"rangerssuck" wrote in message
...
On Friday, May 19, 2017 at 12:33:52 PM UTC-4, Jim Wilkins wrote:
I've finally acquired enough equipment to measure the remaining
Amp-Hour capacity of my Lead-Acid and Lithium battery collection.
The
first result that jumped out is that older batteries suffer from
rising internal resistance as they discharge, enough that the
automatic low voltage cutoff trips short of rated capacity, and then
the battery slowly recovers to well above the full discharge voltage
given in the specs.
http://www.power-sonic.com/images/po...hManual-Lo.pdf
The 5-year-old 12v 4.5Ah UPS battery I tested this AM delivered
2.45Ah
at 3A, which is the average current my laptop draws while browsing.
Table 2 shows in the 1 Hour Rate column that it should be good for
2.75Ah at 2.75A current.
Does anyone know a good reason why I can't measure the true
remaining
capacity in two steps by first discharging to 10V at the fairly high
current of my typical loads, then continuing at the 20 hour rate AGM
batteries are specified for until the voltage drops to [the
appropriate endpoint] again?
The run time for a typical load tells me how useful the battery
still
is, but it combines the effects of capacity and resistance. I'm
wondering if also knowing the Amp-Hour capacity at the 20 hour rate,
with less interference from the internal resistance, would indicate
how well my long-term maintenance procedures work.
-jsw
I think that doing the 20 hour test my provide you with academic
indication of the condition of the batteries, but only at the 20 hour
rate. In my recollection (based on 25 years ago designing a 100
station lead acid charger), there is surprisingly little correlation
between capacity at different discharge rates.
So, I would suggest you test at your normal load and perhaps with a
"normal minimum load" assuming that those rates are pretty far from 20
hours. Anything else is, as I said, purely academic.
BTW, while I was buying the voltage reference, I also bought a USB
power meter (Drok). The Amazon add and the user's manual keep
referring to "capacitance" measurement. What they really mean is
capacity of USB battery packs. Pretty funny. Sort of. You can actually
buy this meter bundled with a USB load bank.
=========================
I want to separate the effects of capacity and internal resistance to
see if equalizing etc improves either or both of them. The internal
resistance of AGMs has some strangely behaved component reputedly
related to an oxide film. Otherwise I discharge them at the current my
laptop draws when browsing as I have them for power-outage backup and
NWS radar is the best indication of storms approaching my house that
I've found. It tells me when to repair roof damage and when to tarp
it.
I bought this which has an easily set low voltage disconnect and
handles up to +/-30A,
https://www.amazon.com/DROK-Display-...ct_top?ie=UTF8
and previously this which is 10x as accurate at low current
https://www.amazon.com/bayite-Multif.../dp/B01D7JGGE4
The first one measures charge and discharge current separately and
counts the Amp-hours up or down accordingly, though the Watt-hours
total is the positive sum of both (???). It has a more accurate
voltmeter and a better timer that counts seconds and stops when the
relay opens, allowing a pause in the measurement and a record of
battery run time. Unfortunately the current resolution is 0.1A despite
the display, so it doesn't handle small AGMs well.
The second one matches other ammeters to 1 or 2 digits and I use both
in series for discharge loads up to 10A. Together they each make up
for the deficiencies of the other. The 12V,12Ah battery is discharging
on them at 0.5A.
-jsw