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#11
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12 volt 7amp/h versus10amp/h ??
One other thing to note with lead acid batteries is that the voltage
will stabilize over about two hours once you let it rest. Resting means no discharge or charge. If you wait a couple hours after disconnecting the load you may see the battery voltage creep back up. The larger battery may gain more than the smaller one. The opposite holds true after charging. The voltage will drop over two hours once you disconnect the charger. Randy ps. My experience is with large battery banks in PV systems. |
#13
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12 volt 7amp/h versus10amp/h ??
Doug,
what are the dimensions of your batteries? I am trying to fit them in a 6"X 2,5" opening. Height is not as critical, but should not be more then 4.5"? Udo The 10AH (20 hour) size DryFit battery is also available as a 13AH battery rated at a 10 hour discharge, BB HR 15-12 (around $30 from internet resellers). I find the 10 hour rated battery is a much better match for glider applications, as I often fly 8-10 hours, but seldom more than 12, and never close to 20. The high discharge version has a little more lead inside, but has the same dimensions. I carry two, and can run a VHF radio, transponder, encoder, SN10, GPS, two PDA's and a logger for around 8 hours without a low battery warning. For longer flights, I limit the transponder/encoder use when below 200' AGL. |
#14
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12 volt 7amp/h versus10amp/h ??
Don't disagree with anything you said. However Udo's test was to
measure voltage after a fixed discharge time, not to measure time to a fixed end voltage. The slower voltage decay of the larger battery will keep the lamp at a higher resistance than the lower capacity battery and I think that will skew the result wont it? Doesn't it make the larger capacity battery look better in comparison than it really is? Perhaps the difference is negligible for the voltage range in question. Andy |
#15
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12 volt 7amp/h versus10amp/h ??
"Don't disagree with anything you said. However Udo's test was to
measure voltage after a fixed discharge time, not to measure time to a fixed end voltage. The slower voltage decay of the larger battery will keep the lamp at a higher resistance than the lower capacity battery and I think that will skew the result wont it? Doesn't it make the larger capacity battery look better in comparison than it really is? Perhaps the difference is negligible for the voltage range in question" Well, we are picking at nits - but that is what soaring pilots do until the "sun's up". The larger battery is supposed to have a slower decay. That slower decay will mean the voltage is higher. Although the bulb will burn brighter, causing the resistance to be higher, the higher voltage will cause more current to flow. The end result will be that the actual current flow with a bulb may be more constant with a lamp than with a resistor. So, it means that we can compare two different size batteries easily. The bottom line is that batteries with greater capacity will cause the bulb to lamp longer. If you compare a 10 A/H with a 7 A/H, you will find that a good 10 A/H battery will cause the lamp to burn approximately 30 to 45% longer. If the current drawn by the lamp is close to what the equipment draws, then you can approximate and compare the effectiveness of the two batteries. Close is good enough. In my experience, people start complaining about their battery when the capacity is down to about 50% of new. Probably a good idea to make a test when the battery is new, then test it maybe every winter when you get bored. Just remember how you made the test and where you recorded the results. Batteries are usually rated for so many charge/discharge cycles. If they are discharged below a minimum voltage, such as 11 volts, they often will not achieve their rated discharge cycles. If they are left in a discharged state, they may not last long. If they are placed on an ordinary charger and left, they will not last as long. A good charger that monitors the voltage and does not overcharge, is a good investment. Usually, you can judge the charger quality by checking the temperature of the battery after the charge cycle is completed. Colin |
#16
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12 volt 7amp/h versus10amp/h ??
Then I think we both agree that Udo should measure time to an end
voltage, rather than voltage after a fixed time, no matter what the load device is. I used to use a computer A/D input to record and plot battery discharge voltage and current. I gathered lots of data. All I found was that in flight battery failure was usually caused by failure of one cell and that could not be predicted, only confirmed after the event, from the discharge plots. Andy |
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