Need some help on this one.

Discussion in 'The Projects Forum' started by carbon arrow, Jul 7, 2010.

  1. carbon arrow

    Thread Starter New Member

    Jul 7, 2010
    2
    0
    Hi all. I'm new to this forum. Glad to have it.
    I am working on a project that has stumped me.
    I have a 1100 Watt inverter that is powered by two deep cycle 79 amp hour batteries giving me almost 160 amp hours in parallel. The inverter runs a mobile workstation consisting of PC, printer, scanner, etc.
    The inverter is rated at 10-15 volts.
    Everything works great. Shut down inverter, re-charge batteries(AGM), fire it all back up.
    But here's my dilema........................
    It has been requested that the mobile cart stays powered up during re-charge.
    What's happening is that when the charger is doing it's thing, the volts can go up to near 15. That's when the auto protection kicks in on the inverter.
    *****The charger is hooked to the batteries and the batteries are hooked to the inverter.
    I need a device or application that I can put between the batteries and inverter that will NOT allow the higher DC voltage, (over 12 or 13 VDC,) to make it to the inverter input.
    Any ideas?:confused:
    Thanks, Marty
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, decrease the output charge voltage threshold on the charger (preferably), or increase the input threshold on the inverter slightly.

    Otherwise, you're going to be wasting quite a bit of power.
     
  3. Dx3

    Member

    Jun 19, 2010
    87
    7
    A couple of fat diodes can be switched into the circuit to waste a volt or so when the charger is on.

    By "fat", I mean diodes that can handle the amps the inverter is using to run the loads. You must measure the highest current draw from the battery to the inverter while running everything you have hooked up, then you wil know what size diodes to buy. Put 2 or 3 of them in series between the battery and the inverter, after the charger is running, by flipping a switch that shorts out the diodes when you are not running the charger.

    A volt meter will probably be necessary to make sure you don't put the diodes into action when the battery only has 11 volts available.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    What is your actual current draw from the batteries?

    If it's any where near the rating of the inverter, you'll discharge the batteries completely in under 2 hours. That's very hard on them. Discharging them 50% will result in cutting their expected service life by 2/3.
     
  5. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    Could you have the system run the inverter on one battery and the charger charge the other battery? Then automatically switch every 90 seconds.
     
  6. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    A Bridge Rectifier might be all you need. Normally used for turning AC into DC but works well in case like this.

    http://www.rectron.com/data_sheets/br1005-1010.pdf

    About $1.20 from Mouser

    Edit2: The diodes in the link are too small by about a factor of 10 or 20 for this project. Charging current plus Inverter Current = 200 Amps would be a good value for the charger side with 100 Amps for the inverter side. The diodes in the link would only work for 120watts inverter and the charger side would still burn out with charging plus inverter currents.

    Connect the AC connections up to the Battery positives,


    HANG ON I think I have this part backwards. Using it sort of backwards is confusing me. :D
    Yes I did but I will fix that. Ok now it is ok.
    -negative connection on the charging supply.+ positive connection goes to the inverter.

    And a picture while I am at it. Never hurts.

    [​IMG]

    This also isolates the two batteries so they avoid discharges into each other.

    Check first because there might already be this exact adaption installed or a version using individual diodes or any other models of bridge diode packs.

    And I hope that the 10 Amps pack is enough - go bigger if there is any doubt because it is a cheap enough part.

    Possible problems _ BIG WARNING _ could be incompatible with the charger and or inverter, which rely on sense circuits that MIGHT be confused by the Diodes.

    Check with the manufacturers for the charger and inverter or their more local distributer since they are quite possibly made in China . You would be amazed how much info an email might get you. You would be even more surprised how important and hard to guess that information could be. It is a good problem that they should have a ready answer for especially if you are clear and detailed.

    But also be prepared for the answer you might get.
    We have another model that will solve that problem and here is a link to where you can buy it.
    There is a strong incentive to make every contact into a new sale.
    Less obvious is that for liability reasons some corporations are not allowed to discuss any technical issues beyond a set limit unless they have a special business relationship with you that requires it, and that does not include most customers.

    Certain matters they refuse to inform even warranty service techs - preferring to replace a whole unit because the repairs are too involved... expensive... risk too much or dangerous of a repeat failure... or other really good reasons.

    It is not a problem - just a reasonable business practice.
     
    Last edited: Jul 11, 2010
  7. John P

    AAC Fanatic!

    Oct 14, 2008
    1,632
    224
    That little bridge rectifier is hopeless--it's rated 10 Amp continuous, and this setup required 1100W. Call it 1200W and then it's 12V at 100A, or 50A per battery, if they're evenly loaded, which isn't guaranteed. Diodes rated for 50A are not going to be cheap, and they'll need heat sinking.

    I'd say the best bet is to find an inverter with a wider input voltage range. It's the cleanest solution, and maybe not even all that expensive, compared with the alternatives.

    Here's a website I found where they specialize in inverters:
    http://www.invertersrus.com/

    Unfortunately 15V does seem to be the standard cutoff voltage for the D.C. input but maybe someone who knows the products could find one that's more versatile.

    One other thing: I'd have thought that the charger would never be able to supply 100A, and if the system were drawing that much current, it would be drawing on the batteries whether the charger is connected or not. In that case, the charger's output voltage would drop or it would cut out completely. So maybe, the idea of operating the inverter while charging will never work, at least not while the full power is being drawn. Do the people who want the charge-while-running option understand this, or is the charger a massive thing that can really do both jobs at once?
     
    Potato Pudding likes this.
  8. Dx3

    Member

    Jun 19, 2010
    87
    7
    I'd chime in again, but there is no response from the OP.
     
  9. carbon arrow

    Thread Starter New Member

    Jul 7, 2010
    2
    0
    Sorry I haven't responded quicker.
    What if I had two 15 amp chargers in parallel?
    Would that decrease my high end voltage while charging?
    Right now I have one 40 amp charger.
    Thanks, Marty
     
  10. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    There are high capacity chargers out there, the kind that will start a car with a dead car battery. Nowdays they use a small SLA to do the same job, but that's besides the point. I'm pretty sure what you need is commercially available from auto stores.
     
  11. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    I don't think they are using the full 1100W of the inverter. How I understood the set up, it is running a computer. Computer and Screen should use between 200 to 600 Watts normal to peak. 600 watts will still need a 50 Amps so yes the 10 Amp bridge is useless. Maybe I was also thinking in terms of a laptop type system which is not mentioned - just that it is portable -but a laptop is worth considering now that I think of it. That would make everything much simpler and new laptops are more powerful and not very expensive. A laptop is cheaper than a big inverter system I will guess.

    If you are using all 1100W then I would definitely think that a new inverter might be a good way to go.

    A higher amperage of Bridge being required was one of the cautions I mentioned in my post, and I encouraged them to go bigger if there was any doubt, but I am glad John P looked at it again. I had also failed to consider that the charger would end up with charging + inverter current flowing through it. That makes me think the whole idea needs a possible rethink because the charger could burn out. The diodes bridge would definitely need much more current capacity than 10 Amps, especially on the charger side. I was looking too much at the isolation and dropping the voltage seen at the inverter, and I didn't consider the whole thing as carefully as I should have.

    Ideas that I would follow up on.

    If this were not a vehicle installation or if you could install an AC cord running from the vehicle and plug in when you get a chance.
    See if the inverter can be run as a UPS with AC Passthrough.
    Try and replace the inverter with one that will run as as a UPS.

    For a vehicle install - create a vehicle on bypass, but first you need to check the alternator capacity if you want to charge the batteries at high amps and run the inverter. Burning out a big alternator will ruin your day.
    The vehicle on bypass would try to run the inverter more directly from filtered and regulated alternator power. It cuts the deep cycles batteries out or reduces draw from them as a source for the inverter. You will need a vehicle mechanic electrical equipment specialist to wire it up for you, and look for an industrial vehicles garage - not your local lube and tires.

    Still seems like it might be worthwhile to check for isolation for the batteries. Do the batteries go dead if they are not used or charged for few days or a week is sure sign.
    Better than diodes for isolation is to have a disconnect switch. Almost no power loss involved compared to possibly having hot 200 Amp diodes.
    Charger and inverter would need to be checked for interference caused by the diodes in the bridge, because blocking the sense feedbacks could set everything up for major damage like overcharging the batteries. Contacting the charger or inverter manufacturers first would be a good start. If you take the work to a professional electrical mechanic, the type that rewires three electrical forklifts on an average day, they would see right away if things like battery isolation are worth a worry.

    Hopefully you are better informed. Even though I now think that this probably needs a professional; talking to the mechanic is easier if you have done some research first.

    If nothing else - I think my first posted idea of a switching module that repeatedly swaps so one battery is always charging and the other runs the inverter would work.
     
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