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.
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.
On Friday, May 19, 2017 at 12:33:52 PM UTC-4, Jim Wilkins wrote:
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 rec
ollection (based on 25 years ago designing a 100 station lead acid charger)
, there is surprisingly little correlation between capacity at different di
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. Anyt
hing else is, as I said, purely academic.
BTW, while I was buying the voltage reference, I also bought a USB power me
ter (Drok). The Amazon add and the user's manual keep referring to "capacit
ance" 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 l
On Friday, May 19, 2017 at 12:33:52 PM UTC-4, Jim Wilkins wrote:
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
I bought this which has an easily set low voltage disconnect and
handles up to +/-30A,
and previously this which is 10x as accurate at low current
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.
Hmm, there are 3 different pictures of the back of those. One has a
built-in shunt, another a pair of relays, and another is bare. Which
is the real meter pic for the "30a w/ relay"?
IF I ever get the weeding done around here, I'll get those panels up
and build the control panel to see how those li'l Bayites work.
You showed another link for a milliamp/millivolt-resolution meter a
few weeks ago, too. How's that working for you?
The former part is cool. Not having proper resolution for decent data
is never fun, though.
Did I ever ask you why you didn't use a real battery for that? <g>
(real being 12v 35-275Ah) I set one up for use with the 45w HF trio
of panels and was able to power a 14" electric chainsaw with the 2kW
modified sine wave inverter, also from HF. It would have taken days
to recharge it (or more panels if needed for continued use.)
--Robert Knight, senior fellow, American Civil Rights Union
The 30A model I received has a shunt and blue NC relay on the base
module. The display module has a small red+black pigtail for external
power if you don't use a USB connection. At first the USB connection
on mine was poor and it intermittently shut off, or switched to
wireless without losing power. The correct accumulated totals
reappeared when it reconnected.
It displays current to 2 decimal places but is accurate only to 1
place +/-, for example 0.478A on a Fluke 8600A reads as 0.48A on the
10A "Electrical Parameter Tester", and 0.65A on the 30A unit.
A layer of Gorilla tape tightened the USB plug in the base unit
against the circuit board contacts and it has remained connected when
The 33.00V/3.000A meter is my favorite for recharging and equalizing
batteries slowly from my solar panels. It clearly shows when a small
AGM's charging current has decreased to 1% of the C/20 capacity, like
45mA for a 4.5A-h AGM battery. Currents around 1% are recommended end
points for trickle charging.
Diagram 4 gives 1-3% for flooded, Diagram 5 gives 0.5% for AGM.
As mentioned, the current rises in older batteries and is an indicator
of declining condition.
I first learned how to make accurate measurements as a chemist whose
results might have to stand up in court, then when building very
precise automatic test equipment for the semiconductor industry.
Analog Devices' op amps and voltage regulators were tested on machines
whose performance I was responsible for.
I do have "real" batteries that will run the fridge for about 20
hours. Once I'm satisfied with my discharge testing setup I'll get to
them. For now I'm testing and risking smaller, older, less valuable
jumpstarter and UPS AGMs. These tests are too long to watch and if the
low voltage disconnect fails the battery could be drained flat before
Previously I was using a rewired Battery Isolator I bought from
Quicksilver Radio for $5 at a hamfest to disconnect the load when the
voltage dropped. This describes the idea:
The rewiring changed it from switching the load to the second battery
when the main one's voltage dropped to switching one battery from the
load to a charger.
As a discharge controller it has the disadvantages of still drawing
current from the main battery to operate the relay after it has
discharged to the disconnect voltage, and needing an adjustable power
supply to set or check it.
I haven't seen that remaining for an hour or so at full discharge
would further harm a battery and want to record the voltage it
recovers to without any load as an indication of true remaining
capacity and a safety check that I haven't set the disconnect voltage
too low and drained the battery too far.
If not for its poor current resolution the 30A Drok unit would be a
fine discharge test controller when powered from an external 12V
supply that separates its operating current from the test circuit. The
circuit board was drilled but not properly connected for an SPDT
version of the SPST NC relay it comes with. I'll set the Battery
Isolator to a lower disconnect voltage as a backup on the load side of
Based on Amazon comments, it seems the 3-wire / 2-wire jumper may
select battery circuit or external power to operate the device.
A DC-AC inverter powering a safe resistive load like a crockpot can be
used as a discharge test load though you can't set the dropout voltage
and it may cycle back on when the battery recovers.
Those I've seen only engage the relay to switch to the secondary
battery. Are you talking about when the secondary battery is
discharged/cutoff and the relay continuing to be engaged? I see that
as a problem, too. Perhaps rig up a kickout relay to disengage when
the cutoff hits on the secondary?
Yeah, that's a fly in the ointment of capacity measurement. Are you
saying "full discharge to cutoff point" there?
There ya go!
It didn't look like that was fully populated in the pic I saw. IIRC,
it had only one pin/solder joint out of the 3.
I started out with nothing and
I still have most of it left!
This one remains powered by the main battery when it switches the load
to the second one, perhaps to avoid the glitch while the
break-before-make relay contact is moving. It drives the relay with an
SCR and won't release and revert to the main battery until the user
pushes a disconnect button, regardless of how high the main battery
may have recovered or been recharged. This means that connecting NO to
a charger won't make the relay turn off when the battery voltage
I cut and jumpered the traces to redefine COM as the battery instead
of the load, which is now NC. Originally COM was the load, NC the main
battery and NO the secondary one. As you said it would simply allow
the secondary battery to die, but retain whatever capacity the trip
point left in the main battery.
Maybe running the anchor light as long as possible is more important
than preserving a battery that sinks when the boat is hit?
Here's the problem:
"To get accurate readings, the battery needs to rest in the open
circuit state for at least four hours..."
The AGM I discharged at a little less than the 20 hour rate (0.5A)
tripped at 10.0V (twice) and then recovered to 12.15V, which is over
40% State-of-Charge on that chart.
The point of knowing the full capacity is to find out why I'm not
getting it, and see if anything I can do makes an improvement. I can't
fix bad interconnects but a discharge - charge - equalize cycle
reforms the active material. Only measurements will show how well
equalizing and desulfating work. I know I can make them last much
longer than usual, but is it worth the effort?
The two-tier DC load method is how the engineer had me test electric
vehicle Lithiums, using a programmable electronic load and a much
better DC current probe than I'll probably ever own personally.
I've downloaded both guides you've mentioned in the past several days
and will have to compare them. Each one will have tidbits of info the
other doesn't. That's the true benefit of research: gleaning tidbits.
That sounds like something I should pay attention to.
Cool! Nice legacy.
I forgot to ask what you meant by that. Are they drawing half an amp
or showing that discharge rate?
That wouldn't be fun. How often do the disconnect programs (or
--Robert Knight, senior fellow, American Civil Rights Union
I suspect it means that the self-discharge rate has increased and they
may need more frequent topping off. I don't know how it relates to
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.
Some of my homebrew test equipment runs on AC power for better
accuracy, some on DC from the battery being tested or the solar panels
so it will still show demand or charging rate during a power outage.
The load current varies too much for accurate measurement and battery
remaining life prediction, but all I really need to know is if the
batteries can run the fridge overnight.
The box whose components I tested this morning will have a switch for
internal or external power. The meters have to be on external power to
measure single 18650 Lithiums to their discharge endpoint which is
below the minimum supply voltage..
The two of these I bought match the Fluke 8800A to 1mV. YMMV.
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.
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...
If I had a solar system, I'd have an arduino or some such
monitoring it at all times.
I have a Palm Pilot monitoring the HVAC system.
Had the installer come fix it before it quit completely.
I gave up. Never got any predictive information.
Most of my testing was done with Lithium batteries in laptops.
I consider a laptop battery bad when it won't run the laptop
long enough. How vague is that? ;-)
Started with bad battery packs and tested cells.
At low current, I almost always got something like specified
capacity. The electrons are in there, but the laptop won't
let you have them.
I don't know what the sampling interval is, but the laptop
wants to shut down at some voltage so you don't lose data
and call the vendor. Subtract the peak voltage across the ESR
from the battery voltage. If it dips below the threshold,
the laptop wants to shut down.
Turn off the power features that warn of impending low voltage
The symptom is that the battery gauge decays slowly for a while
then drops instantly to a much lower number. The laptop senses
impending doom, but you've blocked that.
I've had laptops run two hours past the point when the battery
gauge hit zero. Problem is that when it dies, you lose whatever
you were working on.
I've never had any success trying to fix the ESR. That's probably
the same problem you have when your car fails to start. Never been
able to do anything about that either.
The higher the peak current, the fewer electrons all those
protection circuits will let you have.
Even with a new battery, capacity is a strong function of
Numbers from browsing the web won't help much when
Microsoft decides to do an update and runs all your cores
Well ... that is not a problem for me. The first thing I do
when I pick up a new (to me) laptop is to remove the virus. The last
virus was called "Windows 10". I replace it with either a linux or an
OpenBSD system, so Microsoft doesn't have a say in when I update, and
updates for the others are based on telling me that updates are
available, and letting me decide whether and when to install them. :-)
The "fuel gauge" IC counts Coulombs in and out to determine actual
Lithium battery capacity, on the assumption that they recharge at 100%
efficiency. It resets its capacity estimate if the battery is nearly
fully discharged and recharged, but if only partly discharged it can't
detect the slow loss of capacity with age and retains the old, overly
optimistic number from the last full cycle. That's why the sudden jump
when it realizes it's wrong.
When you give the battery another full cycle it can measure and update
the battery capacity to its new, lower value.
As an experiment I reduced the low voltage trip as far as possible and
got almost as much run time from an old Dell battery below the 5%
level as from 100% to 5%. It appears that Li-Ion cells can be
discharged down to or even below 2.7V briefly without much damage. The
normal settings are above 3.0V.
This gives you the battery voltage:
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