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Old 08-03-2019, 02:53 PM   #1
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Do I need to replace batteries and with what?

My recent cruising trips have brought up an issue that is bothering me. I have to run my generator for hours every morning and every night. I'll run them for lets say 3 hours in the evening then wake in the morning and be down to 60% or less. I have four D8 wet cells. Two to start and two for the house. My electrician told me that my port start battery was separate but the stb start was part of the house bank. To be safe there is a battery combiner, and I have jumper cables in an emergency, and I have two generators, one with its own battery. My big loads are a residential refrigerator freezer with 120v 6.5 amp draw. So that is like, what?, 70 amps through the inverter? Then I have a newer small cooler in the sun deck with a very small ice cube tray area. I can't remember the draw right now. Is this enough to drain my batts in eight hours or less?

Ok, next question. Assuming the batts are getting weak, what should I replace them with. I have Cat 3208s that take some CCAs to get going. I started looking into Lithium-ion but not much cranking power. What about all 6v gold cart stuff?
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Old 08-03-2019, 03:09 PM   #2
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Yes, your 120V fridge could definitely be pulling down your batteries overnight. Yes, it draws 70A DC from your batteries when running on the inverter.

First I think you need to look at doing a bit of rewiring: separate both starting batteries from the house and use the combiner to connect the house bank for charging from the propulsion alternator.

Then I would look at what kind of shore power charger you have. It needs to be a big one- 100A or bigger to reduce genset running time.

Finally replace the two 8Ds with four 6V golf carts which will fit in the space of the 8Ds. More batteries would be better unless you replace that fridge power hog. You can buy Danfoss compressor based 120/12 V full size refrigerators for about $1,500. They will run on 120V when available but switch to 12V when not. The Danfoss compressor draws a fraction of what your 120V fridge draws.

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Old 08-03-2019, 03:38 PM   #3
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I agree with what David said. You currently have 2 8Ds along with another 8 D for starting and part of the house bank. How are they cabled? For one bank you want all the jumper cables between the batteries to be the same lenght. My concern is that your 2 8Ds are next to each other and the starting/house 8 D is somewhere further away. That will cause problems since the connecting cables will not be the same length. The extra resistance in the longer cables will make the battery hooked to them ďdisappear ď. I would do what David suggested and maybe if you have physical room add a couple more 6 volt batteries. Make sure the connecting cables are robust enough and the same length. You want to hook the positive cable at one end of the bank and the negative at the other end of the bank. Donít hook both positive and negative cables to the same battery. I hope what I wrote is understandable...
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Old 08-03-2019, 09:57 PM   #4
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What is charging your batteries when the generator is running? Is it the inverter? Whatever it is you should be able to pump in over 100 amps when charging that big of a bank. What is your inverter set to?
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Old 08-03-2019, 10:29 PM   #5
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I have 3208 cats. They start on the first turn. I do not have a separate Bank, the motor start from the house Bank which are 6v gc batteries
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Old 08-04-2019, 12:47 AM   #6
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What you need is a complete picture of all that is going on. Sure, the generator is running, but is it backed by a puny 15 amp battery charger?

You appear to have a pretty big load, but no mention of how quickly you can put that energy back. You have a good start on measuring how much energy is being consumed, but you really need to pencil this out in its entirety. To do that, you need some actual measurements. I would start with a proper ammeter and measure exactly how much your loads are drawing before trying to factor run time. Then measure your charge current both initially and over time. I donít think we even know if your batteries are even getting charged. Better would be an amp hour meter to make that measurement easier, but with enough diligence a simple current meter will do just fine.

Everything else is opinion.
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Old 08-04-2019, 02:26 AM   #7
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I wouldn't guess on the load. I'd put a clamp meter on the house wire leaving the battery and see what it does. Turn the stat on the refrige to make it come on and see. Thats a lot of battery to drain overnight if it is good.

Also, is your frige building up a lot of frost or, if in an arid area, just running a lot? Maybe the seals are bad or the door isnt closing all the way and it isnt keeping cold well. It'll only draw a lot when the compress or is actually running, which shouldn't be too often. Also,,what is the thermostat setting? It could also need a charge. Is it very empty? If so, filling it up with, for example, water jugs will helo it hold cold better, lose less to leaks, and run more efficiently.

Regardless, remember thatvaint 70A continuous, just as needed.

At any rate, I'd make sure I knew the consumption overnight before replacing batteries -- door seal or recharging is cheaper.

You can also check specific gravity on the batteries.

I agree with everyone who says that is a long time to charge up the batteries. I'm wondering if you run the generator 3 hours because you are doing other things, e.g. enjoying air conditioning,,or because the batteries need that much time to charge.

If the batteries need that time to charge, you really do want a bigger charger (and/or to address the batteries, if they aren't chargin up or heat,limiting) That is wasting much fuel, noise, etc.

If it is due to the charger size -- I agree with everyone and suspect you'll be happier with a new one with "more amps".
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Old 08-04-2019, 06:36 AM   #8
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Do I need to replace batteries and with what?

All good suggestions. To help assess power consumption, especially for the fridge, I recommend a Kill-o-watt meter. They are about $20. Connect your fridge to it and let it run normally for 1-2 days, then see what the kWh per day consumption has been.

Fridges are particularly difficult to assess since they cycle on and off. And the name plate rating is the max and really gives almost no insight into actual daily power consumption.

But I would be suspect of your fridge estimates. Itís still likely your large consumer, but I had a sub zero on my last boat and it didnít draw anywhere near what you are estimating in actual use.
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Old 08-07-2019, 10:22 PM   #9
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You may not have an issue if you have 60% charge left in the morning.

It looks like you have three 8Ds for house use ( 2 house plus one engine batt) roughly about 660 Ah total. Since they should not be discharged below 50%, you only have about 330Ah available over night. Recharging the last 20% of capacity takes a long time (assuming flooded lead acid), so you typically won't get to more than 85% in three hours of generator time. So you really only have about 220Ah to play with over night.

More amp hours are always good, but again, if you have 60% left in the morning after being on the hook for several day, you are doing fine.
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Old 08-08-2019, 11:12 AM   #10
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Originally Posted by gsholz View Post
You may not have an issue if you have 60% charge left in the morning.

It looks like you have three 8Ds for house use ( 2 house plus one engine batt) roughly about 660 Ah total. Since they should not be discharged below 50%, you only have about 330Ah available over night. Recharging the last 20% of capacity takes a long time (assuming flooded lead acid), so you typically won't get to more than 85% in three hours of generator time. So you really only have about 220Ah to play with over night.

More amp hours are always good, but again, if you have 60% left in the morning after being on the hook for several day, you are doing fine.
Also, if you are only charging to 85% regularly and not getting them to 100% on a regular basis, their life expectancy will be significantly degraded and the amp hour capacity will diminish more quickly than they would if you were regularly charging them to 100%. A solar system or wind generator to 'top off' each day would be highly recommended.
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