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Old 14-08-2013, 06:27   #1
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Battery Charging Puzzle

I keep my boat on a mid-river mooring in the Hamble River, and when I'm out cruising, I spend most nights at anchor. As a result, I am not very often connected to shore power.

My power system works very well and I really don't worry too much about electrical power, either AC or DC. I have 420 amp/hours * 24 volts of Trojan batteries, a Victron 3kW, 70 amp charger/inverter, and a Kohler 6.5kW heavy-duty generator. On the hook, one (or at most, two) charging run a day for a couple of hours is generally enough to keep us in electrical power and hot water, even with five people on board as we had for this year's summer cruise.

I am especially pleased with the batteries, which have been abused by being run out dead flat two different times and left for weeks like that (once a stupid mistake of mine, another time a shore power failure when the boat was on the hard being refit). At the moment, they are testing better than they did when new with my Argo capacity tester, and they just go and go, despite living a very hard life. I do avoid discharging them below 50%, and I do equalize them from time to time, but without shore power, they are not brought up to 100% charge very often. They are Trojans, not the true heavy duty ones, but the 12v ones which look suspiciously like car batteries/leisure batteries (these were the only ones which would fit my battery boxes).

Now I'm back on the boat for a few days getting her cleaned up and ready to be left alone for a month while I go back to work. And here's my puzzle:

Say you are charging from a generator, and not from shore power, like I am. You do not want to run your generator 24 hours at a time, or even 10 hours at a time, as this is a waste of diesel fuel and engine hours. You are trying to gradually get your batteries up near 100% charge before leaving the boat for a month. How do you do it? Run the genset for 10 hours (or whatever to really get them to 100%, them top them up just before leaving? Or is it better to charge them a little bit at a time every several hours?

It seems to me that the batteries are accepting more charge per hour of genset run if I charge them a little bit at a time. It seems to me that the surface charge dissipates between runs, extending the bulk phase of charging. The less charge being accepted, of course, the less efficient is your genset run.

I do have a small Honda petrol generator which I bought specifically to put a finishing charge on the batts the last day before leaving the boat. However, the carburetor is clogged again (I bought from a guy who had let it sit unused for five years) and I really don't have time to faff around with it.

I'll be interested in your views on this.
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Old 14-08-2013, 06:33   #2
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Solar. Even you not being in an optimal location for solar, you should still be able to fit a temporary 100 watt panel on deck with a regulator to top and maintain your batteries while you're gone.
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Old 14-08-2013, 06:48   #3
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Re: Battery Charging Puzzle

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Originally Posted by Sailmonkey View Post
Solar. Even you not being in an optimal location for solar, you should still be able to fit a temporary 100 watt panel on deck with a regulator to top and maintain your batteries while you're gone.

+1 In complete agreement.

We have two 130W kyoceras mounted on the stern frame, plus we're hooked up to shore power. Our electricity bill for the last 6 months (which indicates how much the charger has needed to pull from the mains) was $0.22!

In addition to keeping your batteries topped off for no-little OpEx, it also ensures that your bilge pumps have a ready supply of power when you're not there....

Just to add: around 400W/m^2 of power hits the earth at the equator. I believe this can be calculated to around 370W/m^2 at Houston (parallel with Cairo!) where Sailmonkey and I are, and around 150W/m^2 on the south coast of Blighty. As SM stated, a good 100W hard panel facing skyward (even with the around 40% of the light hitting the earth compared to the Gulf Coast) should give you sufficient charge to keep your batteries ready for action.

I also had a Honda EU2000i for emergencies and extra power while at anchor, but this will be going on eBay in the next couple of weeks: I've come to the realization that a 3rd solar panel (so 390W total) with a "backup" solar charger for contingency would mean that I wouldn't have to carry petroleum onboard nor service the generator!
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Old 14-08-2013, 07:05   #4
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Re: Battery Charging Puzzle

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Originally Posted by Sailmonkey View Post
Solar. Even you not being in an optimal location for solar, you should still be able to fit a temporary 100 watt panel on deck with a regulator to top and maintain your batteries while you're gone.
Yes, thats the best solution.
However as you dont have this, do the following.

You say that you do not bring the batteries to 100% often and now you have a few days preparing for leaving.

Make sure your battery monitor is in sync.
Ensure your charger does 29.6v in bulk mode and make sure the bulk mode is more than the default of 4 hrs (I think default is 4 hrs).

Run the generator 24 hrs before to bring the batteries up to say 90%. Switch of all device connections to the batteries.
Then stick the charger on equalise mode which is about 31volts and give it some until the monitor shows positive amps (this means that the batteries have accepted more amps than the last 100% charge). Or let them rest when you think they are full and then again give them a mormal bulk change and see how many amps they accept, if its around 15 then they are full.
If not full, put back on equalise mode and repeat until you think they might be full on a normal charge.
As a guide...(full means accepting around 15a at 29.6 volts)
.... a bit of a guess in Equalise mode they might be accepting 30a at 31v when they are full at 29.6v

So now give them a last equalise for maybe 1 hour and then they are ready to be left.

Now do all your other jobs with the batteries disconnected... if you have to be there into the evening use a head torch.
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Old 14-08-2013, 07:09   #5
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Re: Battery Charging Puzzle

Yes, of course -- you are all exactly right about solar. That is certainly the silver bullet for people in my situation -- no shore power.

I don't have a good place to put solar panels, which is further complicated by the fact that I have 24v so need to mount them in series (which means I have twice the cell shading problem).

I want to mount solar panels on my mooring. But we are going to replace our pontoon soon (I hope next spring) so can't do anything before then.

So meanwhile, it's diesel and diesel. So back to the original question?
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Old 14-08-2013, 07:18   #6
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Re: Battery Charging Puzzle

The simple answer is to charge them, however you can, up to 100% before you leave. Period.

The long answer provided by Fuss is a very good way to do it.

The reason charging a little at a time shows higher acceptance is I'm guessing you have loads running, so whenever you are charging the battery acceptance is higher.

Just do it once and be done with it.

Good luck.
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Old 14-08-2013, 07:52   #7
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Re: Battery Charging Puzzle

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Originally Posted by Dockhead View Post
...

Say you are charging from a generator, and not from shore power, like I am. You do not want to run your generator 24 hours at a time, or even 10 hours at a time, as this is a waste of diesel fuel and engine hours. You are trying to gradually get your batteries up near 100% charge before leaving the boat for a month. How do you do it? Run the genset for 10 hours (or whatever to really get them to 100%, them top them up just before leaving? Or is it better to charge them a little bit at a time every several hours?

....
Firstly, if you are going to be off the yacht for a month or so, it would seem that the only power the yacht should need would be for an automatic bilge pump and, perhaps, an anchor light (LED) if that is necessary or advisable in your mooring field.

As for running the generator, if it is only used for charging, it will be underloaded, which is not advisable for the machine any more than underloading is/would be for your primary auxillary. That aside, at roughly 60%-70% of capacity, that generator should be burning something on the order of 1/4 gallon per hour. With a 10 hour charge cycle that's only 2-1/2 gallons of fuel which, it the grand scheme, really isn't much, eh? And the generator is certainly capable of such a duty cycle assuming you're doing the regular maintenance. Frankly, I'd be more concerned about leaving the yacht unattended, rather than simply unused, on a mooring for such a long period than over-working the generator.

FWIW...
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Old 14-08-2013, 08:50   #8
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Re: Battery Charging Puzzle

Think again about solar panels.

Two 30 watt nominal 12 V panels plus a controller should cost maybe $100-150. They can be temporarily connected (through a 5 amp fuse) to the controller and battery with alligator clips. You can use cheap 18 gauge zip cord for the wiring. Tie the panels down with nylon cords to withstand winds. Then you can remove them in a few minutes when you want to go out.

A 30 watt panel will put out 2 amps or so in full sun which should be plenty to top off your batteries in a few days.
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Old 14-08-2013, 08:54   #9
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Re: Battery Charging Puzzle

You don't need to series connect panels. Buy a 24 volt panel and literally just stick it in the cockpit......hell, just strap it down on the deck even, and run wires through a cockpit locker. It's not pretty, but will keep your batteries happy while you're out.
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Old 14-08-2013, 09:15   #10
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Re: Battery Charging Puzzle

I'm sure you would have a place for some mini solar panels. Maybe a couple of these might do the trick. They are only 11 x 8".
Amazon.com: Instaparkģ NEW All Black 5W Mono-crystalline Solar Panel with 12V Solar Charge Controller: Patio, Lawn & Garden
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Old 14-08-2013, 09:52   #11
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Re: Battery Charging Puzzle

dockhead-
You *do* have a place to put solar. Take a look online at www.solarstik.com
They aren't cheap, but they are incredibly well made, best-of-breed, from folks who are really committed to their product.
You can mount one stick in the aft quarter of your cockpit, or entirely outboard of the hull/transom, and set it up to provide 24V to your batteries since they conveniently use two panels.
And while they are expensive and it looks like it would be simple to copy the idea for way less, that's not likely. A lot of careful detailing (like captive leashes on all the bits that might get dropped) and the build quality are way beyond what most local shops could copy at twice the price.

If you think there's no room for solar, really, take a hard look at this before dismissing it as impractical or expensive. I don't own stock, I'm just impressed.
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Old 14-08-2013, 10:54   #12
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Re: Battery Charging Puzzle

The generator will be a lot happier with one long run than with many starts/stops. But do get the honda fixed to use for the topping charge.
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Old 14-08-2013, 11:17   #13
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Re: Battery Charging Puzzle

I hate to be one of those people that ignores the original question and goes off and suggests something entirely different, but I have to agree that my concern would not be getting a 100% charge before leaving but putting in place a means of continually charging while I was gone.

A number of things can go wrong in 30 days on an unattended boat that would lead to dead batteries...and this time they might really be dead and gone. A small solar panel would protect the investment in your batteries as well as ensure that your bilge pump had enough juice to function reliably while you were gone.
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Old 14-08-2013, 11:59   #14
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Re: Battery Charging Puzzle

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Originally Posted by Suijin View Post
I hate to be one of those people that ignores the original question and goes off and suggests something entirely different, but I have to agree that my concern would not be getting a 100% charge before leaving but putting in place a means of continually charging while I was gone.

A number of things can go wrong in 30 days on an unattended boat that would lead to dead batteries...and this time they might really be dead and gone. A small solar panel would protect the investment in your batteries as well as ensure that your bilge pump had enough juice to function reliably while you were gone.
No argument for me. I've had a couple of battery-killing episodes as a result of this. You guys have inspired me to think again about solar.
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Old 14-08-2013, 20:20   #15
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No argument for me. I've had a couple of battery-killing episodes as a result of this. You guys have inspired me to think again about solar.
I have 400w of solar.
This has made my generator redundant.

Optimised main engine charging and then running the main engine for 30 minutes a day plus the output from the solar replaces my normal daily use on a boat with no gas only electric.
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