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Old 08-24-2021, 08:40 PM   #21
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Maybe get Nigel Calders book on electrical and mechanical boat work. Great reference manual.
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Old 08-24-2021, 10:28 PM   #22
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I think a lot of the group 24 batteries are a deep cycle AGMs. Typically they may have a Ah rating of around maybe 80 Ah.

So if you have 320 Ah in your bank, do the math from there. If you identify the actual manufacturer, you can likely find the specific Ah rating for those batteries.

I agree thought that the PO likely cheaped out on the batteries unless that was what was in there when you looked at the boat.

If you have good access to the battery location, you can’t go wrong with 6v Golf Cart batteries. In the mean time, try to keep from discharging your current batteries below 50% SOC.
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Old 08-24-2021, 11:36 PM   #23
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The first thing you actually need to do is positively identify what batteries you have installed. Until you do that we are all just guessing. Get the manufacturer and any numbers that are on the battery.
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Old 08-25-2021, 03:45 AM   #24
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I found a boat once with a battery issue. His Inverter powered the entire ac panel, INCLUDING his battery charger. You know where this went. Shutting off that breaker to the charger solved his power issues.
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Old 08-25-2021, 04:00 AM   #25
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Agree that actual specs would help.

But even if you have super high quality Lifeline AGM's, Group 24 are only going to hold about 80 ah apiece (we're talking of the various lead acid type batteries here, such as flooded, AGM, or gel.... not LFP).

3 batteries at 80ah apiece is 240 amp hours. So about 120 ah usable even if they are the best kind. That's a pretty minimal house bank for most people (not everyone, but most).

BTW, if your boat has the under-counter style fridge, you probably have something no larger than 130 liters or about 4.5 cubic feet. Perhaps using the Danfoss/Secop BD35 compressor. If it's a "drink" model it may not even have a freezer.

I've been running a fridge of that size and living basically on DC power for some years (solar), so I have a pretty good idea of how many ah it takes per day. Obviously this is dependent on ambient temperature, and your installation; but I'd say something from 30-60 ah per 24 hour day would be a reasonable guestimate (presuming not in the tropics). I just mention it because some others who estimate more may have larger trawlers with full height refrigerators that have larger freezers. Of course they draw more.

Another thing to note is that due to something called Peukert effect, the batteries react worse to a large load/small time than they do to a small load/large time. So for example running something that draws 20 amps for two minutes, exhausts the batteries more than running something that draws 4 amps for 10 hours (even though both add up to 40ah). Just a note in case you are using anything like a microwave, or anything that makes heat via electricity.

Voltage is not typically a very reliable way to gauge your state of charge (SOC), especially if you are relatively new to the care and feeding of a house bank. Reason is that unless the bank has been resting (no loads) for many hours it won't be accurate. If you are using power or charging then the bank is not resting.

Hence using a battery monitor is useful. They either use a shunt and count amps, or there are some that do use voltage but they do it by checking voltage thousands of times and applying an algorithm. (For example, original Smart Gauge.)

Given your plans, I would say you are most likely going to want to re-do your battery bank completely, and possibly parts of your DC system (depending on what you have now). But as you say, you can get through the rest of the season on what you have, and then take your time researching and getting squared away for your loop trip (sounds fun!) over the off-season. Pressure's off on "not wrecking" those batteries if so

I don't know the details of your situation or setup, but until you find out your specific details, I wouldn't assume your system is set up for a relaxed loop trip. I only say that because I can't imagine doing a big electrical job for someone and not noticing that the main house bank is a bit small (or at least mentioning it).

Of course a lot depends on how you travel. If you go marina to marina and so are motoring most days and plugged in most nights, then you may be fine. I'm an anchorer so I approach my system with that in mind. Many ways can work, just depending on your style. But as you are realizing, knowing what you have and how it works can make things a lot more enjoyable, whatever your setup or boating style.
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Old 08-25-2021, 04:43 AM   #26
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Everyone has correctly made suggestions on the storage side of the equation. However, before buying new batteries, I'd strongly suggest that you need to know what these batteries are expected to do. Use your meter to calculate how many ah you need for each item you are powering and add these all together to get a total. It sounds as if you don't have a large system so this shouldn't be too difficult. If in doubt just use your meter to measure the whole boat over a 24 hour period. Double that number to get your minimum bank size to give you 50% DOD on days when there is no charge source available.

Can you fit enough batteries to cater for this? Can you fit more for peace of mind and longer battery life? No one ever complains about having too much battery storage.

Good luck
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