> So far I've restricted myself to ~12v batteries because I don't fully understand the safety procedures required for high voltage applications. Eventually it's something I want to get into as well.

To add a bit of context for you and anyone willing to experiment with batteries:

First, the danger isn't just the voltage. Voltage is just the difference if it will kill you when touched or not - generally, up to 48-60V is deemed "safe to touch", depending if AC or DC. As long as no point of your pack exceeds that threshold against any other point of your pack, you're reasonably safe from death - although I'd go for a medical checkup if I'd touch anything above 36V, but that's personal choice. If the current path goes across your heart (e.g. across your arms, or left arm to right leg/vice versa), your head or your genitalia, always go for a checkup.

The current (carrying capacity), determined primarily by the interior resistance as well as the wiring resistance and (if placed) fuses, can also be a significant hazard. Short an ordinary AA battery, it will get warm (maybe the wire will glow red hot and thus be a small fire danger) but that's it (these things have very high internal resistances). Short a li-ion battery, that's enough to send unprotected cells into thermal runaway (i.e. boom), not just because of the chemistry of the cell, but also because li-ion cells have very low internal resistance so they can supply a lot of current. Short a pack of li-ion cells or a car starter battery? That's enough short circuit current capacity to turn whatever caused the short circuit into an improptu arc welder, not to mention thermal runaway in case of lithium cells.

Now, for some recommendations:

Always fuse off cells or packs as close to the batteries as possible. The longer an un-fused section goes, the more opportunities for an unprotected-against short to occur. Fuses have not just different current capacities (i.e. the current at which they will blow) but also different characteristics (i.e. how fast they'll respond to a given amount of overcurrent). Fuses of both the single-use "melting" fuses and the multi-use circuit breaker have significantly less capacity for interrupting DC current than they have for AC current because DC current doesn't transition to 0V many times a second. Select your fuse(s) to match appropriately!

Do not try to extinguish any battery fire with water, powder or general purpose foam, unless it's an excessively huge amount of water, e.g. a bathtub, small pond or more (and I'd only throw a burning battery in a pond with fish if there isn't any alternative, because the byproducts will probably kill the fish). This risks making the fire much much worse, or turning it into an explosion. CO2 isn't harmful, but it's useless. Your best bet are dedicated fire extinguishers for metal fires (here in Germany, "Class D"), or in a pinch, sand - the point is primarily to drain the burning battery of thermal energy to stop the runaway.

Whenever you are working with batteries, or if you're smoking with e-cigarettes/vapes and charging them, keep a bucket of sand nearby for a first/immediate response to a developing fire.

Never expose a lithium cell to strong heat, e.g. a soldering iron. This can and will send the cell into thermal runaway. Use sockets or, if you absolutely have to make a pack, a spot welder.

Always design battery packs with adequate protection: charge/discharge current, overvoltage (including current spikes, e.g. from motors that undergo external power input or from coils being turned off!), undervoltage, temperature (best: per cell!) and pack voltage/balance. If you can, protect it against ripple load both from charging (=bad chargers) and from intended usage, both are bad.

Leave cells "room" to breathe and to absorb external shock, unless you want to end up like Samsung's last infamous Note series.

If possible, design your battery pack to have some extra voltage headroom - don't (routinely) discharge it to whatever is the minimum operating voltage, don't charge it right up to the maximum voltage. General best practice to ensure longer life is 20% on both ends. The sort-of exception are lead-acid batteries in low-power (!!!) solar powered applications, they'll just turn excess current from the panel to heat.

Design your battery pack in a way that allows for safe disconnection under load - e.g. by using a mechanical, shorter "pilot contact" that triggers a MOSFET or dedicated DC relay. Otherwise, the user may pull it under full load and you'll get arcing. That is just as valid for general high-current electric connectors - if you have CEE sockets for example, go for the more expensive ones with a dedicated internal relay.

[1] https://en.wikipedia.org/wiki/Extra-low_voltage#Regulations