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Thread: DC-DC, should I and brand comparo

  1. #1
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    DC-DC, should I and brand comparo

    Hi all,

    Having only a simple relay (ie not a proper one) between my main and secondary batteries which has proven to be barely adequate for testing purposes I feel the need to fix and upgrade this setup in my P38.

    I have:
    • fixed alternator output afaik, a minimum of 14volts
    • lead acid main battery, considering replacing it but not sure with what
    • hydraulic winch, ie. I do not need stupid amounts of power in the front
    • 270ah of gel batteries as secondaries, might replace with different technology
    • no electrical setup to speak of as of yet


    I would like:
    • hook up HF and other radio's to secondaries, have a full ham license so will be using it stationary
    • run my fridge, of course
    • decent charging of secondaries, both stationary and driving.
    • keep the ever so sensitive P38 main electronics to retain their preferred voltage levels.


    I am probably going to:
    • travel for very long periods of time so no maintenance charge from the wall possible
    • have a solar panel or blanket
    • bring an ordinary switching power supply for charging


    My questions:
    • should I go DC-DC
    • what size
    • what brand


    The long version below

    It seems that with certain battery technologies simply bridging the main alternator output to the secondaries through a relay (heavy duty) and a voltage sensing circuit is simply enough. My current testing with my admittedly lacking setup has shown that just dumping power into the secondaries might not maintain them well enough.

    This is where a DC-DC charger comes in, these were designed to provide the same charging regimes that a multistage "from the wall" charger would normally provide. I like the thought of this since it would maintain my batteries as they were meant to. In the land of DC-DC there are a lot of differences and by now numerous brands on the market. In Australia you get that wonderfull redarc stuff and what not but down here our choices are more limited. A "local" brand here is ctek however and I have seen them being used here on the forum.

    The main difference between a direct connection (relay or manual isolator) and a DC-DC charger is the amount of potential current that can be delivered to the secondaries, given enough wire diameter to handle it. On the other side of this same coin; smaller-ish cables from the front to the rear, since voltage drops across the wiring are handled by the charger in a DC-DC setup.

    Currently I have gel batteries as secondaries. They were secondhand but free so who's complaining This type of battery is not suitable for high current charge or discharge. Large gauge wires and a large charging current are not applicable and a DC-DC charger would work nicely in this situation, I think. However... Seeing as these batteries were free, not new and I do have other needs from my electrical system it seems reasonable that I would change battery type at some point in the future. Lithium is only a consideration when weight really becomes a factor and in all other circumstances I think AGM or LeadCrystal would be the first contenders.

    Both these battery types can handle large currents a common DC-DC charger quickly becomes to light to handle these currents. Redarc "recently" (could be over a year but I can't recall) brought out a 50Amp model but they are quite expensive and the only brand that produce one AFAICT. most common units from other brands are in the 20-25 amp range. So far, without scouring out every option on the intarwebs, I have found that ctek offers a nice option in their 140A offroad kit. 140A OFF ROAD

    The concept of this kit is that it has a DC-DC charger up to 20 amps in the form of the D250SA and it has the ability to simply hook the mains up to the secondaries through the SMARTPASS 120. The way I currently understand how this works is that under normal circumstances, when the secondaries are empty the smartpass bridges over the D250SA and full current from the alternator will go into the secondaries, charging them with as high a current as they want/can absorb and by the time the voltage has raised and the amperage has dropped to a certain point the D250SA will kick in, taking over the rest of the charging. On paper this sounds to me as the best of both worlds.

    Since this unit also has a MPPT controlled solar input that makes life even easier when stationary for a long time. I also figured that since solar is nothing more than a DC voltage source I would bring a switching mode power supply that could provide any voltage inside the input range of the solar input and I could charge the system when stationary and have mains power available. (disconnecting the solar panels at that time of course). A modified HP DL380 server power supply is small, reliable, auto sensing input voltage and can crank out a whopping 80 amps!

    Now, I am not sure of redarc or national luna or any of the other manufacturers have similar solutions but in general I wonder what the "best" brand in this business is? ctek is widely seen around here with their 240V chargers and they seem to pretty much own this market, making me believe that since the product is not cheap it must be doing something right. One would assume that this translates well into the mobile solutions. The housing is plastic though and I would think a heat sink type housing would be preferable.

    In any case, is there any information out there that my google-fu could not find or does anyone here have any experience on this subject?

    Thanks for any input

    Cheers,
    -P

  2. #2
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    Hi and the best DC/DC charger for your needs would be the Sterling 60 amp DC/DC unit and you can get them from the UK. These are usually cheaper than Redarc and do a lot more that most other brands of DC/DC

    But first off, where are your batteries located?

    What size cabling are you using?

    Is you solar fixed to your P38 or a portable panel?

    Does you solar already have it’s own solar regulator?

    Regards, Tim

  3. #3
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    NO, faff the DC-Dc off unless you're stepping across voltages.

    mount your solar to the aux batteries

    get a traxide to handle the charging while you're driving side or running the engine while stationary.


    even a dead simple 200A relay with a VSR will do a better job than most DC-Dc chargers for getting the bulk into your batteries.
    Dave

    "In a Landrover the other vehicle is your crumple zone."

    For spelling call Rogets, for mechanicing call me.

    Fozzy, 2.25D SIII Ex DCA Ute
    Tdi autoManual d1 (gave it to the Mupion)
    Archaeoptersix 1990 6x6 dual cab(This things staying)


    If you've benefited from one or more of my posts please remember, your taxes paid for my skill sets, I'm just trying to make sure you get your monies worth.
    If you think you're in front on the deal, pay it forwards.

  4. #4
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    Quote Originally Posted by Blknight.aus View Post
    even a dead simple 200A relay with a VSR will do a better job than most DC-Dc chargers for getting the bulk into your batteries.
    Yep, especially with that size of battery capacity.

    The alternator voltage is fine and should not be the problem.

    Sounds like another problem, such as thin cabling.

  5. #5
    DiscoMick Guest
    A DC-DC would limit the amount of charge going between your batteries to maybe 20 amps as you mentioned. A DC-DC is only needed if you have lithium, which have different charging requirements.

    Make sure your cable is thick enough.
    My Defender just has a fat cable with 60 amp fuses each end and a Traxide unit linking the starting battery and secondary AGM. Works fine. No fuss.
    Another cable with 50 amp fuses runs from the alternator to an Anderson plug on the rear bumper for charging our campers batteries. An auto electrician did this work.
    A 12 pin plug powers other trailer needs. A fridge in the back runs off the secondary battery.
    An 80 watt solar panel on the roof runs to a PWM regulator next to the start battery, because you always want the starting battery to start the vehicle.
    This is a no fuss set and forget system. So I suggest you keep it simple. Maybe also carry a solar blanket as a backup.

  6. #6
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    Thanks for the replies!

    My batteries are currently located in the former spare wheel well but since I will be fitting an extra fuel tank there they will move to behind the front seats in the foot well of the rear seats.

    The current size cable is quite small, 6mm2 I believe. I know this limits the current while charging at this moment but since the gel's can't absorb that much anyway it is less of a problem. It'll just take longer. In the end when the batteries are nearly full the current will drop far enough that the voltage drop over the cable is negligible. When moving to the permanent setup however I have a large quantity of 50 or 70 mm2 neoprene cable laying around for free so I'll be using that. (I also have the tools to crimp connectors etc. for this cable)

    I still need to purchase a solar solution so I can go either way. I think a blanket is more flexible (pun intended) and will allow me to get the most out of the sun. An mppt charger sounds like the best way to go as well?

    Just to be clear, not to drone on, I am well aware that a DC-DC will limit the current. A traxide or similar device will pass all the available current to the secondaries. The CTEK setup does both as I understand it. (but can't handle more than 120A but I guess that is hardly a problem) The alternative could be the sterling DC-DC and a traxide for example, bridge the alternator to the secondaries for bulk charging and switch to DC-DC once they are nearly full.

    Let me put it this way: Am I worrying to much about sticking to charging regimes for batteries? Multistage chargers "maintain" the battery, dropping the voltage once the batteries are full whereas the alternator will keep on pushing 14.x (whatever is left at the end of the cable) and potentially overcharge the batteries on very long trips. This is my main reason for looking into DC-DC chargers with dedicated charging profiles. The reason I worry about it is that PSOC seems to be bad for batteries and when taking very long trips (think a year in one go) the batteries may be subjected to long periods of PSOC due to staying in locations for a long time or driving short distances whilst at other times I might be traveling for days crossing large distances which will top the batteries up for sure but will also keep the (too high?) voltage over the batteries for many hours on end overcharging them.

    Cheers,
    -P

  7. #7
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    specifically which exact gel batteries are you using.

    a very simple rule with the gel batteries is you charge them as fast as you pull from them.

    IF your charging requirements are so specific as to warrant a DC/DC Id be looking at a decent inverter and the proper 240v charger for them.

    if you want to field craft it...

    12v20-30W bulb, SPDT relay, SPST relay and a VSR

    Wire the SPST relay to close on ignition on
    Wire the SPDT relay with the VSR on the Gel side of the batteries, Wire the gels to the common, the Main batteries to the NC contact bridge the NC and the NO with the light bulb.

    When the ignition is onthe batteries connect, if the gel batteries are low then full current passes to the batteries through the NC contact when the batteries are charged the VSR opens the relay and the bulb acts as your current limiter.
    Dave

    "In a Landrover the other vehicle is your crumple zone."

    For spelling call Rogets, for mechanicing call me.

    Fozzy, 2.25D SIII Ex DCA Ute
    Tdi autoManual d1 (gave it to the Mupion)
    Archaeoptersix 1990 6x6 dual cab(This things staying)


    If you've benefited from one or more of my posts please remember, your taxes paid for my skill sets, I'm just trying to make sure you get your monies worth.
    If you think you're in front on the deal, pay it forwards.

  8. #8
    DiscoMick Guest
    Quote Originally Posted by prelude View Post
    Thanks for the replies!

    My batteries are currently located in the former spare wheel well but since I will be fitting an extra fuel tank there they will move to behind the front seats in the foot well of the rear seats.

    The current size cable is quite small, 6mm2 I believe. I know this limits the current while charging at this moment but since the gel's can't absorb that much anyway it is less of a problem. It'll just take longer. In the end when the batteries are nearly full the current will drop far enough that the voltage drop over the cable is negligible. When moving to the permanent setup however I have a large quantity of 50 or 70 mm2 neoprene cable laying around for free so I'll be using that. (I also have the tools to crimp connectors etc. for this cable)

    I still need to purchase a solar solution so I can go either way. I think a blanket is more flexible (pun intended) and will allow me to get the most out of the sun. An mppt charger sounds like the best way to go as well?

    Just to be clear, not to drone on, I am well aware that a DC-DC will limit the current. A traxide or similar device will pass all the available current to the secondaries. The CTEK setup does both as I understand it. (but can't handle more than 120A but I guess that is hardly a problem) The alternative could be the sterling DC-DC and a traxide for example, bridge the alternator to the secondaries for bulk charging and switch to DC-DC once they are nearly full.

    Let me put it this way: Am I worrying to much about sticking to charging regimes for batteries? Multistage chargers "maintain" the battery, dropping the voltage once the batteries are full whereas the alternator will keep on pushing 14.x (whatever is left at the end of the cable) and potentially overcharge the batteries on very long trips. This is my main reason for looking into DC-DC chargers with dedicated charging profiles. The reason I worry about it is that PSOC seems to be bad for batteries and when taking very long trips (think a year in one go) the batteries may be subjected to long periods of PSOC due to staying in locations for a long time or driving short distances whilst at other times I might be traveling for days crossing large distances which will top the batteries up for sure but will also keep the (too high?) voltage over the batteries for many hours on end overcharging them.

    Cheers,
    -P
    I still don't think you need a DC-DC. Nothing you said makes me see any need for one.

    Your wiring definitely sounds too light.

    MPPT regulators can achieve some extra in limited situations, but most of the time a simple and much cheaper PWM does all that is needed.

    I also don't think your alternator will over charge the batteries. Remember that your vehicle is drawing from the batteries while the engine is running, while the alternator is replacing what is drawn. Any excess is simply dissipated. I think you're over-thinking it.

    Flexible solar panels are compact, but only work when you remember to set them up. In contrast, a panel mounted flat (not tilted) on the roof rack just works all day, no matter which way the vehicle is pointing, without requiring any human intervention. Set and forget.
    Keep it simple.

  9. #9
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    I have this model batteries: HZY12-100

    3 in parallel at this time and I have some more coming my way, not that I will mount them all in the car mind you I'll just test them and use the best.

    Sorry for overthinking, I have some experience in power systems on board pleasure yachts and with victron equipment but although largely similar, it's different to overlanding. I have seen the trouble some sailors had with underpowered alternators, battery banks that have remained in PSOC to long/often and what overcharging can do to a battery that can not be refilled so that's why I am a bit cautious. Also, most automotive alternators are not rated to deliver full current at 100% duty cycle so I have seen some burnt out examples or the alternator simply derates. That is not something a DC-DC charger would solve btw.

    Thanks for your patience and advice though!

    edit: soooo I was too soon in pressing post

    Regarding the solar panels, I agree that forgetting is worse and I will be in situations where I would park a car with a fixed solar panel but not throw out the blanket now that I think about it. I am not sure about the alternator not overcharging though, yes there will always be accesories drawing current but as long as the alternator can keep up with demand, the voltage should stay up at 14.x volt

    The wiring is to small, yes. At this time it functions as a current limiter since the automotive relay can't handle more than 40A anyway. It will be changed whatever route I chose to go.

    -P

  10. #10
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    Quote Originally Posted by prelude View Post
    I have this model batteries: HZY12-100

    3 in parallel at this time and I have some more coming my way, not that I will mount them all in the car mind you I'll just test them and use the best.

    Sorry for overthinking, I have some experience in power systems on board pleasure yachts and with victron equipment but although largely similar, it's different to overlanding. I have seen the trouble some sailors had with underpowered alternators, battery banks that have remained in PSOC to long/often and what overcharging can do to a battery that can not be refilled so that's why I am a bit cautious. Also, most automotive alternators are not rated to deliver full current at 100% duty cycle so I have seen some burnt out examples or the alternator simply derates. That is not something a DC-DC charger would solve btw.

    Thanks for your patience and advice though!

    edit: soooo I was too soon in pressing post

    Regarding the solar panels, I agree that forgetting is worse and I will be in situations where I would park a car with a fixed solar panel but not throw out the blanket now that I think about it. I am not sure about the alternator not overcharging though, yes there will always be accesories drawing current but as long as the alternator can keep up with demand, the voltage should stay up at 14.x volt

    The wiring is to small, yes. At this time it functions as a current limiter since the automotive relay can't handle more than 40A anyway. It will be changed whatever route I chose to go.

    -P
    alternators will output their rated wattage all day long.

    overloading them simply drops the volts as the amps rise and most alternators if spun hard enough will happily run past 125% some have peaks over the 200% mark.
    Dave

    "In a Landrover the other vehicle is your crumple zone."

    For spelling call Rogets, for mechanicing call me.

    Fozzy, 2.25D SIII Ex DCA Ute
    Tdi autoManual d1 (gave it to the Mupion)
    Archaeoptersix 1990 6x6 dual cab(This things staying)


    If you've benefited from one or more of my posts please remember, your taxes paid for my skill sets, I'm just trying to make sure you get your monies worth.
    If you think you're in front on the deal, pay it forwards.

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