Using a 3.6 Volt Battery in an IBM 5170
'modem7' at the Vintage Computer Forums
Instead of the standard 6 volt Lithium backup battery (sometimes called a
CMOS battery), some IBM 5170s have been fitted with a 3.6 volt Lithium
battery (by ?????).
Some experimentation by me indicates that some
5170 motherboards may not function as designed if a 3.6 volt battery is
used. Specifically, on power off of the 5170, whilst the battery
voltage is adequate to preserve the 5170's configuration information
(base RAM amount, expansion RAM amount, drive type, etc.), the 5170's
real-time-clock (RTC) will intermittently stop updating.
Some owners of 5170s will not be concerned by that.
A new 3.6 volt Lithium battery will probably measure about 3.7 volts.
That figure is based on three unused 3.6 volt Lithium batteries
that I possess.
Refer to the diagram here.
The circuitry that the backup battery supplies power to is two
chips: a Motorola MC146818 and a Motorola MC14069. The MC146818 is
the CMOS/RTC chip, and the MC14069 is configured to generate the
oscillator signal for the clock within the MC146818.
The 'Advance Information' datasheet for the MC146818 indicates "3 V to 6 V operation". The datasheet for the MC14069 indicates "Supply Voltage Range = 3.0 Vdc to 18 Vdc"
Between the battery connector and the MC146818/MC14069 chips, are two
isolation diodes (designated CR1 and CR2). A side effect of those
diodes is that the MC146818 and MC14069 chips receive something less
than the battery voltage.
MEASUREMENTS / RESULTS
Using three 5170 motherboards, and a regulated DC power supply in place
of a battery, I noted at what voltage I saw each motherboard starting to
lose time (after being powered off for about an hour).
||Approx. battery voltage
at which time loss
|Approx. voltage drop
over CR1 + CR2
|Approx. voltage at
MC146818 and MC14069
( spec'ed at 3 volts minimum )
||- 0.8 volts
||- 0.8 volts
||- 0.8 volts
The combination of the two isolation diodes, CR1 and CR2, consistently dropped about 0.8 volts.
Three motherboards is nowhere near a statistically significant sample.
However, the measurements for motherboard #3 suggests that some
5170 motherboards may lose time even with a new 3.6 volt backup battery
(measuring 3.7 volts) fitted.
Given that 1. the MC146818 and MC14069 chips are spec'ed for 3
volts and above operation, and 2. the voltage drop over CR1/CR2 is
consistently about 0.8 volts, then that suggests that for guaranteed
operation, a battery that measures 3.8 volts or above is required.
The use of a 3.6 volt backup battery (measuring up to 3.7 volts) on a
5170 motherboard, appears to be taking advantage of the fact that the
MC146818 and MC14069 chips continue to fully operate at a supply voltage
slightly under their specified minimum voltage of 3 volts. The
actual minimum voltage (versus specification of 3 volts) will probably
vary from batch to batch of chips.