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Old 19-08-2016, 05:58   #1
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Bank Size and Charging Capacity

We are now full time cruisers- WooHoo!- enjoying Newfoundland currently and getting used to running the boat with all new electronics and electrical gear following a lightning strike last year. It seems like my battery/charging situation is not under control - and/or the controls are lying to me!

I think my 3 year old battery bank maybe too large - 4 X 8D - about 900AH while my charging sources too small - Generator with 60A charger, shore power (infrequent) with 100A charger, 100A Altenator, 260W solar and an AirX wind generator. My "monitoring" unit is a Blue Seas Vessel System Monitor (VSM)...

We are taking advantage of being at a dock right now and have been plugged in for 2 days and I still can't bring the batteries to full charge (according to the VSM)... (I tend to think all this stuff is too clever for it's own good!)

The last time I was plugged in, I attempted to equalize the batteries - but the new charger wouldn't go into equalize mode - apparently because the batteries couldn't get into full float mode... So, I'm considering bypassing one and possibly two of the 8D batteries - any thoughts?

I should add that we don't tend to discharge too deeply - normally to 65 - 70% and can quite easily get back to 80+% with green energy and perhaps an hour on the generator...
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Old 19-08-2016, 06:19   #2
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Re: Bank Size and Charging Capacity

Assume you can check the specific gravity of your 8Ds. Start there to confirm actual state of charge. Let them rest for hours to get a good baseline.

Have you checked the settings of your charge controller? Might be worth confirming it's what you expect it to be.

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Old 19-08-2016, 07:15   #3
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Re: Bank Size and Charging Capacity

It is just about time when charging to get to true full charge. But I don't think the reason your aren't getting to full charge is that your battery bank is too large. Your chargers have more than enough capacity to to do a finishing charge and even with a house bank of half of what you have it should take the same amount of time as you would only be using a percentage of the charger capacity.

The only time really a large bank with a smaller charger is an issue is if you are at low charge where the charger capacity is the limit because it can not put out what the batteries can accept.

Have you checked your charger settings? Are they based on voltage or just time? Is the charger dropping into float too early?
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Old 19-08-2016, 07:26   #4
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Re: Bank Size and Charging Capacity

Installing A Battery Monitor Photo Gallery by Compass Marine How To at pbase.com

This link should be helpful. It is from Mainesail's web site. Very informative. The piece to start with is the lines about what is a full charge

"*Voltage at 14.4V+ - Check

*Net "accepted current less than 1.5% of Ah capacity - Check" then battery is full.

If the charger is at or above 14.4v and the current going into the batteries is less than or equal to 1.5% of the overall bank capacity then they are full.
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Old 19-08-2016, 07:35   #5
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Re: Bank Size and Charging Capacity

The only thing you can change, with larger charge sources, is to shorten your time spent in bulk charging or constant current.. The time spent in absorption is determined by the batteries not really the charge sources. Shrinking the bank, and keeping the discharge the same, will simply result in deeper discharges and more sulfate formation.

Your batteries may never be getting to a low enough charge current to be considered "full" by the VSM. This can be due to chronic PSOC use and chronic under charging or "premature floatulation" where the voltage regulation goes into float based on a time clock..

On most boats with lead acid batteries, & typical CC/CV controllers, regulators and chargers, the time to 100% SOC from 50% SOC is in excess of 7 hours of charging with healthy batteries.. As the batteries age and sulfate, from partial state of charge use (PSOC), this time only gets longer..
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Old 19-08-2016, 08:35   #6
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Re: Bank Size and Charging Capacity

The bank will be sized to your consumption. No bank is to big, but a bank can be mis-sized.

Start with measuring your daily consumption, this will tell you what size your hose bank should be.

The charge capacity is related to the bank size and to how you charge.

I like 25% charge capacity for our style of cruising, but this will not be sufficient for closer inshore cruising by a power hungry generation. We are mostly solar with alt as a back-up.

I have 15% charge capacity on a big boat I work for - from a major luxury segment manufacturer. Marginal, but no issue since her main charge source is a genset/shore side charger.

If in doubt, CREATE A MAJOR charge overhead.

As shore chargers above 200A (12V) are not very common, banks above 800Ah may pose some challenge to manage in smaller boats.

To sum it up: measure your daily consumption, allow for your cruising mode and charging options, add days between charges overhead, this is the bat size you want.

PS Talking normal batteries, not lithiums.

Cheers,
b.
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Old 19-08-2016, 09:16   #7
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Re: Bank Size and Charging Capacity

FWIW, we side with the "never" large enough group. Charging system has little to do with bank size, just affects the charging time. We used 2 volt tractor cells and could draw almost 2000 amps. Basically spot welding if needed. They kept going for over ten years just using various low amp recharging methods(wind, water, engine, solar). Having a huge supply of energy certainly was comforting, especially with high capacity water pumps that did not have to rely on the engine.
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Old 19-08-2016, 10:27   #8
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Re: Bank Size and Charging Capacity

I would swap the genset charger and the shore charger, assuming the genset has the output power required. You want to run the genset for as short as practical, so using the 100 amp charger there will reduce the bulk time of charging. When on shore power you have lots of time to charge, so the smaller one would be fine.

This will not change your issue with getting to full charge, but should make genset use more efficient.
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Old 19-08-2016, 10:40   #9
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Re: Bank Size and Charging Capacity

Thanks all!

Looks like I'm doing OK - just shouldn't pay too much attention to the VSM - certainly getting to 14.4V and < 1.5% charge current. Basically been sitting like that for 24 hours!

Paul, good suggestion on reversing the chargers - but unfortunately, the 100A is also the inverter and runs the 110V side of the boat. The generator and 60A charger operate 240V...

Also checked the SG - looking good!

Thanks for all the reassurances!
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Old 19-08-2016, 10:54   #10
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Re: Bank Size and Charging Capacity

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Originally Posted by Bill Balme View Post
Thanks all!

Looks like I'm doing OK - just shouldn't pay too much attention to the VSM - certainly getting to 14.4V and < 1.5% charge current. Basically been sitting like that for 24 hours!

Paul, good suggestion on reversing the chargers - but unfortunately, the 100A is also the inverter and runs the 110V side of the boat. The generator and 60A charger operate 240V...

Also checked the SG - looking good!

Thanks for all the reassurances!
Then the 60amp @240v charger is the bigger one and should stay on the genset.

Did you do a capacity test on the batteries after the strike?
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Old 19-08-2016, 11:07   #11
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Re: Bank Size and Charging Capacity

No bank is to large. It is an out vs in. If 4 8Ds are running down you must have quit a load with all of your charging methods. You may want to check that you don't have one battery that has bought the farm.
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Old 21-08-2016, 07:12   #12
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Re: Bank Size and Charging Capacity

If you have 8 x 8D AGM batteries then you are about 100A too small on your alternator. If they are flooded batts, you may be ok.

A 100A shore charger would be about right, since anything larger is going to be prohibitively expensive. You could simply supplement an additional charger to go with your 60A existing unit. If the voltages are programmed correctly they will interact just fine.
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Old 21-08-2016, 08:46   #13
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Re: Bank Size and Charging Capacity

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If you have 8 x 8D AGM batteries then you are about 100A too small on your alternator. If they are flooded batts, you may be ok.

A 100A shore charger would be about right, since anything larger is going to be prohibitively expensive. You could simply supplement an additional charger to go with your 60A existing unit. If the voltages are programmed correctly they will interact just fine.
I was assuming he had flooded batteries, but you raise a good point. Old fashion flooded batteries can be trickle charged to 14 volts; not so the new computer chipped high tech batteries. One can polish off a lead acid battery by over charging for a short time(half hour or less) to keep the plates clean. A good quality charger with an adjuster will let you do that. Or just keep reconnecting till the volt meter reads 14.8 or higher. Can not do that with the chip installed batteries. You will damage the chip.

PS, most docks limit their systems to 30 amps.
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Old 21-08-2016, 11:53   #14
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Re: Bank Size and Charging Capacity

As many battery questions that get posted you think people understood the basics. Yet people still post crap that shows they don't as they post it as "advise"
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Old 21-08-2016, 13:38   #15
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Re: Bank Size and Charging Capacity

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As many battery questions that get posted you think people understood the basics. Yet people still post crap that shows they don't as they post it as "advise"
The problem is, so much of the advise is BS.
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