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Old 02-04-2019, 10:04   #1
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How much battery time when sailing?

Hi,
I'm sure this is one of those, "Depends on a ton of variables" question, but I'm going to throw this out anyway. I have two 4D house batteries. How long should I expect to be able to run on them for the following:

Instruments - chartplotter, wind indicator, AIS, Radar (on standby), Autohelm (on standby)
Running lights
Steaming Light
Compass light

I'm just trying to set my expectations; I believe I have bad batteries because I was running only running lights (one is LED, other is incandescent), steaming light (incandescent), AIS (because it is new, I couldn't figure out how to turn it off, so I assume it was on even though all my instruments were off), and the compass light.

I lost all power (starter battery is separate) about 4 hours in. My alternator apparently is putting out less than 12 volts, so I'm going to replace it with something that can charge both batteries and the starter batter. I know I have issues with my charging system, but when I left, my batteries showed full charge (having been charged from shore power). At sea, they measured 11.5v for three hours, dropped to 9v for about an hour, and then went to 3v followed by 0.

I'm going to load test the batteries to see if they are good (I expect not, this is a new to me boat and the batteries are several years old) and get the alternator issue resolved. Once that is done, though, I was looking to get some idea as to how long I can/should go before charging with the engine.

Thanks for reading to the end of this missive. Any / all input is appreciated! Rich
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Old 02-04-2019, 10:07   #2
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Re: How much battery time when sailing?

An Automatic Charging Relay would be helpful for charging both the house and starting banks from the altenator.

For draw you would simply add the total draw of the devices.
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Old 02-04-2019, 10:17   #3
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Re: How much battery time when sailing?

To set your expectations, 4 hours is totally unacceptable in any circumstance. Even on an electrically intensive boat, which yours is not, you should be able to run for at least 12 hours, better yet 24 without charging while underway.

If your batteries have a shorted cell, that would explain why your alternator never gets to 12 volts. Check the batteries BEFORE you replace the alternator. Based on your report that the batteries started at 11.5 volts, it is highly likely you have a shorted cell. Even worn-out batteries should start at 12.8 volts, or so before tapering off.

Have them checked promptly. Having a shorted cell can lead to serious overheating of the batteries during charging, even to the point of melt down and fire.
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Old 02-04-2019, 10:29   #4
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Re: How much battery time when sailing?

That is exactly what I was looking for. Thanks! I'll get on checking the batteries ASAP. rjw
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Old 02-04-2019, 11:06   #5
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Re: How much battery time when sailing?

yes batteries likely shot

due to (also likely) the charging system being sub par

fix both
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Old 02-04-2019, 11:48   #6
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Re: How much battery time when sailing?

well if you were steaming you should be ok to go till you run out of fuel

I use about 10 amps while underway sailing in day light and around 12 amps at night

and before I started blaming my alternator for not charging, waaaaaaay before, I would be checking and changing the engine belt as any working alternator is well up to the loads a boat is using
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Old 02-04-2019, 11:52   #7
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Re: How much battery time when sailing?

To echo what others have said, with the minimal loads you describe, 12-24 hours should be achievable.

But if your steaming light is on, your engine is running, and you should never run out of power.
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Old 02-04-2019, 12:11   #8
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How much battery time when sailing?

Based just on voltage one or both batteries are dead. Coming off of charging (100%) batteries should be around 13.0v.

By the time you get to 12.0v you are at 30-40%.

For best longevity you should recharge at 50% or greater.

My guess is that you have at least one dead cell in 1 battery. Charge 1 battery at a time and see which if either will charge up near 13.0v.
If you are lucky one battery will still be good for reduced capacity. Leave the damaged battery disconnected. The 4Ds are 200a-hr batteries of which you should be using 100a-hr or less before recharging.
100a-ah should get you thru 1 or more nights with the loads indicated.

The dead battery in the system is drawing down the good battery.
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Old 02-04-2019, 13:47   #9
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Re: How much battery time when sailing?

I know Arabic numbers have pointy corners and sharp edges and lots of people try to avoid them, so in the spirit of the OP's original question, I think the best answer is "They'll last a while."

Avoiding all those nasty numbers.

Really, though, I'd suggest buying one of the many excellent books on boat electrical systems and learning how to check *all* the components of the system (alternator, regulator, cables, switches, batteries, shoe charger, inverter, who knows what else) because unless you know the capacities and ratings of ALL PARTS of the system? You're just throwing spaghetti against the wall to see what sticks.

The alternative is trying to find a well-recommended marine electrician and getting their hand$-on opinion of what needs to be done. Either way, someone has to run the numbers if you ever expect the system to run gracefully.
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Old 03-04-2019, 08:27   #10
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Re: How much battery time when sailing?

Sounds like the batteries are shot. I would isolate each one, then test them before and after charging, use a hygrometer to test for bad cells. Be careful to keep them well vented while charging, I have seen batteries explode twice from the hydrogen gas collecting.

Boat is new to you, I would check all positive and negative connections for corrosion everywhere. How is the alternator output split between starting and house bank? Cheap battery isolators will drop voltage going to the batteries.
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Old 03-04-2019, 08:35   #11
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Re: How much battery time when sailing?

Boat is new to you, I would check all positive and negative connections for corrosion everywhere. How is the alternator output split between starting and house bank? Cheap battery isolators will drop voltage going to the batteries.[/QUOTE]


2nd on this. I thought I had a set of bad 4D's and it turned out I had a bad (marginal) ground that kept them from getting a full charge. Check for corrosion and tightness.
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Old 03-04-2019, 17:36   #12
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Re: How much battery time when sailing?

If your batteries died when you were motoring, make sure that your batteries are topped off , and then have your alternator rebuilt. You should never run out of battery power when steaming, if your alternator is working...ever. I had mine rebuilt for about $100US. Once I did, the vast majority of my battery issues disappeared.
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Old 03-04-2019, 18:05   #13
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Re: How much battery time when sailing?

other causes are possible
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Old 03-04-2019, 20:17   #14
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Re: How much battery time when sailing?

2 things you need to do:

1) Hire a competent marine mechanic / electrician to figure out what is what before your electrical system and batteries start a fire.

2) Educate yourself thoroughly about how to test and maintain your charging system.

You need to do #1 ASAP as #2 is likely to take some time.

Checking for a shorted cell is excellent advice. There is also a possibility that your problem originated with nothing more than corroded connections or a loose alternator belt. This assumes that your boat's systems were safely designed and installed in the first place. However with boats one never knows.

And this leads us back to #1.
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