IBM supplied a 6 volt lithium battery.
In the 5170's technical reference, the pinout diagram for the battery connector (J21) on the motherboard shows "6 Vdc".
Pretty much, any 6 volt non-rechargeable battery with the required connector, is going to do the job.
Fitting a rechargeable battery is pointless, because the 5170 motherboard does not contain battery charging circuitry.
If you are changing the battery because of a 161 error, then note that after battery replacement, the 161 will remain until you complete the SETUP procedure.
Motherboard connection
The battery connects to J21 on the motherboard.
6 volt battery
Below is pictured one of the 6 volt batteries supplied by IBM during the lifetime of the IBM 5170. There were others that IBM supplied (some examples here).
Click on picture for larger view
It is lithium (not the same as lithium-ion), and IBM part number 8286121
Understandably, IBM do not sell the battery anymore.
There used to be many companies that sold a replacement as part number 8286121, but that is no longer the case.
The pictured battery has a resistor on top (possibly for external short-circuit protection). The replacement battery does not need to have a resistor.
An example of a third-party replacement battery is shown here. Batteries like that, found on eBay and elsewhere, are possibly well depleted.
6 volts via a CR-P2 battery and holder
Soldering skill required, because you will need to bring across the wires and connector from the old battery. Pay attention to polarity. It is prudent to use heat shrink tubing on the solder tabs, in case the unit falls from the 5170 case onto the motherboard.
6 volts via four 1.5 volt AA cells
Using the battery holder pictured below, four standard 1.5 volt AA cells can be used to provide 6 volts.
Click on picture for larger view
A search of eBay using battery holder aa 6v may find this battery holder. Pay attention to the connector.
I have not seen these (ones with the required connector) for quite a while. Most of the AA holders that I see now, have no connector at the end of the wires. If you have soldering ability, you could bring across the connector from the old battery. Or perhaps use single-post connectors as pictured at here.
If you are in Australia, modem7 at the Vintage Computer Forums can supply these (with the required connector attached). Limited quantity.
4.5 volts via three 1.5 volt AA cells
As above, but three cells instead of four, to provide 4.5 volts.
3.6 volt lithium
Some people have used a 3.6 volt lithium battery.
I consider a 3.6 volt battery in an IBM 5170 to be borderline. With a 3.6 volt battery fitted, some 5170 motherboards may keep the CMOS settings but lose time. (technical info)
Pictured below is a particular model of 3.6 volt battery that has the required connector. Batteries like that, found on eBay and elsewhere, are possibly well depleted.
Click on picture for larger view
Warning: Batteries of greater than 6 volts nominal
Refer to the related diagram at here.
The combined voltage drop over diodes CR1 and CR2 is approximately 1 volt (as measured by me on multiple unpowered IBM 5170 motherboards, using a 6 volt battery).
The datasheet on the Internet for the MC146818 chip indicates "3 V to 6 V operation".
So that means that if a battery with a measured terminal voltage of over 7 volts is fitted to an IBM 5170, then the published MC146818 rating of "3 V to 6 V operation" is exceeded.
That is why I do not support fitting a battery of more than 6 volts nominal to the IBM 5170. Note the 'nominal'; a new 6 volt lithium battery will measure more than 6 volts.
But maybe Motorola's "6 V" was put in the datasheet only because Motorola did not expect users of their MC146818 to be using a battery that measured more than 6 volts. Who knows !
But maybe later versions of the DIL packaged MC146818 chips had greater voltage ratings. Who knows !
I suggest that you 'play safe' and do not use a battery that measures over 7 volts.
