Byron,

Don't forget my disclaimer at the bottom of the last message.  There are
temperature issues that as the battery gets warmer the voltages change.
You're doing the safe thing by the limits you've chosen.

I tend to be a bit more consertive though.  I have one of those 5AH
Lantern batteries with the spring posts that I use for a six volt
Flashlight.  I've written on the side of the battery with an indelible
ink marker.

Charge till 7.05V at 2.0A.

Hold at 7.05 till 0.1A

Float at 6.85.

I've had this Lantern battery for over 7 years.

I use a current limited supply (Radio Shack 1A 0 to 24V) and I just dial
in 7.05V and leave the battery on this for a number of hours till the
current reaches 100ma or it's been a few hours and then remove the clip
lead, adjust the voltage to 6.85 and put the clip back and leave it
overnight.

On my to do list (#2,327) is a PIC based charger for this puppy although
the UC3906 would do it too.  Problem is I'm only on project #210;
there's a few to go yet.


Sigh....

John Dammeyer


Wireless CAN with the CANRF module.
http://www.autoartisans.com/documents/canrf_prod_announcement.pdf
Automation Artisans Inc.
Ph. 1 250 544 4950


> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Byron A Jeff
> Sent: Sunday, July 21, 2002 6:13 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]:speaking of gellcell's ?
>
>
> On Sun, Jul 21, 2002 at 05:21:26PM -0700, John Dammeyer wrote:
> > Hi,
> >
> > >
> > > [ BAJ Note. This thread is getting to long. I'm going to do
> > > some editing...]
> > >
> > > > Charge the battery,  in a constant current mode to C/3 until the
> > > > terminal voltage reaches 2.45V per cell.  (BULK CHARGE) At
> > > that point
> > > > switch over to a constant voltage charger holding the
> > > terminal voltage
> > > > (ABSORPTION CHARGE) until the current drops to:
> > >
> > >
> > > Here's where I still want to press the point I asked earlier:
> > > Does this need
> > > to be voltage limited in any way? What input voltage should
> > > be presented to
> > > the battery? Is there an input voltage that is harmful as
> > > long as the current
> > > is constant that the charging is transferred to contant
> > > voltage mode after
> > > reaching the specified cell voltage?
> >
> > You don't need to limit the voltage (within reason of
> arcing over inside
> > the battery I guess so 2kV power supplies are out).  The point is if
> > your power supply was capable of producing 50V at 100A and
> you put that
> > 50V across the battery it might only be milliseconds before
> the battery
> > terminal voltage of 2.45V per cell was reached. At that
> point you have
> > to switch over to constant voltage mode.  Plus if you put too high a
> > voltage across a really dead cell you may find the voltage
> stays at 1.8V
> > for a period of time but your C/3 charge setting may be severely
> > exceeded and you have to once again reduce the voltage to reduce the
> > current.
>
> That's so enlightning! I had always read that SLA batteries
> needed constant
> voltage charging with current limitation. The fear of
> overcharging dominated
> my thinking.
>
> So there's no problem with dumping 10A of current limited power into a
> 33Ah battery so long as the terminal voltage doesn't exceed
> 14.7V. Got it.
>
> I think I'll open up my battery charger and bump up the
> voltages to 14.4V and
> 7.2V for my 12V and 6V lines respectively. While it still
> isn't perfect (2.4V
> per cell) it'll bring a big charging boost to my batteries.
>
> BTW both lines are current limited anyway.
>
> thanks,
>
> BAJ
> >
> > Recall on an earlier thread I've posted questions about alternators.
> > The Open Circuit no load voltage on the BMW D12 Marine Diesel engine
> > alternator is 200V measured in AC volts.  It's listed under
> test #3 and
> > there is a curve published in the workshop manual for checking the
> > output open circuit.  However,  once loaded, it will drop to a more
> > reasonable value and the purpose of the regulator is to
> hold the charge
> > voltage it sees at the battery terminal to 13.85V.  (Crappy
> regulator).
> >
> > Think of the battery not as a fixed resistor but as a
> special kind of
> > resistor that changes over time.  Use Ohms law to simulate
> this resistor
> > at time t to t+n.  If the resistance becomes to high the
> voltage across
> > is will increase.  If the resistance is too low for your
> power supply
> > capability then it will suck down your power supply voltage
> resulting in
> > either a) current limit, b) a voltage drop, c) fuse pops or
> something
> > smokes.
> >
> > In other words,  whack 50 volts across the battery but as soon as it
> > hits either 2.45V per cell or current exceeds C/3 reduce
> the voltage.
> > In the marine market the simple way to fast charge
> batteries was to put
> > a rheostat in series with 12V into the alternator field along with a
> > SPDT switch.  Run the engine up to some RPM, flip the switch and add
> > current into the field till the C/3 value is reached on the ammeter.
> > Then once the voltage reaches 14.4V start cranking back the rheostat
> > until the current dropped to about 15A.  Then flip the
> switch and let
> > the regulator top it off.  This one guy I know recharged
> his double bank
> > of 8D wet cells in just a few hours.  Alternator got really warm.
> >
> > John Dammeyer
> >
> > WARNING!!!
> >   There are safety issues with fast bulk charging batteries.  The
> > battery temperature must be monitored if you are operating
> on the edge.
> > Batteries can explode if charged to quickly as the gas is
> hydrogen.  For
> > 12V Gel Cells you can buy 3 Step Chargers from West Marine that are
> > rated at 12A and will top up a battery or let you leave them on the
> > charger forever without damage.  Please do be careful.  Think
> > Safety!!!!!
>
> I always do. That's why I've only float charged up until now.
>
> Thanks again for the info.
>
> BAJ
>
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