Sorry, I was busy, but here we go!
To do maintenance, fixing and all sort of DIY stuff with batteries certain devices are necessary, mandatory even, and some others are helpful. All of them exist in different price ranges.
1. Multi-Chemistry Charger / Smart Charger / Hobby Charger / RC Charger / ...
Different words for the same thing, a charger for all kinds of batteries you can possibly come across. While lithium is certainly the main thing here they also support Lead Acid (SLA/LA/PB) and Nickel Metal Hydride / Cadmium (NiMh/NiCd). For lithium batteries they have an integrated balancer. Many support three lithium variants, LiIon, LiPo and LiFe. I'm not sure where LiIon comes from as this mode is usually preset to a end of charge voltage of 4.1V per cell (Vpc). I can only assume that there used to be lithium batteries that used this lower end of charge voltage. But it is still useful to charge lithium batteries not all the way to extend their cycle life on chargers that don't offer adjustable voltages. LiPo then is the general lithium mode for the usual 4.2Vpc and LiFe is for LFP/LiFePO/LiFePO4 with 3.6 or 3.65Vpc.
LA, NiMh/NiCd, LiIon/LiPo/LiFe is the minimum every device should support, some higher end devices might have adjustable programming to support lithium titanate / LTO (2.4Vpc) or offer a separate mode for LTO by default.
Also all of them offer a discharge/test function but the discharge power is often very limited. Higher discharge power needs a bigger heatsink so this is usually directly connected to the size of the device.
Companies/Brands to look for: Junsi/iCharger, SkyRC, EV-Peak, Hitec, ...
All of them offer a range of products and are generally high quality. You might find suspiciously identical looking devices under different brands. That's because there is an OEM building them and putting different labels on but the devices are the same.
Cheap: IMAX B6ACv2 (DC and AC, built in power supply), IMAX B6 Mini (DC only) or one of their clones
Although some clones are better than others I generally advise to buy the real stuff and not the cloned devices.
The IMAX B6 are small units with a balancer for up to six cells, the charging power is limited to 50W or 5A/6A, whatever comes first. Discharge power is very small, only 5W.
Prices might vary from country to country, generally I'd say around 50 EUR for the real ones.
Not so cheap anymore: Lots of devices are available, for example certain iChargers (1- and 2-series), SkyRC D250/D400, EV-Peak A8, ...
They basically do everything the smaller units do but just more of it. Higher currents, more charging power, more discharging power, balancer for more than six cells (usually seven and eight). Also many feature a dual port design to work with two different batteries at the same time. Most of them run on DC, but there are also some with an integrated power supply to operate on AC. Some of them might offer advanced features like regenerative discharge (discharge a battery into another battery or into the power source of the charger, if it's a battery)
Prices up to 200 EUR, more or less.
Not cheap at all: iChargers (3- and 4-series) up to the biggest one, 4010, and just very few others like the Pulsar 3/3+
They do all of what has been mentioned before and again more of it. Bigger balancer (eight and ten cells and more), more power, higher currents, more access to the inner workings to change all parameters, create own charging programs and so on.
300 EUR and more, much more...
2. Charger/Discharger/Tester for cylindrical cells
Like the chargers above, but not for entire batteries. Instead they take single cylindrical cells, usually four. Often limited to NiMh/NiCd and lithium cells, often 4.2Vpc exclusively for lithium (so no LFP, no LTO). No LA because there are no small cyclindrical LA cells. Some support NiZn, but that's as rare as the batteries. NiZn is not very important as of now.
Biggest cell possible is usually 26650, some might also take 32650/32700, but either of them usually not in all slots at the same time because of the larger diameter.
Cheap: Usually the ones that only do charging. These come from the flashlight (and nowadays vaping) community like the Nitecore D4. Prices are around 20 EUR.
Not so cheap anymore: The Opus BT-C3100 which is very popular among the DIY community because it can discharge and test four 18650s at the same time. Costs between 30 and 50 EUR, depends on the store of your choice.
Not cheap at all: SkyRC MC3000, like the Opus but much better quality and much more features (configurable settings). Earlier they were available for 60-70 EUR (China) to 100 EUR (everywhere else) but it seems that everyone raised the prices so it's more like 80-100 EUR now.
3. Lab Power Supply / DC-DC converter with CC/CV
Unlike the chargers they can put out any voltage you like (within reason and their specs) and with CC/CV function they can be used to charge cells or batteries. But unlike chargers there is no automatic cutoff so you have to manually stop the process. Because they aren't chargers and don't have any intelligence built in they do what you want. But that also means there are no safety mechanisms in place. This is useful for reviving overdischarged cells, for example.
Cheap: Any chinese DC-DC converter from eBay, Amazon, Banggood, Gearbest, Aliexpress, you name it. Many are under 10 EUR, some might be a bit more expensive depending on power and the store you buy at. Run on DC only.
No so cheap anymore: Ruideng DPS (Buck) and DPH (Boost) units. You can find them at their store at Aliexpress directly from the manufacturer but many 3rd party stores sell them as well, at higher prices. They offer them with various specs and they come with a display for ease of use. Case are also available. Costs between 25 and 50 EUR depending on the model, excluding a case. Run ony DC only.
Not cheap at all: Any semi-professional or professional lab power supply / bench power supply. Overkill for most DIY applications. Might become worth it if your diving deep into electronics. Usually run on AC only.
4. (AC) Power Supply for the DC units (like chargers and DC-DC converters)
You can run them on DC from batteries or your solar system, or from an AC supply to run them from the grid.
Cheap: Existing solar system / battery to run them from DC, alternatively an old ATX computer supply (the 12V rail is a decent start)
Not so cheap anymore: Dedicated AC supply for a fixed DC voltage. Cheap ones from China start around 20 EUR.
Not cheap at all: Industrial DC power supplies, Mean Well is one of the biggest names in the business. Also Seasonic and Mascot, just to name a few. Industrial could also mean IT, especially telecommunications. Especially telco stuff often runs on 48V so there might be specialized units for this use case that we could use as well. If you can find them at a store or company that is being liquidated it might even be cheap but that's rare.
5. Soldering iron / Spotwelder
If you have no soldering iron yet it might be the right time to get one. Sooner or later you will have to solder something. I'd skip the cheap options here, a cheap USB-powered soldering iron is no fun. Get either one of the better ones (TS80) or its bigger brother TS100. The TS100 isn't USB-powered anymore, but runs directly on 12V-24V DC. Both of them are also a good choice if you want the soldering iron to be portable. The TS80 can run from a powerbank but needs QC3.0 support, the TS100 runs from any battery within its voltage range (ideally 6s lithium).
For a stationary unit I'd recommend one from Hakko, like the FX-888/FX-888d.
A spotwelder is purely optional at this stage. There are the Sunkko units from China, they are based on a transformer and are generally not the best quality but get the job done. They run on AC and have a tendency to trip your breakers if your installation is a bit on the weak side. You can also build your own based on the transformer from a microwave oven if you know what you are doing.
The alternative is a DC powered spotwelder. They usually use MOSFETs to switch the short circuit current of a battery like a LA battery or a RC LiPo battery. I prefer these over the cheap AC units. There are decent AC units available but they are professional and very expensive equipment.
A spotwelder might become a thing if you start making your own batteries using nickel strip.
I think that's it for a start. I've given you lots of names to look for, check your local stores or on the internet that send to your place. Addionally you will need cables and stuff but you'll find out what exactly as you go along. It depends on what you want to do so it's hard to give some sound advice.
A digital multi meter / DMM should also be on this list but as you could measure the voltage of the cells I guess you have one already. But for anyone reading this who is interested in this as well, the cheap solution is a Uni-T while the not so cheap solution is obviously a Fluke.
All of this is based on my personal experiences, there might be other options I haven't mentioned here.
