IIRC this is even a loading screen hint, but just in case anyone missed it (or is interested in the actual ratio), I decided to quickly test it.
| Zonai Charges | Capsules | Ratio |
|---|---|---|
| 1 small | 1 | 1 : 1 |
| 2 small | 3 | 1 : 1.5 |
| 3 small | 5 | 1: 1.6 |
| 4 small | 8 | 1 : 2 |
| 5 small | 12 | 1 : 2.4 |
| 1 big | 10 | 1 : 10 |
| 2 big | 22 | 1 : 11 |
| 3 big | 34 | 1 : 11.3 |
| 4 big | 47 | 1 : 11.75 |
| 5 big | 60 | 1 : 12 |
Conclusion: if you find a Zonai Device Dispenser with stuff you really like, feed it 5x Big Zonai Charges at once for maximum payout. The smaller Zonai Charges are best kept for other stuff, like checking what exactly a newly discovered dispenser spits out or refilling your battery in a pinch.
EDIT / ADDITION:
If you mix-and-match, you get the following output in capsules:
| Zonai Charges | 0 small | 1 small | 2 small | 3 small | 4 small | 5 small |
|---|---|---|---|---|---|---|
| 0 big | 0 | 1 | 3 | 5 | 8 | 12 |
| 1 big | 10 | 12 | 14 | 17 | 21 | |
| 2 big | 22 | 24 | 27 | 31 | ||
| 3 big | 34 | 37 | 40 | |||
| 4 big | 47 | 50 | ||||
| 5 big | 60 |
The blank squares are impossible as you can only put 5 items at once into the dispenser.
The Ratio:
| 1:X Ratio | 0 small | 1 small | 2 small | 3 small | 4 small | 5 small |
|---|---|---|---|---|---|---|
| 0 big | 1 | 1.5 | 1.6 | 2 | 2.4 | |
| 1 big | 10 | 6 | 4.6 | 4.25 | 4.2 | |
| 2 big | 11 | 8 | 6.75 | 6.2 | ||
| 3 big | 11.3 | 9.25 | 8 | |||
| 4 big | 11.75 | 10 | ||||
| 5 big | 12 |
What about Construct Horns?
These are always a 1:1 deal. Even if you put 5x Captain Construct IV Horns into the Device Dispenser at once, will you get 5x capsules in return.
Can you do anything with the crystalized charges other than upgrade your battery? I bought a bunch of charges to upgrade my battery before exploring the depths fully. Now after beating the temple bosses and stopping by various mines and yiga hideouts I have hundreds of charges that are just collecting dust in my inventory.
IIRC once you completely upgraded your battery (including turning them all blue) the Constructs will instead sell Zonai Device Capsules to you, so you won’t have to rely on the RNG of the Device Dispensers anymore. I can not personally confirm yet tho as haven’t maxed out the battery yet.
This is pretty much inline with the gameplay. You can accumulate them without having to use them but keep in mind, I am involved in a grinder mindset set. ex. I have thousands of Poe on hand just in reserve for future weapon purchases.