Thanks Lee - as usual, top notch advice.
----- Original Message ----
From: Lee Hart <
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To: Electric Vehicle Discussion List <
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Sent: Tuesday, December 18, 2007 12:01:39 PM
Subject: Re: [EVDL] Buck/Boost rectification question
Frank John wrote:
> I'm looking for a badboy approach for an onboard opportunity charger
> for my 120 VDC pack of T125's... If a nominal 120-12 = 108 VAC (rms)
> configuration is used it peaks at about 152 volts (or a bit higher
> where I am, as the local utility tends to run at 122-124 volts). I
> think I need somewhere around 145 VDC to charge this pack.
I would aim for 2.5v/cell, which is 150v for a 120v pack. Your 120-12 =
108v is just about right as-is. When you're right at the peak, charging
current has fallen to zero amps.
> Will 152 peak drop down with a bit of capacitance on the output side
> of a bridge?
No. Capacitance on the DC output of a bridge *increases* the average
voltage -- it makes it track the peak voltage. But the batteries
themselves have more capacitance than any real capacitor you might add.
The right place to put capacitors in such a charger is across the AC
line. This type of charger has a poor power factor. Capacitors across
the AC line will spread out and phase shift the peak current spike,
improving the overall power factor. This means you can get a little
more
charging current from an AC outlet without tripping the breaker.
> Could anyone suggest capacitance values on the output?
0 uF. You only add capacitors if the ripple current is too high. This
will sometimes happen with small batteries on a large charger, or when
trying to float charge AGMs or gels.
> Would a capacitor/inductor approach to limit current on the input
> also drop output voltage a bit?
It depends how you do it. In general, here's how it's done:
1. Set the final voltage and current into fully charged batteries
with the AC voltage. Adjust this voltage with a variac, or tapped
buck/boost transformer.
2. Limit the maximum charging current into dead batteries with series
inductance. (Could also use resistance, but it wastes power and
things get hot. Could also use series capacitance, but capacitors
are bigger and more expensive for a given current.)
3. Add capacitance across the AC line to improve the power factor.
This has no effect on charging, but reduces the AC line current
needed for a given charging current, so you can get more current
from a given outlet before tripping its breaker.
Note that you still have a non-isolated unregulated charger. It doesn't
correct for changes in AC line voltage or battery temperature. Add a
GFCI, input and output fuses, and a timer so it won't murder your
batteries the first time you forget and leave it unattended!
--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net
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