Dc to dc - how to ground

Discussion in 'OnBoard Electronics & Controls' started by Mick Rogers, Jan 13, 2023.

  1. Mick Rogers
    Joined: Jan 2023
    Posts: 4
    Likes: 0, Points: 1
    Location: Queensland

    Mick Rogers New Member

    Hi. I have installed life po4 house bank batteries on my boat. I am installing a dc to dc charger to protect the alternator. The alternator and starter are floating ground. How would I best ground the dc charger?
     
  2. fallguy
    Joined: Dec 2016
    Posts: 6,770
    Likes: 1,385, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    the dc to dc charger may not protect the alternator

    in most boats the starting batteries and house batteries are on common grounds

    the charger needs to be defined as isolated or non-isolated to answer the question

    here is some reading for you

    Isolated vs Non-Isolated? What DC DC Charger to get. https://shop.pkys.com/isolated-non-isolated-dc-dc-charger
     
  3. Mick Rogers
    Joined: Jan 2023
    Posts: 4
    Likes: 0, Points: 1
    Location: Queensland

    Mick Rogers New Member

    Thank you for your reply. We have a 20a isolated dc/dc charger. We are currently stuck on a small island in Indonesia that has virtually no resources and extremely high import tax. All we want the charger for is to protect the alternator until we can get back to Australia to get the wiring sorted properly. We paid to have work done on the wiring, it is now a floating earth which we believe after reading up, is totally wrong. The alternator and starter were changed to suit floating earth, this should never have been done. The charger is not necessary for charging the life po4 battery bank, we have ample solar. Will it work by just joining the crank with pos and neg and the lithium with pos and neg?
     
  4. fallguy
    Joined: Dec 2016
    Posts: 6,770
    Likes: 1,385, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    So, the concept that the charger will protect the alternator is sort of incorrect, generally.

    The reason this is so is because the chargers are not current regulated; except to their ratings. And so, for example, a charger that is rated for 15amps out and 24v from a 12v source is really pulling 30 amps. And it is nonstop on a deeply depleted bank. So, for a 33amp alternator; easily fried..

    Since I have no details, I can only answer generally.

    In your case, on a boat, it is proper that ALL grounds are shared between the bank and the starting system. So, on most boats, non-isolated are used. And, you can make it non-isolated as Peter Kennedy shows on the link. You may already have common grounds, I do not know. But you can continuity test if u r unsure. Set a meter for continuity beep and run a jumper wire if distance restricted between house neg and start neg and see if it tones.

    Back to the bad concept..

    An alternator that runs too long can get warm and overheat. The way to best monitor alternator output is simply to keep a voltage meter on any start battery. Settings on the charger don't generally offer much other than voltage options for when to turn on or off.

    So, current limiting is generally not available on most dc-dc chargers.

    A hack method you can use is to put a 3-4' piece of #10 wire between the start pos and the charger. This will effectively current limit the charger to the carrying capacity of the wire. Of course, this can cause the wire to overheat, so you will need to fuse it large enough for the output of the charger. For example, if your alternator is delivering 50 amps at 12v, the charger would produce its rating of say 20amps at 24v, and the wire reduces the current the alternator can push and you can try a 40amp fuse. If the alternator is 40amps; then you'll need to fuse down to say 15amps and see if she holds.

    A less hack way is to run the charger with bluetooth for 5 minutes on and 5 minutes off if you get behind on solar, but this assumes bluetooth is available.

    I have two Orions in my boat and 33amp alternators on each engine. I lost one of the Orions last fall; and need to pull it and test it. I also bought a Sterling unit that will only deliver 6amps to the 24v bank, which means it will only be pulling 12amps @12v. Not installed, but will if I killed an Orion somehow, no idea how, my install is as per manufacturer. House was really never deeply depleted last year, so the Orions were only doing top off work on short boat rides.

    But if you are in the boonies, disable/enable the charger and/or use the wire hack method to protect the alternator. The charger itself would only do so under the right conditions where it is current limiting at say 40amps input with a larger alternator of say 120amps.

    If you provide more details, I can give you better advice. Like actual charger, if bluetooth is available, and the alternator available. If you have a small o/b and not much else, then use the wire hack, bring a range of fuses and have the first mate down by the wire the first time you run it to monitor wire heat..
     
    Last edited: Jan 15, 2023
  5. Ike
    Joined: Apr 2006
    Posts: 2,613
    Likes: 418, Points: 83, Legacy Rep: 1669
    Location: Washington

    Ike Senior Member

    I am curious about your statement that the alternator and starter are floating rather than grounded. On most installations the starter is bolted directly to the engine, which connects it to ground. Same for the alternator. Generally there is no necessity to run a ground wire from the starter or alternator because of this. How did they accomplish isolating these two from the engine?
     
    fallguy likes this.
  6. Rumars
    Joined: Mar 2013
    Posts: 1,561
    Likes: 865, Points: 113, Legacy Rep: 39
    Location: Germany

    Rumars Senior Member

    If I am understanding you correctly you have an isolated alternator and starter, like you would use on an aluminum boat. This isn't good or bad, it's just another a way of wiring. Grounding isn't necessary, you don't have to provide an alternate path to the negative terminal of the battery, or to the water. All you have lost by going to an isolated engine circuit is the ability to start the engine on the house battery bank by flipping a switch, so buy jumper cables long enough to connect the house with the starting battery.

    The DC-DC charger only protects the alternator from the high loads imposed by the lithium bank, not from a disconnect under load. To protect the alternator from that you need another battery or a dedicated protection device. I presume you have a lead acid starting battery in the circuit, if not, get one.
    The circuit should look like this: alternator/starter - lead acid battery - DC-DC charger - LiFePo4, all wires of the same length.

    In short, you connect the DC charger to your starting battery. If you have a battery combiner switch you keep it in the starting battery position, if an ACR bypass it. The alternator only ever sees one load, the starting battery. When you or the BMS starts the DC charger, the alternator sees an increased load, just as happens if you switch on the lights in your car. When the BMS disconnects the charger, the alternator still has a load (the battery) and all is well.
    Everything else, meaning the house and navigation loads are only connected to the lithium battery bank.
     
    Last edited: Jan 17, 2023
    baeckmo likes this.
  7. baeckmo
    Joined: Jun 2009
    Posts: 1,522
    Likes: 496, Points: 83, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Excellent advice, Rumars!
     
  8. Mick Rogers
    Joined: Jan 2023
    Posts: 4
    Likes: 0, Points: 1
    Location: Queensland

    Mick Rogers New Member

    Our visas have expired and we have to leave in the morning so I disconnected the alternator from the house bank. I should have enough solar in by day to keep amp hours up enough to get me through the nights.
     

  9. fallguy
    Joined: Dec 2016
    Posts: 6,770
    Likes: 1,385, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    The best approach is to stay topped off, so rather than disconnecting, if you get foul weather you'll want to keep the system up and not deeply discharge.

    Much depends on what alternator you have. It is really hard to give you great advice absent the details.

    But deeply discharging and then using the alternator at near its maximum output for long periods, whatever that is, needs to be avoided.

    If, the alternator output is over 100amps; things are very different as others have said as the charging system will limit current flow.

    The only reason I said to disconnect was for on off cycling in certain scenarios that I identified absent ANY details other than the current rating of the charger.

    And as Rumars wisely mentions, I also assumed you had a l/a starting battery-another unknown detail.
     
Loading...
Similar Threads
  1. sdowney717
    Replies:
    10
    Views:
    2,822
  2. fallguy
    Replies:
    8
    Views:
    1,050
  3. sdowney717
    Replies:
    12
    Views:
    4,384
  4. fireman_bob
    Replies:
    21
    Views:
    9,137
  5. cgoodwin
    Replies:
    38
    Views:
    9,107
  6. sdowney717
    Replies:
    18
    Views:
    6,713
  7. rasorinc
    Replies:
    2
    Views:
    2,094
  8. sdowney717
    Replies:
    4
    Views:
    12,769
  9. papabravo
    Replies:
    4
    Views:
    1,831
  10. sdowney717
    Replies:
    22
    Views:
    12,856
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.