What does reduced Spell Damage mean

Hi am i new to the game a friend asked me what reduced Spell Damage does/means as he is looking to buy a void battery weapon i looked up on google could not find an answer to his question so here i am if any one could kindly tell us :)

As you probably assumed, it's the exact opposite of 'increased spell damage'. So if you got, say, 120% inc spell damage, 100% increased elemental damage and 80% reduced spell damage your elemental spell would have 140% increased damage in total.

The 80% reduced Spell Damage is a negative number added (or positive number subtracted) from your damage multiplier from Increased/Decreased Damage.

Suppose you have a Spell that deals 100 base damage, and for simplicity, you have no Passives or other gear on, and a 20% implicit on the Void Battery. Then, at 0 Power Charges, your Spell will deal 100 * (1 - 0.8 + 0.2) = 40 damage.

Now, suppose you somehow gained 9 Power Charges, but with no other Passives or gear.

Your spell will deal 100 * (1 - 0.8 + 0.2 + 0.25*9) = 265 damage

That's Void Battery in a nutshell. You tend to deal less damage at 0 Power Charges when wielding it, but once you Charge it up with Power Charges, it gets pretty powerful.

The entire purpose of void battery is to run it with many power charges... you need to have at least 4 power charge up to start benefiting from the wand. But after 4 power charges everything is free damage...

If you just look at the weapon mods itself (excludes everything else and ignore the implicit) the bonus increased spell damage per power charge is a direct counter to the 80% reduced spell damage.

At
0 charge you have 80% net reduced.
3 charges you have 5% net reduced.
4 charges you have 20% net increased.
6 charges you have 70% net increased.
8 charges you have 120% net increased.

This is why many high end builds run 2 void batteries. Properly constructed character and items you will gain 240% increased spell damage pretty much all the time. Which is pretty insane. And after that you are looking as all the "more" spell damage modifier. Say you have something like pain attunement which gives 30% more spell damage when you play low life, this will scale up the 240% to 312%... there's a reason why void battery is so damn expensive, and it's not just because it's rare. It's just too good for both self casts and coc.