Re: FW: (ET) batt's, new gauge points, and a faq draft for discussion

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

My 2 kilowatts' worth on charging and state of charge measurement:

Unless you want to keep your ET 100% original or you don't want to spend 
any money on it, I think you're better off not investing too much time or 
effort in fine-tuning the original charging and SOC (state of charge) 
measuring equipment.  It just isn't that great to begin with.

First, the charger, which is fairly crude.  It has some nominal voltage 
regulation provided by the ferroresonant transformer.  But basically it 
is just a battery boiler.

For longer battery life and more efficient use of electricity (if that's 
important to you; many people don't care since it's not charging all that 
much of the time), rip the GE charger out and replace with a modern 
microprocessor controlled charger.  

Use a "I U Ia" charge profile:

1.  Bulk phase: as much as the charger can produce (within reason) until 
the battery reaches 2.4 to 2.45 volts per cell while on charge.  This is 
a constant current phase.  Twenty to 30 amps is fine for this.

2.  Hold voltage at this point until current drops to 2-4% of the 
battery's amp-hour capacity - for our batteries, about 4-8 amps.  This is 
a constant voltage phase.

3.  Hold current at this point for 1-2 hours more, or until voltage 
reaches 2.5 volts per cell (perhaps 2.6 with a brand new battery), or 
until voltage stops rising, whichever occurs first.  This is a constant 
current equalization phase.  

If you get rid of the taps for the lights and lift and power them from a 
14v DC:DC converter instead, you'll stop imbalancing your pack on every 
use of the tractor, and then you won't need the equalization phase on 
every charge cycle.  You can then apply it perhaps only on every fifth 
cycle or so.  The other 4 times, the charger can just shut off at this 
point instead.  But as long as some batteries are getting more discharged 
than others, you have to equalize every time (and consequently somewhat 
shorten the battery life).

BTW, Lester "Lestronic" golf car chargers accomplish a similar objective 
using a different charge algorithm.  They measure the voltage and control 
the charge based on a flattening of the rising voltage curve (dv/dt).

Should you actually do this -- that is, dump the GE charger for a more 
modern one?  It probably depends on how much you use the tractor and thus 
how many years your batteries last.  If you're satisfied with your 
battery life now, I'd say don't sweat it.  

But apart from making sure the GE charger puts out about the right 
voltage and the timer works, I wouldn't put much effort into it.  And I 
think I'd be inclined to replace the entire charger with a more modern 
one if the transformer quit.

Now, about the "fuel gauge."  As Larry suggests, the GE voltmeter is an 
expanded scale voltmeter.  Most likely it has a zener diode in series 
with a basic meter movement which (from Larry's description) reads 20 
volts full scale.  Thus it reads roughly 20 to 40 volts instead of 0 to 
20.  

This is much more useful information than 0 to 40 would be, since it 
concentrates the meter range into the area where the interesting stuff is 
happening.  (Actually, I think it would be more useful still if it read 
30 to 40 volts, since 31.5 volts is 1.75 volts per cell.  When it reads 
that under load, you should stop and charge.  But I digress.)

Be that as it may, as Larry as observed, these old meters are often out 
of calibration.  And they were never all that great for their intended 
purpose in the first place.  It takes a practiced eye to understand what 
they are saying about the battery's state of charge.

For an accurate and immediately understandable indication of how much 
energy really remains in your battery while you are working, rip the GE 
voltmeter out and replace it with an E-meter.  This tracks the actual amp-
hours or watt-hours used and replaced.  

A less accurate but cheaper alternative is a Curtis battery gauge, which 
works much the same as the GE voltmeter but with (1) a time constant, 
which helps average out the variations in voltage; and (2) a lot better 
calibration (it is a bargraph type electronic meter).

FYI: I don't do any of the above.  

I charge my tractor's batteries, six 12v gel batteries in series-
parallel, with more-or-less automatic 12v chargers which are a cut above 
taper chargers.  I have to; the GE charger would trash these batteries in 
a matter of weeks by not throttling back enough as they come up to 80% 
charged.

The nice thing about charging my 12v batteries with individual 12v 
chargers is that they get rebalanced every time (each battery gets as 
much charge as it needs).  So I don't have to worry too much about not 
having that DC:DC converter, and using the battery taps for the lift 
(24v) and lights (12v).

As for the metering, I really don't worry about the state of charge. I 
just don't have that much area to mow or plow, so I've seldom used enough 
of the energy in the pack to notice any loss of pep.  While I work, the 
voltmeter bounces around in the red and green ranges, and I just ignore 
it since it's most likely grossly inaccurate anyway.  But one of these 
days I'll probably put an E-meter where the voltmeter is, just so I know.


David Roden - Akron, Ohio, USA
1991 Solectria Force 144vac
1991 Ford Escort Green/EV 128vdc
1979 General Engines ElectroPed 24vdc
1970 GE Elec-trak E15 36vdc
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