Lead Acid Solar Charger

[Adrian Hamilton]

Hello All

I was wondering if there is someone out there with a bit of knowledge on
solar cells and charging wet lead acid batteries. Basically I have an
application where I will have a 16.2 volt 1.3A solar panel on a lead acid
battery. Some documentation I have found implies that during the bulk
charge the battery should have a constant current and this charge should be
cut off when the battery voltage reaches around 14.55 volts . At this
stage, the voltage should be held at 14.55 volts until the current drops to
a low enough level and then the charger should go into float mode with a
constant voltage of 13.1 volts to maintain the battery's charge and this is
where I am a bit confused. How can I efficiently drop the solar panels
output voltage for these stages or are there better ways than
dropping/wasting voltage? I have found a circuit based on a 555 timer to
control the float voltage (13.1V) and I was hoping to use its theory to
control both the Absorption and float stages. The way this circuit works is
by setting a low and a high threshold and when the battery voltage drops
below the lower threshold the solar panels full voltage is switched to the
battery, when it goes above the high threshold the solar panel is
disconnected. I was wondering what your thoughts/experiences are on using
this process and what would the voltage thresholds would be. i.e. 10 - 100mV
from the centre voltage.

I was also wanting to double check the best way to detect the end of charge
for the absorption phase (constant voltage of 14.55), is it better to
monitor the current or does holding the battery voltage at 14.55 volts for a
certain period of time work O.K. If so, does any one have any idea on how
long I should leave it in the absorption stage?? The documentation I am
referring to seems to indicate about 1 hour!

Thanks in advance for any assistance offered.

Best Regards,


Adrian Hamilton

 

[Bernd Felsche]

I was wondering if there is someone out there with a bit of
knowledge on solar cells and charging wet lead acid batteries.
Basically I have an application where I will have a 16.2 volt 1.3A
solar panel on a lead acid battery. Some documentation I have
found implies that during the bulk charge the battery should have a
constant current and this charge should be cut off when the battery
voltage reaches around 14.55 volts.

At this stage, the voltage should be held at 14.55 volts until the
current drops to a low enough level and then the charger should go
into float mode with a constant voltage of 13.1 volts to maintain
the battery's charge and this is where I am a bit confused.

How can I efficiently drop the solar panels output voltage for
these stages or are there better ways than dropping/wasting
voltage?

You need what's called a maximum power point tracker (MPPT). [Google
is your friend.] This device places a synthetic load on the solar
cells such that they maximum power (V*I product) is maintained while
at the same time providing an appropriate output voltage to the
storage battery probably using a buck-boost convertor.

I have found a circuit based on a 555 timer to control the float
voltage (13.1V) and I was hoping to use its theory to control both
the Absorption and float stages.

You may well find that it's much easier to implement nowadays with a
small microcontroller, running FETs on both sides of the MPPT with
voltage and current sensing using an internal ADC.

I was also wanting to double check the best way to detect the end
of charge for the absorption phase (constant voltage of 14.55), is
it better to monitor the current or does holding the battery
voltage at 14.55 volts for a certain period of time work O.K. If
so, does any one have any idea on how long I should leave it in the
absorption stage?? The documentation I am referring to seems to
indicate about 1 hour!

It all depends on the type of storage battery.

 

[Adrian Hamilton]

Hi Bernd

Thankyou very much for you reply. I was planning to use a PIC with some
FETS instead of a 555 and now I know I should be using a MPPT aswell.

Thanks again

Best Regards

Adrian Hamilton

I was wondering if there is someone out there with a bit of
knowledge on solar cells and charging wet lead acid batteries.
Basically I have an application where I will have a 16.2 volt 1.3A
solar panel on a lead acid battery. Some documentation I have
found implies that during the bulk charge the battery should have a
constant current and this charge should be cut off when the battery
voltage reaches around 14.55 volts.

At this stage, the voltage should be held at 14.55 volts until the
current drops to a low enough level and then the charger should go
into float mode with a constant voltage of 13.1 volts to maintain
the battery's charge and this is where I am a bit confused.

How can I efficiently drop the solar panels output voltage for
these stages or are there better ways than dropping/wasting
voltage?

You need what's called a maximum power point tracker (MPPT). [Google
is your friend.] This device places a synthetic load on the solar
cells such that they maximum power (V*I product) is maintained while
at the same time providing an appropriate output voltage to the
storage battery probably using a buck-boost convertor.

I have found a circuit based on a 555 timer to control the float
voltage (13.1V) and I was hoping to use its theory to control both
the Absorption and float stages.

You may well find that it's much easier to implement nowadays with a
small microcontroller, running FETs on both sides of the MPPT with
voltage and current sensing using an internal ADC.

I was also wanting to double check the best way to detect the end
of charge for the absorption phase (constant voltage of 14.55), is
it better to monitor the current or does holding the battery
voltage at 14.55 volts for a certain period of time work O.K. If
so, does any one have any idea on how long I should leave it in the
absorption stage?? The documentation I am referring to seems to
indicate about 1 hour!

It all depends on the type of storage battery.
--
/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
\ / ASCII ribbon campaign | I'm a .signature virus!
X against HTML mail | Copy me into your ~/.signature
/ \ and postings | to help me spread!

 

[John Harvey]

I was wondering if there is someone out there with a bit of knowledge on
solar cells and charging wet lead acid batteries. Basically I have an
application where I will have a 16.2 volt 1.3A solar panel on a lead acid
battery. Some documentation I have found implies that during the bulk
charge the battery should have a constant current and this charge should be
cut off when the battery voltage reaches around 14.55 volts . At this
stage, the voltage should be held at 14.55 volts until the current drops to
a low enough level and then the charger should go into float mode with a
constant voltage of 13.1 volts to maintain the battery's charge and this is
where I am a bit confused.

It's not exactly what you're contemplating, but Oatley have a
regulator kit for around $30 that operates between 13.8 and whatever
you set for the full charge voltage (I use 14.2V). It's very
efficient with virtually zero power loss, unless you need to limit the
maximum panel output, as might be the case for a very small
accumulator. (Also bear in mind that all charging will cease
overnight, unless it's bright moonlight.)

I have car batteries permanently connected to 37W panels which are
virtually trouble free. Average load on the batteries is only about
100mA (fence energisers).

 

[Adrian Jansen]

The battery manufacturers publish very good specs on charging their
batteries. Ask your supplier for the specs for the battery you are using,
or look on the Web.

--
Regards,

Adrian Jansen
J & K MicroSystems
Microcomputer solutions for industrial control

 

[Dave]

Hello All

I was wondering if there is someone out there with a bit of knowledge on
solar cells and charging wet lead acid batteries. Basically I have an
application where I will have a 16.2 volt 1.3A solar panel on a lead acid
battery. Some documentation I have found implies that during the bulk
charge the battery should have a constant current and this charge should be
cut off when the battery voltage reaches around 14.55 volts . At this
stage, the voltage should be held at 14.55 volts until the current drops to
a low enough level and then the charger should go into float mode with a
constant voltage of 13.1 volts to maintain the battery's charge and this is
where I am a bit confused. How can I efficiently drop the solar panels
output voltage for these stages or are there better ways than
dropping/wasting voltage? I have found a circuit based on a 555 timer to
control the float voltage (13.1V) and I was hoping to use its theory to
control both the Absorption and float stages. The way this circuit works is
by setting a low and a high threshold and when the battery voltage drops
below the lower threshold the solar panels full voltage is switched to the
battery, when it goes above the high threshold the solar panel is
disconnected. I was wondering what your thoughts/experiences are on using
this process and what would the voltage thresholds would be. i.e. 10 - 100mV
from the centre voltage.

I was also wanting to double check the best way to detect the end of charge
for the absorption phase (constant voltage of 14.55), is it better to
monitor the current or does holding the battery voltage at 14.55 volts for a
certain period of time work O.K. If so, does any one have any idea on how
long I should leave it in the absorption stage?? The documentation I am
referring to seems to indicate about 1 hour!

I understand the voltage trip points can get you into trouble if you
have an imperfect battery. Might be good to also use a time-out
scheme.

I'm looking for two things; a reasonable model of a solar cell and the
ultra-safe trickle-charge current that can be applied indefinitely to
an automobile lead-acid battery. Perhaps this is what you call "float
mode."