On 1/28/2013 4:54 PM, Chuck Hursch wrote:
I think the Elcon gets set up with IUI for floodies, with perhaps even a
fourth-stage trickle charge.
The final xxI in an IUI charge cycle is generally for equalization.
After the battery is nominally "full", it keeps charging at some low
current until some cutoff condition is satisfied. It might be for a
fixed time (1-2 hours), or until a particular voltage is reached, or
some other condition. But this final xxI stage should in no case last
more than a couple hours.
A lead-acid battery should never be trickle charged. It is bound to
overcharge it. "Trickle charge" means continuing to charge at some low
current forever, regardless of what the battery voltage is.
It's acceptable to end a lead-acid charge cycle with a constant
*voltage* step (i.e. a "float" voltage).
Knowing how Greg McCrea set up my K2 after
he did the one repair back in 2006, it's going to be IUI. I think
US Battery once upon a time recommended IU, but it now seems to be IUI.
I'm not at all clear if there's an advantage one way or the other.
Third-stage `I' may give you more assurance of getting the amp-hours
into the pack.
I don't think the particular algorithm used is all that critical. The
important thing is to charge until it is full, and then shut off.
Failing to fully charge, or going well past fully charged, is going to
shorten the life of the pack.
Occasionally not fully recharging (such as opportunity charging) is OK.
Occasionally overcharging (equalization) is also OK. The problems come
when you *chronically* undercharge or overcharge.
I've had the best luck with chargers that use some form of dv/dt or
di/dt algorithm... you could *add* something to the charger to
produce this behavior. It could detect the old battery's "full"
point, and just shut off the charger early.
Yeah, maybe that's what has to happen. Something to watch the dV.
Some people think this requires computers and complexity, but it
doesn't. A simple "add on" scheme is just a DC meter in series with a
big electrolytic capacitor. As long as the voltage keeps rising, the
meter reads a positive voltage. Set up your circuit to turn off the
charger when the meter falls to zero.
50 years ago, they would have used a real analog meter, with a tiny
switch contact operated by the pointer. Today's equivalent would be a
MOSFET transistor, with a 10v zener diode from gate to source (to clamp
the voltage between -0.6v and +10v). Also put a high-value resistor or
potentiometer (1 megohm or so) from gate to source. Use the drain to
operate a relay coil, wired so the relay drops out when the gate voltage
falls too low to keep the MOSFET on.
Then, connect a big capacitor and diode in series from your pack to the
MOSFET gate and source. As long as the voltage is rising, the capacitor
couples the rise to the gate of the MOSFET. The gate stays on, so the
relay is on. When the voltage stops rising, the capacitor current falls
to zero, and the MOSFET and relay turn off, stopping the charger.
As I recall, Lester's dv/dt chargers shut off when the rate of change of
voltage is less than 0.005v per cell per 15 minutes. On a 120v (60-cell)
pack, that would be 0.3v in 15 minutes. If the gate resistor is 10
megohms, and the MOSFET turns off at Vgt=1 volt, then the series
capacitor needed is:
dv/dt = 0.3v/15min = 0.02v/1min = 0.000333v/sec = 333uV/sec
i = v/r = 1v / 10megohms = 0.1uA
For a capacitor, i = C dv/dt, so
C = i / (dv/dt) = 0.1uA / 333uV = 0.0003 farads = 300uF
This is a significant amount of capacitance, so it will be an
electrolytic. Use a good one, with a voltage rating well above your pack
voltage. It will still leak current, which is why the resistor across
the MOSFET gate-source should be adjustable.
However, an aging battery is probably going to be dumped in the bulk
charge stage, 13A to nothing in a heartbeat.
It's hard to say. An old battery has a lower end-of charge voltage, so
the charger may remain in bulk charge mode until some internal timer
shuts it off. Or, the old battery may also have higher internal
resistance, so the voltage rises too *high*, and the charger shuts off
before it's actually full.
--
First they ignore you; then they mock you; then they fight you; then you
win. -- Mahatma Gandhi
--
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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