>> I looked at your schematic and was wondering what you are using to control
>> the battery voltage.  Is Vsupply regulated or do you have something else to
>> set the battery float voltage.
>
>Yep, the supply is pretty well regulated at 6.7V (or something, I'll have
to check)with a switcher.
>
>>  Do you worry about equalize mode?  My lead
>> acid chargers stay in equalize mode (7.5 Vdc on a 6V gell cell) until they
>> drop out of current limit, whereupon they drop down to float mode (6.75
>> Vdc).  They switch to equalize mode both on powerup and anytime the charger
>> goes back into current limit.
>
>I'm not sure what equalize mode is. Please tell me more.

Lead acid batteries have 2 different charging parameters.  The voltage can
stay permanently at the float voltage: if the battery is heavily discharged,
it charges at that voltage and reaches probably 75% of full charge
relatively quickly.  Once the charge current starts to drop off, the battery
takes many hours or days (at float voltage) to reach 100% charge.

Equalize mode causes the battery to hold the charge voltage (and current)
high until the battery becomes near to 100% charged.  In essence, if you put
a current limited voltage supply set at the equalize voltage across a
discharged battery, the slope of the charge curve (% full charge) remains
relatively constant until the battery is full charged.  BUT - if you KEEP
the voltage at the equalize level for an extended time, the battery
overcharges, begins to vent, and becomes permamently damaged.

You say that you are using a regualted 6.7 Vdc switch mode supply.  I think
that should be just fine.  Just remember that the last 25% or so of the
battery's capacity will take substantially longer to be charged than the
first 75% or so.

Couple of other thoughts: how does your 6.7V switcher react when being
backfed by the battery?  Will that cause startup problems for it?  What kind
of loading does the supply have internally - how much current is it going to
draw from the battery?  Just something to check out.

Scott mentioned another concern: do have or need any mechanism to prevent
the battery from becoming too deeply discharged?  If this is a concern, I
have a couple of suggestions.  First, put another N-channel FET in series
with the (-) lead of the battery.  This FET is driven by your low voltage
dropout detector (another section of op-amp using VDD as the refernce).  Now
that you have this normally on FET in the negative lead of the battery, use
it as your current sense.  It would reduce the problem of getting rid of the
PWM swings your existing filter has to deal with.  Of course, if deep
discharge is NOT a concern, don't do this . . . ;)

dwayne


Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(403) 489-3199 voice     (403) 487-6397 fax