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Battery capacity calculation
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stulk75 View Drop Down
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Joined: 25 November 2020
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Points: 72 		Post Options Post Options   Thanks (0) Thanks(0)   Quote stulk75 Quote  Post ReplyReply Direct Link To This Post Topic: Battery capacity calculation
    Posted: 20 April 2025 at 02:50
We need to replace our house batteries. 4x AGM 115 amp hour.  I’ve done an inventory and calculation on some max reasonable usage and I’m coming up with a daily usage of close to 400 amp hours, which means we’ve been seriously under powered.  We have a solar array and I’d prefer to avoid running our generator or engine. 

I’d appreciate any views as to if I’m grossly missing anything, or if there is a better way to determine our battery bank needs.  If my analysis holds, I think we have to move to lithium, as we don’t have space in our current layout to put more than 4 batteries of similar size as the ones we have in there (group 31).

uploads/6889/Electrical_Inventory_Audit_and_Planner.pdf
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Wayne's World View Drop Down
Admiral
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Joined: 18 July 2012
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Points: 1434 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Wayne's World Quote  Post ReplyReply Direct Link To This Post Posted: 20 April 2025 at 07:04
Stulk75,

Not sure what model your Hanse is or how you use it but we currently use about 400AH a day and that includes all you have plus Starlink (Gen 2), watermaker (about 2 hours per day when at anchor), water heating (via inverter) and washing machine ( one load every 1.5 days). We have 600ah of lithium and 1025W of solar. We are liveaboard and used the boat basically 365 days a year and for the last 5 years spend little time in marinas. Like most liveaboard cruisers we spend most time (95%) at anchor. On cloudy days we need to top up the batteries with the generator. 
Wayne W
Cruising, currently in the Pacific until the end of 2026.
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stulk75 View Drop Down
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Points: 72 		Post Options Post Options   Thanks (0) Thanks(0)   Quote stulk75 Quote  Post ReplyReply Direct Link To This Post Posted: 20 April 2025 at 10:27
Thank you.  We have a 445 and use it for week+ cruising trips, long weekends and day sails. No more than 5 hours at a time sailing, the rest at anchor or a mooring.  

We have 520W of solar, and then the existing batteries are 460aH (but they are 4+ years old, and we probably have shortened their lifespan by overdischarge).

Up until this season we've had pretty consistent battery reserve - rarely needing to use the generator/engine to charge the batteries.  We've added a cooler and then increased use of power hungry devices for charging (phones/laptops).  So, now I'm trying to size for the future.  Here's a summary:

CategoryAvg Daily UseSum of Ah per DayQty.
Charging and Power Management21.020.99
Device Charging3.563.66
Heavy Duty / Momentary Loads0.670.03
Lighting, Fans, and Misc8.469.28
Navigation and Sailing Electronics4.865.810
Plumbing / Pumps0.25.44
Refrigeration / Cooling12.0103.63
Grand Total7.7398.543

Sailing electronics includes the autopilot drive motor, heavy duty is winches and windlass.  Misc includes radios (VHF & entertainment)
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Black Diamond View Drop Down
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Points: 1275 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Black Diamond Quote  Post ReplyReply Direct Link To This Post Posted: 20 April 2025 at 10:40
How do you figure you are underpowered in your bank?    On my 575 I have 800watts of solar on my bimini but I expanded my battery bank (added 2 more batteries) to make an 800AH bank as well. My solar can dump up to 50amps into the batteries on a good day.

Granting the occasional cloudy or rainy day (and I'm not sure what latitude you are at), but with your solar you will (much of the time) we only begin drawing down on our bank later in the day, and have a surplus of power much of the day.   So much so that I was going to add the option of running the hot water heater off the inverter.  So prior to that you are basically running off your solar.

Or are you saying you need to support all that concurrently? 

Rick
S/V Black Diamond
Hanse 575 Build #192, Hull# 161
Newport, RI
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stulk75 View Drop Down
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Points: 72 		Post Options Post Options   Thanks (0) Thanks(0)   Quote stulk75 Quote  Post ReplyReply Direct Link To This Post Posted: 20 April 2025 at 20:26

Thanks Rick and Wayne — really appreciate the feedback.

To clarify, I don't mean that we're underpowered from a solar generation perspective day-to-day.  I think its the battery storage that's the constraint.

But I'm not sure if I'm really calculating things correctly either.

We have 520W of solar (2x260W panels), and in good Caribbean sun (~8 hours), I calculate that we can generate about 260–270Ah/day @ 75% efficiency. That should do a good job replenishing much of our usage during the daytime sailing load.

Where I think we're struggling is that with 4 × 115Ah AGM batteries (460Ah total), we only have ~230Ah usable capacity (staying within 50% depth of discharge to preserve AGM lifespan).


The 398 aH usage is over a 24 hour period for all things.  Not concurrent.

I've inventoried what we have, how long its on for, about when its used and the amp hours to get that overall number.  

I think roughly, we use 130 a/h for sailing (electronics, autopilot, momentary winching), 120 a/h for background loads (coolers, power management), 75 a/h at anchor and in the evening for lights, fans and stereo, and 60 spread across whenever for charging all the devices everyone has nowadays.   

Is my thinking right?
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Wayne's World View Drop Down
Admiral
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Points: 1434 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Wayne's World Quote  Post ReplyReply Direct Link To This Post Posted: 20 April 2025 at 20:39
Stulk75,

It seems the "standard" batteries last up to about 7 years but less when they are over discharged. After 5 years of use we found our original battery bank to be not supplying the "normal" supply. We checked by charging the bank then disconnecting all the batteries, waiting say one hour then checked the individual voltage of each battery and found one had dropped a lot and it was obvious that battery was failing.
Wayne W
Cruising, currently in the Pacific until the end of 2026.
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Mike2145 View Drop Down
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Joined: 18 October 2014
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Points: 278 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Mike2145 Quote  Post ReplyReply Direct Link To This Post Posted: 27 April 2025 at 18:56
I would agree that the current bank may be a little under specified if you want to avoid running a generator or engine every day
If I read this correctly then your net consumption from the batteries is about  200Ah/day. (I think  the solar may be a bit optimistic) If you like to keep within a 50% discharge then you'll need to have an additional power source pretty much once a day. 
Without adding more solar there isn't a lot you can do about that. The only real option is to add capacity to the bank.  If there is room with lead batteries.  Alternatively lithium could easily double, probably more, your capacity for the same volume and these days probably not a lot of additional cost.  You will still need to add an additional charging source to make up the shortfall of the solar but at least not every day.  

Just some thoughts. 

Originally posted by stulk75 stulk75 wrote:

Thanks Rick and Wayne — really appreciate the feedback.

To clarify, I don't mean that we're underpowered from a solar generation perspective day-to-day.  I think its the battery storage that's the constraint.

But I'm not sure if I'm really calculating things correctly either.

We have 520W of solar (2x260W panels), and in good Caribbean sun (~8 hours), I calculate that we can generate about 260–270Ah/day @ 75% efficiency. That should do a good job replenishing much of our usage during the daytime sailing load.

Where I think we're struggling is that with 4 × 115Ah AGM batteries (460Ah total), we only have ~230Ah usable capacity (staying within 50% depth of discharge to preserve AGM lifespan).


The 398 aH usage is over a 24 hour period for all things.  Not concurrent.

I've inventoried what we have, how long its on for, about when its used and the amp hours to get that overall number.  

I think roughly, we use 130 a/h for sailing (electronics, autopilot, momentary winching), 120 a/h for background loads (coolers, power management), 75 a/h at anchor and in the evening for lights, fans and stereo, and 60 spread across whenever for charging all the devices everyone has nowadays.   

Is my thinking right?
Cant take a joke, don't buy a boat.
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Black Diamond View Drop Down
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Joined: 24 October 2015
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Points: 1275 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Black Diamond Quote  Post ReplyReply Direct Link To This Post Posted: 28 April 2025 at 02:58
Did you say how old the 4x115 house bank batteries are?    

I wouldn't expect the same performance out of a 8 year old battery with several hundred deep cycles under its belt versus a new bank.   Its the difference between 50% discharge before the bank drops to 11V versus it only taking 30% discharge (counting amps that is).

Even with a new bank, most battery monitoring systems tell you to size it at no more than 80% of the total AH for the bank.     My point being that age matters.
Rick
S/V Black Diamond
Hanse 575 Build #192, Hull# 161
Newport, RI
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