A few years ago I scored a used Opus BT-C700 on an auction site, very cheaply because it looks like everybody wants Li-Ion support nowadays. Nice little charger (and discharger), my only complaint is that there is no way to force it to start charging a cell depleted down to zero, it just thinks the slot is empty.
More than you ever wanted to know about battery chargers:
https://lygte-info.dk/info/indexBatteriesAndChargers%20UK.html
I use the LaCrosse branded BTC-700 which is the same as OPUS with a different branding.
There is a way to charge cells depleted down to zero. You need a good cell (one that requires charging seem to work better), and a metal paper clip (or a breadboard jumper wire).
You insert your depleted cell in a slot, and the a non-depleted cell in the next slot. The good cell will begin charging while the bad cell say NULL. Now find a metal paper clip. Bend the metal paper clip to create a contact between the depleted cell's + and the non-depleted cell's +. When the contact is good, you should see that the depleted cell begin to charge. You can remove the paper-clip and it should keep charging. I keep a short breadboard wire by my charger specifically for this use.
If after you remove the metal paper clip and it turns NULL again, your cell was too depleted (to hold voltage long enough), you will need to maintain the contact a bit longer before removal. A couple of seconds for the first retry. If it still go back to NULL, try even longer. 5 seconds or so is the longest I've needed, but a cell
is in very bad condition may well need more.
After full charge, you should do a Charge-Test to see what the capacity is. If appropriate (you got time), you should do a Discharge-Refresh. Discharge-Refresh mode will charge full then discharge and show mAH capacity. If this cycle has better capacity than the last time, it will do the charge then discharge cycle again
until it doesn't improve.
The user manual describes improvement as the only measure it use to terminate cycling and it doesn't have a cycle limit. It
typically works very well, but this approach has a short coming -- it could be running for weeks, and use up the remaining charge-discharge cycles the cell can do before it totally dies. Even the Eneloop has a charge-discharge cycle limit of 2000 (first gen) to 2500 (later gen). So infinite cycling will degrade your cell. May be after a few days, terminate it manually when you feel further improvement is no longer worth the cost of lost cycles.
Look for a charger on a site like Amazon. They have a lot of different ones at different prices.
Look for one that also TESTs cells not just charge them. That way you can test a cell after it has been charged to get an idea how well it works as it gets older. These are typically 4 bay chargers that have an LCD display.
They also have different current settings for NiMH cells such as 500ma, 1000ma, 1500ma, 2000ma, etc.
They also sense the condition of the cell and if the resistance is too high they automatically cut back on the current so the cell does not get too hot.
They also show the terminal voltage while charging, the ampere hours, the time, and of course the actual charge current which may vary for each cell.
The discharge test tells you if the cell is getting too old. For example if the cell can hold 2000mAh when new and it is tests as low as 1000mAh then it must be getting old and so wont run a device as long as when new.
A typical charger that does all that is the OPUS BT-C3400 but i am sure there are others in that price category which is probably around $40 USD.
I also use the OPUS BTC-3400. The BTC3400 firmware/hardware look and work
exactly like the BTC700 except it also do LiIon batteries. I like the BTC3400 a lot.
Two other differences:
1. The Discharge-Refresh has cycle limit of 3 for the BTC3400, so it wont cycle your cell to the point of degrading it. I actually would like it to be a selectable limit but it cant do that. It is fixed at 3.
2. The BTC3400 has back-lighting for the LCD screen. Press DISPLAY and it will light up for a few seconds, great help when the room is dim. The back-light has a hidden feature. If you hold down the DISPLAY button for 5 seconds, it stays ON.
I purchased the BTC3400 after having the BTC700 for a bit over a year. Both are in use -- The BTC700 is for family use and BTC3400 is for me only -- I am the only one dealing with LiIon.
EDIT:
re: "...The BTC3400 firmware/hardware look and work
exactly like the BTC700 except..."
I worded the sentence wrong which will likely lead to confusion. The "hardware" there I mean
only the user interface hardware. Same words on the LCD display, same buttons, so on. The guts are not "exactly" the same since the BTC700 does NiMH only whereas the BTC3400 does NiMH and LiIon accepting cell size up to 18650 (with a few mm extra for protected cells). The BTC3400 also has higher current capability.
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