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Old 08-07-2009, 12:44   #1
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Question Adding 2nd Battery Bank

I presently have 2 banks consisting of 2 Rolls 6 volt "golf cart" type batteries each. Total of 4 batteries adding up to 440 amp hours. I am planning to combine these two banks into one bank, and add a second bank in another location. Unfortunatly, space considerations will not let me use these same size batteries. My thought was to install 4,Rolls 12 volt, group 24 batteries in parallel, giving me an additional 400 amp hours. The batteries will be charged from my engine alternator and from a 110 watt solar panel. All will be accessed through the main battery switch,(#1, #2, Both, or Off) I am thinking that the difference of only 40 amp hours will not be a problem. All batteries will be flooded cell type, so no problem there as to different charging perameters.
Does anyone have any input as to yay or nay on this plan????

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Old 08-07-2009, 14:22   #2
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I dont see any problem at all.. except keeping them full of amps!

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Old 08-07-2009, 20:49   #3
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If I understand you the plan is to have 840 a/h in to banks.

My only comment is that if you plan to discharge to 50% (420 a/h) deficit. The solar panel will take 46 hours to fill it at 100% efficiency.

You should evaluate your "daily usage plan" and develop a charging system to restore that amount (or greater) first.

Then you evaulate time between charges and decide how much storage you need to last between charges.

Then you have to develop a charging capacity that recharges your deficit in the time window for charging.


Daily usage = 200 amps
Time between charging = 2 days
Capacity @ 50% = 800 a/h min
2 day deficit = 400 amp
Charge window -
Solar 6 hours / day average continuous
Solar panel 110w X .8 efficiency = 7.3 amp
Solar capacity 7.3 amp/hr = 44 amps per day = 88 amp over 2 days
Remaining deficit = 312 amps
Alternator rating 60amp X 90% efficiency = 54 amp
Alternator running time each 2 days 6 hours
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Old 09-07-2009, 08:57   #4
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Big battery banks are nice, however, once you get behind.... it's a lot of amps to get back in... The advantage I see though is if you are traveling and end up motor sailing overnight etc, you can fill 'er up!
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Old 09-07-2009, 13:51   #5
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Are the 2 battery banks far from each other? If so how long are the cables to the battery switch/alternator for each bank?
If one bank has significantly longer cables and are not sized correctly it will not charge equally with the other bank. It “sees” a lower voltage due to the voltage drop in the cable. When you shut off the alternator the closer bank will discharge into the farther bank till the voltages are equal.
There could be some long term issues (battery life) with this arrangement unless the two sets of cables are designed to give the same voltage drop taking into consideration the different capacities of the batteries.
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Old 09-07-2009, 14:05   #6
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I would divide the two banks and never use both at the same time. They can share similar charging profiles but relative to each other are very different. Age alone is enough but there may be type differences too. The 1, 2 both would be to use the both when you ain't got nothing. Rotating multiple banks can be more manageable when you divide and never combine them. Over 800 amp hours total won't ever be charged back 100% and your 110 solar panel is almost worthless combined. I run 429 amp hours on twin 65 amp panels and it works pretty well. If I motor a few hours every 2 days I can do pretty well. If you need that much power over say a 48 hour period you are screwed. You'll be charging by engine a whole lot every day.

Look at it this way if you had 10,000 amp hours you would eventually reach a point where the bank was dead or useless. You would be floating some place less than the bank was optimum. Adding more bank yields the need for more charge. You want the entire set of banks fully charged on a regular interval.

It's like a car loan. Sooner or later the note is due. You can borrow all you want but you always make the payments. It has to include principle on the payments unlike a credit card.

Unless you can show you need that much power I would say it's money down a rat hole. You'll abuse the extra batteries and take the others along on the ride. When the power out is not made up by the power in in a reasonable period of time you trash the batteries. A power boat running engines a fair amount is no problem as there is surplus charging ability. A 38 ft sail boat just ain't got the alternator. You are borrowing from the bank more than you can hope to ever pay back. The answer is probably - genset!
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Old 09-07-2009, 14:59   #7
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I beg to differ with you Paul. It should be a boat loan, real cruisers don't have cars!

(See, I think I a funny, just no one else does!)

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