Re: Battery Question

Jobst Vandrey (onlinenow@stlnet.com)
Sat, 17 Oct 1998 20:17:46 -0500

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West Wight Potter Website at URL
http://www.lesbois.com/wwpotter/
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>Then I connected a bow light (7.5 watts) to the battery overnight for 16
>hours. At the end of the 16 hours, the voltmeter said the battery was
putting
>out 11 volts. It's an analog meter, so my readings of the gauge lack
>precision.
>
>If amps = watts/volts, amphours used = 7.5w/12v * 16 hrs= 10 AH. (please
>check my physics on this -did I use the right formula and should I use 12
>volts or the 13 volts shown on the meter.). For an 86 AH battery, that's
11.6%
>discharged.
>
>The charger manual says that the approximate time to recharge is given by
the
>following formula:
>HOURS = [(Battery capacity in ampere-hours) * (depth of discharge in
>percent]/900
>According to this formula, the batter should have recharged in about 1.1
hour.
>(86*11.6)/900 = 1.1 hours.
>
>However, it took somewhere between 4 and 6 hours for the battery charger to
>recharge the battery -- which makes me suspicious of the condition of the
>battery. After charging the second time, the voltmeter read 13 volts
again.
>
>Can I make any conclusions about the state of the battery from this info?
If
>the battery is good, would a 7.5 watt bulb use up 11.6% of the amphours of
the
>86 amphours over a 16 hour period?
>

Judy

Below is the battery voltage vs depth of discharge info:

Your voltage readings indicate an almost completely discharged battery (gel
cell 90% discharge is 11.22 volts at 77 degrees F). When you check the
voltage of a battery after a charge, you should let the battery sit for a
few hours and better yet, put a small load on the battery while you check
the "full charge" voltage. Otherwise, you may be reading an artificially
high surface charge. A digital voltmeter will give you a better picture,
however, I suspect that the battery in question has gone belly up and will
need replacement if you expect to run any significant loads. Remember, if
you want to get a reasonable life from the battery, you should not discharge
it much below 50% of capacity - and recharge it as soon as possible.

>From the Battery FAQ at www.windsun.com

On the table below, you have to be careful that you are not just measuring
the surface charge. To properly check the voltages, the battery should sit
at rest for a few hours, or you should put a small load on it, such as a
small automotive bulb, for a few minutes. The voltages below apply to ALL
Lead-Acid batteries, except gelled. For gel cells, subtract .2 volts. Note
that the voltages when actually charging will be quite different, so do not
use these numbers for a battery that is under charge.

Voltage at DOD
Here are some no load typical voltages vs depth of discharge.
(figured at 80% max discharge and 77 degrees F)
1% = 12.91 Volts.
10% = 12.8
20% = 12.66
30% = 12.52
40% = 12.38
50% = 12.22
60% = 12.06
70% = 11.90
80% = 11.70
90% = 11.42
100% = 10.50.

Regards

Jobst