Dresda, The 386 M/B came with a internal battery. It looks like a blue electrolitic cap. Next to this battery is a jumper for int or ext battery. This selects where the battery voltage will come from. set to ext and install a external battery + to pin 1 - to pin 4.
you will need to run a CMOS set up. contact Milltronics service and ask for a copy of the CMOS set up entry.
Milltronics also has a new battery available if needed.
Check the old battery for corrosion. clean or remove the old battery. If corrosion gets into the mother board. It's time for a control update.
I belive the latest update for the C1 and C5 -B and newer controls is $1875 US. this will get you a lot of new functions and capibilities.
BTW C5 controls with serial #'s that are 3 digits ie: 178 or that have a -A IE 178-A are not readily updatable. these controls have acro and sime disks that can not be used with newer Mother boards and software and will need to need updated to a C7 control. If the iron is good it may be worth the costs of a C7 update. Be careful of what is for sale on flebay I see alot of no usable -A and earlier stuff out there.