Roger has some excellent advice here. It reflects a lot of what I do
myself. :-)
Roger Stockton via EV wrote:
Thought I'd mention that while a programmable load is an *excellent*
way to discharge when testing batteries, it can be pricey...
Indeed. Programmable loads get very expensive if you are dealing with
high power (high voltages and/or high currents). They are also effective
room heaters. Good in Minnesota winters; but bad in Arizona in the
summer when you're already paying to air condition.
If you already have a data logger or other means of
measuring/controlling a load and charger, then perhaps consider using
a passive load in parallel with a smaller electronic load, so that
you can save money on the electronic load purchase.
A simple QBasic program on an old laptop controlled the relay for
the load and another for the charger via the parallel port.
This is the approach I use. I have a couple of $50 DAQ118 analog/digital
I/O modules that plug into my PC. They have several 12-bit analog inputs
(to read voltages, currents, and temperatures), outputs (to throttle the
charger, and digital output channels (operate relays to select various
loads).
They are run by a PC with software written in QuickBASIC. I have an old
PC that is too slow for anything modern; but has a parallel port and is
rock-solid-reliable and can run tests for days.
These modules were bought in 2002. I'm not sure if they are available
today, but there are lots of equivalents.
I also have a Keithley 576 Measurement and Control system. This is a
stand-alone data acquisition controller. Rather tedious to set up, but
lots of channels, high precision, and very versatile.
I use these with a commercial battery charger for charging, and a custom
made load box that basically consists of a bunch of big power resistors,
in a box, with a fan, and relays to select the load resistance in a
stepwise 1-2-4-8 sequence.
I started out with a battery cycler setup consisting of a bank of 12VDC
Edison-base (household screw-type) light bulbs
Light bulbs are a good *and* cheap load resistor. :-) As Roger says,
they have the useful feature of drawing a roughly constant current
despite changes in voltage. This is a simple way to get an approximately
constant load current despite the sagging voltage as the battery discharges.
I've used car headlights (about 4 amps each) and taillights (about 1 amp
each) as load resistors for 12v battery testing.
A simple QBasic program on an old laptop controlled the relay for
the load and another for the charger via the parallel port.
An E-meter with the RS232 comms option provided voltage, current,
etc. measurements to the QBasic program.
This describes the most common load tester I use. I have a few E-Meters
/ Link 10 / ProLink meters (all the same meter, but sold respectively by
Cruising Equipment / Heart Interface / and now Xantrex). These measure
volts/amps/watts/amphours/watthours/time/temperature and send the data
to a PC via an RS-232 serial port. I have a QuickBASIC program that
logs, plots, and prints the data.
The "controller" is an old Manzanita Micro "Rudman Regulator". It has an
output that switches a relay to select between charge and discharge, and
trimpots to select the max and min voltage at which to switch between
charge and discharge. This setup has been documented in the EVDL archives.
I also built my own stand-alone battery tester/cycler, which includes
the charger, constant-current load, and metering for volts, amps,
amphours, temperature, etc. It is documented at
http://www3.telus.net/nook/balancerland/cycler/index.htm
This stuff is not exactly "hard" to build yourself, either from scratch
or cobbled together using whatever you have on hand. But it does take a
fair amount of time and effort to sort it all out and figure out how to
use it.
--
A designer knows he has achieved perfection not when there is
nothing left to add, but when there is nothing left to take away.
-- Antoine de Saint Exupery
--
Lee Hart's EV projects are at http://www.sunrise-ev.com/LeesEVs.htm
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