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  1. #1
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    Default DC-DC Buck boost convertors

    I have a couple of projects in mind that require the use of a buck boost converter.
    Granted possibly an unanswerable question.
    Can these things fail High? if so how high?
    How long is a piece of string?

    One is a MPPT charger that requires more than 13.4V but a Max of 16V.
    Its not a cheap charger and you would like to think it was smart enough to look after itself over 16V, but I'm not so sure. Given that it is only about 6 months old yet appears to not play well with smart alternators.

    The other uses 12VDC but says a 12V car battery is ok as long as the car isn't running. A 12V buck boost would mean I could trickle charge at the same time.

  2. #2
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    Hi Stuart,

    Its very unlikely to fail with an over voltage output ! They generally have both under and over voltage lockouts. Also if the oscillator stops there wouldn't be an output voltage anyway !

    The one that I built for the camper, using a 56 volt 300 watt solar panel, basically doesn't care unless the output voltage rises above 14.2 volts or the battery voltage drops below 10 volts. Between those values it can provide upto eight amps output. Normally the batteries are trickle charged at about 100 ma with no load.

    MPPT doesn’t like input voltage spikes, normally the battery filters those out, but long leads don't help or poor electrical connections. I used to know a guy who thought it was a good idea to put his battery in the boot, it completely ruined his radio reception.

    If you think about it trying to trickle charge the battery that you are boosting from is akin to lifting yourself by your boot laces !
    Best Regards:
    Baron J.

  3. #3
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    Revesby - Sydney Australia
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    Quote Originally Posted by BaronJ View Post
    I used to know a guy who thought it was a good idea to put his battery in the boot, it completely ruined his radio reception.

    Minis used to have the battery in the boot. No radio issues when they had generators. Not so sure in the later, alternator, models.

  4. #4
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    Mar 2011
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    What’s the application because it reads to me like you’re trying to charge one battery from two sources, which is fine, can be done. Also reads a lot like you want to condition the input of a MPPT regulator, which will work conditionally in that it’ll charge but you’re likely going to lose the power point tracking. Some info on application would help a lot.

    Baron I currently have the secondary battery in my 4WD in the rear of the tub, there is pretty much the entire length of the car between the main battery and the secondary, being a VW Glamarok that’s the best part of 6m. Even with some relatively sophisticated electronics controlling charge and isolation of the two I have no issues with radio reception.

  5. #5
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    Thanks Baron, seems I am "pretty safe".

    Quote Originally Posted by BaronJ View Post
    If you think about it trying to trickle charge the battery that you are boosting from is akin to lifting yourself by your boot laces !
    No sadly I haven't be able to over turn our current the laws of physics as yet, maybe one day

    Filling in some blanks on the applications

    The first one is a caravan "system" it controls everything in the van; pumps, fans, lightening, also MPPT of the solar to the batteries +240V charging of the batteries and then the problem child, the charging from and TO the vehicle battery(batteries in this case). Its oh so clever but not smart enough to get out of its own way lol.
    If the vehicle battery is below 13.4V the van tries to charge it. Depending on one thing and another the smart alternator wont fire up because its happy for the V to be that low. So the van batteries don't get charged.
    One work around that has yet to be tried(and may not be acceptable to the owner) is to fit a V gauge to the vehicle and drive with the headlights on until the alternator kicks in. That may work at least when the van batteries are low so they are charging at 20Amp.
    The other thing that has be suggested is to fit a DC-DC battery charger(I'm not so sure that will work as who knows what V it will see coming from the "system".)
    So I'm just looking into 13.8VDC output buck boost. With just a simple set up that would lose the "van charges vehicle" function bit the owner is ok with that.

    The second one is effectively a 12VDC UPS (I think its like an 18W load) so trickle charger-battery-12VDC buck boost converter-load

  6. #6
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    Nothing is ever easy lol
    Just dawned on me the buck boost I was thinking of using for the van because is claimed ip68 doesn't have an adjustable low V cut off. So if left connected to the vehicle would happily drain its batteries to 8V . Well once

    So much for "plug and play"

  7. #7
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    Mar 2011
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    A Meanwell RSD series 9-36V in 12V out will sort no 2 out. We use the RSD60G-12 at work a lot on heavy vehicles as a 24-12V converter, and on light vehicles where LV Fleet maintenance refuse to believe the alternator reg packs are breaking down and injecting intermittent AC into the supply for the two way radio . About $60ish, very robust unit.

    The caravan.. It’s a battery charger, it doesn’t need o be that complicated. I would really be looking to simplify it. As I suggest to my colleagues at work, not everything requires microprocessor control or Bluetooth

  8. #8
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    Quote Originally Posted by racingtadpole View Post
    The caravan.. It’s a battery charger, it doesn’t need o be that complicated. I would really be looking to simplify it.
    Yes it does seem to be to clever for its own good, sure with "old" alternators its pretty neat idea, but how long is it since a car had an "old" alternator?
    With smart alternators I cant see a way around needing a "ignition sense" wire to the van. So far it appears there isn't a standard pin on a 12 pin plug for "ignition sense".

    At first glance I could setup the Anderson plug to only have power with the ignition, but I would rather leave the tow vehicle alone.

    In a perfect world the van could be moved to any other vehicle and have it function correctly.

    I really don't want to get to involved, the reason for the ip68 rated unit was I could just fit Anderson plugs to the input and output and be done with it. Outside near the hitch, little chance of it setting fire to anything, if it fails just unplug it and you aren't any worse off than you were before. Putting it inside the van under a seat out of sight I'm not so sure I want anything to do with.


    Quote Originally Posted by racingtadpole View Post
    As I suggest to my colleagues at work, not everything requires microprocessor control or Bluetooth
    I assume you've heard about the AEG microwave ovens that had an over-the-air update that made them think they were steam ovens? With bonus point for disabling over-the air-updates at the same time. It cant be fixed without a service call, I wonder how many of them end up in the bin?

  9. #9
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    Stu, could you just fit a suitably sized diode between the car and the van systems, so that current can only flow from the car battery to the van battery and achieve what you have in mind? I know about the voltage drop of diodes, but a Schottky diode has a very low voltage drop. A lot cheaper and simpler than a DC-DC converter. And no risk of draining the van battery.
    Cheers, Joe
    retired - less energy, more time to contemplate projects and more shed time....

  10. #10
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    Hi Joe
    Quote Originally Posted by jhovel View Post
    Stu, could you just fit a suitably sized diode between the car and the van systems, so that current can only flow from the car battery to the van battery and achieve what you have in mind?
    Possibly
    Quote Originally Posted by jhovel View Post
    And no risk of draining the van battery.
    Draining the van batteries wasn't really a worry, it was draining the vehicle battery(if for no other reason than I assume the van is smart enough not to over discharge its batteries, I'm not even sure if it will use power from the batteries to charge the vehicle, but it certainly uses solar power).

    A diode would stop the van charging the vehicle which would mean the smart alt would have to come on. A quick look(and I am no expert) shows Schottky diodes with the amp rating I am looking for have a drop of about 0.5V. So I guess a start might be to get the owner to see how often the V goes above 13.9V.

  11. #11
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    Quote Originally Posted by jhovel View Post
    Stu, could you just fit a suitably sized diode between the car and the van systems, so that current can only flow from the car battery to the van battery and achieve what you have in mind? I know about the voltage drop of diodes, but a Schottky diode has a very low voltage drop. A lot cheaper and simpler than a DC-DC converter. And no risk of draining the van battery.
    The trouble is Joe as I understand it anyway, it is practically impossible to charge a van house battery from a tow vehicle fitted with a smart alternator, without using a DC to DC charging device. A schottky diode while having a lower forward voltage drop than a silicon diode would just make the task harder.
    I must admit that I have a difficulty in understanding why they want any such charging device to be as close to the vans house battery as possible, instead of being as close to the tow vehicles alternator or battery. That arrangement would require two more connections for sense wires to read the voltage at the house battery which would control the charge device, but the upside would be that any dc to dc charger fitted close to the power source would have a robust supply, and the sense cables would be in a high impedance circuit essentially transferring a voltage reading but carrying negligible current. Smaller cable cross sectional areas could be used because the charger would just up the voltage to get the required charge current. My thoughts anyway.

  12. #12
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    Most 'smart' alternators are controlled by the ECU. I've had several work vehicles returned to dealers to have the smart/eco/PITA charging turned off. It was particularly problematic on the PX II Ranger I drove in my last job.

  13. #13
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    Late last night I realised I had forgotten something in my last post, this morning I cant recall what that was


    Quote Originally Posted by Ropetangler View Post
    The trouble is Joe as I understand it anyway, it is practically impossible to charge a van house battery from a tow vehicle fitted with a smart alternator, without using a DC to DC charging device. A schottky diode while having a lower forward voltage drop than a silicon diode would just make the task harder.
    But wouldn't the diode would stop the van charging the vehicle battery forcing(sooner or later) the smart alt above 13.9V?(ok I have no idea what the output of this smart alt is like. I know VF coms were happy down as low as 12.1V, unless the dash display wasn't that accurate as the battery was in the boot)
    Quote Originally Posted by Ropetangler View Post
    I must admit that I have a difficulty in understanding why they want any such charging device to be as close to the vans house battery as possible, instead of being as close to the tow vehicles alternator or battery. That arrangement would require two more connections for sense wires to read the voltage at the house battery which would control the charge device, but the upside would be that any dc to dc charger fitted close to the power source would have a robust supply, and the sense cables would be in a high impedance circuit essentially transferring a voltage reading but carrying negligible current. Smaller cable cross sectional areas could be used because the charger would just up the voltage to get the required charge current. My thoughts anyway.
    I assume the location is fixed because it is a "one box" solution. Apart from the 240Vin, solar in, 12Vvehicle in and has 4 output buses(I guess they are called), fuses for 20 odd circuits, with the control panel conveniently located by the door and of course blutooth. Having the charger in the vehicle would also mean that were a bit of a "matched pair", trying to avoid that if possible.
    Quote Originally Posted by racingtadpole View Post
    Most 'smart' alternators are controlled by the ECU. I've had several work vehicles returned to dealers to have the smart/eco/PITA charging turned off. It was particularly problematic on the PX II Ranger I drove in my last job.
    What a coincidence. I'm dealing with a BT50. I must say I hadn't thought turning off the smarts would be an option. I will pass that along. There was talk about fitting a "dumb" alt. I guess it may well be possible to "cut the right wire" and just fit a V reg.

  14. #14
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    Remembered

    If I was to add a diode.
    What is the result of the van seeing 12.9V from the vehicle and trying to charge something that isn't going to take any current?

  15. #15
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    Not much. The electrons will smack into the back of the diode and stop.
    You could just wire the Anderson from the car to go straight to the caravan battery, the tow car still charges the battery but you basically eliminate anything smart in the caravan out of circuit then

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