Where to connect the jumper cables

Volvo 850 jumper cable location

If the booster battery to be used is in another car, check that the cars are not touching to prevent premature completion of negative circuit. Note the position of the battery terminals and using jumper cables:

First connect the booster battery positive (+) terminal (1) to car battery positive (+) terminal (2).

Then connect the booster battery negative (-) terminal (3) to a stationary solid metal part on the engine at a point away from the battery (4).

Do not connect booster cable to any part of fuel system or any moving parts. Avoid touching hot manifolds.
After the engine has started, first remove the negative (-) terminal jumper cable. Then remove the positive (+) terminal jumper cable.

Volvo 850 Charging problem

 On cars of this type and age, the crimp on the positive terminal can fail turning the positive wire to the battery into a resistor rather than a wire.  This is diagnosed by measuring the charging voltage on the battery and giggling the wire (it will jump around, and not remain at the required 13.8 V), and by noticing that the wire itself is hot.  Volvo recently released a fix for this, and previously the shop had to remove the entire wiring harness, at great expense, to fix it.