Adding a battery bank

Discussion in 'OnBoard Electronics & Controls' started by kapnD, Mar 6, 2024.

  1. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    My single engine boats primary electrical system is 24 volts, currently utilizing a pair of group 27 dual purpose marine batteries charged by a 55 amp alternator. Most of my electronics, lights, and pumps are 24 volts, with exception of the VHF and a USB outlet, which are 12 volts, and gets its power from a 20 amp 24/12 step down device.
    The system works well, with autopilot, depthfinder , washdown pump, baitwell pump running whenever the boat operates, and the batteries never require external charging.
    I’m about to extend the range of my operations, and will install a mid sized chest freezer, a microwave, and a coffeemaker, all 120 volt powered through a 3000w inverter, as adding a generator is not an option.
    Although I will run the 120v items on shore power at the dock, it looks like a second bank of batteries will become necessary for time offshore.
    Will I need a bigger alternator? Or a second alternator?
    Will an ACR distribute the charging currents effectively?
    Will I need a shore powered charger?
    I’m hoping to choose the simplest path here, any input will be appreciated!
    fallguy likes this.
  2. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    We need more info.
    1. Is the engine always running during while offshore or not?
    2. What is the time spent on batteries alone (anchored, or other engine off situations without shore power)?
    3. What's the desired on battery autonomy time.
    4. What is the total wattage of the additional appliances?
    5. What's the typical current on your alternator at the moment? (If there is no ampermeter present borrow a clamp type).

    From your description I will presume a powerboat operating from a marina without any meaningfull time on batteries alone. In this case you probably don't need anything as long as the current alternator still has sufficient capacity.

    Inverters are inefficient at low power and bigger inverters have larger standby current draw. The usual strategy for fridge/freezers is to have the smallest inverter that can run them dedicated to this task. This isn't a concern on powerboats with enough generation.
    BlueBell likes this.
  3. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I hope you don't mind. I'd like to piggyback a bit.

    My boat also has a 24v system. I started with 2 GR27 batteries, but I have room to add two more. I know that adding another bank means I cannot charge the two banks the same way.

    I have a Victron 2000w inverter.

    To the OP.

    Were you planning to add four new batteries or were you trying to run dual banks?

    I'd like to run two banks so I can purchase two batteries at a time vs 4. I think the Victron Multiplus can run a second bank as the charger; not 100% sure. I have two solar panels and could also see adding an MPPT to run solar to each bank.

    But I think a second bank needs to either be discrete discharge/charge or else be part of the same bank.

    I'll try not to hijack real bad. Or at all if you have these questions answered.

    I also can charge off my outboards with 32A alternators and have an Orion on each engine. One of them seems to have died.
  4. rangebowdrie
    Joined: Nov 2009
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    Location: Oregon

    rangebowdrie Senior Member

    Dual purpose batts are generally neither fish nor fowl.
    You'd generally be better off with 4 good 6V batts like a T-105 and 1 group 27 or 31 for a starting batt.
    Biggest bang for the buck for charging is to have your alternator re-configured so that it can use a decent
    3-stage external regulator.
    You can program them for the parameters you need and some use sensing to prevent the alternator from overheating.
    The stock internal regulators are very inefficient at charging, they taper-off the charge and take endless hours to recharge a battery bank that's been drawn down much, they're designed to replace the few watt-hours needed to start and then just carry some load.
  5. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    A typical day trip is from 8 to 14 hours, engine running all day.
    I do infrequent outer island multi day trips, keeping battery use to a minimum when the motor is not running.
    Running on batteries alone will be just overnight on those trips.
    My big concern is having adequate charging capability for the second bank, and smart charge management, as the battery banks will not be similar.
    So far, I’m still in the planning stage, and don’t have any hard data re added power requirements, but I will try to approximate that soon.
    Thanks for the feedback!
    fallguy likes this.
  6. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    In my opinion you don't need a second battery bank. Of course you may still want one, so I'll adress it.

    The real question is if you need a bigger alternator or not. For that you need to know what the average charging current is right now, before any upgrades.
    The existing alternator can produce 55A x 28V = 1500W for about 5 minutes, then it heats up and derates itself to a max. of around 40A x 28V = 1120W. Let's call it an even 1000W for simplicity. Unless you're already pulling a constant 35A out of it, you don't need to upgrade.
    Batteries are fine as they are, depending on the installed brand they range somewhere between 66Ah and 100Ah. 66Ah x 24V = 1584Wh, of wich you want to use half, that's 792Wh. If you have 100Ah batteries the usable capacity is 1200Wh.

    Let's see what happens with the added loads while the engine is on:
    The alternator carries all the running loads, plus whatever the batteries take to replace the engine starting and top off. The added loads are all intermittent, they are only applied for a few minutes. When a new load is switched on the alternator increases production and if needed the batteries supplies the rest. Once the load it's switched off the alternator reverts to charging the batteries until full.
    Example: you pull 20A total from the alternator, then start microwaving with a 1000W machine. The total load is now 60A, 55 from the alternator and 5 from the batteries. You microwave more then 5min, now it's 40A from the alternator and 20 from the batteries. When you stop microwaving you revert to the 20A constant load, but the alternator has to replace whatever you pulled from the batteries, so for a time the total load will be 30A instead of your usual 20A.
    This happens regardless of how much battery capacity you have, or in how many banks or what chemistry the batteries are.

    Overnight it's simple, you have between 800Wh and 1200Wh to run a freezer, radio and a few lights. The freezer should not take more then 1kWh/day even with the inverter, so that's 500Wh for 12 hours. That leaves you with 300-700Wh to run some lights and the radio. Just remember to start the engine before making coffee and microwaving the milk.

    Now if for whatever reason you want to add another bank, there are two big options: use the same brand and type of battery and add them in parallel to the existing ones, or use something different (brand, capacity, chemistry). The "something different" route is simplest with a DC-DC charger and a dedicated load. That means the only load you hook up to this new battery is the inverter, and it's only charged via DC-DC. I would buy a 12V 100A lithium battery, hook up the VHF and a small inverter for the freezer to it, and keep the big inverter for the microwave and coffee maker on the 24V system. Replace the 20A step down converter with a smart DC-DC charger and you are done with minimal headaches. The converter can stay in place as a redundancy, if something happens you can always switch the radio back to the old setup. When you are overnighting or at the dock, just switch off the DC-DC charger.
    baeckmo and BlueBell like this.
  7. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    My main concern is reliability in that offshore environment. Yes, you need more battery capacity for sure. If you wake up after a long haul, where you refueled in the afternoon and then went on to a destination 14 hours from your home port, and you alternator decides to crap out then, you should have the battery capacity to make the run home. That probably means 30kWh in reserve after a night on batteries. You should either have a complete spare alternator aboard, or do what most people do and carry an Honda 2000 around for emergencies.

    As far as load goes, the engine needs a battery. The boat needs a battery. Each person aboard needs a battery. Even the smallest cooler or icebox or holding plate unit needs a battery. That is for basic boat-camping with maybe one 150 inverter available for personal devices and recharging stuff.

    So the simplest path is add a couple 31s and a pickled Honda 2000. Alternatives for generation include solar in lieu of the generator - at least 500 W rated, or the second alternator. Guests must not have unsupervised access to inverter power. And mistakes like leaving the shower sump pump on have gotten people killed out there. Start battery isolation is really important for overnighting setups.

    The 55 amp alternator is marginal, but serviceable if the engine rpm is above idle for most of the time. The solar is great for topping up batteries, because running the diesel to top a battery is a stupid expensive way to do things. If you want to look seriously at solar, capacity equal to about 20% of the battery bank's normal charge acceptance rate works well for topping after the diesel is shut down.
    DogCavalry likes this.
  8. comfisherman
    Joined: Apr 2009
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    Location: Alaska

    comfisherman Senior Member

    I love the acr setups. Have two circuits on my current boat, they work great. My last boat got one somewhere around 2018, sold the boat in 2022 and probably put around 5k hrs on the system. That system has probably another 5k on it from the guy who bought the boat and to my knowledge it's still going strong.

    They're just nice cuz they kind of simplify your thinking of battery management. Have the segregated starting and 24hr system batteries and a house bank connected to everything else.

    It manages the load as advertised.

    My inverter is also a charger with a dual circuit combiner to bypass. Not sure if it's best for battery hygiene, and to be honest I only rarely use it as I've redundant separate 120v chargers for each bank.

    Can't speak to alternator size, 55 amps is at peak rpm more than likely. Find your alterbator curve at your normal rpm and then add up your normal daily consumption. That will tell you if your alternator is up to the task.

    I've found I'm much harder on my house bank now, used to be very careful with battery cycles. Acr has enabled me to kill them a bit faster, it's a trade off for me as swapping out house bank every 3 years vs 5 isn't a big issue compared to rarely thinking about batteries during the season.

    Be careful with sizing. Go up one size if you're on or near the amp rating. I did kill my first one with over amperage, dialed back the alternator pulley and fixed it. (This was on a giant 12v leece Neville that could pump some real juice.)
  9. Nidza
    Joined: Nov 2016
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    Location: Belgrade, Serbia

    Nidza Senior Member

    Well you got a quite a few good answers, but here is my contribution, if it can help.

    As I have understood, the key change is that you want to add three new loads to existing setup - freezer, microwave and coffee maker. Then even though not your initial intention, you want to add also inverter to cover these three new loads since you have picked all three devices to be 120VAC.

    Here is how I would start analyzing it and maybe if you go through the analysis you can figure out and calculate the key data for yourself:

    1. Someone already mentioned, I think Rumars, coffee maker and microwave are only short lasting loads (low duty cycle) and so they are less critical in terms of using energy on the long run, similar to fresh water pump. That would be our Assumption 1: low duty cycle of coffee machine and microwave (e.g. total usage of both together only ~1h in every 24h). Probably, there is no big choice of 12/24VDC options for those machines, and those few that exist are quite expensive, so I understand your choice.

    2. Now you need inverter to cover those widely available, cheap and easily re-placable coffee machine and microwave. To cover your needs and save unnecessary costs, you need the smallest inverter possible to provide sufficient power to those two appliances. For the sake of saving costs we could make our Assumption 2: You will not use microwave and coffee machine at the same time. That way, the inverter size must only cover the power required for the appliance which requires more power (coffee maker or microwave), but not both at once.

    3. Now before continuing technical topics, let's turn back to economy again - compare the price of cheap 120VAC coffee machine + 120VAC microwave + selected inverter. Compare that price with appliances on 12V if available and check if it makes sense. To the price of inverter add some 50% due to required cables, connectors, mechanical components for installing, circuit breakers and mounting all those and time required for all that. Of course, this 50% could be more or less, depending if you do the work or someone else or on the prices in your region, etc., but you get the point. So before continuing to next step, please check if you want to change your decision which appliances to use in step 1.

    4. (we continue here assuming you decided to proceed with inverter) Again, as already commented, when inverter is turned on, even without any load, it is using some power itself - for inverting. So, if it would be turned on whole day, this would increase the required energy, meaning required battery capacity. So, for the sake of saving costs on battery, we could make our Assumption 3: You will turn on inverter only when you really need coffee maker and microwave and then inverter will also become low duty cycle load, not requiring big battery capacity increase.

    5. Now, finally, we get to freezer. Here, I would really challenge your choice of 120VAC freezer instead of 12VDC. And there are multiple routes to solve this: a) buy new one on 12/24VDC which is available and produced, not only for marine use, but also for automotive use in e.g. trucks, but this is the most expensive route (but then again, mind the price of required inverter if you want to compare it); b) buy it used, much cheaper, most often it works out of the box or it can be repaired for peanuts; c) find a repair guy for refrigeration devices and he could easily exchange AC compressor for 12/24VDC one in cheap dead 120VAC freezer. d) etc. So, why I am pushing for 12V? Because freezer is not low duty cycle, it works 24/7 (even though periodically during the day, but still this can reach 50%, i.e. 12h in every 24h). That means that freezer is really affecting the required battery capacity and therefore the cost. This duty cycle would not change in case of 120VAC freezer, but now your inverter would have to work also 24/7, even in periods when freezer compressor is not running. That would mean that your inverter would not be low duty cycle anymore and it would even more add up to the required battery capacity. In addition, your inverter would have to be higher power than proposed at step 2, because now it has to be sized to cover the power required for the higher consuming appliance (coffee maker or microwave) plus the power of the freezer, making the inverter more expensive. I would not count that it would be possible to know when freezer is not working and then use coffee machine or microwave at that time. Turning on and off time of a freezer can be really considered random.

    6. From this step onwards, we could make our Assumption 4: you have decided to proceed with 12V freezer :) . Now you need to calculate the battery capacity required with new equipment compared to what you currently have and check if what you have is sufficient or not (I assume everybody here already knows to calculate this, so I will skip that; Rumars already gave some examples and calculations above, also regarding calculation if existing alternator is sufficient). My guess, without calculation is that you will be fine with existing alternator. It would be quite a good feature to have alternator with external smart 3 or 5 phase regulator, but I think most of today alternators do come with integrated regulators so someone needs to change it and in case you are somewhere out and it dies, you better have a spare fancy regulator. Regular alternators you can find and service anywhere. But, I am interested in e.g. would the car battery last longer in vehicles with this regular alternators or with that 3/5 phase alternator and what is the lifetime of each of these alternators (but this is out of scope here)?

    7. Now it is required to decide if it is sufficient to have single battery bank or additional battery bank. Without the freezer, if you can guarantee that you will never-ever forget to turn off inverter after usage of microwave or coffee machine, I would place those appliances on the same existing battery bank (Hey, but you already have step down which can also deplete the bank if you do not turn it off and you do not complain about it). Even with the freezer, according to your description how you use the boat, it seems that the second battery bank is not required (only overnighting without engine when the freezer works the least). But, the main question is what do you do with freezer when the boat is not used - do you turn it off, or keep it constantly on? In case you use it only when sailing, then no issues, but if you want it to stay on while you are away, then I would strongly propose separate battery bank. The reason is, the freezer is constantly eating the power of batteries and cycling them, also reducing their life and sooner or later that bank will be depleted. In that case, your start batteries are not affected and you can start engine and go home (without required e.g. Honda generator or...). Added functionality - in case the start battery bank dies for any reason, you can start from the second bank, again without required e.g. Honda generator or... In case you add this second battery, I would then think to reroute the coffee maker/microwave inverter to that new battery bank as well in case you forget to turn off inverter, so that you cannot deplete the start battery bank by accident.

    8. Do you need shore power charger? In case you want to leave the freezer on while you are away from boat, then it is recommended either to have a shore charger or to have solar panels (with everything required around the panels, e.g. controller/charger, cabling, mounting, etc.). Mind that in both cases, you could probably find somewhere somewhat more galvanic corrosion than before having any of these setups (no system is perfect, even when you do everything right).

    9. Do you need ACR? Well, in case of two battery banks you need some mean of separation, each having its advantages and flaws. You mentioned if you would need additional alternator, well that could also be one type of separation method, that each alternator is charging dedicated battery bank (still mind that if you want to use different battery chemistry of one bank, then alternator needs to have correct voltages for that battery bank). If you are more mechanically inclined than elctrically, maybe two alternators is better solution for you and also you do use a motor a lot according to your desciption. It depends what do you want to achieve. ACR is also simple, but can be used only if both battery banks use the same chemistry (e.g. lead acid/AGM on both sides (these two are actually the same chemistry and can be mixed), Gel on both sides, Lithium on both sides, but you cannot mix those three groups, e.g. lead with Gel or Gel with Lithium or...). If you want to mix different chemistries, you have to use something else. Mind also that the different flaws could come from different designs between the brands (I have seen one such issue, but it is out of scope now). If the system is designed OK, ACR will "distribute charging currents effectively", it does not matter if banks are not similar in size as long as they are the same chemistry.

    Bonus: The simplest and cheapest path is to not add anything and bring the thermos bottle with prepared coffee and use the gas stove to heat the pre-prepared food or buy it outside of boat on the day of sailing. It just depends on the use case of the freezer in the end which actually complicates it the most. An additional feature of not adding anything is that when you are off the boat, you only worry about the seacocks, same as before, and not about additional systems running when you are away and potential expensive accidents like dead batteries, fire hazards, etc. There is always that debate between inboard/outboard preference, well I can credit them for not worrying even about the seacocks when they leave the boat. Hmm, maybe renting the boat with crew is even better, but then my brain could probably be bored, so some of us are just cursed to suffering.

    Sorry for long post, hope it helps. You really clarified the crankcase ventilation topic recently for me kapnD, thanks one more time.
    Last edited: Mar 26, 2024
    BlueBell likes this.

  10. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    IMG_4508.jpeg Thanks for all the help, I’m coming along with the project.
    So far, inverter/charger is installed, second battery bank (lead acid for now, I already had them), selector switch and shore power hookup. So far, everything is working, a miracle in my most definitely mechanical mind!
    Next step is an AC distribution panel.
    Coffeemaker has been eliminated, instead bought some delicious Via instant coffee to use with hot water in the microwave.
    Melted the cup on the first try, so I’m looking into microwave safe cups…
    fallguy likes this.
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