NXP Semiconductors | UM10301 |
| User Manual PCF85x3, PCA8565 and PCF2123, PCA2125 |
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.
VSUP
D1
R1
C1
3.6 V to 4.8 V 100 nF NiCd/NiMH
(1)Due to the low RTC current consumption, a parallel diode over R1 (directed from the battery to the RTC) will not be of any use. The voltage drop over R1 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.
UM10301_1 | | © NXP B.V. 2008. All rights reserved. |
User manual | Rev. 01 — 23 December 2008 | 32 of 52 |
