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Fire up your cordless drill
WITH A
LITHIUM BATTERY PACK
Do you have a perfectly good cordless drill or other tool with a
dead battery pack? Repacking it with sub-C Nicad cells used to be
the standard approach to recycling but now you can do much
better – use a lithium polymer battery pack. You’ll get more battery
capacity for less weight and you can discharge them more deeply.
by LEO SIMPSON
B
ack in December 2006 we featured an article on how to bring
dead battery packs for cordless
power tools back to life by re-packing
them with sub-C Nicad cells. Fast forward seven years and the problem still
exists, although the cost of sub-C cells
12 Silicon Chip
makes the proposition not as attractive
as it was. And if you don’t upgrade
the charging circuit with our Cordless
Power Tool Charge Controller, you can
look forward to the same battery pack
failure in a few years time.
But now there is a better way – up-
grade to a lithium polymer battery
pack. With the rising popularity of
radio-controlled helicopters, Li-Po
battery packs have become a relatively
cheap and very potent power source.
They are far more energy dense than
anything based on Nicad or nickel
siliconchip.com.au
metal hydride cells and don’t suffer
from “memory effect” which is a (bad!)
feature of Nicad-based battery packs.
This trend has been reflected in the
rising sales of Li-Po cordless drills.
Almost all 18V, they are lighter and
more powerful than the superseded
Nicad-powered drills but there is a
big drawback – they are expensive.
And what do you do about a perfectly
good old drill with a dead or dying
battery pack?
This problem confronted me recently when I went to use a 24V cordless
drill which I purchased some years
ago from Dick Smith Electronics. It’s
a powerful but heavy beast which can
also be used as a hammer drill and
indeed I have used it many times with
masonry bits.
Sadly, its battery pack has been
gradually losing capacity, to the point
where it would only drill a few holes
in timber after a recharge, whereupon
it would just wimp out.
Clearly, a replacement battery pack
for this drill is highly unlikely to be
available now and even if it were, it
is likely to cost more than the original
price of the drill plus its battery pack,
carrying case etc. Yes, you can get battery packs re-packed but again, the cost
would be very unattractive!
However, the drill is quite ruggedly
built and has a gearbox with metal
gears rather than plastic so I was loath
to discard it.
OK. So I decided to follow the lead
The original dead (and as-yet unmodified)
24V Nicad battery pack from my cordless
drill with the more-powerful 22.2V
Li-Po replacement alongside, showing
the relative sizes. The Li-Po is an
easy fit inside
the old battery
case.
given in the June 2013 issue of SILICON
CHIP in an article on Lithium batteries. In fact, that article was devoted
to LiFePO4 (lithium iron phosphate)
rechargeable batteries but this article
is devoted to the more potent lithium
polymer batteries used by aero-modellers which are available in a wide
range of capacities and in 11.1V and
22.2V packs.
In my case, I wanted to replace the
original 24V 1.2 amp-hour Nicad battery pack with an equivalent or better
Undoing the (in this case four) screws allows the top of the
battery to be lifted off, revealing the 20 Nicad cells packed
inside (20 x 1.2V = 24V). Note the two cells on top which
hold the battery connector – this part is important!
siliconchip.com.au
Li-Po battery.
To cut to the chase, the one I chose
is a Zippy 35C series 6-cell 22.2V pack
with a capacity of 2450mAh, almost
double the energy rating of the original
battery pack. As an aside, to calculate
the energy rating, just multiply the battery’s nominal voltage by its amp-hour
rating. Multiplying 22V by 2450mAh
gives an answer of 54.39 watt-hours.
Even better, the nominated Li-Po
battery pack weighs 390 grams which
is significantly less than the original
The original cells are now effectively garbage (but note
the symbol telling you not to dispose of them in the trash).
However, you do need to carefully remove the battery
connector from the top two cells, noting + and – connections.
October 2013 13
DRILL CONNECTOR
BATTERY LIFESAVER PCB
–
Fig.1: using the Battery
Lifesaver makes connection
pretty straightforward.
Make sure you use
heavy duty cable to
cope with the high
charging currents
and even higher
discharge currents
B+
involved.
11108131
10V
+
B–
L–
+
CHARGING
CONNECTOR
Reproduced from our September
2013 issue, this is the tiny “Battery
Lifesaver”, a very worthwhile addon for any battery powered tool. It
prevents the battery being permanently
damaged by over-discharging.
and its discharge rating is very high:
35C constant; 45C burst.
This means that it is rated to deliver
a current of 85 amps with a peak of
110 amps!
Now it is highly questionable
whether the output leads of the battery
would survive more than a few seconds
at such huge currents but does indicate
that it would be more than adequate to
cope with the discharge current of a
24V cordless drill, even if you stalled it.
So why did I choose that particular
model? The main reason is that it will
fit into the battery holder of the drill
– pretty important. A smaller battery
could have been fitted but that would
mean less battery capacity.
Now a search will quickly reveal that
there are numerous sources of these
batteries via the internet but many of
these are a questionable proposition.
There are plenty of reports where
people have purchased batteries online (did someone mention China?)
only to find that their stated capacity
is mythical, to say the least.
You also need a suitable charger
and this is where Li-Po batteries are
far more demanding than Nicads, although most drills with Nicads would
give far better life from their battery
packs if they had better designed
chargers.
There are plenty of chargers for LiPo batteries but most of these are not
intended for use with 230VAC mains
supplies – this seems to be related to
the fact that most of these Li-Po battery packs are used away from mains
source.
Having said that, most Li-Po chargers
are designed for an input of around 11
14 Silicon Chip
30A BLADE
FUSEHOLDER
CELL
BALANCING
CONNECTOR
to 18V at 5A or more, ie, 85W or more.
Which means that these chargers are
fine working from 12V batteries in cars
or 4WDs but that means they need a
charger and a 230VAC supply which
can deliver 12 to 18V at about 6A.
With all of those questions to be
answered, I decided to purchase the
battery, a suitable charger and 230VAC
switchmode supply from the one
source: Hobby King. www.hobbyking.
com
Refer to the Battery Lifesaver article (Sept 2013)
for setup instructions
LITHIUM–POLYMER
BATTERY PACK
To be specific, in addition to the
above Li-Po battery, I purchased a
Turnigy Accucell 6 charger and a Hobby King 7A power supply ($18.16). The
charger is capable of charging Li-Po or
LiFe (lithium iron phosphate), Nicad
or NiMH plus lead acid batteries.
Furthermore, it can charge lithium
batteries up to six cells, Nicad/NiMH
up to 15 cells and lead acid up to 20V.
All up, including accessory leads, the
total cost was around $82.
It’s not even a real tight
fit to get the new
Lithium battery,
Battery Lifesaver
PCB and the
cabling inside the
old case. At top you
can see the power
tool connector
salvaged from the
old cells – it’s glued
in position where it
would have sat in
the old battery.
siliconchip.com.au
I hasten to say that this is more than
I paid for the drill originally but probably less than a replacement Nicad
battery pack, if one to suit the drill
was actually available.
It is also considerably less than the
cost of an equivalent new drill with
an 18V lithium-polymer battery pack.
Fitting the battery
Superficially, fitting the lithiumpolymer battery in place of the old
Nicad batteries is straightforward;
unscrew the battery case, remove old,
bung in the new.
In practice, it is quite a bit more
involved because the new battery
pack has nine leads; two for the main
battery leads and seven for sensing the
individual cell voltages.
This last feature is most important
because all the cells must be equalised
for correct charging, something which
the purchased charger will do.
I also wanted to fit a high current
fuse because lithium-polymer batteries do represent a fire hazard if their
output is shorted. Hence I fitted a 30A
blade fuse holder and 30A fuse (Jaycar
SZ-2040; SF-2139).
However the most important part
of the new battery installation is a
method of avoiding over-discharge.
Lithium-polymer batteries will be
damaged or destroyed by being overdischarged.
In normal applications such as in
model aircraft, the speed controllers
contain circuitry to prevent overdischarge but in this drill application
we have to provide it separately. We
assume that all 18V lithium-polymer
drills also incorporate over-discharge
protection.
Fortunately, we have an ideal means
of preventing over-discharge in the
form of the Battery LifeSaver featured
in last month’s issue. This has very
small PCB and has a profile so that
it can be squeezed into tight battery
compartments.
You will need to purchase a kit for
the LifeSaver (available from Jaycar;
Cat KC-5523 <at> $29.95) or otherwise
obtain the components and assemble
it according to the instructions in last
month’s article (September 2013).
The PCB is available from SILICON
CHIP.
drill’s battery pack. In my case, it was
pretty straightforward.
Just remove four self-tapping screws
and the old Nicad cells slip out easily
– they are all spot-welded together to
make up the 24V pack.
Two cells are in the small cylindrical section which carries the contacts
for the power connection inside the
handle of the drill. Pulling out those
last two cells also pulls out the springy
contacts which are also spot-welded.
Now it is most important that the
polarity of the leads connecting drill
connector are correct.
If the polarity of the supply connecting to the drill is reversed, the drills
inbuilt speed controller will not work
and there is even the possibility that
if will be damaged.
So it is important to check the polarity before you make the connections.
Have a look at the cylindrical section
of the battery pack and you should
see + and – symbols moulded into it.
These can be clearly seen in a number
of the photos in this article.
The contacts, still on their moulded
plate, must be clipped off and ultimately be connected to two leads
which will connect to the Battery
Lifesaver, according to the diagram
of Fig.1.
We used some of our old friend, JB
Weld, to ensure the contacts stayed
in exactly the right spot. (Just make
sure you don’t let any glue get on the
outside of the contacts themselves, as
this would make a very nice insulator.)
Commercial Li-Po Tools:
Do they check cell status?
When preparing this article, we wondered: do commercial battery-operated
tools using Li-Po batteries have the ability
to equalise cells or even check individual
cell status, as this project does?
The easiest way to check this was to
scour the shelves at a couple of major
hardware stores – and we have to report
that we didn’t find a single tool that had
any form of charger multi-connector,
as would be required if it was capable
of monitoring and equalising individual
cells. That’s not to say none have such a
feature – but we couldn’t find any!
Therefore, we have to assume that the
battery life of most Lithium battery power
tools, even those in the stratospheric
price range, may be just as compromised
as would much cheaper/older tools using
Nicad or NiMH batteries.
All it takes is one cell below par – and
that is not just a possibility, it’s a certainty
as the tool ages – and you would find that
battery life will very quickly start to fall.
And without individual cell monitoring,
there’s nothing you can do to eliminate
this or even minimise the problem.
A stupid boo-boo to be avoided
Before going any further, you need
to check how the new battery pack
and other components will fit inside
the case. Now there is a trap here and
I fell right into it.
The completed “new” battery, offering
much more “grunt” and with individual cell
monitoring/equalising, should last much
longer than the original Nicad pack. If there
is enough space in your battery, you might
even be able to glue the two sockets virtually
inside the case for an even neater finish.
Assembling the new battery pack
The first step in the process is to
remove the old Nicad cells from the
siliconchip.com.au
October 2013 15
You may have noticed that there is
a difference between the photo in the
opening shot (and the one below) and
the photos on pages 14 and 15. Haven’t
twigged to it yet? Look at where the
charging and monitoring sockets
emerge from the battery pack . . .
I did a trial assembly to see how it
all fitted together comfortably. I (foolishly) reasoned that it would be best
to have the two sockets emerging from
the back of the battery because that
seemed to be the logical place for them.
So I cut two slots in the back of the
compartment; one slot for the 2-way
lead for the battery charging connector and the other for the 7-way lead
from the lithium-polymer battery pack
which is necessary for monitoring and
equalising the cells during charging.
I then assembled everything, including anchoring the leads with silicone
and screwed it up.
The two battery connecting leads
need to hang out from the finished
battery pack so that it can be charged
when necessary. (Your battery might
have the room to mount them deeper
inside and glue them in place, which
would look a little neater).
After charging the new battery
pack, I duly clipped it into the drill,
switched it on and it all worked.
Beauty!
Later that day I went to put the drill
into its carrying case and then my
boo-boo became abundantly clear. As
the two connecting leads exited from
the rear of the assembled battery pack,
they effectively stopped me closing
the lid of the carrying case. Naughty
words were uttered.
So I had to do it all again, with the
leads correctly exiting from the front
of the case. Most drills of this type
come in a carry case so would need
to be treated in the same way. Learn
from my mistake.
You can see how the various wires
are tucked into the case. You need to
use fairly thick hook-up wire to ensure
good current carrying capacity. However, if they are too thick it becomes
difficult to make them sit inside the
case while you clip it together.
In use
As our photo on p12 shows, charging a Lithium-Polymer battery properly is a little more complicated than
bunging the battery in the charger, as
you would have done originally. Apart
from the obvious need to plug in both
the charging socket and balance socket
Hobby King’s $15 “LiPro Balance
Charger” will also handle Nicads,
NIMHs, SLAs, and more.
to the charger, you need a charger designed specifically for the purpose of
charging LiPos.
And while this might set you back
a few bob, once you’ve switched to
LiPo batteries you’ll wonder why you
ever persevered with Nicad or NiMH.
After we purchased our setup, we
even found a cheaper charger (~$15
at Hobby King). They even admit it is
a copy of the real thing but we bought
one anyway and found it worked just
as we would have expected.
It will charge LiIon, LiPo/LiFe (1-6
cells), Nicad/NiMH (1-15 cells) and
even lead-acid (2-20V) with charge
rates from 0.1 - 6A, from a DC input
of 11-16V.
Hobby King “CELL CHECKER”: a great investment!
While gathering together the bits’n’pieces for this article, we happened to notice this USB flash-drive-sized cell
checker, also at Hobby King. It certainly looked interesting
from the description, so we added it to our order.
We’ve got to say it is simply brilliant for checking LiPo cells with a multi-pin monitoring “port”. And at the
price – a princely $3.15 (plus P&P) if you use Lithium
Polymer batteries, you cannot afford NOT to have one of
these in your toolkit. It’s an incredibly cheap investment
for what amount to relatively expensive batteries.
16 Silicon Chip
All you need to do is connect the integral plug on the
Cell Checker to the balance socket of your battery pack.
The Cell Checker then steps through each cell (up to six
in the battery), displaying the individual cell voltage on
the digital readout then the total voltage of the battery.
If you check the battery before and after charging, it will
give you a very good indication on the state of all cells.
Weighing just 13g and measuring 70 x 24 x 13mm, it
can attach to your keyring so you’ll always have it handy.
Visit www.hobbyking.com for details.
SC
siliconchip.com.au
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