Using a 3.6 Volt Battery in an IBM 5170

'modem7' at the Vintage Computer Forums
June 2010


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.


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).  

Type 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 )
#1 1 3.65 volts - 0.8 volts 2.85 volts
#2 1 3.6 volts - 0.8 volts 2.8 volts
#3 3 3.7 volts - 0.8 volts 2.9 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.