Aldi's tool range is specifically designed such that batteries and chargers are sold separately so you can purchase as many of each as you required
It's a good idea, the tool+battery+charger costs less than the more typical model where you buy a body, battery and charger in one case, and you don't end up with multiples of things you don't want. If you want 5 tools, 10 batteries and 2 chargers, Aldi make that easy
In fairness to them also, they make it blindingly obvious on the outside of the box that it contains no battery:
If you bought it recently, there may still be a big pile of batteries and chargers in an adjacent basket to where you found the tool
Given that they market batteries as 20v/40v I expect that one of the pins will be 20v positive, another will be 40v positive and one will be negative, with some internal trickery used to have two banks of 20v either wired in series or parallel with a mechanical change over or diode strategy.
It's not marked on the tool because it doesn't need to be; you're supposed to buy the fitting battery, not jury rig your own.
That said, it may be marked on the battery or charger, so if you're dead set against buying one you may be able to visit Aldi, take one out of a box, use it to work out which pin is which and jack 3 car batteries together (or whatever you were planning) for a chance at success.
The other pins are for the tool to communicate with the battery and it may or may not work without them connected, but there is no harm in giving it a go (I have some Aldi gear that I run off adapters that fit my Bosch batteries, and the adapters have only 2 wires)..
..but I'd just pick up a battery and charger off eBay if Aldi is out of stock, and move on with setting the job started