I previously created a float charger to maintain a lead-acid battery.
After that, I bought a module with an LCD that can display current as well as voltage because it was sold on Aliexpress. And I created an enclosure to hold this foundation.
[Dual Display DC-DC 5-23V To 0-16.5V 3A Max Step Down Power Supply Buck Converter Adjustable LCD Step-down Voltage Regulator] (https://en.aliexpress.com/item/DC-DC-5-23 -0-16-5-3A / 32919584748.html) (This link is not an affiliate)
If the diameter of the nozzle is 0.4 mm, sufficient accuracy can not be obtained, so it may be necessary to scrape the rod for pushing the button or its hole.
This explanatory note machine-translated Japanese to English with Google Translate.