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,
(Amazon.com product link shortened)
and previously this which is 10x as accurate at low current
(Amazon.com product link shortened)
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.
The HF carbon pile performs the test as they specify, with a 15 sec
timer to warn you to turn the current down before it overheats. So far
it seems to be a good and relatively inexpensive tool to measure the
margin your battery has over the starting current your vehicle
Here is a good, simple overview of battery charging:
I use simple dumb chargers for the high current bulk phase and
smaller, more efficient and better controlled ones for the final ones.
It costs nothing to leave a battery on a solar powered charger. The
big chargers draw idle power, transformer magnetizing current, that
may be more than goes into the battery. I've added an input tap after
the rectifier to let a charger operate on solar if available or grid
power if not.
"The charging parameters
discussed here are applicable to ?ooded lead acid batteries. Be aware
some available smart chargers may
not be suitable for other applications."
However the makers post their products' parameters, which aren't hard
to meet with a voltage-adjustable power supply with meters. Some of my
AGMs have the charging conditions printed on the cases.
And you thought the tripping was from the internal resistances, didn't
you? But the deeper the DOD, the shorter the battery life. (Loved
the Hot Tip on the Fridge and Solar site. Batteries love to be
charged but don't much like being discharged, etc.) Speaking of
which, what's the difference (other that price) between the standard
Ford style starter relay @ $12.99 delivered and the $80 Enerdrive VSR
super-duper battery disconnect switch? As I look at it again, I see
that it has voltage-sensitive engagement. ($0.37 worth of old 7400
IIRC, I recently read that EQ can be good, but frequent EQ shortens
Good question. Perhaps with a more expensive battery, it would be, or
in a top-down situ where the grid is and stays out. I rather doubt it
with UPS batteries otherwise, though. Q: are the internal plates and
connections in the larger glass mat batteries the same as the smaller
AGM UPS batteries? I do know that the larger, PV-rated LA batteries
are more up to the task, and they're really heavy (massive lead
plates) and pricy.
It seems like this would have been done and written about by many a
battery manufacturer by now, or by their ad people. "Our batteries
and chargers are better because..." But I grok the "need to know"
function, too. I believe I'll be getting a lot of experience and
experimentation in the next decade, too, playing with solar.
I started out with nothing and
I still have most of it left!
Thus taking the battery down to 10V cutoff at the 20 hour rate wasn't
a full discharge, so I couldn't answer. There was still capacity left
that was unavailable for some reason, perhaps one higher resistance
cell that I might be able to bring back by slow equalizing. I've had
some luck restoring a weak cell in a flooded battery and had popped
open that 12V 12AH AGM to add water, but it didn't appear to help
I had bought some LM324s and a relay to build one before I found the
Battery Isolator for $5. It's a hand-drawn circuit board in a Radio
Shack grey aluminum box, like the stuff I built as a kid, though it
seems to work well enough.
Since I don't cycle my batteries daily I can afford to experiment with
slow charging from the solar panels at a few percent of the Amp-Hour
rating current. Rather than adding a current limiter which would cut
into the already minimal voltage overhead of solar panels, I've been
charging with simple, rugged LM317 and LM350 regulators with meters
and bumping the voltage up a little when I walk by and notice the
current has dropped. Before long the battery charges high enough that
an AGM draws only C/100 current at 15V and a flooded battery at around
14.0V, though they all are different. The current lost to electrolysis
seems to decrease, as shown by the battery drawing little more current
above 14V than at 13.6V.
I built a homebrew power supply whose current limiter adjusts from
<1mA to 0.4A. I use it to restore old electrolytic capacitors at about
5mA and to desulfate free batteries that need to be hit with over 16V
to accept any current. For them the current needs a limiter to avoid
pegging the ammeter as they recover. I've been using one such free
"dead" battery in my tractor for two years.
I'm equalizing as gently as appears effective. The goal is to
determine if a variable voltage source with high resolution volt and
amp meters is enough to prolong and restore batteries IF operated
properly, which is the tricky part; the hardware is cheap except for
the Variacs that I already had.
It may be the sort of gizmo that only the inventor can make work, too
troublesome to be commercially valuable. I've come up with several
ideas that work fine for me but no one else.
AFAIK flooded batteries can be nursed to live longer than
maintenance-proof ones, so I lack the experience to answer that. I
don't own batteries larger than I can carry down the stairs and
outdoors to let them gas freely when I equalize them.
This isn't New Orleans; after a natural disaster the local governments
quickly clean up and repair and only expect FEMA to arrive afterwards
and write checks to cover the cost. My father was the CFO of one of
the state's departments that participated.
The dump trucks and loaders the towns need to clear snow can repair
flood washouts and push fallen trees off the roads, really everything
except paving and building bridges. My one-week storm preparations
could stretch to two weeks but I don't think any longer is likely with
the high level of response I've seen here.
Neon John posted a good reference to actual experience maintaining
backup batteries. I haven't found much else that gives hard technical
details instead of wishful copywriter promises. I did some work once
on 48V telco battery banks, otherwise my industrial experience is
mainly with Lithiums which are still overly expensive.
There are some nifty hall-effect sensors with almost zero drop.
Used one in a solar/wind system with a shunt regulator.
Wind generators don't like being unloaded by a series regulator.
I'd like to hear more about your desulfation successes.
High voltage didn't help. Other crazy ideas I'd read about, like
AC at various frequencies to 'ring' the plates
and shake off sulfation, etc. Got absolutely nowhere.
How stable is the zero reading? This is the best DC Hall effect
ammeter I've seen and its zero drifts by several mA per minute if held
still. When moved the Earth's magnetic field throws it way off.
I've fixed several "dead" U1R lawn tractor batteries and used them for
several years. The symptom was not accepting more than a few milliAmps
from a commercial charger, although the resting voltage seemed OK. The
fix was using the variable power supply to force 16V to 17V which
caused the current to very slowly increase and then the required
charging voltage to drop, an unstable condition that requires current
limiting, such as with a low end lab-type supply like these if you
can't rig up your own.
(Amazon.com product link shortened)
After the battery had accepted enough charge to raise the electrolyte
level I checked specific gravity and found one or two low cells.
Charging at a current that didn't make the others bubble excessively,
around 0.5A, eventually brought up the low cells. The resurrected U1R
in the tractor now can put 150A into my HF carbon pile load tester.
I've read that salvaging batteries this way can take up to a week. I
saw progress with salvageable flooded batteries in a few hours but
haven't had much luck with AGMs.
The neighbors who give me these batteries know I can fix them, and
that they will need frequent attention afterwards. I have to top up
the charge at least monthly or their internal resistance will rise
I don't know if the cause is literally lead sulfate recrystallization
or not but it's a handy suspect to blame. There's a theory that it's a
thin oxide barrier between the grid and the active material.
Whatever the cause, the effect is a very high internal resistance.
Automatic chargers see the voltage rise as though the battery was
fully charged and shut off. Some people have reported success using a
Harbor Freight manual charger on a Variac as the variable supply.
You added water to an AGM? How did you do that? And how had the AGM
I've seen soldered IC, resistor, capacitor forever encased in clear
RTV which, while not purty, was fully functional.
OK, I think I now have the full picture.
So "dead" batteries go into meltdown mode once they do finally start
accepting a charge? I see why most people have no success at it.
Even with the "save your battery" goo scam.
I wish I'd grabbed the cheap Variac which was offered to me long ago.
For most people, plug-n-play is the only way. For Makers like those
of us on RCM, fiddlin' is the only way.
Interesting. Remember seeing the difference between OK after a
tornado and elitists in NY after a hurricane? Everyone came out to
help in OK, while elitists sat surrounded by mess and complained to
the Press about their blocks, all while not one single person came out
to work to clean it up. Absolute night & day differences, wot?
NO: Save the city in a hole! Don't fill in the ground and bring it
above levee level so it never happens again, just put in bigger pumps.
Crom, those folks are smart...not. I wonder what genetic strains will
come from those folks now living where thousands of fracking ponds
(full of 600 different hazardous chemicals) were strewn by Katrina.
I tend to not buy anything from LA nowadays.
Were I there, I'd have months worth of supplies for everything.
New Atlas had an article recently on doubling the density of lithiums.
I hope Tesla Gigafactory takes advantage of it. That should drop the
I started out with nothing and
I still have most of it left!
I pried off the cover plate over the rubber caps, which pull off
easily. I don't know why some of the cells were dry. It's a
replacement from Batteries Plus I bought in 2009 to revive a dead UPS.
When new it delivered 82 Watt-hours; 12V times 12 Amp-hours is 144
Watt-hours, though only at or below the C/20 rate.
I measured computer run times on it a few times and plugged it in
occasionally to recharge. It wasn't on continuously because I had
changed from desktops to laptops that consume about 1/4 as much power
and serve as their own UPS, and after I found the free APC1400 and
rigged two external 105A-h batteries to it the small UPSs weren't
needed. In 2015 I noticed that its run time was very short and began
testing suggestions on how to salvage the battery.
I suspect that pulse desulfators are a simple way to apply a high
enough voltage to break through sulfation while reliably limiting
current with capacitive and inductive impedance instead of active
control. Personally I prefer DC because an ammeter that reads
milliAmps shows how much the battery is improving, or not. My own
results and advice I've read suggest that a battery which needs over
17-18V applied before it accepts current can't be saved.
Dumb transformer + rectifier battery chargers have a moderate output
impedance that makes the current decrease as the voltage rises and
conversely increase as it drops, but not as sharply as a regulated
The charger I was using then was an old manual 6 Amp Schauer with a
small 3 Amp Powerstat added to adjust for the current I wanted. The
current didn't change all that much as the voltage rose or fell, so
the battery didn't actually run away.
A tightly regulated power supply could possibly let a battery run away
unless its current was limited by circuitry, its transformer or the
solar panel source such as the HF kit which is a good match to slow
The power supply I built from a Variac and an arc welding transformer
has a relatively high output impedance to give the arc its constant
current characteristic, enough that the rectifier/cap output ripple at
20A is roughly a 1V sine wave. It puts out over 50V no load and drops
rapidly to around 35V as the current increases, with 120V AC in. The
output voltage then holds steadier up to around 25A out, its
experimentally determined 100% duty cycle limit. It will briefly
exceed 50A which is useful for testing components.
As a 20A 24V battery charger it holds its output current quite steady
during the bulk charge phase but then becomes dangerous because it
won't automatically decrease the current enough when the battery
voltage rises to the gassing level. The simple fix is to pass the
output through a P20L or similar cheap solar controller set to reduce
the current when the voltage per battery reaches 13.6V. Light fizzing
that varies a little between cells becomes visible around 13.8V.
My two series-connected batteries, of different ages and sometimes
topped off to different levels, automatically self-balance to 13.6V
each on the APC1400's float charging current. Each has its own
voltmeter to watch that, also if one discharges faster.
Cheap Variacs may need new brushes, which are far from cheap. I bought
a brass bar and some larger carbon brushes and machined my own
replacements because the right ones were out of stock. The original
brush geometry was too tricky to mill and assemble so I made the
brushes thicker and thus less fragile and will have to trim the end
contact width as they wear.
They use silver epoxy to glue in replacement carbons instead of
pressing tightly fitted ones in like the originals, and after pressing
mine in I understand why. I had better luck tediously cutting them to
size with a razor saw and files than milling them because they are so
I think two weeks is probably enough in an area where people and local
governments have the tools to recover, though not in cities where
there's no place to run a generator. Germany and Japan kept their
societies functioning pretty normally until the very end.
I assume my biggest need will be roof repair to prevent further
damage. When a fallen tree top punctured my roof in over a dozen
places I quickly covered the holes with sheet metal shoved under the
shingles at the upper end. I lost the shiny finish on my 6061 aluminum
but saved the house from water. Plywood covered with a large tarp
would also have worked, unless everyone else had the same idea and the
stores were empty. I happen to have the machines to work sheetmetal
and the need to make electronic enclosures.
When Jay Leno asked New Yorker actress Lea Michelle how she handled TS
Sandy she told him she didn't even own a flashlight. In my Mensa
experience that's fairly typical of the complete dependency they
accept as natural, can't change and don't think about. Let the good
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