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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 that
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 two
of the cells are identical; internal resistances are different.  If you
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 gets 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 full pack is only 38V - that seems like way too much "extra" voltage. Sure, it 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 the
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 38V (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 pack 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 is, 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). 

Mike



-----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 see
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 charger

needs to put out 42+ V to fully charge the pack, since presumably fully
charged is only around 38V. 42V+ will charge quicker (greater potential
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