adrian said:
We have a charger that chargers a lead acid battery to 90 percent
after 4 hours and 100 percent after 8 hours. Does anybody know what
formula we should use for plotting a graph of Voltage versus time?
please reply to
[email protected].
Terminal voltage can be very useful for charge determination of Lead Acid
batteries, but you have to take care :
1. If the battery is left standing for several hours, you can use tables
from manufacturer's websites - stuff like this :
<Q>
Open Circuit Voltage vs. State of Charge Comparison*
====================================================
(taken from EAST PENN VRLA (sealed) Manual)
% Open Circuit
Voltage Charge Flooded Gel AGM
100 12.70-12.60 12.95-12.85 12.90-12.80
75 12.40 12.65 12.60
50 12.20 12.35 12.30
25 12.00 12.00 12.00
0 11.80 11.80 11.80
NOTE: Divide values in half for 6-volt batteries.
* The *true* O.C.V. of a battery can only be determined after the battery
has been removed from the load (charge or discharge) for 24 hours.
(taken from exideworld.com checking_battery_condition.pdf)
100% 12.75 & higher
85 to 100% 12.60-12.74
75 to 85% 12.40-12.69
50 to 75% 12.20-12.39
25 to 50% 12.00-12.19
12.00 & below fully discharged
</Q>
After standing a few hours and at a specified temperature, SG and terminal
voltage are locked together, so voltage is a quality indicator of charge.
For a sealed battery, voltage is the only indicator of charge. For all your
further development, you can check other estimates of charge by taking out
the battery, resting it and measuring voltage. Temperature compensating
terminal voltage is possible when measuring capacity, but you are never
certain, so I recommend you sit the battery in a water bath at 26.7 C (18 F)
temperature - a good reference point for battery measurements.
2. It gets more complicated if you can't stand the battery. If your
application has a steady load which is not too heavy, you can still use
voltage, but offset the voltage a bit - record voltage with load on, then
pull out the battery, rest it and measure voltage.
3. If you just want to know when your battery is fully charged, then forget
all of the above. Get a decent voltage controlled charger and leave your
battery on for as long as you can.
If your cycle is brief charge followed by discharge, then your battery will
have a short life - the longer you can charge on a voltage controlled
charger the better.
Don't use of a cheap, simple charger which just pumps current in until you
turn off - your battery will die young, because you inevitably overcharge or
undercharge.
Roger