QI have a great sounding old “boombox” that runs on eight D cell batteries. I would like to convert it to run on a rechargeable system like LiPo. Do you have a schematic for doing this?
Dale D. Thomas
AFirst, we’ll start with calculating the required voltage. Then, we’ll see if we can find a suitable LiPo voltage. With eight D cells at 1.5V each, the nominal supply voltage is 12V ( 8 x 1.5V).
That’s a convenient voltage for lead-acid batteries, but not
so much for other chemistries.
Let’s look at the LiPo battery side of the equation.
While the voltages vary a bit
with construction techniques and
charge state, the nominal voltage
for this chemistry is about 3.7V.
This doesn’t divide nicely ( 12 /
3. 7 = 3. 25). However, I think your
boombox might be okay with the
voltage being a bit low. This may
require a little experimentation.
I think three cells in series
for a total of 11.1V would be
the first thing to try. There are a
number of options for the cell
type and form-factor. Amazon
has a large selection of LiPo cells
in different capacities. D cell
capacity is actually quite large;
approximately 20,000 mAh. I
doubt that equivalent capacity
is needed to still get reasonable
life — particularly since lithium
batteries have very stable voltage
throughout their discharge. Probably 5,000 mAh will be
adequate to allow for peak current and reasonable running
time. Amazon and other retailers have such cells available.
That is the easy part, though. The real problem will be
finding a proper charging circuit or pre-assembled charger
for this configuration of cells. It’s possible to build a single-cell charger using parts like the Microchip MCP73833.
There are nice boards for not much money, though,
that already use this device with a USB port to implement
a single-cell charger. A great example is the Adafruit USB
LiIon/LiPoly charger v1.2. Making this work as a multi-cell
charger requires some work.
The Adafruit website has a nice tutorial on making
a multi-cell charger using their board. You can find it
overview. I think they explain it in more detail than I could
easily cover here, but the key concept is switching the
batteries out of the boombox circuit and reconfiguring
them from series to parallel for charging.
A somewhat more radical configuration change would
be separating all of the cells and routing their connections
to three individual chargers. This gives the fastest charge
time and best balance charge, ensuring that each cell is
optimally charged for longest life.
The tutorial shows two cells, but this could easily be
extended to three. The switching can be done with relays,
where the relay coils are powered from the USB charging
power. Figure 2 shows a reproduction of the balanced
charging approach from the Adafruit site.
I did find one other balance charger that might work.
It supports two to three cells and has balance charging
connections. You can find it at Trossen Robotics at www.
The specs would have to be examined carefully to
see how the balancing connections should be made. The
picture seems to indicate that the cells might not have to
be reconfigured, with the balancing connections being
made where the cells connect to each other in series. That
would need to be carefully checked, though.
What if three cells don’t provide enough voltage for
this to work? An additional cell could be added in series,
boosting the voltage to 14. 8, but that’s a bit high. Three
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n FIGURE 2. Balance lithium chemistry charger (CC BY-SA 3.0 license, Bill Earl
author, adafruit.com, unmodified).
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