Modern car battery test gear
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Battery chargers
have been around for donkeys years (click to see my collection) |
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Back in the 1920s up to
the 1940s radio receivers used battery valves. Many pre-war examples
used valves with 2 volt filaments and those had an accumulator
to power them. In those days radio equipment was horrendously
expensive and very few listeners could afford to charge their
own accumulator so two were employed with one usually absent,
being charged at the local cycle shop.
Car batteries were charged by
a dynamo driven by the engine and later of course by the more
efficient alternator. If the battery was in good condition it
didn't need extra charging, but once it began to degrade it needed
to be removed from the car and charged, especially in cold weather.
Additional charging was also necessary if the vehicle wasn't
used regularly or if a light was left on. A battery charger was
designed to provide an intial high charging current which dropped
gradually as the battery voltage was restored. Once a decent
voltage had been reached the charger output would just "trickle
charge" the battery. Charging entailed the first step of
loosening a set of plastic screw caps so the gassing could commence.
This resulted in the escape of hydrogen so of course one had
to prevent any sparking and also keep a windows open or at least
ensure good ventillation.
Later, batteries would be marketed
as "sealed" or "maintenance free" and one
didn't have to mess with screw tops. Although this was a good
idea it did result in a reduced life because once the battery
started to degrade internal gassing might result in a lot of
internal pressure which resulted in the battery expanding and
shorting its lead plates. This is what happens in UPS equipments
which use lead acid batteries to take over if your mains supply
fails.
Basically proper battery charging
is a complicated affair. True its easy to just remove a car battery
and connect a 10 Amp charger for a few hours, but once the battery
gets close to being fully charged it's easy to forget the time
and damage it. So, with the advent of cars with more and more
complicated electronics their battery would be continuously drained
when just standng in ones drive. Wait a fortnight and you may
be unable to start the engine because all the battery power has
been drained by the alarm system and the keyless remote feature
not to mention electrical leaks from minor faults.
Things can get even more complicated
once one's car battery has degraded. For example one cell of
the six used in a 12-volt battery might fail. A charger designed
to behave correctly based on a proper terminal voltage seeing
say 11 volts may just bash on pumping lots of amps into a near
short-circuited cell. Years ago a garage would test a car battery
by firstly removing the screw caps then pressing a large steel
flat file across the terminals. This would be accompanied by
lots of sparks and smoke, and if a cell was bad, a lot of boiling,
fizzing and release of gas from the hole in the top of the battery
corresponding to the bad cell.
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Once very low cost microprocessors
entered the market people came up with all sorts of clever ideas
to make money. I'm going to describe two here. One is a car battery
tester and secondly a car battery charger. In both cases the
designers needed to solve two separate puzzles. Both are solvable
once full knowledge of the way a battery is designed to be used
is understood. From this understanding a program can be written
which controls the way the charging circuit is operated. In parallel
with the production of the program code an electronic design
engineer must devise a circuit capable of handling the task.
Of course it's not that easy because there are lots of different
battery designs, lots of different power ratings and many degrees
of battery condition. Add to that the actions of users who may
be totally incompetent when it comes to using technology and
basic user mistakes such as connecting red and black leads the
wrong way round. As a buyer of test gear how can we be sure of
getting a reliable and properly performing purchase? I suppose
one could read the manufacturer's claims and see if there's any
customer feedback. If possible I suggest you find the operating
instructions and read these before purchase, |
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My first example
of battery test gear is the Ancel BA301
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Over the last few years
I've had problems with car batteries. I replaced two without
properly testing them and tried to charge several others with
limited success. I have a small 6 volt lead acid battery that
was absolutely dead that I managed to charge over a long period
using quite a high voltage to get any current to be drawn. It
ended up with about 6-volts across its terminals but is it any
good? I also have a larger one for stabilizing a 6 volt DC supply
to my T1154 transmitter. I don't have any requirement for 24
volt batteries so I chose a tester for 6 and 12 volts. I can
hopefully test both my 6 volt batteries plus a selection of 12
volt UPS batteries as well as ordinary 12 volt car batteries.
Figuring out precisely what a typical tester can do isn't easy,
but I believe I worked out in advance that this Ancel tester
would do exactly what I wanted... and it does. My "dead"
6 volt battery shows up as well nigh perfect after a really long
charge starting at something like 20 volts at a relatively low
current from a bench power supply, one of my high power start-stop
car batteries is also excellent and with a second in poor shape.
My campervan leisure battery from 2005 is pretty useless without
mains charging in progress. |
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This is typical of an
old-style battery charger. It uses a very heavy mains transformer
and brute force charging. |
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Compare this with a modern
battery charger.
This one weighs only a few ounces
and uses a switch-mode power supply although its compact design
does rely on a pretty noisy fan to keep it cool. This example
is being used to resurrect a very large car battery originally
designated as having an "SOH" of only 10%. After several
hours of pulse charging "Repair" this figure has risen
to 14%. Whether the battery will ever get back to a usable condition
is as yet unknown.
I wonder who makes this charger?
It was sold by ISEA Store in China and it's made by Shenzhen
Haitan Technology Co. Ltd or HTRC and goes by their designation
"P10".
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On the left, I've shown a typical
car battery label which indicates details of its design and rating.
The "EN" or "European Norm" is a more stringent
figure than the alternative "Cold Cranking Amps or CCA"
found on cheaper batteries. In fact 760A is much the same as
CCA 800A. The spec is supposed to provide the maximum drop in
terminal voltage after cranking for 30 seconds. This will equate
to the internal resistance of the battery at a specific temperature.
This label says the battery
was made in week 33 of 2008
Below is a copy of the charger
manufacturer's specification.
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When the Ancel tester
(shown above) measures the battery
it indicates its voltage and internal resistance as well as its
state of charge, terminal voltage and potential cranking amps.
To accurately measure a battery, clearly the tester needs to
know about its own design in respect of the resistance of its
internal wiring and leads as well as the quality of its connection
to the battery under test. A lead-acid battery relies on a couple
of main things in order to keep well. These are the electrolyte
state, whose specific gravity depends on the amount of water
it contains, and the condition of its plates. Obviously if the
volume of the electrolyte decreases, say through gassing, the
lead plates may not be completely covered which reduces the battery
capacity. Also, the condition of the plates is important. One
likely problem is one or more of the battery cells might short
and reduce the terminal voltage from say 13 to 11 volts. Before
these new fangled testers arrived on the scene a garage mechanic
would hold a large file across a suspect battery terminals. A
shorted plate would show itself by tremendous gassing at the
bad cell.
Older batteries always had screw
caps which allowed topping up and these needed to be removed
before that test with the file.
Another plate problem, particularly
if the battery has been left uncharged for some time is the build-up
of undesired lead salts. The repair feature of a battery charger
is to get rid of these lead salts, or at least render them ineffective.
The process is something similar to reviving an old NiCad cell.
The modern battery below doesn't
have those individual screw caps too visible, but you can see
under that plastic covering give-away indications. All that's
required is to cut around the indentations and using a large
wood chisel as a screwdriver remove the caps. In my case there
were several cells showing up as low in electrolyte because the
top of the plates were clearly visible. I used over a pint of
filtered boiled water to top all the cells up. Immediately the
internal resistance dropped from 26.7 to 24.9 milliohms and the
terminal voltage dropped from 13.97 to 13.31 volts so back went
the battery into repair mode. The voltage change must be due
to the dilution of the electrolyte?
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Below is a table showing
the steady improvement of this old (2008) battery as measured
by the Ancel tester. After reaching a plateau in the battery
figures I decided to add boiled filtered water and a big improvement
resulted although it still recommends the battery is changed. |
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Test |
Resistance
Milliohms
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Voltage |
Amps |
State of
Charge
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State of
Health
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Comments |
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1 |
33.67 |
12.83 |
82 |
100% |
10% |
Initial condition |
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2 |
30.81 |
13.67 |
90 |
100% |
11% |
After charging for an hour |
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3 |
29.32 |
13.76 |
95 |
100% |
12% |
Repair mode |
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4 |
26.73 |
13.94 |
104 |
100% |
13% |
Further repair mode |
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5 |
26.13 |
13.97 |
109 |
100% |
13% |
After 12 hours or so |
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6 |
24.90 |
13.31 |
111 |
100% |
14% |
After topping up with water |
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7 |
13.46 |
13.28 |
206 |
100% |
27% |
After several hours of repair |
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in progress... pending |
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