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Re: (ET) charger voltage

"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.

The ET also has several sets of more inter-cell connections, and several
times more sets of external connections.  Each has the potential to be a
.1 ohm contact.  More contacts; more chance to have problems.  Cheap
fix; add voltage.

Over charging CAN cause problems; eventually you boil off electrolyte.
At 45V after 1 hour you won't get any visible boiling.  Leave it there a
week and you have a problem.  That's why the ET uses a timer.  Again;
cheap fix.

Engineering IS compromise.  Cheap fixes always win, provided they work.

Larry Elie

-----Original Message-----
From: Michael S Briggs [mailto:msbriggs alberti unh edu] 
Sent: Friday, July 28, 2006 11:24 AM
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:

> Than you don't understand how charging flooded series connected
> batteries work.  There are a number of web pages that explain it, but
> those that design chargers are not a bunch of dummies.  Your car
> charging circuit regulates at 14.2 to 14.4 V, and charges a battery
> when new and fully charged at room temperature shows 13.8 V.  Those
> 'extra' .6V on the 6 cells helps the battery reach equilibrium.  No
> of the cells are identical; internal resistances are different.  If
> tried to charge at 13.8 (all you 'need') and if one cell is 'weak', it

I didn't say you only "need" exactly the max voltage the batteries can 
hold - you need the charger to put out at least a bit higher voltage 
than that otherwise it would take essentially an infinite amount of time

to get the battery pack up to that voltage (since as the pack voltage
closer and closer to the charger voltage, current will keep decreasing).

What I said is that I don't see why you need something like 45V if a
pack is only 38V - that seems like way too much "extra" voltage. Sure,
will charge the batteries quicker, but could cause severe overcharge 
problems, especially with hot batteries, I would think.

>From your numbers, a fuly charged car battery can maintain 13.8V, and
alternator puts out 14.2-14.4 V, only 2.9-4.3% higher than the batteries

max voltage. That's far less "extra" voltage than what some of you are 
saying the ET charger needs to put out, if the max battery voltage is
(that's the value I was initially trying to get, to then know what the 
charger needs to put out based on the sustained pack voltage). If the
voltage fully charged is 38V, an extra 5% (more "extra" voltage than 
the alternator puts out) would only be 39.9V. So, unless that 38V figure

(for the voltage of a fully charged pack) is off, I still don't see why 
we need chargers putting out 42-45V.

> will prevent all of the other cells from reaching 'full' charge.  The
> more cells, the bigger the potential problem.  Add to that battery and
> cell age, temperature differences and so on, and it's easy to see why
> the extra potential is used.

Yes, I know why extra potential is used - the issue is *how much* extra 
potential (and the issue of what the potential of a fully charged pack
to know how much extra potential there really is at a certain charger 
voltage). I've never said you only want the charger to put out the same 
voltage as a fully charged battery pack - I explicitly said you want the

charger to have a higher voltage so you'll have a potential difference 
between the charger and pack, so I don't know where you're getting this 
idea that I said the charger should be the same voltage as the pack. It 
should be more - I don't see why it should need to be 20% more though 
(essentially what it is if at 45V, and full is 38V).


> -----Original Message-----
> From: elec-trak-bounces cosmos phy tufts edu
> [mailto:elec-trak-bounces cosmos phy tufts edu] On Behalf Of Michael S
> Briggs
> Sent: Friday, July 28, 2006 8:54 AM
> To: David C Robie
> Cc: elec-trak cosmos phy tufts edu
> Subject: Re: (ET) charger voltage
> Dave,
>       Thanks for the feedback, that's essentially what I wanted to
> know
> - what the voltage is these 6 volt batteries will hold. As far as I
> it, if they'll typically hold 6.3 V each (so just under 38V combined),
> there's no reason the charger NEEDS to put out 42-45V. That high of a
> voltage will charge it quicker, but won't give a more complete charge,
> since the pack itself can't stay above 38-39V or so, depending on the
> batteries. My charger is putting out around 39.5V roughly, which to me
> means it should be able to fully charge the pack - it will just take
> longer than it would if the charger was putting out a higher voltage.
>       Since the Landis controller stops charging once the pack reaches
> 38V, it wasn't making any sense to me why people were saying the
> needs to put out 42+ V to fully charge the pack, since presumably
> charged is only around 38V. 42V+ will charge quicker (greater
> difference between the charger and battery pack, so more current will
> flow
> into the pack), but quicker isn't necessarily better, especially when
> dealing with charging batteries.
>       I'll measure the capacitance of the charger capacitor anyway
> (have
> a multimeter that can measure capacitance) to see if it's ok or not
> though.
> Thanks,
> Mike