Re: (ET) Dave Roden battery lesson

["David Roden (Akron OH USA)" <roden ald net>]

Bill's reply is pretty much spot on.  I have only a couple of 
clarifications 
on equalization and sulfation.

Be careful not to take away from this the idea that equalization is only 
necessary for a multi-battery pack.  Bill says in his first sentence

> Over time, some cells will become less fully charged than others.

and this is the key.  Not just individual batteries, but the cells in each 
battery, get out of balance over time.  This is because manufacturing 
tolerances are not perfect, temperatures differ a bit, and so forth.  As 
the 
battery ages, the cells get even further apart in state of charge.

The way the ET taps the pack makes this appreciably worse on the battery 
level.  The three batteries that run the lift usually discharge the most, 
then the two that run the lights (maybe), and finally the one that just 
runs 
the tractor gets the least discharge.

> the voltage is run up to slightly above the fully
> charged voltage ...

Just to be clear, here we're talking about the voltage as measured while 
~still on charge~, not the voltage you measure after the battery has been 
removed from the charger.  I don't want to get into charge algorithms 
right 
now, but fully-charged on-charge voltage is typically 2.4 to 2.45 volts 
per 
cell (43.2 to 44.1 volts for our packs).

Equalization normally calls for holding the charge current on a fully-
charged pack at a stable level of 1/40 to 1/20 the battery's 20-hour amp-
hour capacity expressed in amps (I prefer the low end of the scale, about 
5 
amps for our batteries).  You stop when one or more of the following is 
true: 

        1. about 2 hours has gone by; 
        2. the voltage has stopped rising; or
        3. voltage exceeds 2.5 volts per cell. 

For point 3, some battery experts give voltage limits as high as 2.55 or 
2.6 
volts per cell, and some say use NO voltage limit.  I tend to agree with 
2.5 
volts as a limit, but if you're trying to restore a tired old battery you 
probably have nothing to lose by raising that limit.

> If this is not done, the batteries that are not
> fully charged will quickly "sulfate" coating the plates with a layer
> of non conductive sulfur which will permanently reduce their capacity.

Just so there's no misunderstanding here, lead sulfate formation is a 
normal 
part of a battery's discharge reaction.  Lead sulfate ~has~ to form on the 
plates, or it can't produce any current.  When you charge the battery, the 
lead sulfate turns back into lead.

Most of it does, anyway, and that's the rub.  What we usually call 
"sulfation" is an irreversible condition (and electronic pulse gadgets can 
NOT truly reverse this, nor can snake oil additives).  

When lead sulfate sits on the plates for an extended period of time, for 
example when the cell isn't ever fully charged, it begins to form 
crystals.  
The crystalline structure resists taking part in the chemical reaction 
caused by charging, so this part of the lead sulfate doesn't turn back 
into 
lead.  This is one source of falling capacity as a battery ages.  You can 
literally measure this as declining specific gravity.  (Some of the snake 
oil additives "correct" this by reconcentrating the electrolyte, which 
causes other damaging conditions in the battery.)

The sulfate crystals may stay on the plates for a time, but eventually as 
the crystals grow they loosen and fall off.  This sludge builds up in the 
bottom of the cell.  When it reaches the bottom of the plates, it can 
create 
a short-circuit.  This is why older batteries sometimes develop shorted 
cells and/or high self-discharge rates.  

Some of the crystals stay in suspension, as a result of vibration and 
gassing during charging.  You've noticed that older batteries' electrolyte 
turns grey?  That's suspended lead sulfate crystals.

One way that manufacturers have of making batteries last longer is to 
allow 
more room in the bottom of the cells, so sulfate crystals can precipitate 
there longer.  The tradeoff is that, for a given case size, the plates 
have 
to be smaller.  This reduces capacity.  And vice versa:  high-capacity 
batteries will often be less able to handle sulfation.  You'll find that 
the 
T-145 type batteries will have a somewhat shorter life because of this 
(among other reasons).

> equalizing charges
> are important if regulaters are not installed on each battery to
> ensure they are charged and discharged equally. 

The golf car batteries most of us use don't provide any way to make 
electrical connections to each individual cell. Therefore, even if you use 
battery regulators or balancers (devices which attempt to even out the 
voltages of the individual batteries in a pack), this doesn't balance the 
cells in each battery.  

Battery regulators help, and they're really important for VRR batteries, 
which can't tolerate the same amount of overcharging that flooded 
batteries 
can. But even with regulators, equalization is ~always~ necessary!


David Roden - Akron, Ohio, USA
1991 Solectria Force 144vac
1991 Ford Escort Green/EV 128vdc
1970 GE Elec-trak E15 36vdc
1974 Avco New Idea 36vdc
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