FPB 78 Battery Bank And Charging Details

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It is a law in the yacht building universe that the sparkies (electricians) are always the last ones off the boat. And with the DC system now almost complete, we can see the light at the end of the long building cycle tunnel. We thought this might be a good time to go through the DC battery bank and related circuits.

We’re looking here at half of the house battery bank. These are Hoppeke12 OPzV1500 batteries, rated at 1570 amp hours based on a C/10 discharge rate, to 21.6 volts. Capacity is a function of temperature (warmer is better in terms of capacity), and how quickly the batteries are being discharged. C/10 rating would mean a discharge rate of 150 amps. We might get this high a small percentage of the time when cooking, but a high average would be more like 40 amps so C/25. This adds 10-to-15% to available capacity.

These are generically called “traction” batteries, and have an expected cycle life of 1200 80% discharge cycles.

Hoppeke DOD amd Temp vs life

Back off from 80% depth of discharge to 65% and cycle life jumps considerably. By habit we all look to minimize DoD so as to maximize life. But does this make sense? Given the solar array, and periodically moving the boat where the engine driven alternators recharge the batteries, it is doubtful whether you would average a full 80% cycle on a weekly basis. But if you did, cruising full time, the batteries should last 20+ years if properly charged and maintained when in storage.

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These 12 cells represent roughly 2400 pounds, 1.1 tons, and live within a sealed (and vented) coffer dam within the central tank of the FPB 78. There is a containment vessel that houses each half bank, and substantial space around the batteries for air circulation.

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Aluminum construction makes placement of overhead lifting eyes easy, for installation of the cells and later removal and replacement.

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In the photo above you can see the wiring exiting through watertight glands.

A large battery bank requires a substantial charging source. The FPB 78 has three means of recharging.

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Each engine has a 250 amp/28 volt (hot rating) alternator, with dual sets of windings, in other words effectively two alternators on a single shaft.

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The AC current produced by the alternators is rectified into DC by remotely mounted rectifier assemblies, shown above. Each pair of these services one alternator (a rectifier for each set of windings). The alternators are controlled with multi-stage regulators, one for each pair of windings. There is a backup system, created by Deon Ogden, that keeps watch on charging voltage. If it gets out of range, a relay breaks the multi-stage regulator circuit and turns on an old fashioned PWM single voltage regulator.

There are also manual switches with which to select the control circuit.

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Three inverter/chargers can charge the house bank when on shore power, or with the genset running. These produce a theoretical 5000 watts of AC power (each) and 120 amps/26 volts DC charging.

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Ten 340 watt solar panels are run through with three charge controllers. Two of these can do the job but a third is wired in, and if one fails the load of the other two can be consolidated.

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All of this power requires substantial switching and fusing. The major circuit protection/switching is shown above. These are remotely controllable switches, with manual backup, of course. The fuses are not yet in place.

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The lighter DC circuits are controlled from the main panel and from a small sub panel in the engine room. Note the generous space between the DIN rail breakers, which makes R and R of the breakers a relatively simple process. The breakers themselves are substantially more expensive, but over the life of the yacht this will be paid back many times.

There’s lots more, but it is a beautiful sunny day, and we’ve got a stack of office work left to do, so we’ll leave the rest till later.

Posted by Steve Dashew  (March 7, 2016)

19 Responses to “FPB 78 Battery Bank And Charging Details”

  1. Carl E Says:

    Hi Steve: Thank you for the detailed update. When do you expect her to launch?

  2. Steve Dashew Says:

    Howdy Carl:
    We are looking at a June launch.

  3. Shannon Says:

    Looks Amazing! The wiring looks clean and organised. I love the eye bolts for use in lifting those batteries. I have read the earlier posts where it is hoped those solar panels in good weather can relegate the gen set to backup duty. Have you received data from FPB 97 ICEBERG on how effective the solar panels are in daily use?

  4. Steve Dashew Says:

    Hi Shannon:
    We have had quite a bit of data from the FPB 97, several of he 64s, and of course Wind Horse regarding the solar panel output. We are confident that 90% of the time the genset will not be required, and when it is, it will typically be when the washer/drier units are in use at anchor.

  5. Wolf Says:

    Should one of the 2V cells fails, the battery bank will be compromised. Why not split the single 1500ah bank into two banks @ 750ah each for redundancy?

  6. Steve Dashew Says:

    Hello Wolf:
    Your point is a good one, and we’ve wrestled with this for years. The single bank is far more efficient in terms of the discharge rate/capacity, and charging cycles. And our experience has been that with proper maintenance, these traction batteries are highly reliable. So, on balance, we usually go with a single large bank.

  7. John Says:

    I don’t understand why having a cell fail compromises the bank significantly enough to warrant splitting it. If one fails then disconnect it from the bank, charge the bank to a max of 26 volts instead of 28 volts (you could quickly recalibrate the Multiplus Victrons to 26 volt bulk charging and 24.5 float charging), order your new cell and in the meantime charge the bank more often as you have a lot less capacity before the low voltage warning come on.
    Don’t like my suggestion, then get yourself a spare 2 volt cell and connect it to a small laboratory power supply, set it to 2.25 volts and maintain the spare alongside the main bank. If a cell fails then replace it with the spare.

  8. Alan Ferris Says:

    I know it can be hard to avoid at times, but running cables across fans can reduce quite drastically the cooling effect of the fan due to reduced air flow. Just something to avoid if you can. Clearly there was not space to run the black cabled above the fan.

  9. Steve Dashew Says:

    Thanks Alan:
    We are always concerned with air flow over the diodes! We think this will work, and will monitor the diode temperature. If there is a problem, the wiring will be rerouted.

  10. Philip Streat Says:

    I have a bank of the same battery type though smaller cells. They have a temperature compensation curve which is a step function rather than linear. In other words the relationship between desired voltage and temperature is not a simple straight line. My Mastervolt charger can only be set up with a simple linear compensation curve. I wonder if this is an issue you have a solution to other than just being conservative with the settings so that the desired voltage is never exceeded.

  11. Steve Dashew Says:

    Hi Phillip:

    Modern gel cells are relatively inflexible in their charge requirements. Charging voltage should be compensated for cell temperature as a start. If you are charging continually, i.e in storage mode or underway, this needs to be at a lower voltage than you use for bulk charging. And when in storage, ideally the batteries will be put through a conditioning cycle on a periodic basis. The Victron inverter/chargers we use have a stepped charge curve, a storage mode, and are temperature compensated.

  12. Paul Says:

    In the picture w/ the 3 Multiplus Victrons what is the 4th Phoenix inverter for?

  13. Steve Dashew Says:

    Sharp eye Paul:
    The the three Victrons are for 230V, the other is 115V.

  14. Endre Lukacs Says:

    Hi Steve,

    excellent work as always! I would like to call your attention to a technical solution by which you can maybe further optimize the rectifier part, at least in theory.
    In modern switchmode DC/DC converters they tend to replace the rectifier diode by a switched FET (field effect transistor) thus eliminating the power loss of diode.
    The tech is called synchronous rectification and should work here as well.

    Company Linear Technologies has this controller IC http://www.linear.com/product/LT4320 which does exactly this for AC mains with the help of additional power FETs.
    As I calculated if we use 4x 10 1mOhm/60V power FETs paralleled the total dissipation can be cut down to 50W @ 500 Arms full wave rectification at a BOM cost <200 USD/EURO.
    Higher peak current can be also handled at the expense of adding more FETs and controllers.

    If the idea works, you can increase efficiency and eliminate the fans thus increase overall reliability.

    Best regards, Endre

  15. Steve Dashew Says:

    Thanks Endre:
    When we get 78-1 into cruise mode we will do some checking on your solution.

  16. Nikolas Says:

    Hi Steve,
    Very informative article as usual. May I ask why you are not using Sonnenschein cells anymore?

  17. Steve Dashew Says:

    Hi Nikolas:
    Both Hoppeke and Sonnenschein are good products.

  18. Nigel Says:

    Hello Steve

    Interested to know what electrics you intend to leave on when the boat is unattended in a marina or ashore for a few months. I always prefer to turn everything off including shore power for safety/galvanic reasons but I have not had solar to panels to consider. Can you leave solar to condition batteries?

  19. Steve Dashew Says:

    Hi Nigel:
    We typically leave on the fridge/freezer, security alarm system, and bilge pumps. Depending on time of year, and weather, there is sufficient solar capacity to carry these items and periodically condition the batter1es.