NXP Semiconductors UM10301 PCF2123 - Manual (Page 32)

NXP Semiconductors
UM10301
User Manual PCF85x3, PCA8565 and PCF2123, PCA2125
UM10301_1 © NXP B.V. 2008. All rights reserved.
User manual Rev. 01 — 23 December 2008 32 of 52
evaporation can be neglected. At elevated temperatures of for example 60 °C electrolyte
evaporation will be much higher. Refer to the manufacturer’s datasheet. Vendors of
lithium batteries include Panasonic, Sanyo and Varta.
13.2 NiCd and NiMH secondary batteries
The well known Nickel-Cadmium and Nickel-Metal Hydride batteries can also be used to
provide backup power to an RTC. In many countries use of NiCd batteries will be
restricted in favour of NiMH batteries due to environmental considerations (cadmium).
NiMH batteries also suffer less from the memory effect than NiCd batteries. As a further
plus, NiMH batteries provide a higher energy density than NiCd batteries, but on the
other hand have a higher self discharge rate (about 20 % per month at room
temperature) than NiCd batteries (about 10 % per month). The timekeeping current draw
of an RTC is so low that often – depending on the selected battery capacity - the self
discharge is the determining factor for the available backup time, which in that case
would make the NiCd more suitable for backup applications. The typical operating
temperature range during charging is approximately 0 °C to +40 °C. During discharge the
permitted operating temperature range is a bit wider, in the order of -10 °C to +50 °C.
Just like lithium cells NiCd and NiMH batteries must be separately soldered or placed in
a battery holder after the board has gone through reflow soldering. The charging circuit
for NiCd and NiMH batteries in this application can be very simple; just trickle charge it
via a resistor or other form of current limiting. Ordinary NiMH batteries are less suitable
for trickle charging than NiCd batteries which is another reason that often NiCd batteries
are better in this application. However, as pointed out before, use of NiCd batteries will
be restricted in many countries due to environmental considerations. Therefore it will be
harder to find NiCd batteries for backup purposes. They are being replaced by newer
NiMH batteries in the same form factor and which are suitable for trickle charging. An
application diagram is given in Fig 13.
001aai848
R1
D1
3.6 V to 4.8 V
NiCd/NiMH
C1
100 nF
V
SUP
V DD
V SS
RTC
(1) Due to the low RTC current consumption, a parallel diode over R
1
(directed from the battery to
the RTC) will not be of any use. The voltage drop over R
1
is small
Fig 13. Backup circuit using secondary cell (NiCd or NiMH)
The capacity of a battery is expressed as C. The charge or discharge current can now
also be expressed in relation to the capacity of the battery. Assume a battery with a
capacity of one ampere-hour (1 Ah). A discharge current of C/10 now equals 1 Ah / 10 h
= 100 mA. The recommended charge current is also specified as a fraction of C.