For your information, the internal battery is glued with the back cover of MS908. So if there is something wrong with the battery, it is not suggested customer to replace the battery themselves. As for the battery replacement, different regions, different policy. So please consult your local dealer for this replacement whether can be ordered.
As for the 2nd question, "Will this external battery used to power a DS708 work to get a Maxi sys"
--> As you know there is no battery configured in DS708, the input voltage of DS708 is 8V-32V. But the input voltage of MS908 can accept is 12V (9V-24V) from power supply. If the battery output voltage in the attached picture is 12V, you can use it to MS908 from the DC Power Supply Input Port. BUT, please don't use it from the inside as the replacement of the internal battery. The specification of the internal battery is 11000 mAh 3.7 V lithium-polymer battery. Otherwise it will burn out MS908.