During bike trips， Ed uses Icom handie-talkies for communication.
To keep the radios‘ batteries working， he uses a three-pack charger
that he designed at his workbench. The charger has three 250-mA
channels and one 100-mA channel so he can charge different packs.
An early version of the GPS radio interface described in my October
column accompanied us on our summer bicycling vacation （"HT Audio，
GPS for APRS， and What Works，" Circuit Cellar 219， 2008）。 I hadn‘t
finished the replacement battery-pack enclosure，so the circuitry
traveled between foam-rubber slabs strapped to the back of the
radio， with everything zipped into a pannier on our daughter's
bike. Somewhat surprisingly， the lash-up worked fine， although
there‘s a bit of RFI that a full metal jacket should cure.
The radios can use any external voltage from 4.5 to 16 V， but
require only 9.6 V to transmit at their rated 5 W. Icom provided
4.8- and 9.6-V NiCd battery packs with ratings in the 650- to
800-mA·h range； the lower-voltage pack produced about 1 W of RF.
Despite an absurd level of pampering，all my original Icom packs
have long since failed， so our radios have sprouted pigtails
terminated with Anderson Power Products Powerpole connectors for
external battery packs.
I made six- and eight-cell NiMH packs from ordinary AA cell holders
with Powerpole-terminated pigtails. NiMH cells have improved over
the years：the current crop sports a 2，600-mA·h rating. The radios
draw about 40 mA while idle and I find that the packs last about a
week in ordinary use. The GPS interface adds another 70 mA， so its
battery packs last a few days.
But on our bicycling vacations we use the radios all day， nearly
every day， and that presents a problem： I needed a three-pack
charger with a dead-simple interface. Given the vagaries of travel，
anything requiring fussy handling just wasn‘t going to be useful.
In this column， I'll describe the trivially simple charger I lashed
together， then explore the history of my battery packs. It turns
out that batteries don‘t always behave the way you expect， even
when you expect odd behavior.
CHARGING COMPLEXITYAs you saw in my April 2007 column，safely charging NiMH cells
presents some challenges and， if your application requires
high-speed charging，you must deploy rather complex circuitry to
eliminate high-temperature failure modes （"Battery
Capacity：Charge，" Circuit Cellar 201）。 Some straightforward
arithmetic reveals the magnitude of the problem.
Most "fast chargers" require an hour or so to bring AA cells up to
full charge， but my Energizer CH15MNCP4 charger does the job in 15
minutes for the quartet of 2.2-A·h NiMH AA cells included in its
package. The array of components on the circuit board shown in
Photo 1 attest to the fact that this is not a dumb charger； a
second board holds a highcurrent DC/DC power converter.
Because NiMH cells have a charging efficiency of about 2/3，
injecting a full charge requires 150% of the nominal capacity.
Charging a cell in 15 minutes means you must deliver：
The data plate embossed on the back of the charger states that it
delivers 7.5 A to each AA cell； it can also charge AA cells at 3.0
A. Working backwards， that gives a charging time of：
If you ignore the charging efficiency，that‘s 17.6 minutes. If you
also figure that most cells aren't completely discharged，the usual
recharge time should be under 15 minutes.
Just for fun， I discharged one of the 2.2 cells at 2.2A·h down to
0.5 V，well below the usual 0.9-V limit， and then recharged it. The
discharge required 54 minutes and delivered 2.0 A·h，which is quite
good performance at that current. The recharge took 17 minutes
until the green “charged”indicator blinked on and 29 minutes until
the fan stopped.
The instructions tell you to wait 10 minutes after the green
indicator comes on for a full charge， so the actual times are
pretty close to my estimates. They are， however， somewhat longer
than the advertised "15 minutes，" so perhaps it‘s no surprise that
the specs for the same charger now indicate that it ships with four
1.85-A·h AA cells. That lower rating means they recharge in exactly
15 minutes at their nominal capacity and 22 minutes if you allow
for charging efficiency.
However， that data applies to a single cell： charging four AA cells
simultaneously requires four times the power. The charger‘s data
plate claims an output of 5.6 V at 7.5 A，which is 42 W， and the
unit's wall wart delivers 40 W （10 V at 4 A）。 So，in round numbers，
the charger dumps 10 W into each AA cell.
Remember that NiMH cells mark their end-of-charge state by heating
up， to the extent that you can actually build a good charger by
noting the rapid temperature rise. Missing that signal， however，
will allow a rapid charger to overheat the cells or，worse， burst
them. That Energizer charger has a fan for a good reason： the cells
get rather warm！
The eight-cell packs for our radios use 2.6-A·h cells， so a true
“15-minute”charger would require：
The Energizer charger evidently uses power FET switches to connect
the cells in series to its power supply and switch them out as
they‘re charged. I'm certain the charger adjusts its nominal 5.6-V
supply to match the number of cells in series.
In round numbers， therefore， an eight-cell charger would require a
12-V source that could deliver nearly 16 A：nearly 200 W. Charging
three packs at once becomes a 600-W proposition，which seems
aggressive even to a guy who built a kilowatt resistance soldering
unit.
A 1-h charger draws a quarter of that power and takes four times
longer， but that‘s still a 150-W supply for three packs. I decided
that a slow three-pack charger made a whole lot more sense： it can
work while I sleep without fuss， bother， fans， or risk of damage.
DEAD-SIMPLE CHARGINGThe dumbest of dumb NiMH chargers consists of a current source set
at 10% of the battery's nominal capacity：260 mA for a 2，600-mA·h
battery. After 15 h， more or less， that current will bring a
completely dead cell back to life. While further charging isn‘t
recommended， contemporary NiMH AA cells include sufficient catalyst
to recombine all the oxygen liberated by low-rate overcharging
without venting. The cells become warm， but will not deteriorate
due to overheating.
Photo 2 shows what I came up with in the week before we left： three
250-mA and one 100-mA current sources in a box， with Powerpole
connectors on short pigtail leads. The thin black wire supplies
14-V power from a surplus laptop charger.
A slightly less dumb charger would incorporate a 15-h （more or
less） timer to cut off the current after the maximum allowed
charging time. An even brighter charger could monitor the pack
temperature to detect the reliable end-of-charge heating. A truly
smart charger would monitor the charging voltage to detect the
end-of-charge plateau. All of those frills aren't really necessary
if your plans include daily charging and constant moving.
Figure 1 shows the trivial schematic for a single charger channel.
The power source， common to all the channels， is a surplus laptop
charger that delivers a regulated 14 V at up to 3.5 A. The 2-A fuse
will blow at about twice the maximum total charging current.
A venerable LM317 three-terminal voltage regulator acts as the
current source. The regulator adjusts its output voltage to
maintain 1.25 V between its Out and Adj terminals， so the parallel
combination of R2A and R2B determines the output current：
I picked 250 mA because it requires a 5-Ω resistor that‘s merely a
pair of 10-Ω resistors. The 100-mA channel，chosen to charge the
slip-on batteries I rebuilt in the original Icom packs，calls for 22
Ω in parallel with 30 Ω。
The main disadvantage of this circuit comes from its profligate
power use： each LM317 regulator must dissipate the product of its
load current and the voltage difference across it. The lowest
voltage across a six-cell battery can be under 6 V at the start of
charging， R2 always drops exactly 1.25 V，and D1 accounts for about
0.5 V： call the total 8 V. The regulator will thus see about 6 V，
making its power dissipation：
The full-charge voltage for a six-cell pack is about 9 V， so the
regulator dissipation eventually drops to：
I selected a 14-V laptop power supply from my parts heap to ensure
the regulator had （just barely） enough voltage headroom for a fully
charged eight-cell pack at 12 V.
A rackmount server power supply in my heap disgorged the four
TO-220 heatsinks and mounting hardware appearing in Photo 2. They
were designed for use in a forced-air environment：the LM317s run
much hotter than I prefer， despite the rows of vent slots in the
aluminum box I used. A thermocouple poked into a blob of heatsink
grease on one of the regulators showed that it hits 80°C at the
beginning of the charge cycle.
LM317 regulators shut down when their junction temperature reaches
125°C. The TO-220 package has a 4°C/W junction-to-case thermal
resistance，so the internal temperature at 1.5 W falls well under
that limit. It‘s too hot for long-term reliability， but at least
the regulator won't incinerate itself.
The charging process requires essentially no thought at all. At the
end of the day‘s ride， I unplug three battery packs from our bikes
and plug them into the charger's three 250-mA connectors. Because
even a full day‘s use doesn't completely discharge the pack，it‘s
generally ready for use in much less than 15 h. Simply touching
each pack tells me when to unplug it as it reaches full charge， but
no harm ensues if I do nothing until the next morning.
A friend points out that building a battery charger with a
touch-dependent charge termination requires a certain，um，
personality. All I know is： it works for me！
PACK PERFORMANCE
I‘ve been using AA NiMH battery packs for about three years now
and，having both method and motivation for tracking their
performance， can relate some interesting real-world data.
Figure 2 shows the results for the oldest cells in my collection：
2.0-A·h cells made by Lenmar Enterprises. As nearly as I can tell，
it‘s reasonable to assume that all non-tier-1 NiMH cells come from
anonymous Chinese factories，which makes the brand name largely
irrelevant.
In March 2006， one of those cells provided just over 1.5 A·h at 500
mA，its C/4 rate， as shown by the black trace （use the right-side Y
scale）。 The stairstep effect reveals the 10-mV voltage resolution
of my West Mountain Radio CBA II battery tester.
A cell‘s capacity strongly depends on the discharge current， with
lower currents extracting more energy from the cell. Different
manufacturers may use C/10 and C/20 rates， but typical uses require
far more current than that； after all， NiMH cell advertising touts
their "high drain" abilities.
The remaining traces show the capacity of a six-pack made from
those Lenmar cells， but tested in recent months. These traces use
the left-side Y scale， where the CBA II's resolution isn‘t nearly
so obvious.
The blue trace corresponds to the first discharge of the pack after
I pulled it from my battery FIFO. I rotate the packs so that that
they're neither freshly charged nor totally dead. NiMH cells
self-discharge up to about 1% per day， so I estimate this pack has
been on the shelf about three weeks.
The red trace shows roughly the same capacity for a 1-A discharge
as it does at 500 mA， which is a good sign. However， the other
traces （made at 500 mA） show that the cells have lost 10% of their
capacity during the last two or three years.
Figure 3 shows the results of a yearold six-pack made from Tenergy
2.6 A·h at 500 mA. I ran the test that produced the black trace
shortly after removing the pack from the bike； you can see that one
cell was almost totally discharged， even though the remainder of
the pack still had nearly 20% of the rated capacity. By
definition，the weakest cell goes flat first.
The blue trace shows the other five cells can provide about 1.3 A·h
after recharging. The final discharge slope has a step at 5.0 V，
revealing the nextweakest cell in the pack. However，they are
reasonably well matched.
Recharging the weakest cell， which I labeled Q， produced the red
trace（use the right-hand Y scale）。 Obviously，it was suffering from
under-charging，the typical cell failure mode，rather than reaching
its end-of-life point.
Putting the pack back together again and charging it for a good
long while produces the purple trace. The pack now has about 1.3
A·h of capacity and the final dropoff shows that all the cells go
down together.
Adding the GPS interface to our radios increases the current drain，
so I bought more AA cells to make up the eight-cell， 9.6-V packs
required for 5-W transmitter power and the TinyTrak3+ voltage
regulator. Surprisingly enough， the new cells sport the same
Tenergy brand and the same 2.6-A·h rating as before. I have no way
to tell if they also have the same internal construction.
The traces in Figure 4 document the first six charge-discharge
cycles applied to one cell. Common knowledge has it that NiMH
capacity increases during the first few cycles，but that‘s certainly
not obvious from the data.
The first discharge， shown by the black trace， had the lowest
capacity，but the other five traces don't show any particular
progression. In fact，under these conditions， the cell delivers only
about 70% of its rated capacity. While that‘s better than the
yearold Tenergy cells， it's not really up to spec.
Figure 5 shows that this need not be the case. Earlier this year I
made up an eight-cell battery from Duracell 2.65-A·h cells; the
additional 0.05 A·h has no significance outside marketing staff
meetings. Although I cannot vouch for how long this pack sat in my
rotation， the initial discharge and subsequent cycle deliver nearly
the entire rated capacity. Notice that this is at 1.0 A， rather
than the 500 mA used in the other tests.
The bottom line， I think， is that there‘s a difference between
Tier-1 batteries and the rest， but that there's an even larger
difference due to the care and feeding the cells receive during
their lifetime.
CONTACT RELEASEWhen we‘re not on vacation， we spend about an hour a day riding on
errands， to stores， and making other trips around the neighborhood.
I change the battery packs on Friday afternoons so that we're all
charged up for our longer weekend rides， and carry a spare pack.
Under those conditions，all of the packs continue to provide good
service， but I plan a major weak-cell purge when I get all three
GPS units running.
Even such a simple project requires analog design， power
management，and EMI control. When you‘re handed a new project，
remember that it's never as simple as it seems and you can‘t work
on any of the pieces in isolation！
Ed Nisley is an EE and author in Poughkeepsie，NY. Contact him at
ed.nisley@ ieee.org with "Circuit Cellar" in the subject to avoid
spam filters.
PROJECT FILESTo download code, go to
ftp://ftp.circuitcellar.com/pub/Circuit_Cellar/2008/221.
RESOURCES
E. Nisley, "Battery Capacity" Circuit Cellar 199 & 201, 2007.
---, "Battery Power, Feeding the Z3801A," Circuit Cellar 155, 2003.
PowerStream Technologies, "NiMH Battery Charging Basics,"
www.powerstream.com/NiMH.htm.
SOURCESPowerpole connectors
Anderson Power Products
www.andersonpower.com/products/singlepole-connectors.htmlTinyTrak3+ Tracker and GPS receivers
Byonics
www.byonics.com
Energizer CH15MNCP4 15-minute charger with four NiMH AA batteries
Eveready Battery, Inc.
www.energizerbatteries.com/productdetail.asp?device=DIGCAM&prodcode=155052.0-A•h Cells
Lenmar Enterprises, Inc.
www.lenmar.com(Distributor: www.batteries.com)
2.6-A•h Cells
Tenergy Corp.
www.all-battery.com
Computerized battery analyzer
West Mountain Radio
http://westmountainradio.com
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