Cor van de Water via EV wrote:
For many years, I have used a "take-out" UPS battery
to keep my internet, WiFi and VoIP powered through regular power
outages, always floating it to 13.5V...
I can second Cor's advice. 13.5v is a good float voltage; better than
the usual 13.8v you'll find in most "float" chargers.
Various sources recommend different float voltages for a 12v lead-acid
battery, ranging from 13.2v to 13.8v. There are lots of "feathers on the
scale" to lean in one direction or another. 13.5v is a good compromise.
1. 13.2v
--------
no life reduction (can last 10 years or more)
best with gel cells and old batteries
best during hot conditions
takes a month or more to fully recharge
does not equalize cells
battery sits at about 80% of its capacity
AGMs never fully recharge
2. 13.5v
--------
long life (5 years or so)
gel cells, or old "sulfated" batteries will gas continuously
best during average temperatures
takes a week or so to fully recharge
does not equalize AGMs
battery sits at about 90% of capacity
3. 13.8v
--------
widely used in consumer gadgets (float chargers, UPS, etc.)
life only 2-3 years
best during cold conditions
takes a day or so to fully recharge
can equalize (slowly)
battery sits at about 100% capacity
So, it appears that a simple LDO regulated at 13.5V output will allow
your solar panel to charge but avoid over-charging.
Since solar panels may go as high as 18V when there is little load,
the regulator may see just over 2 Watts of dissipation 1 hour a day
so it should have a small heatsink or be bolted to a metal piece/case
if that does not create a short circuit.
NOTE: to avoid back-feed, you might want to set the LDO to 13.8V and add
a Schottky diode after it or test that the LDO does not sink current
when input is removed or insufficient, you don't want to add an extra
drain on this battery!
These solutions work fine. Here are some others.
You don't need precision regulation for a float charger, so a precision
regulator IC isn't strictly needed. One of the simplest is simply a
resistor, two 6.2v zener diodes, and a series blocking diode.
PV panel+____/\/\____________|\|___12v Battery +
. . . . . . . R1 . . | . . . |/|
. . . . . . . . . . _|_/ . . . D1
. . . . . . . . . .//_\ . Z1
. . . . . . . . . . .| . 6.8v
. . . . . . . . . . _|_/
. . . . . . . . . .//_\ . Z2
. . . . . . . . . . .| . 6.8v
PV panel-____________|___________12v Battery -
D1 can be any general-purpose diode (1N4001 etc). It's there so the
battery can't discharge back into the PV panel or zeners, even if you
connect some higher-voltage charger to the battery.
R1 is chosen to limit the peak zener current in full sun. If you are
using 1 watt zeners (1N47xxx or equivalent), limit the current to about
100ma. With 5 watt zeners (1n53xx or equivalent), limit it to 500ma.
Most small PV panels (1 square foot or smaller) can't deliver more than
500ma, so R1 may not even be needed.
Z1 and Z2 are zener diodes. Choose them so the sum of their voltages
minus 0.6v for D1 is about your desired "float" voltage. For example, (2
x 6.8v) - 0.6v = 13v. Note that zeners usually have a +/-5% tolerance,
so any given 1N5342B actually ranges from 6.8-5% = 6.46v to 6.8v+5% =
7.14v. The purpose of using two in series is that you can hand-pick them
to get exact the voltage you want.
You can also add another forward-biased diode in series with the zeners
to bump it up 0.6v. For example, if the two zeners you bought just
happen to both be at exactly 6.8v (giving you 13.0v), then add another
diode like D1 (1N4001 etc.) in series with the zeners to get 13.6v.
--
"IC chip performance doubles every 18 months." -- Moore's law
"The speed of software halves every 18 months." -- Gates' law
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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