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
