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vidaliaman
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# Posted: 2 Oct 2014 09:13
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lets say i have 7 deep cycle batteries in parallel, is it better to run a constant trickle charge or run them down and recharge when depleted and to what percentage.
thanks again
dave
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MtnDon
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# Posted: 2 Oct 2014 09:34
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Lead acid batteries prefer to be fully charged all the time, for best longevity. The # of discharge / recharge cycles and the depth of the discharge also affects their lifespan. So, no, you don't want to un-necessarily cycle them. Use them but no need to run them down "just because".
If left on long term float be sure the voltage is not so high as to cause excessive water use, resulting in running the cells dry, or exposing the plates to air.
And if you have seven batteries in parallel you really should be using a higher voltage system, or much larger capacity batteries or both. Seven in parallel is pretty much guaranteed to invite early battery failure. It's a matter of internal resistance differences that results in over charging some batteries or undercharging others. It gets worse as time goes on and also is worse the deeper the discharge cycles. Three parallel should be the absolute limit and two is better and no parallel is superb.
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vidaliaman
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# Posted: 2 Oct 2014 11:14 - Edited by: vidaliaman
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ah, good info
lets say hypothetically i would use 2 12v batteries at 115 AH per battery, all i want to run off these 2 batteries is a refrigerator that uses 725 watts lets say 12 hours a day, first of all per calculation is this feasible? what size inverter should be used? second, can a low rpm generator powered by constant stream flow(hydro power, bicycle tire belt driving generator).
third what equipment would be used to make sure the batteries don't over charge with constant turning of waterwheel?
i know it's not enough however would like to see the math worked through
help me walk through the math if you would please
thanks
dave
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MtnDon
Member
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# Posted: 2 Oct 2014 14:28
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Some basic electrical math needed for off grid calculations;
volts x amps = watts
volts x amp-hours = watt-hours
A basic electrical truth; inverters have loses. Ten to 20% of the energy going into an inverter is lost in the DC to AC conversion
Basic battery info; batteries have loses too. If you take 100 watts out you need to put more in via the charging. Maybe 20% more, maybe more.
Batteries should not be discharged more than 50% of the capacity. Battery ratings are usually done at a 20 hour rate. That means if the rating is 115 amp-hours that much energy was withdrawn over 20 hours. Discharge the battery faster and you get less total energy from it.
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So; 115 amp-hours at a nominal 12 VDC = 12 volts x 115 amp-hours = 1380 watt-hours total. Cut that in half to preserve battery life. 1380 / 2 = 690 usable watt-hours.
IF that refrigerator really uses 725 watts while running, in one hour it will consume 725 watt-hours. (Watts x time in hours = watt-hours.) But remember the inverter inefficiency. For arguments sake lets say the inverter is 85% efficient at that load. Rough math is to add 15% to that 725 watt-hours; 725 x 1.15 = 834 watt-hours.
690 watt-hours available from battery. 834 watt-hours needed. 690 / 834 = 0.82 ..... approx 82% of one hour, or 0.82 x 60 minutes = about 49 minutes run time from one battery.
Doesn't look like 2 batteries will run the fridge very long.
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If you have more battery capacity and enough PV power to recharge them quickly you could then basically run the fridge off the sun while the sun shines brightly. You do need a good sized array of PV panels to do that.
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MtnDon
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# Posted: 2 Oct 2014 14:32
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I doubt that a low speed water driven water wheel has sufficient power to do much battery charging. Don_P ran some calcs in another thread that did not look too promising. I'd trust his math.
I've never had much interest in hydro power; we live in a desert climate. Water comes a few times a year, usually in too large a quantity, and then goes away. However, I do understand that turning a generator to produce usable quantities of electricity takes a lot of energy going in.
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MtnDon
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# Posted: 2 Oct 2014 14:34
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Quoting: vidaliaman third what equipment would be used to make sure the batteries don't over charge with constant turning of waterwheel?
A charge controller that has a dump or diversion load for when the battery is fully charged. Unlike PV panels where the CC can simply stop accepting power from the panels, a water or wind system needs something / someplace for that excess power to go.
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vidaliaman
Member
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# Posted: 3 Oct 2014 08:32
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great info, just for a learning experience i am going to build a waterwheel from a bicycle wheel and a low rpm generator(200 rpm), want to check out what that will give me, even if it charges only lights for my camper it will be fun and a great learning experience, then i have to think seriously about solar, my goal is to live off grid in upper michigan, thanks for the help, ill be back for more
dave
michigan
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