Charging lead acid batteries.

Discussion in 'Electronic Design' started by adrian, Nov 11, 2004.

1. adrianGuest

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 .

2. BanGuest

You guys cannot use any formulas, because the actual voltage will depend on
temperature, age of the battery, charge history and on the charger used.
To charge a lead acid battery you can do the following: a power supply of
14.4V (best is a corrected value according to a temperature sensor) will
charge pretty fast with the max. current (Ah/3h). When the current starts
dropping to 1/20th of the Ah-rating, you continue with that current until
15.0V(formation), then trickle charge at 13.8V. So here we have 4 phases:
constant current/constant voltage/constant current/trickle. The formation
bit is done only once a month. Your charger will probably do something like
this. Just measure with your voltmeter, take a reading every 10min. or so
and draw that diagramm, which is valid only for that time.
To find out the state of charge of the battery, you have to constantly
monitor incoming/outgoing currents and calculate the charge with an
efficiency multiplicator. There are dedicated ICs available for this kind of
stuff.
Look up the different charging methods on google.

3. budgieGuest

How have you determined that it's 80% after 4 hours and 100% after 8 hours?

The only "real" way to determine SOC is temperature and electrolyte SG for
non-sealed non-gel lead acid batteries. And then refer to the manufacturer's
data - it does differ. And it also depends on how precise the electrolyte mix
and fill were at the factory.

Which voltage are you seeking - the actual terminal voltage on charge, or the
rest voltage for that SOC? Either way, you are going to need to measure the
voltage rather than trying to determine a formula. If you really want to
determine the rest voltage you are going to have to disconnect the charger (and
any load) and allow a rest period of several minutes minimum before measuring.

You need to explain exactly what your objective is.
Nope, replies go to the group.

4. Rich GriseGuest

If you want to plot it, you won't get a formula until you do a
curve-fitting on your plot. To _get_ the plot, measure the voltage
every 10 or 15 minutes (the same interval each sample), and draw
a graph.

What is it you're trying to accomplish?

Thanks,
Rich

5. Roger LascellesGuest

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

6. martin griffithGuest

as others have said, quite difficult to quantify, but here are a few
links

A few links
http://www.varta-automotive.com/eng.../antrieb_beleuchtung/antrieb_beleuchtung.html
http://www.batteryuniversity.com/parttwo.htm
http://www.uuhome.de/william.darden/
http://www.yuasabatteries.com/pdfs/TechMan.pdf

martin

Serious error.
All shortcuts have disappeared.
Screen. Mind. Both are blank.

7. MacGuest

I guess you meant 80 F?

--Mac

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