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Old 22-07-2009, 17:43   #31
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Use it for only fifteen minutes, and it comsumes 1/4AH. Which could be called fifteen amp-minutes but never ever is.
Indeed, but this can be called 250 mAH (250 milli-amp-hour)

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Old 22-07-2009, 18:49   #32
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All those electronic meters for measuring how much you have used and how you used them are great BUT they will never tell you how messed up or degraded your batteries are.


If you have a 100 AmpHr battery that is partially sulphated or whatever a test of it capacity is the only way to really know beyond any doubt.




In Battery Lingo the AmpHr Rating of a battery is its 20 Hour Rate.



100 ampHr rating in battery lingo means the battery will be dead in 20 hours at rate of 5 Amps.


200 ampHr rating means it is dead in 20 hrs at 10 amps.



This means at 10 hours at 5 amps a new fully charged 100 ampHr battery will be ½ charged. Let it rest with no loads on it for a couple of hours and it should read 12.10 volts. You do need a calibrated voltmeter not one of those Micky Mouse deals. You could use one of those density measurement devices. The problem is stratification of the acid, dense on the bottom and thin on top.


So if you had that 100 ampHr battery and after 10 hrs at 5 amps, and then waiting a couple hours the voltage is 11.96 volts means 40% remaining. This means what used to be 100 AmpHr battery is now a 80 ampHr battery. You have lost 20% of its capacity.



Does that make sense?
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Old 22-07-2009, 19:23   #33
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Even a calibrated multimeter is only calibrated "here and now" and it may be out of calibration a month later. Ignoring the % accuracy rating, and ignoring the float in the rightmost digits (often 2-3 digits of float) making "calibrated" only the icing on the cake.

For practical calibration, you can measure the voltage of any new coin cell (i.e. CR2016, CR2032) and reset the meter to match the rated voltage of the cell. Silver cells or zinc hearing aid cells are all fiarly precisely manufactured, and a relatively good source for a calibration voltage when new. Compared to anything else you can conveniently find.

Of course if you have a known good alternator which puts out a fairly precise 14.3-14.4 volts (as some but not all models do) that's another standard that can be used. For the non-techie...either one beats grabbing an old cheap meter out of a drawer and guessing how far off it might be.
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Old 22-07-2009, 19:44   #34
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Zener diodes work great as a reference voltage.
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Old 22-07-2009, 19:45   #35
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Unbusted: There's no stupid questions just stupid answers. Anyway there is a book by Don Casey "This Old Boat" that answers a multitude of refrigeration and power questions in a very easy to understand format. I found it on Amazon for $5 used. Good Luck!
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Old 22-07-2009, 19:56   #36
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Zeners are great--but the average layman won't have one, the average dockside mall won't have one, and the guys at Radio Shack will probably show you a tool case in the back room and tell you to go find it yourself, if you dare to ask for them. Not something readily available to most folks in most places, but handy to buy one when you can and stick it in the case with the spare fuses for the meter.
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Old 23-07-2009, 02:46   #37
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So basically the only thing about batteries you can be sure about is that you can't be very sure about anything about batteries? Thank you for all of your help. I am going to go dig into that xantrex manual.
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Old 23-07-2009, 04:11   #38
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Unbusted,

If you weren't so damn lazy, and had done that in the first place, we wouldn't have wasted all this bandwidth!!!
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Old 23-07-2009, 05:09   #39
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So basically the only thing about batteries you can be sure about is that you can't be very sure about anything about batteries? Thank you for all of your help. I am going to go dig into that xantrex manual.
Get a copy of Nigel Calders Boat owners mechanical and electrical manual as well. Tells you all you need to know and more. With pretty pictures
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Old 23-07-2009, 07:43   #40
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I love this site! What a great amount of info. I actually think after reading everything here I'm getting a good handle on electricity. Along with some exciting post!
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Old 23-07-2009, 07:55   #41
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And there´s more.....
Boat Electrical
Plenty bedtime reading there
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Old 24-07-2009, 12:36   #42
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when my fridge comes on it draws about 75 dc amps and takes about 45 minutes to get the freezer to go from 40 to 32 degrees. It runs about once every 6 hours. We also have a clutch that allows the compressor to run from our engine.
Sound pretty good what is the ambient temperature, 95 degrees?
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Old 24-07-2009, 15:14   #43
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Sound pretty good what is the ambient temperature, 95 degrees?
no, about 80 all the time
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Old 27-07-2009, 16:20   #44
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About those cheap and/or calibrated multimeters: just get over it and buy a Fluke. Yes, they cost a lot. Every time you switch it on, it calibrates itself, using an internal reference voltage. No need to have zener diodes and resistors to check it.

My old Fluke (say 20 years old now) is still dead on accurate. I think Sperry meters are also decent but just as pricey. So throw away those cheap meters and buy one that can be trusted.

About measuring battery capacity; the best way is measuring the density and stratification is not a problem when you measure it during and after equalization (which mixes up the electrolyte again).
If you want to measure the Ah rating, you need a test load which draws the exact amount of amps plus a decent voltmeter.

cheers,
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Old 27-07-2009, 17:49   #45
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"Every time you switch it on, it calibrates itself, using an internal reference voltage."
Nick, can you expand on that a little? I've never seen Fluke mention it for their handhelds, and they only seem to refer to outside and optional calibration and certification.
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