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Old 20-10-2009, 08:21   #1
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Battery Capacity - Reserve

It's been a long time since I had to figure reserve electrical capacity on anything. So here is the question: Figuring on a "worst case" scenario how much reserve capacity do I need to run 2 360 gph rule bilge pump for 72 hours with no charging going on. I think I read that the rule pumps draw about 2 amps per hour. I could be wrong on that though. I have 1 deep cycle battery that has 700 amps. Will I need to install a second battery? I plan on using a solar battery maintainer, but it is possible to not have sun for a couple of days in a row. Thanks in advance
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Old 20-10-2009, 09:47   #2
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you might want to check your battery capacity first. I've never heard of a 700 AH battery.
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Old 20-10-2009, 09:59   #3
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The 700 amps is probably a cold cranking amps specification, and if your battery is an automotive size, it probably has less than 100 amp-hour capacity. You have the amp draw of the pump right, and if you really want to run 2 of them for 72 hours worst case, you need a (4 x 72=288) 300 amp-hour battery bank. You will be needing about 250 lbs of batteries for that--say 2 4D truck batteries.
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Old 20-10-2009, 10:04   #4
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My bad, 700 was the cranking amps, it's a 85 amp-hour battery.
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Old 20-10-2009, 10:09   #5
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LOL, okay maybe my "worst case" scenario is to worst case. I don't think I want 250 lbs of battery in my 21 foot vessel. I don't think it's possible to need two 360 gph pumps for 72 hours straight anyway.
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Old 20-10-2009, 10:22   #6
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Reserve Capacity is the time, in minutes, that a new, fully charged battery will deliver 25 Amperes at 80 degrees F, and maintain a terminal voltage equal to, or greater than, 1.75 Volts per cell, or 10.5 Volts for a 12V nominal battery.

Your battery appears to have a 700 minute Reserve Capacity, meaning it will deliver 25 Amps for 700 minutes, or 11.66 Hours.

The Rule 360 Non-Automatic Bilge Pump draws 2.1 Amps at 13.6 Volt (12V Nominal).

I’d expect this battery to deliver 5 Amps (2 x 2.5A) for about 50 to 60 hours.

I'd expect this pump to discharge UNDER 200 GPH (3-1/3 GPM), as installed ( 5 feet of total head), and as little as 60 GPH as the battery voltage drops below 12.3 V actual.
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Old 20-10-2009, 10:50   #7
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What are you expecting from the pumps?

I'm not sure what you are expecting these bilge pumps to do for 72 hours but as written above by GordMay your best case is 400gph. And as he states your actualy pumping rate will be much lower over time.

Those pumps are made to keep the bilge dry from rain and small slow leaks. The usually cycle or have a float switch. So they would not be on all the time.

However.....

If you are thinking of using them for 72 hours of a serious problem say a 1 inch hole at 3 feet deep, you’re looking at 2000 GPH coming in!



Borrowed from GordMay other post.....



Approximate Flooding Rates, for smooth holes at specific depths, are calculated using the formula:

Flooding Rate in gpm [ Q] = 20 x d x h

where:
d = diameter of hole in inches
h = depth of hole underwater in feet (head)
h = square-root of h

Hence, for a 2" diameter hole at a 3 Ft depth:

Q = 20 x 2" x root 3 = 20 x 2 x 1.73 = 69.2 gpm (4,152 Gal per Hour)

The actual initial flow rate ('Q') will be less than calculated above, due to frictional & turbulence losses at the aperture.
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Old 20-10-2009, 11:04   #8
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Well like I said my "worst case" may be more worst case than I could / would ever expect to need but that's why I called it the worst case. The hull is not water proof. I do expect to take on a little water from 3 or 4 pin holes in the hull, and I do mean "pin holes" that are no more than 1-2 feet below the water line. I do not know how much I will take on in a rain storm, but I have re-caulked everything on the deck, around every screw and fitting. But from the info given to me in this post, I think I may install a second battery, so that I have a greater reserve. And may opt for the larger of two battery maintainers available from West Marine.
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Old 20-10-2009, 11:17   #9
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Well if you have a battery charger on then the battery capacity changes. Is that in addition to the solar?
Either way the pumps either cycle or use a float switch. If you have a constant flow of water in your pumps will run all the time with the float switch. (given you pump it out as fast as it comes in or less).

If you pump faster than the flow of water the float switch will shut the pump off and your battery will last longer.

If the pumps reqularly cycle you will use a more even amount of capacity but maybe not enough to stop the flooding.

How are you configuring the pumps as thats the place to start. Are you using two because there are two compartments? Or have you thought of mounting one float switch higher?
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Old 20-10-2009, 11:43   #10
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No charger, just a solar battery maintainer. These pumps are configured in separate compartments,and each is mount at about the same level, and each one has it's own float switch and three way auto/off/manual switch.
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Old 20-10-2009, 11:55   #11
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Well then they will only be on when the water floats the switch.
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Old 20-10-2009, 12:02   #12
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Yes, I know that, and I can't see taking on 400+ gph for 72 hours straight, which is what my worst case was. So given that I think I will be safe with two batteries similar to the one that I already have. Which will amount to about 170 amp hours, which should cover my needs.
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