RE: (ET) Batteries

[David Roden <roden wksu org>]

> 1). What is the best way to charge at a low (10-15 amp) rate? ... I
> notice that my smaller 8 amp charger puts about 4 amps into the batteries
> after charging for 12 hours or more. I have a big big charger which does
> not have an ammeter, but in the fast charge position (80 amp) these same
> batteries get bubbling pretty well. 

The easiest way to answer the first question is probably to describe an
ideal charging regimen.

It almost doesn't matter a lot how much current you put in between 20% and
80% charged.  It can go into the hundreds or even thousands of amps as long
as you watch battery temperature.

Above 80%, more and more of the energy going into the battery is wasted in
disassociating the hydrogen and oxygen in the water.  This is where
controlling current becomes very important to prevent overcharging.

The simplest way is to regulate the charge voltage so it doesn't exceed 
some
value, typically between 6.9 and 7.5 volts per 6-volt battery.  As the
battery charges, its voltage increases.  This fights the voltage of the
charge, and thus the charge current slowly tapers off.  Hence this type of
charger is called a taper charger.  It is the most common type of charger
sold to charge auto batteries at home.  Taper chargers usually are rated to
charge at 4 to 10 amps, but most of them will charge at that rate only into
a dead battery.

The good news about taper chargers is that they can't overcharge the 
battery
(as long as they  are correctly calibrated for the type; it's also highly
desireable if they automatically adjust the voltage as the battery
temperature changes).

The bad news is that the battery voltage begins to rise well before that
magic 80% point is reached, so a taper charger cuts back sooner than it
needs to.  If set for float voltage (6.9 volts), it can literally take days
to complete the charge.

What if you turn up the voltage, and regulate the current (to prevent
burning up the charger)?  Then you can charge much faster, but you risk
damage from overcharging in that last 20%.

The ideal charger delivers a regulated current (usually as much as it can)
up to 80% charged.  This is a constant current phase.  Then it switches to 
a
constant voltage phase.  Now it works like a taper charger, regulating
voltage.  Now it watches the current going into the battery, and when it
falls to a small level (usually under 5% of the battery's amp-hour
capacity), it shuts off.

So, to answer your question, you might use your big charger until the
battery reaches some threshold voltage (say, 7.4 to 7.5 volts), then switch
to the small charger.

If you are pouring 80 amps into the batteries and they are "bubbling pretty
well," you are most likely shortening your batteries' life.  Overcharging
corrodes the grids which hold the battery's active material.

> I can only get the specific gravity 
> up to spec with the big charger.

You shouldn't expect to (or try to) get it "up to spec."  

As a battery ages, its fully charged specific gravity declines.  Here's 
why.
When you discharge a battery, the sulfate ions in the electrolyte combine
with the lead in the plates to create lead sulfate.    Charging reverses
this chemical reaction.

Over time, some of the sulfate crystalizes and fails to take part in the
reaction when the battery is charged.  This degradation is called 
sulfation,
and it's accelerated by over-discharging (below 20% state of charge) or by
allowing the battery to sit for an extended period in a discharged state.

As the sulfation process continues, the electrolyte becomes less
concentrated.  This, with the loss of active material from the plates,
contributes to the battery's capacity loss.  

Of course you can easily re-concentrate the electrolyte (as I mentioned in
the old-timers' trick the other day), but then your capacity is limited by
the active material only, rather than both active material and electrolyte.
So you have to be very careful not to use too much of your new capacity.  
If
you do, you'll be over-discharging the plates more often, which accelerates
the sulfation.  You get some capacity back, but if you're not careful, the
battery dies even faster.  

> 3). Why does the battery fluid get dirty looking? Does this hurt 
> anything?  When I charge to get the specific gravity up to spec on old
batteries, 
> the fluid gets very dirty looking. After a month or so this seems to
settle
> out and the fluid is clear again. 

Those sulfate crystals break off the plates.  They are grey, and make the
electrolyte like dirty.  Normally, they fall to the bottom of the cell.
Then the electrolyte is clearer, but if they accumulate sufficiently they
can short the cell.  (Battery makers allow some space below the plates for
this reason.  But the high-capacity batteries need more lead, so they can't
build in so much space.  This is one reason high-capacity batteries don't
last as long.)

When you overcharge with your big charger, the excessive gassing stirs up
the electrolyte and brings up the sulfate crystals so the electrolyte looks
grey again.


2). 70 amps of auto headlamps is like 12 of them (aren't they 35 watts
apiece?). Are there any other ways to discharge?

The ideal way is with a battery discharger.  These machines are designed to
test batteries by monitoring them as they discharge, usually at 25 or 75
amps.

For testing electric vehicle batteries, I have a homebrew load that I made
with the heating element from a derelict heat pump.  

You could modify a 1500 watt nichrome-wire portable space heater for 36
volts by dividing the element into 3 segments and paralleling them.   This
will draw about 40 amps.   Don't forget to run the heater's fan on 
household
AC when you use it.

Other hobbyists  have used such amp hogs as electric water heater elements
in buckets and chunks of metal dunked in barrels of salt water (don't do
this indoors -- it generates chlorine gas).


> Is this dirty stuff what I should be
> rinsing out? Maybe charge the cells up to the corrrect gravity with the
> big charger, stir up all the gunk in the process, then drain and rinse 
> the batteries. 

You could do that, but it might be better to take the ET out for a nice
bumpy ride.  You'd accomplish much the same thing without the risk of 
damage
from overcharge.

> By the way, how much is new battery acid, and how much does one
> of these batteries hold?

I used to have a few gallons I got from a battery dealer about 10 years 
ago.
I don't recall what I paid, but I don't remember it as being expensive.  

As for quantity, I've never measured it.  Maybe 3-4 quarts???  I have this
in some Trojan specs somewhere, I'll see if I can find it tonight.

4). What about these pulse things which claim to prevent/remove battery
sulfation?

Some folks who've used them swear by them.  But one electrical engineer I
know says you can get much the same effect by carrying out a long, slow
equalizing charge -- which is pretty much what they do.

David Roden
Akron OH USA