87 Mits Montero electrical issue

Have an 87 Mits Montero 5 spd single ohd cam. Just replaced a broken rocker arm everything else mechanical appeared ok. Battery will not hold a charge, I know it is bad. But tried to jump this vehicle to see it start. First turn of key got all warning lights (as expected) and cranked about a second. Then totally dead. Now no electrical at all while still hooked up to jumpers. Could this be a fuse or the alternator?

Unless you have really heavy gauge jumper wires, the ‘jump’ is more a boost than starting from the donor battery. A running donor car will charge the donee battery to the point where that will crank the donee engine. Monitor the the Montero battery at the posts using a DVM during the charging and when the key is turned ‘on’ and to ‘start’. If the voltages are below 9 volts DC during either of these, service the battery and try again.