"mike" wrote in message
news
On 5/22/2017 9:34 AM, Jim Wilkins wrote:


I kept the current from the battery close to 0.5A by tweaking the
rheostat load. Surplus phone chargers power the meters and the
relay
independently from the battery, since I won't be wasting my backup
by
running these discharge tests into a dummy load during a power
outage.


One consideration for backup is the cost of failure.
if the worst that can happen is a gallon of milk has shortened shelf
life,
that's a lot less serious than a mechanical fridge compressor that
stalls when it tries to start under brownout conditions.

I'm more concerned with being able to replace the lost food after a
hurricane or ice storm when the roads are blocked by fallen trees
tangled in possibly live power lines (generator backfeed) and the
stores don't have power either. The area I'm in typically stays dark
for a week before the line crews get to us.
https://en.wikipedia.org/wiki/Decemb...ates_ice_storm

If you're serious about these kinds of measurements, wire up
a LM317 (maybe with a booster transistor) as a current load.

If the voltage doesn't vary too much, an incandescent light bulb
makes a current source that's more stable than a resistor.

A computer controlled dual-output power supply is a useful tool.
Use one output to charge the battery and read back the voltage.
Use the other output to drive a voltage to current converter for the
load.
Makes it very easy to control and log and graph and...

I had very nice, and expensive, equipment like that when I was the
battery tech at Segway. While running the tests I found out what was
important and what wasn't, and saw that I could obtain adequate
results at home with much cheaper surplus equipment like old Ohmite
rotary rheostats. Tubular variable resistors are difficult to adjust
when hot, around 600F near their rated power.

The only time I programmed a load to change automatically was for
two-level battery impedance measurements. Otherwise the voltages and
currents remained constant until I changed them, which can be done as
easily with a knob. We used our lab power supplies to recharge vendor
sample Lithium cells to the appropriate voltage at a constant current.

The best representation of a real-life active load isn't constant
resistance or current but constant power such as a DC-AC inverter
draws, and I can just use one with a hotplate on a Variac for the
adjustable load. Inverters usually have a functioning low voltage
disconnect to protect the battery. However a fixed resistance load is
fine for tracking battery ageing and easier than a switching load to
measure accurately.

These connected to a laptop make a good data acquisition system for
slowly changing parameters like battery voltage.
https://www.amazon.com/Tekpower-TP40.../dp/B000OPDFLM
Since they are optically isolated they can be connected anywhere in
the circuit without creating unwanted current paths through their
cabling, a major concern with grounded instruments like scopes. You
can combine their separate datalog files into a spreadsheet by
aligning the timestamps.

A laptop makes it easy to set up in the best place to run the test.
For me that's in the laundry room where I have running water to deal
with battery acid spills. The laptop itself can be the representative
load on the battery and its own internal battery will keep it running
when the battery being tested drops out, so it doesn't lose the most
important data point. The time that happens, saved in the system
Event Log, may be all you need to know. Older, thicker laptops with
CardBus or ExpressCard slots for port adapters to read multiple meters
are cheap.


My pure sine inverter reports status over a hand-wired non-standard
cable to a monitoring program I wrote but I haven't found much use for
the results. This is more valuable:
https://www.amazon.com/bayite-6-5-10...yite+100v+100a
If that link doesn't work it's a Bayite PZEM-051.

This VAC-1030A could be great if they fixed a few minor bugs and wrote
a decent manual. It's good enough already.
http://www.ebay.com/itm/VAC1030A-The...-/262562495499
I haven't tried the 100A model, VAC-1100A, because I normally recharge
at a current too low for it to measure accurately, though the
VAC-1030A isn't much better.

If I had a solar system, I'd have an arduino or some such
monitoring it at all times.

Monitoring my solar system showed that clouds pass randomly and I
still need a backup generator since I can't depend on a consistently
adequate solar output, especially in the kind of weather when I'd need
it most. I occasionally check the output and wiring drops on bright,
clear days. A cheap HF DVM gives the short circuit current.and an RC
wattmeter and variable resistor can find the maximum power point.
https://www.amazon.com/Tenergy-Preci.../dp/B017YCTRKK
That's just an example, not the discontinued meter I own. It shows
that the power doesn't fall off much on either side of the maximum,
meaning there's little benefit from an MPPT controller on a small
system.

-jsw