Lead acid batteries and float charging question

[Mike Harding]

It's normal to float charge a lead acid 12V battery at 13V8
the idea being that the battery can be left connected to the
charger indefinitely in order to keep it fully charged and
avoid problems which occur if a lead acid battery is allowed
to self discharge too much.

Question: If instead of float charging I applied a PWM
voltage to the battery, at some frequency, which altered in
width according to battery terminal voltage (over, say, a
30 minute period) anyone have any idea of the
consequences, if any, on battery life etc? Also any
suggestions for the frequency? My guess is around 200Hz?

Mike Harding

 

[Phil Allison]

"Mike Harding"

It's normal to float charge a lead acid 12V battery at 13V8
the idea being that the battery can be left connected to the
charger indefinitely in order to keep it fully charged and
avoid problems which occur if a lead acid battery is allowed
to self discharge too much.

** The above scheme is correctly called "constant voltage - current
limited charging". The open circuit voltage of the charger is regulated to
13.7 volts +/- 0.1 volts ( for a 12 volt SLA ) while the maximum available
current is limited to a value that is safe for both the charger itself and
the size of SLA battery concerned. Initially, with a flat SLA, the current
flow will be quite high ( requiring limiting) then it will fall to some
intermediate ( safe) value and finally to a trickle when the SLA has charged
fully.

Question: If instead of float charging I applied a PWM
voltage to the battery, at some frequency, which altered in
width according to battery terminal voltage (over, say, a
30 minute period) anyone have any idea of the
consequences, if any, on battery life etc? Also any
suggestions for the frequency? My guess is around 200Hz?

** Your question as posted makes little sense unless the *peak* PWM voltage
is regulated at 13.7 volts and the duty cycle is restricted to limit the
current flow during the early stages of charging expanding to full duty when
the current falls.

Remember, the *average value* of the current is what *charges* a cell or
battery - so simple RC filtering of the PWM voltage dropped across a
current sense resistor will suffice to control the PWM duty cycle.




.............. Phil

 

[Johnny]

On Sat, 17 Jul 2004 17:46:45 +1000, Mike Harding
<mike_harding1@nixspamhotmail.com> wrote:

It's normal to float charge a lead acid 12V battery at 13V8
the idea being that the battery can be left connected to the
charger indefinitely in order to keep it fully charged and
avoid problems which occur if a lead acid battery is allowed
to self discharge too much.

Question: If instead of float charging I applied a PWM
voltage to the battery, at some frequency, which altered in
width according to battery terminal voltage (over, say, a
30 minute period) anyone have any idea of the
consequences, if any, on battery life etc? Also any
suggestions for the frequency? My guess is around 200Hz?

Mike Harding

Raw PWM should not be used uless you have some kind of output filter.

This could be done with a SMPS adjusted to 13.7V and a light globle.
The use of a light globe is an old-tech way of limiting the charging
current.

Since light globes may be unreliable, you might be better to use a
nominal 16V adustable power supply and a 3 terminal regulator such as
LM350 on a suitable heatsink. The voltage regulator can be used as a
current limiter with only one external resistor.

best regards,
Johnny.

 

[Phil Allison]

"Johnny" <john_wr@NOSPAM.hotmail.com

Raw PWM should not be used uless you have some kind of output filter.

** What do you imagine a battery is Johnny ??




............ Phil

 

[Johnny]

On Sat, 17 Jul 2004 18:48:40 +1000, "Phil Allison"
<philallison@tpg.com.au> wrote:

"Johnny" <john_wr@NOSPAM.hotmail.com

Raw PWM should not be used uless you have some kind of output filter.


** What do you imagine a battery is Johnny ??

I imagined that a batter is a device with an low internal resistance.

Just as you need current limint with using a DC charging supply, the
same applies with PWM charging.

regards,
Johnny.

 

[John Tserkezis]

Johnny wrote:

Since light globes may be unreliable, you might be better to use a
nominal 16V adustable power supply and a 3 terminal regulator such as
LM350 on a suitable heatsink. The voltage regulator can be used as a
current limiter with only one external resistor.

If you're using an adjustable supply, if it has current limiting, set the
voltage 13.5v with no load, limit the current to whatever the battery will take
safely, and forget about it.

--
Linux Registered User # 302622 <http://counter.li.org>

 

[Phil Allison]

"Johnny" <
"Phil Allison"

Raw PWM should not be used uless you have some kind of output filter.


** What do you imagine a battery is Johnny ??


I imagined that a batter is a device with an low internal resistance.

** Nah - batter is what you have on your sav.

Just as you need current limint with using a DC charging supply, the
same applies with PWM charging.

** Just like a micro fiche needs a bicycle.


??????????



............. Phil

 

[Roger Lascelles]

It is important not to draw current back out of the battery during part of
each cycle, even if the net current is into the battery. Lead - acid
batteries have a short life when you do this. This means you need a diode
somewhere.

Mike, you don't want to apply a PWM *voltage*, because you don't want the
battery voltage to go up to a voltage then back down to zero. PWM battery
chargers dump bursts of *current* into the battery and let the battery
determine the voltage. Slow response feedback checks the average battery
voltage and controls the current pulse width - or omits whole pulses if a
very small average current is required.

Many chargers have little or no filtering of the pulses, because it has been
found that the pulses help prevent and reverse sulphation. The pulses are
often produced by building a current up through an inductor, then dumping it
through the battery. Larger chargers use a push pull PWM which requires
filtering in order to work at all.

best wishes
Roger



"Mike Harding" <mike_harding1@nixspamhotmail.com> wrote in message
news:kolhf0p3gk7qd9t0a47ucn8dabbpov5vsh@4ax.com...

It's normal to float charge a lead acid 12V battery at 13V8
the idea being that the battery can be left connected to the
charger indefinitely in order to keep it fully charged and
avoid problems which occur if a lead acid battery is allowed
to self discharge too much.

Question: If instead of float charging I applied a PWM
voltage to the battery, at some frequency, which altered in
width according to battery terminal voltage (over, say, a
30 minute period) anyone have any idea of the
consequences, if any, on battery life etc? Also any
suggestions for the frequency? My guess is around 200Hz?

Mike Harding

 

[Mike]

Several things to watch if you want your battery to last !

a. Peak instantaneous voltage, dont exceed abs max of
2.75v/cell depending on type of lead acid, alloy
variations - especially take note of manufacturere notes !
Rarely 2.75v/cell is used to 'equalise' but needs great
care, a safer max is 2.5v/cell for a sealed battery,
some manufacturers suggest lower, like 2.25v/cell and
even one French manufacturer suggested 2.2v/cell.

b. Avoid plate stress caused by high instantaneous current,
so limit your current - measure it via equipment that
can detect an instantaneous current to at least a 1KHz
bandwidth, I use a cro and shunt where possible, otherwise
a meter with a 10KHz to 100KHz bandwidth,

c. Take care with frequency selection, depending on the
type of construction you might cause some plate to
oxide sympathetic resonance, this can damage a plate
where it joins a bus bar or cause oxide to fall off
for regular wet cell unsealed lead acid not that
much of an issue with SLA but resonance could still occur.
I have seen a pwm setup that caused a noticeable
hum on one side of a battery which coincidentally ran
10deg hotter - even though all cells were ostensibly
the same voltage !

d. Watch your temperature, dont exceed 50deg C, some suggest
no more than 45deg C, depending on type of manufacture
even others suggest no more than 35deg C, it all depends
on how long you want the battery to last. I've seen some
wet cell stuff go at 60deg C for 24hrs as a last resort to
recover from sulphation - only then with close attention
to specific gravity !


e. Some Sonnenshine batteries can take really high charge
currents when flat, this can be misleading because
if you measure average or RMS current you arent seeing
instantaneous, this could be much higher depending on
your pwm ratio and with a high intantaneous current you
can get hot spots on the plates which you wont easily
see from outside - even with a ir temp probe,

If I was coming up with a specialist charger I might consider
pwm to attempt to handle issues such as light sulphation or
use it to attempt to 'fix' an old SLA battery that wont take a
charge, if you are going to throw out 7c/Kg of lead anyway,
then you might as well try it to destruction, but for serious
application for reliability you want to control everything
and instantaneious currents and voltage distribution across
a multi cell SLA (if you can get to the cells) is a must !

Take a look at a charging system I developed in the jungles
of Malaysia a few years back, link below (Sabah), all wet cells,
big 80Kg 1metre tall 2v cells all in series to 300v,
charged at 250 to 300Amps, all field controlled,
avoided pwm like the plague ! Blowing a DC fuse when charging
was an interesting lightshow I can tell you, on that occasion
we didnt exceed 60deg C - but needed to equalise since they
had been left for some time discharged - that brought them
back from near death and they worked for months after that
before the local authority pulled a political tantrum...

Rgds

Mike
Perth, WA
http://www.members.iinet.net.au/~erazmus




In article <kolhf0p3gk7qd9t0a47ucn8dabbpov5vsh@4ax.com>,
mike_harding1@nixspamhotmail.com says...

It's normal to float charge a lead acid 12V battery at 13V8
the idea being that the battery can be left connected to the
charger indefinitely in order to keep it fully charged and
avoid problems which occur if a lead acid battery is allowed
to self discharge too much.

Question: If instead of float charging I applied a PWM
voltage to the battery, at some frequency, which altered in
width according to battery terminal voltage (over, say, a
30 minute period) anyone have any idea of the
consequences, if any, on battery life etc? Also any
suggestions for the frequency? My guess is around 200Hz?

Mike Harding

 

[Albm&ctd]

On Sat, 17 Jul 2004 21:01:10 +1000, "Phil Allison"
<philallison@tpg.com.au> wrote:

"Johnny"
"Phil Allison"

Raw PWM should not be used uless you have some kind of output filter.


** What do you imagine a battery is Johnny ??


I imagined that a batter is a device with an low internal resistance.



** Nah - batter is what you have on your sav.

I've found in practice most normal wet pussy type 12v lead acid

batters need a float of about 14.3 volts. What have you found with
batters? Actually fish (shark) done in a nice yeasty home brewed beer
batter is delicious.

Al
2004 insult page awaits your contribution
http://kwakakid.cjb.net/insult.html