Project Float Charger

RogerN wrote:
I'm interested in doing a project both for the practice and to be useful.
It would be something like a Battery Tender automatic float charger.

The basic idea is a microcontroller to control based on battery voltage
and current. If I understand correctly, you charge at maximum current
until you reach 14.4V on a 12V battery, then you lower the current to
prevent the voltage from going over 14.4V.

That's oddly stated. Normally a lead-acid battery charge is described as
"constant voltage with current limiting", and for all but absurdly large
chargers the current limiting is there for the charger, not he battery.
The _behavior_ is the same, but the normal description captures the fact
that the _important_ part is the constant voltage.

Note, too that the optimal charge voltage varies by battery temperature. I
don't know how critical this is, but my understanding is that if you're
going to be charging in an unheated space you want to monitor at least
ambient temperature and adjust the charge voltage accordingly.

After the current reduces to perhaps 0.1A,

As mentioned, this magic current depends on the battery capacity to a
great extent, and (probably) to a lesser extent on the battery
construction.

the charge is terminated and float mode keeps the voltage at 13.2V, if
the voltage goes under 12.6V, the charge cycle is restarted.

My question is what to use to interface the microcontroller to the
battery? Voltage regulator circuit or what. I need to sense voltage and
current and control the output. I can come up with something to work but
I'm looking for ideas that might be better than mine.

That depends on how much work you want to load onto the microcontroller.
You could just have a current-limited voltage amplifier (i.e. a
microprocessor-controlled regulator) that you set and forget, or you could
drive the base (or gate) of a transistor directly from a pin on the micro
and let the micro do _all_ the work, or you could go somewhere in between.
It depends on your abilities, and what you're trying to do.

There's are books on the care and feeding of batteries -- you may want to
get one. When I get downstairs I'll try to remember to send you info on
the one I have.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

"Tim Wescott" <tim_at_seemywebsite.now> wrote in message
news:COqdnVEHvKVrkBnWnZ2dnUVZ_tGdnZ2d_at_web-ster.com...
RogerN wrote:
I'm interested in doing a project both for the practice and to be useful.
It would be something like a Battery Tender automatic float charger.

The basic idea is a microcontroller to control based on battery voltage
and current. If I understand correctly, you charge at maximum current
until you reach 14.4V on a 12V battery, then you lower the current to
prevent the voltage from going over 14.4V.
That's oddly stated. Normally a lead-acid battery charge is described as
"constant voltage with current limiting", and for all but absurdly large
chargers the current limiting is there for the charger, not he battery.
The _behavior_ is the same, but the normal description captures the fact
that the _important_ part is the constant voltage.

Note, too that the optimal charge voltage varies by battery temperature. I
don't know how critical this is, but my understanding is that if you're
going to be charging in an unheated space you want to monitor at least
ambient temperature and adjust the charge voltage accordingly.

After the current reduces to perhaps 0.1A,
As mentioned, this magic current depends on the battery capacity to a
great extent, and (probably) to a lesser extent on the battery
construction.

the charge is terminated and float mode keeps the voltage at 13.2V, if
the voltage goes under 12.6V, the charge cycle is restarted.

My question is what to use to interface the microcontroller to the
battery? Voltage regulator circuit or what. I need to sense voltage and
current and control the output. I can come up with something to work but
I'm looking for ideas that might be better than mine.

That depends on how much work you want to load onto the microcontroller.
You could just have a current-limited voltage amplifier (i.e. a
microprocessor-controlled regulator) that you set and forget, or you could
drive the base (or gate) of a transistor directly from a pin on the micro
and let the micro do _all_ the work, or you could go somewhere in between.
It depends on your abilities, and what you're trying to do.

There's are books on the care and feeding of batteries -- you may want to
get one. When I get downstairs I'll try to remember to send you info on
the one I have.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

I was wondering about using a linear voltage regulator to scale the output,
perhaps 0-5V from the microcontroller could result in up to 15V output from
a regulator (or amplifier circuit?). I thought perhaps PWM out to a R/C,
switch the OUT pin to IN , read voltage and current, calculate new PWM
value, repeat.

I could buy a Battery Tender but I could use several and a microcontroller
project sounded fun. Plus I want to be able to do some things that the $50
Battery Tenders don't do.

RogerN

"Tim Wescott" <tim_at_seemywebsite.now> wrote in message
news:COqdnVEHvKVrkBnWnZ2dnUVZ_tGdnZ2d_at_web-ster.com...
RogerN wrote:
I'm interested in doing a project both for the practice and to be useful.
It would be something like a Battery Tender automatic float charger.

The basic idea is a microcontroller to control based on battery voltage
and current. If I understand correctly, you charge at maximum current
until you reach 14.4V on a 12V battery, then you lower the current to
prevent the voltage from going over 14.4V.
That's oddly stated. Normally a lead-acid battery charge is described as
"constant voltage with current limiting", and for all but absurdly large
chargers the current limiting is there for the charger, not he battery.
The _behavior_ is the same, but the normal description captures the fact
that the _important_ part is the constant voltage.

Note, too that the optimal charge voltage varies by battery temperature. I
don't know how critical this is, but my understanding is that if you're
going to be charging in an unheated space you want to monitor at least
ambient temperature and adjust the charge voltage accordingly.

After the current reduces to perhaps 0.1A,
As mentioned, this magic current depends on the battery capacity to a
great extent, and (probably) to a lesser extent on the battery
construction.

the charge is terminated and float mode keeps the voltage at 13.2V, if
the voltage goes under 12.6V, the charge cycle is restarted.

My question is what to use to interface the microcontroller to the
battery? Voltage regulator circuit or what. I need to sense voltage and
current and control the output. I can come up with something to work but
I'm looking for ideas that might be better than mine.

That depends on how much work you want to load onto the microcontroller.
You could just have a current-limited voltage amplifier (i.e. a
microprocessor-controlled regulator) that you set and forget, or you could
drive the base (or gate) of a transistor directly from a pin on the micro
and let the micro do _all_ the work, or you could go somewhere in between.
It depends on your abilities, and what you're trying to do.

There's are books on the care and feeding of batteries -- you may want to
get one. When I get downstairs I'll try to remember to send you info on
the one I have.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

I was wondering about using a linear voltage regulator to scale the output,
perhaps 0-5V from the microcontroller could result in up to 15V output from
a regulator (or amplifier circuit?). I thought perhaps PWM out to a R/C,
switch the OUT pin to IN , read voltage and current, calculate new PWM
value, repeat.