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Battery charging

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Stinkpotter
Master Marine Consultant

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Djibouti
9075 Posts

Response Posted - 06/23/2010 : 18:23:26

Good information--even for a $+!nkp*++er.

Edited by - on

TakeFive
Master Marine Consultant

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2272 Posts

Response Posted - 06/25/2010 : 14:16:34

I finally got a few minutes to set up a capacity test. I used a high intensity emergency spotlight connected directly to the battery (which is still in my garage). I could not find enough contiguous time to run one long test, so I ran two tests of approx. 2 hours each, with several hours down time in between. I measured amperage and voltage under load several times during the test, and no-load voltage immediately after stopping and after 1-2 hours recovery. Amperage varied from 6.72 to 6.62 amps (which means I need to be concerned if I would blow out a fuse if I tried to use this thing), dropping exactly in proportion to the load voltage. The calculated resistance was 0.552 ohms in all cases - too low for my cheap little multimeter to measure accurately. This means the bulb was about 80 watts.

The battery is a West Marine Seavolt Dual Purpose 685, rated at 65 Ah. When I calculate all the exact numbers, I consumed 25.0 Ah and no-load voltage (after recovery time) dropped to 12.34 v (67% remaining). So based on this test it looks like the battery has about 77 Ah of capacity vs. its rated capacity of 65 Ah.

The battery is fine, as I would expect for one that is <1 year old and has been stored on trickle. It may not be the most high-end battery out there, but it's fine for the daysailing that I currently do.

Next I will test the two ~10 year old batteries that I keep on trickle in the basement (connected to a back-up sump pump).

Edited by - TakeFive on 06/25/2010 14:33:32

Voyager
Master Marine Consultant

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USA
5352 Posts

Response Posted - 06/26/2010 : 20:15:41

Looks like your battery is operating better than spec. When you were running the test, drawing 6+ amps, what was your measured battery voltage? For example, if open circuit voltage started around 12.7V, and you connected your load, what was you loaded voltage? You would be able to determine your battery's internal resistance - R(i) - if you knew the voltage drop and the current.

Edited by - on

TakeFive
Master Marine Consultant

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2272 Posts

Response Posted - 06/26/2010 : 20:46:02

Bruce,

I had meant to calculate the internal resistance but did not get around to it. Here goes:

After 1.95 hours of drain: 

12.17v under load; 6.72 amps; 12.42 v immediately after removing load, recovered to 12.49 next day

Ri = (12.49 - 12.17)/6.72 = 0.0476 ohms

After 1.76 hours of additional drain (3.72 hours total):

12.00v under load; 6.62 amps; 12.21 v immediately after removing load, recovered to 12.34 90 minutes later

Ri = (12.34 - 12.00)/6.62 = 0.0514 ohms

Looks like the plates are pretty clean!

I put the battery back in yesterday and took a sail this evening. I discovered that the battery box was cracked so I replaced it, and added a strap which was missing when I bought the boat. Before putting the battery back there I installed additional threaded post adapters onto the "fat" posts. I wanted to separate the charger/alternator wires from the wires to the control panel so I could easily disconnect the house electric while leaving the charger connected to the battery. I also replaced most of the nuts with wingnuts, since I don't like using metal wrenches that can short circuit across the posts.

One thing I meant to do was measure the resistance of the house system before reconnecting the battery. I have a feeling there are a few things wired directly to the master line, bypassing the switches in the control panel. (I'm virtually sure that the stereo's memory retention wire does this.) I'll have to leave that for next time I'm working back in that area.

Edited by - TakeFive on 06/28/2010 10:15:59

Voyager
Master Marine Consultant

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USA
5352 Posts

Response Posted - 06/29/2010 : 21:42:30

Looks like the battery checks out fine! Better than mine, hmmmmm.....

The stereo's memory retention wire from the battery should bypass the house switch (or else you'd lose your settings), but it MUST be fused with a 2-5 Amp fuse.

Also, your engine's alternator (and starter) must also be a direct link to the battery, and should be wired with a fuse, but without a switch.

Everything else should go through the house switch and through the electrical panel, nothing else should be wired directly.

Never run any wire from the battery direct to anything without a fuse - not even a solar panel - otherwise you risk your boat burning up. Yes it's made of fibreglass, but its impregnated with flammable plastic!

That includes a 30A fuse to the main house switch, AND a 50-60A standard fuse to the engine (starter). You could also use a 40A slow blow fuse to the starter.

Some people use a thermal cut-out or circuit breaker, but these are too easily prone to failure (in a closed position), especially when there's a short elsewhere.

You just never know when or where you'll get a short on any circuit. It's a bet you can't afford to lose.

Edited by - on

Stu Jackson C34
Admiral

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844 Posts

Response Posted - 07/01/2010 : 16:54:50

For some very good information on batteries, charging and using them, plus energy monitors, go to www.amplepower.com, tech tab download and read (memorize?) the Ample Power Primer.

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cwstrang
1st Mate

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USA
78 Posts

Response Posted - 07/31/2010 : 07:15:40

Guys:
Good morning and greetings from St. Helens! A couple of follow up questions. I pulled both of my batteries this week and I am thinking they were orginal or at least one of them as it had a sticker dated 7/03 and the other had no date information. Popping off both caps showed one battery was dry and the other was low with the metal plates exposed. It seems like an simple decision to replace them both, I now have two 700A 140 reserve min deep cycle marine batteries. My solar panel is rated at 10 watts. Now my question is how best to take care of them? Three Bs stays in the water year round, do I pull the batteries during the winter and if there is the ocasional sunny just take one battery to go out for a short sail? Just want to get best life out of these two. Thanks in advance, Craig

Edited by - on

Nautiduck
Master Marine Consultant

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USA
3704 Posts

Response Posted - 07/31/2010 : 10:10:31

Do you have shore power and charger? If so, I'd leave the batteries in the boat all year and charge as needed. If not I'd leave one on the boat over the winter and take it home for a charge when it gets low. Alternate the batteries back and forth.

These batteries are heavy and I'd do whatever I could to reduce the times you need to transport them.

Edited by - on

Voyager
Master Marine Consultant

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5352 Posts

Response Posted - 07/31/2010 : 10:34:41

Welcome Craig,
As you may have seen, this forum can be very helpful on a variety of issues. Based on the condition of the batteries, I'm surprised they worked at all.

You replaced them with two new "700A 140 ... deep cycle marine batteries". Are they AGM (glass mat) batteries?

In order to answer your question, a bit more info is needed.

Sounds like each one could be rated for 100-150 Amp Hours. That is a lot of capacity to have, and a lot to recharge.

Beside from the solar panel, are you able to charge them from your engine's alternator on the boat? Barring that, do you have shore power to charge them using a conventional battery charger?

Here's why I ask. The solar panel is tiny compared with the Amp Hour capacity you have. Assuming you have 120 AH each, and you discharge each battery half way, you'd have have to recharge them at 6 amps for 20 hours (alternator), or at 10 Amps for 12 hours to recharge (battery charger).

The solar panel would take 360 hours of full sun to recharge the batteries. That's an awfully long time. Ok for keeping them topped up, but not for recharging.

If you have shore power, what type of charger do you have? A standard automotive single-stage or two-stage charger? You may need a three-stage charger, depending on the battery type. Others may be able to advise on that.

How are your batteries wired in? Do you have a four-way switch on them? E.g.: Battery 1, Battery 2, Both, OFF switch?

If you have an alternator or shore-power charger, how does it connect to the batteries? Hopefully, both batteries are always connected to the alternator.

The more you can describe about the charging system, the better.

Edited by - on

cwstrang
1st Mate

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USA
78 Posts

Response Posted - 08/01/2010 : 06:31:55

Randy & Bruce:
Good morning on a cloudy Sunday.
I don't have shore power. I do have an alternator on my 15hp Honda and that combined with the solar panel sounds like I have very little chance in keeping the batteries charged. The battery switch is a four-way. I am not completely sure how the alternator is hook in; I will have to investigater further. Randy's suggestion of switching out the batteries over the winter doesn't sound too bad.
Thanks, Craig

Edited by - on

Voyager
Master Marine Consultant

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USA
5352 Posts

Response Posted - 08/04/2010 : 20:54:21

If you have two 20W solar chargers, you can get 2A for about 10 hours a day, so that's 20AH per day.
If you dropped 40AH over the course of a weekend, you would be able to recharge the battery on two panels over the course of three to five weekdays.

If you run your engine for a few hours while you're out, you can get 6A x 2hours, or 12 AH each time you're out.

In order to check the state of your charge, measure your battery voltage with all circuits turned off.
For AGM, a full charge is approx 12.8 volts. If your batteries are discharged by 1/2 (120AH), they will read somewhere around 12.0 volts. If your batteries are 1/4 dischaged (60 AH), the batteries will read approx 12.4V.

If you don't think that your alternator or solar panel will adequately make up for the discharge losses, you should, from time to time, take the batteries home and recharge them on your bench battery charger. Do both at the same time to keep them balanced. Don't overcharge them.

You can also purchase or borrow a gasoline-powered 120 VAC generator, and plug in a conventional battery charger to charge up your batteries. You may want to use a 10A or 20A three stage charger. A 20Amp charger will recharge your batteries from half charge in approx 8 hours.

Once you recharge your batteries, remove the charger for about an hour to accurately read their state of charge. The batteries will be fully recharged if they read 12.8V.

Good luck!

Edited by - on

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