Well, if you assume your battery is in good condition and actually has it's rated 90ah capacity, then we know that theoretically you can pull out 90ah. In the real world you don't want to draw more than 50%, or 45ah. The standard rate of discharge that these are rated at is c/20 (capacity over a 20 hour time period), or in your case 90ah/20h = 4.5a. So if you can get a 4.5a load on your battery, then discharge it for ~10 hours and you will be near 50%.
and FWIW, as a super general approximation a battery at a c/20 load (in your case, 90ah/20h = 4.5a) will read about 11.6v when at 50%.
So, what you could do is turn on a light or two until you are drawing 4.5a (do you have any way to measure current? a simple clamp on meter from Radio Shack on a main cable will work) then measure voltage. When it's down to 11.6v, disconnect the battery and let it sit overnight (or at least a few hours). Measure the voltage after it's rested to get a more accurate state of charge reading to see where you are.
a battery, assuming it's in good shape, should take ~10 hours to go from fully charged to 50% state of charge when being discharged at the c/20 rate, but it may be closer to 8 hours or less depending on condition.
1998 Ford E350 extended van, V-10, 3.73
1998 Fleetwood Terry 22lw, 24' long, 5000 pounds
Equalizer, Prodigy, Link-10, CPE 2000w generator
While discharging the battery, I have been using a DC lamp fixture to monitor the voltage instead of directly off the battery (pita to get to).
also, this is not a good way to measure the voltage, it's too far from the battery and the wiring losses are going to be significant when you are talking about measuring voltage to the tenths. You need a good connection to the battery itself.
camperdave wrote: Well, if you assume your battery is in good condition and actually has it's rated 90ah capacity, then we know that theoretically you can pull out 90ah. In the real world you don't want to draw more than 50%, or 45ah. The standard rate of discharge that these are rated at is c/20 (capacity over a 20 hour time period), or in your case 90ah/20h = 4.5a. So if you can get a 4.5a load on your battery, then discharge it for ~10 hours and you will be near 50%.
and FWIW, as a super general approximation a battery at a c/20 load (in your case, 90ah/20h = 4.5a) will read about 11.6v when at 50%.
So, what you could do is turn on a light or two until you are drawing 4.5a (do you have any way to measure current? a simple clamp on meter from Radio Shack on a main cable will work) then measure voltage. When it's down to 11.6v, disconnect the battery and let it sit overnight (or at least a few hours). Measure the voltage after it's rested to get a more accurate state of charge reading to see where you are.
a battery, assuming it's in good shape, should take ~10 hours to go from fully charged to 50% state of charge when being discharged at the c/20 rate, but it may be closer to 8 hours or less depending on condition.
Perhaps installing and using a battery monitor that isn't so dependent on voltage draw causing an extra drop in voltage would help. The installation of such a device like a Xantrex LinkLITE will make monitoring easier. This device uses a shunt to monitor capacity that is not dependent on sudden voltage drops during periods of high current draw.
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