Re: (ET) charger voltage

["Elie, Larry \(L.D.\)" <lelie ford com>]

Actually, the more cells, the greater the likelihood of one bad actor.
I've seen them actually reverse charge.  Let's see how well you can
bring the pack up when one cell is at -2.3V....  You are lucky to get
the others to 60% charge.

Mr. Landis and I have disagreed on this in the past.  His charger works.
The ET charger works.  They don't work in exactly the same way.  If the
ET charger stopped at even 40V you will not get a full charge.  Harry's
charger will pump it up again a number of times after the battery cools
which gets the job done.  It won't be as fast, but if you want to just
leave it on for months it's superior.  There are other techniques
available as well.  Remember, the ET charger can put out lots of current
when the batteries are low because it is a simple constant voltage
design; you dropped the peak voltage the current it would put out at
lower voltages is also reduced.

Another factor you are leaving out is that 38.2V is the room temperature
voltage 90 minutes or more after charging.  The voltage at the pack at
the end of the cycle on an ET charger will be over 42V, with the
batteries well above room temperature.  If you really want to see where
to cut off, measure each battery with a DVM and see if they are all at
least 6.4+V.  Frequently, you have one still at 5.9V with others at 6.5V
at the 38V level.  By the time you are at 42+V, the worst battery is
usually near 6.4V.  Try it yourself and see.  For the ET charger to
fully charge the deck it needs to be able to get to these high voltages.
It may not be the best design, but it works and it's cheap.

Larry Elie


-----Original Message-----
From: Michael S Briggs [mailto:msbriggs alberti unh edu] 
Sent: Friday, July 28, 2006 12:53 PM
To: Elie, Larry (L.D.)
Cc: David C Robie; elec-trak cosmos phy tufts edu
Subject: RE: (ET) charger voltage


On Fri, 28 Jul 2006, Elie, Larry (L.D.) wrote:

> "Car" batteries have 6 cells.  The ET has 18 cells.  The more cells,
the
> more potential for not getting the cells fully charged.  If I need an
> extra 2.9% on 6 cells, I'll need several times that for 18 cells.
Cheap
> fix; add voltage.

As a percent, you shouldn't need more voltage for a larger pack. The 
amount of extra voltage will be more, but I wouldn't think the
percentage 
needs to be greater.

The issue I'm still trying to get answered is what exactly is the
voltage 
of a fully charged pack. The reason I initially asked this is because of

the discrepancy between the recommendations that a charger should put
out 
around 45V, compared to the Landis controller which has a default set 
point of 38V for ending charging (so what I'm trying to figure out is if
I 
should raise that set point, and if my charger needs to put out more
than 
39.5V).
        What I'm thinking is - if the pack NEEDS to be charged to 
something like 42-45V, then presumably the set point on the Landis
charger 
needs to be higher. It seems to work by shutting of the charger every
few 
minutes, measuring the voltage of the pack, and only turning the charger

back on if the pack voltage is below 38V. So, if the pack can be charged

up to something lik 42V, then if you don't raise that set voltage of 38V

on the Landis controller, I'd think it wouldn't be possible to fully 
charge the pack, since it won't turn the charger back on once the pack
is 
up to 38V.
        If, on the other hand, a fully charged pack has a voltage of
38V, 
then I would think that a charger voltage of 39.5-40V would be perfectly

fine, since you would be able to charge it up to its full voltage, and 
would have some additional potential there to account for weak cells.
        So, what I'm still not clear on is what the voltage of a fully 
charged pack is. If it's something like 42V, then I would think I'd need

to adjust that cutoff point on the Landis controller, as well as
replacing 
the cap on my charger to see if that gets its output up higher. If a
fully 
charged pack only has a voltage of 38V though, then I'd think that a 
charger voltage of 39.5V would be fine. No?

Mike