC & ATX PSUs etc - what cheaper is there? A few years back I went though the exercise of finding a suitable 12V supply for a water system (my "sump pump" which sat in gully-traps to pump waste water onto the garden etc during drought - a $20 marine pump, an SPST relay and 2 (brake fluid) level switches - all under $30). Even 12V 2A supplies were >$30, and I think I needed 3A. Then the obvious struck - a surplus PC PSU - often available for $5 - $10. Way more than 5A @ 12V & 20A @ 5V in a well grounded/earthed case complete with "remote" power up option. That can be housed indoors for easy power-off or on as needed, and maybe controlled by the pumps/switches too (not that I've considered that yet). Anyhow, many modify for other voltages - usually boosting the high current 5V rail to 12V (13.8V etc) for audio amplifiers etc. It has been done/mentioned over at mp3car.com and elsewhere. In SMPS, voltage boosting is easy (unlike linear/transformer PSUs). Rating-wise, current is current, and voltage is merely insulation (unless there is overvoltage or spike protection like Zenors or MOVs etc). And despite whatever circuit complexity for the SMPS, they all usually depend on a single sample/feedback line from the output for regulation purposes. Find that feedback - usually it's a resistive voltage divider - and insert a few diodes or whatever gives the required drop. Knowing the SMPS (dc-dc converter) chip helps heap - then you know which pin is the feedback and can trace the circuit form there. The are typically 8 or 14 pin DIP IC packages. (MC33363, MC34063, TL496, etc.) Of course some manufacturers disguise their chips, but it's usually easy for figure out. Of course I cannot stress the safety issues enough. Not only the AC side, but SMPS usually involve HVDC - ie, >1.4 times RMS, hence over 170VDC & 325VDC for 120 & 230 VAC respectively). And if you are a 12V installer, you may be aware that HVDC occurs at a voltage far lower than HVAC - ie, 80VDC is far more dangerous than 120VAC etc. [ Just last night I forwarded a youtube link with an Indian electrocution (on top of an electric train). Though the title claims "40,000V", I suspect it's the standard 500VDC to 1500VDC train supply. I like the way the, er... person "petrifies" and.... You may survive 220VAC and 220,000VAC shocks, but not DC! ]

I should probably advise of my level of understanding lol. I know what a blocking diode does and what a resister does... I can only use them in the simplest forms like lowering voltage to operate a light emitting diode etc. I've used relays a bunch for changing + to - (ie door locks) and using them as a switching solenoid to use a smaller switch on a higher amp circuit. I used to install alarms, gps trackers, vehicle monitoring type stuff and a myriad of other accessories. Actually working with board level circuitry is greek to me. I can usually follow a diy instruction or read a basic schematic. I can tell your level of understanding far exceeds mine so the following quote left me drooling.

oldspark wrote: Knowing the SMPS (dc-dc converter) chip helps heap - then you know which pin is the feedback and can trace the circuit form there. The are typically 8 or 14 pin DIP IC packages. (MC33363, MC34063, TL496, etc.) Of course some manufacturers disguise their chips, but it's usually easy for figure out.

Now that being said I'm going to ask a noob question lol. If the accessories in my vehicle are designed to operate within a given range of 12v, why would the 12.6 volts not suffice with enough amperage? I realize an alternator pumps out up to 14.5v at a certain rpm which keeps the battery charged with a load on it, but wouldn't the 12.6v from the psu maintain low load accessories without draining the battery? I would understand if I was running high power amps and a high wattage head unit but I've got a simple 200w head unit that would never be turned up loud while not traveling. The cooler and the 12" tv would be the highest draw and I havent checked what they use. The tv turns itself off if the battery voltage drops below 10.5v and the cooler I'm not sure. It has run 3 or 4 hours (i forgot to unplug it) without running the battery out so it can't be too high of a draw. I know if I can separate the conversion accessories that the psu will offer plenty of power to operate them, I was just concerned about what kind of damage I would do the vans systems if any.

Oh forgot to add that I've been burned a time or two with low amperage DC. Its ugly lol. 110a/c sucks, 240ac will throw ya across the room but even low voltage high amp dc can cook ya.

If your PSU is 12.6V, it may be ok, but 12.6 seems a bit high - they are supposed to be 12.0V with (as I recall) a 5% tolerance, though maybe it is 10%. First, some "benchmarks" or theory: A (rested) 12V battery varies from maybe 11.4V (flat) to 12.7V (fully charged). A battery taken off a charger will have a "surface charge" that can take it to ~13.6V (even if it is flat). Surface charge can take many minutes of full beams (>100W) to dissipate - ie, there is lots of "charge" in that surface charge. Normally surface charge dissipates itself within 24 hours.      Chargers (aka rectifiers, alternators etc) are set for ~13.8 to 14.4V to "adequately" charge 12V batteries. ERGO: A 12V system is rarely 12V. You cannot determine a battery's SOC (state of charge - ie, %age of full capacity) whilst charging or after charging (ie, battery/charger may be 14.4V and the battery might measure 13.2V if the charger is removed, but the "actual" battery may be 12.3V (~50% discharged). Why not a 12V or 12.6V PSU? If the PSU is 12.6V and the battery is 12.7V, will the battery discharge into the PSU? (Ditto for alternator at 13.8V etc.) Will a battery blow the PSU anyhow? (IE - a PC PSU was never designed to connect to a battery, therefore it may not have protection diodes etc unlike battery chargers. However that protection may not be required...) Even if ok, the nominal battery fully-charge voltage is 12.67V (but in practice this varies) hence at least some discharge before the 12.6V PSU takes over. Partial discharge causes sulphates to form (sulfation), hence better to disconnect the battery and leave as fully charged as possible.    As to finding those chips, it's just a case (ha!) of removing the PSU cover and searching. You seem well qualified (ie, understand kicks, and can access a crew driver). Normally PSUs are constructed quite well and only a few Philips-head screws need be removed. Then search for the chips which involves the mandatory slicing of hands and fingers on sharp edges as you manipulate cables out of the way etc. Some chips will merely be 4 or 6 pin opto-couplers, and others may be on fan control or temp-sensing boards. The SMPS chips are normally in with the big chokes (coils) and capacitors, though the chips will dive FETs which "buffer" the coil switching. What brands and models are your PSUs? You may be able to find pics or conversions on the web.

Ok I see the issue with a back feed of a battery that is charged at a voltage higher than what is being put out by the psu. the 12.6 volts was what I measured on the 300w that I did a couple of years ago. I haven't converted anymore but am going to do so tonight just for the fun of it and get this project rolling. So a blocking diode wouldn't correct the issue of back feed? I learned what I know on the internet and read a lot using the fluid method... the blocking diodes I always pictured as a one way valve... it would let current flow one way but not back into the device. Thats why I suggested that. I see your point though. I need to get a couple of psu's modified tonight so I can see what realtime output is on them. I do have a conversion bus up under the hood that runs a short 10guage wire direct to the battery. I need to find out what all it controls as it would be easy to disco what I need from the battery using relays and just run that stuff off the psu. I'll post my findings soon.