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CD-ROM Aktief 1995 #3
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BATTERY.002
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1994-09-18
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Subj : Solar charge controller.
>Their problem was that lead/acid batteries need a bit of careful current
>control in order to make them last longer. So I set out to design one that
>would fit her needs. So far it works pretty good and is sensative to the
>batteries charging rate. It charges a full current untill 'gassing' level
>has been reached then adjusts the current down to a level that is just below
>the 'gassing' current. It is designed around one cell but could easily be
>modified to work with multi cell setups.
Sounds like you just came up with Unitrod's UC3906 battery
charger IC. I don't have the data book with me here, let me know if you
want more info on the part, I'll dig it up. These 3906's make for
very simple lead-ascid/gel-cell battery chargers....
Subj : Solar charge controller.
> How does your device determine if a cell is 'gassing'? Does it
> actually measure gas production or specific gravity or is it able
> to infer these quantities from voltage and amperage levels?
Just thought I'd give a guess as to the answer:
The US Navy uses lead-acid storage cells for (among other things) the
main storage batteries in most submarines. These cells are huge
(typically more than 120 "man-sized" cells in series, contained within
a swimming pool sized compartment) and charged manually using a "TVG"
curve. This "Temperature-Voltage-Gassing" curve is based on the factory
("Gould") testings of an individaul cell. Essentially, a graph of
"magic number" voltages is given, based upon cell temperature. The charge
process "cranked up" charging amps until the desired voltage was reached,
at which point the current would be reduced to maintain constant voltage.
Once the charging current "drops" to a suitably low value [(say 50 amps
or so) (But that's not LOW.) (It is compared to roughly 2,200 amps
at the start of the battery charge.) (Uh, how many Amp-Hours did you
say this thing was rated for?) (I didn't say, but it's 4,000 A-Hr.)
(Oh, BIG Battery)] the charge was declared "done."
Anyway, the only data inputs used were:
Cell Temp
Bus Voltage
Charging Current
Obviously, the manufacturer must have monitored the H2 levels when it
arrived at the magic TVG numbers, but we didn't have to.
(H2 is the major gas produced during a lead-acid battery charge -- it's
from electrolysis of water in the electrolyte of the cells. About half as
much O2 is also produced, but much of that stays water soluable within the
cells.)
CRUDE ASCII Batt charging graphs:
| |
| |
V| C| *
o| u|
l| r| * *
t| r| *
a|TVG _______________ e| *
g| / n| * **
e| / t| *
| / | **
| / | * **
|--/ | ***
| | ****
|_________________________ |_*_______________________
0 0
Time Time
The amount of time it took to reach TVG depended upon charging capacity
(how many other DC loads were in use) and battery depletion (empty meant
longer). The amount of time it took from reaching TVG to completion
depended upon battery depletion and how quickly the battery had been
depleted (fast discharge meant fast charge).
Hurn Smith
> ... The faster you try to do something the longer it takes.
Subj : Solar charge controller.
JM> BP> Sounds like you just came up with Unitrod's UC3906 battery
JM> BP> charger IC. I don't have the data book with me here, let me know if you
JM> BP> want more info on the part, I'll dig it up. These 3906's make for
JM> BP> very simple lead-ascid/gel-cell battery chargers....
JM>Yup...sure do...like sources and prices and data sheets.
From Unitrode's Linear Integrated Circuits data book:
UC2906/UC3906
Sealed Lead-Acid Battery Charger IC
Features:
* Optimum control for maximum battery capacity and life.
* Internal state logic provides three charge states.
* Precision reference tracks battery requirements over
temperature
* Controls both voltage and current at charger output
* System interface functions
* Typical standby supply current of only 1.6mA.
Description:
The UC2906 series of battery charger controllers contain all
of the necessary circuitry to optimally control the charge and hold
cycle for sealed lead-acid batteries. These intergrated circuits
monitor and control both the output voltage and current of the charger
through three separate charge states; a high current bulk-charge
state, a controlled over-charge, and a precision float-charge, or
standby, state.
Optimum charging conditions are maintained over an extended
temperature range with an internal reference that tracks the nominal
temperature characteristics of the lead-acid cell. A typical standby
supply current requirement of only 1.6mA allows these ICs to
predictably monitor ambient temperatures.
Separate voltage loop and current limit amplifiers regulate
the output voltage and current levels in the charger by controlling
the onboard driver. The driver will supply up to 25mA of base drive
to and external pass device. Voltage and current sense comparators
are used to sense the battery condition and respond with logic inputs
to the charge state logic. A charge enable comparator with a trickle
bias output can be used to implement a low current turn-on mode of the
charger, preventing high current charging during abnormal conditions
such as a shorted battery cell.
Other features include a supply under-voltage sense circuit
with a logic output to indicate when input power is present. In
addition the over-charge state of the charger can be externally
monitored and terminated using the over-charge indicate output and
over-charge terminate input."
[You can also design things so there is a low battery cut off,
so the battery won't kill it self by going below 'dead'.]
The UC2906 is good for -40'C to +70'C. The UC3906 is 0->70'C
Also ask for Application Note U-104: "Improved Charging
Methods for Lead-Acid Batteries Using the UC3906"
You'll have to ask them where you can get them in your area.
Unitrode Intergrated Circuits
7 Continental Blvd.
Merrimack, NH 03054
(603) 424-2410
I have no connection with Unitrode. I do work for a
electronic company as the company librarian, among other things, so I
get to see all of these neat parts in the data books.
Subj : relay wanted
The current issue of Home Power magazine (Feb/March 1993) has a simple
circuit to turn on a relay when the battery voltage exceeds a certain
point. The intended use is to turn on a fan to ventilate the battery
room if the battery voltage exceeds the point where the cells start to
outgass. You could use the circuit by setting the trip point at your
minimum battery voltage and wiring the changeover relay (a non-latching
relay is fine) to be on grid power when the coil isn't energized. If
the battery drops below the minimum voltage the relay will turn off and
return you to the grid. It may sound backwards from what you're trying
to do but energizing the relay when you want to be on solar has the
advantage of falling back to