4 Silicon chip Australia’s electronics magazine siliconchip.com.au
Questioning use of LiPo charger IC
with LiFePO4 cell
I am questioning the use of an
MCP73831 charger IC to charge the
LiFePO 4 cell in the Solar-powered
data repeater design published in the
May 2019 issue (siliconchip.com.au/
Article/11615). This chip is intended
to charge Li-ion and LiPo cells with
its output regulated to a maximum
of 4.2V.
It is unsuitable for LiFePO 4 charging
because these cells have a full charge
voltage of 3.65V. Under charge, the cell
will never rise to the module’s cut-off
voltage, and the cell will fail due to
over-charging.
I have used one of these modules
to charge a LiFePO 4 cell by inserting
a forward-biased silicon diode and a
schottky diode between its output and
the cell’s positive terminal, to drop the
charge voltage to something like 3.65V.
I have also successfully charged three
NiMH cells in series using one of these
chips, though I would not recommend
either of these schemes.
Also, I think there may be a typo in
the last paragraph under “power sav-
ing features” (page 49). The quiescent
current draw is quoted as 9.4mA. I
think this should read 9.4μA consid-
ering all the power saving measures
in the circuit.
Bob Temple,
Churchill, Vic.
Response: you are not the only person
to point this out. We were aware that
the MCP73831 is intended for charging
LiPo cells and has a charge termina-
tion voltage of 4.2V. It is also true that
the fully charged voltage of a LiFePO 4
cell is usually around 3.65V. However,
LiFePO 4 cells will not be damaged by
being ‘over-charged’ to 4.2V (although
that is the maximum safe voltage).
For example, see the following web
page of a battery manufacturer: http://www.
powerstream.com/LLLF.htm
To quote them: “A [lithium ion] bat-
tery has a very narrow overcharge tol-
erance, about 0.1V over the 4.2V per
cell charging voltage plateau, which
also the upper limit of the charge volt-
age. Continuous charging over 4.3V
would either damage the battery per-
formance, such as cycle life, or result
in fire or explosion.”“A LiFePO 4 battery has a much
wider overcharge tolerance of about
0.7V from its charging voltage pla-
teau of 3.5V per cell. When measured
with a differential scanning calorim-
eter (DSC), the exothermic heat of the
chemical reaction with electrolyte af-
ter overcharge is only 90J/g for LiFe-
PO 4 versus 1600J/g for [lithium ion].”
“A LiFePO 4 battery can be safely
overcharged to 4.2V per cell, but high-
er voltages will start to break down the
organic electrolytes.”
You could add a series diode from
the charger IC’s output diode to the cell
to reduce the charge voltage if you are
concerned. The maximum charge cur-
rent is only 100mA, and once the cell
voltage reaches 4.2V, the charger drops
its output to 5mA and waits for it to fall
to 3.9V before resuming fast charging.
Another option would be to use a
14500 type Li-ion cell, such as Jay-
car Cat SB2300 (800mAh), as the
MCP73831 designed for this type of
cell. But we have more faith in LiFe-
PO 4 cells as they have a wider range
of voltage tolerance and are far less
prone to catching fire.Letters and emails should contain complete name, address and daytime phone number. Letters to the Editor are submitted on the condition that
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