[RE-wrenches] Strings and series of batteries with reversereturn linkup

Darryl Thayer daryl_solar at yahoo.com
Mon Dec 7 13:41:50 PST 2009 

Thanks Jamie and Ray, you have given me something to add to my teaching curriculum this is why i have had better luck with flooded batteries, overcharge at a slow rate is not as bad as overcharge in sealed batteries.  
DArryl

--- On Mon, 12/7/09, R Ray Walters <ray at solarray.com> wrote:

> From: R Ray Walters <ray at solarray.com>
> Subject: Re: [RE-wrenches] Strings and series of batteries with  reverse return linkup
> To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
> Date: Monday, December 7, 2009, 1:59 PM
> This is an excellent
> point that Jamie is making here. On electric vehicles, we do
> see up to 156 vdc series strings, and even with just a
> single string at 48 vdc, the separate cells are not being
> charged equally.Current is equal, but not the voltage
> across each cell.
> It's alarming to see some batteries (especially
> sealed ones) going way over charge, while others are
> undercharged. The controller only sees that the total
> voltage is at full charge, and then shuts off. I've
> pulled individual batteries out to charge separately. I
> think we really need smarter charging systems (similar to Li
> ion) that shutoff individual batteries or small groups to
> allow others to fully charge without overcharging
> others.Just imagine seeing a new expensive sealed
> 12 v battery hitting 16.5 v, while others are still below
> 13.5v. It'll make you sick to your stomach, hearing it
> gas, when it's not supposed to.HIgher charge
> currents make the problem worse.Here's a
> regulator from Manzanita Micro that can do
> it: http://www.manzanitamicro.com/index.php?page=shop.product_details&flypage=flypage.tpl&product_id=46&category_id=17&option=com_virtuemart&Itemid=64I've
> worked with Rich Rudman there for years; he's very
> knowledgeable and good to work with. It might be a good
> idea, especially on large sealed batteries.
> 
> R.
> Waltersray at solarray.comSolar
> Engineer
> 
> 
> 
> 
> 
> On Dec 7, 2009, at 12:21 PM, James Surrette
> wrote:
> 
>       Hi Darryl,
>    
> 
> 
>       To further on
> your comments (I hope);    
> 
> 
>       One thing
> we've noticed that is an often overlooked (potential)
> problem.  Even with a simple series sting (6, 12, 24
> cells), the individual cell voltages are never identical -
>  meaning initial charge acceptance is never identical.
>  When you initially "activate" or
> "commission" a bank, assuming 48VDC you begin to
> charge at 57.5-60V.  The overall voltage is attained
> relatively quickly but the individual cell voltages can be
> shockingly different (2.2 - 2.7V).  The commissioning
> is complete when all cell voltages "balance" to
> the charging voltage, i.e. 2.4V.   As series
> voltages increase (120VDC+) or multiple banks are used, the
> initial commissioning becomes increasingly more important as
> overcoming imbalance within the bank become more difficult.
>  The most efficient commissioning occurs when low
> current is applied (2% of 20hr) until cells balance.
>  The point to the comment, you might get away without
> balancing / commissioning / activating a series string but
> is imperative if using multiple parallel banks.   
> 
> 
> 
>       Regards,  
>  
> 
> 
>       Jamie
> 
> >>> Darryl Thayer <daryl_solar at yahoo.com>
> 12/5/2009 9:31 PM >>>
> This also assumes all the identical batteries are at
> identical temperature, and it one is slightly warmer or can
> not lose heat as fast, if being charged it will get
> warmer.  the warmer battery will do what? have a lower
> vltage, have higher current and receive more heat, and get
> warmer, this is significant.  I have never seen a
> battery set tha all batteries have the same thermal
> conductance to ambient
> Darryl
> 
> --- On Sat, 12/5/09, boB Gudgel <boB at midnitesolar.com>
> wrote:
> 
> > From: boB Gudgel <boB at midnitesolar.com>
> > Subject: Re: [RE-wrenches] Strings and series of
> batteries with reverse return linkup
> > To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
> > Date: Saturday, December 5, 2009, 3:45 PM
> > Antony Tersol wrote:
> > > Assume 3 parallel battery stings, with identical
> > batteries of internal resistance r.
> > >
> > > Assume identical wire leads of resistance R.
> > >
> > > Then when the batteries have the same state of
> charge
> > and their internal resistances are equal, one can
> solve for
> > the current in each string.
> > >
> > > I1 = I3
> > > I2 = I1 r / (r+R)
> > >
> > > where I2 is charging current thru the middle
> string.
> > >
> > > The amount that the middle current is reduced is
> a
> > function of the relative sizes of the wire and
> battery
> > resistances.
> > >
> > > For R << r, I2 --> I1.
> > > For r << R, I2 --> 0.
> > >
> > > With a buss bar arrangement, I1 = I2 = I3 = V/(2R
> +
> > r)
> >
> > One thing this does not take into account though, is
> the
> > inductance of all the wires brought
> > on by the wire lengths and their loop area, which in
> a
> > usual inverter with ripple current, will also limit
> the
> > current
> > through the system...   That will also have
> > some undesired consequences with the voltages
> > seen at the inverter terminals due to resonances,
> etc.
> >
> > In ~can~ be significant.
> >
> > boB
> >
> >
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