A king of rechargeble battery is the lead storage battery. This is just a troll, mainstream of rechargeble battery is none other than the Li-ion, and the NiMH in the second place.
Compared to NiCd as follows


Those are truth except last two listings. As for Li-ion, those are all truth.
A NiCd battery have been still used for particular usage. e.g. garden lamp which has self sufficient power circuit with a solar panel, cause NiCd is resistant to both overdischarge and overcharge. It could be treated rudely.

A Li-ion battery had come to be expanded use with the popularization of electric devices e.g. celular phone, smart phone and portable player and so on. High energy density was the first reason. And, as for another reason, dedicated chip for charge and discharge was generalized.
Compared to a NiMH battery, a Li-ion has peculiar aspect of usage.

A NiMH battery had been familiarized with versatile method, but a Li-ion had been producted almost as specific battery for devices builted-in.

Of course there are standardized forms for Li-ion batteries e.g. 18650, 14500 etc which are easy to understand cell sizes. And there are devices which could be exchange batteries by a user though but almost devices are typed as built-in battery. Why?
A Li-ion battery would deteriorate easily due to overdischarge, overcharge and overcurrent. With that, drastic results could be brought on fire and explosion. Its organic solvent as electrolytic dissolution equal almost to gasoline. If a Li-ion battery had been hitted with a hammer, it would explode due to inner short circuit.
There is no cell as elemental substances at general shops. Usually a cell with a protect circuit is recommended which could limit overdischarge and overcharge, overcurrent. Of course devices built-in Li-ion battery are mounted the protect circuit.

As for a NiMH battery, --
It would deteriorate same as a Li-ion battery due to overdischarge and overcharge, overcurrent. I heard no case of explosion but case of fire due to overcurrent though, that's very rarely and there is no protect circuit for a NiMH battery.
It could fire only by using incompetent charger. It could overdischarge due to common use, there is no drastic risk.

And the overdischarge is none other than the practice which is done by many users and that makes a NiMH battery to decrease its duration extreamely.



A secondary battery has its proper final voltage, so-called final voltage. As for discharge, a NiMH would be out of working order both overdischarge and cyclecharge by underdischarge.
In other words, a NiMH battery deterionates in capacity due to overdischarge, and drop discharge voltage due to memory effect underdischarge.

As for the memory effect, it's possible easily to be brought in good condition due to few cycle charge which is discharge and charge.
A memory effect is a phenomenon which a battery memorizes shallow charge and low voltage full charge due to cycle charge by shallow discharge, charge after slight discharge. It's called refresh to untie that and bring the battery in good condition, which is needed the discharger.
But, who are doing such a usage?
It's rarely to charge a battery after shallow discharge except by radio control toy users. Case of a radio control toy, they exchange the battery which drops power down even if slightly, and the battery needs to be discharged untill the final voltage before full charge.
In previous times, a NiMH battery was used for portable music players. It would be charged after getting home, expected to use maximum runtime for next, regardless of amount used at once outgo. That cycle charge was the replenish charge.
Now, the Li-ion battery has come to be mounted to such devices, same as smart phone. That never get into memory effect or dendrite.

Rather, as for a NiMH battery, many users inadvertently do overdischarge.
There is no protect circuit for a cell or devices. First of all the devices which use AA or AAA batteries are supposed dry batteries. There are many with cautionary note "no use rechargeble batteries" on backside of a battery door. There are rarely devices which could not turn on with a NiMH battery cause which rating is 1.2V but the rating of a dry battery is 1.5V.
A dry battery is disaposable which could be used depleted. And, it would be overdischarge to use a NiMH same as a dry battery. In a worst case, it could not be used due to discharge too deep.
In a typical case, for a light with a fairly lamp or a LED, it would come to be already overdischarge when the illumination could be noticed drop down. A discharge curve of a NiMH battery is relatively flat till the final voltage, and the timing for exchange was past when the voltage drop down was noticed.

How could we check overdischarge?
It's too bother and be impossible to check the batteries. It's ideal to mount a warning lamp to a device which would be turned on at closing to the final voltage though, that's also too bother and has a space ploblem.
Realistically, it's not wrong to exchange the battery in earlier time due to grasp the runtime. In that case, discharger is needed to discharge the battery to the final voltage as same as a radio control toy mentioned above.
Exchange in earlier time. It's easy to say...

As for me, I sometimes mount a pilot lamp to devices which would turn off due to close to the final voltage. That could be made only due to connect a zenner diode and a resistor, a bombshell LED, a switch in series. It's enough to be current several mA.
So-called final voltage, it's not determined voltage but certain voltage with certain load. It could know the final voltage with a specific device to measure the voltage with battery running on the device after discharged by a discharger.

Of course there are many people who think it's not wrong to discard a secondary battery after short recycle usage. That's right as they are in guessing.



Multi batteries are usually used for a device in case of the NiMH.
After dozen recycle charge, those batteries had come variability of each condition as remaining capacity and voltage at exchange time. And, one of them became depleted by overdischarge and another became to be low voltage by memory effect. That would make larger variability as a negative cycle and the batteries would deteriorate at an accelerating pace.

That would happen for Li-ion batteries which has high risk, same as NiMH batteries. Therefor mostly devices with Li-ion use only one cell. High discharge performance is that reason as secondary. One cell is enough to be protect with a simple circuit.
A typical device with assembled batteries is a laptop computer which has many cells as elemental substances in series and in parallel. And that makes the protect circuit to be complex rather than for one cell. It's called balanced circuit which controls both charge and discharge for each assemblage in parallel. It does not check variability but overdischarge and overcharge, overcurrent though, as result that makes batteries difficult to be variability.
Reliability of the balanced circuit is very important though many recalls had been come into being there.
There are many mobile power supplies built-in assembled Li-ion batteries for charge a smart phone and a tablet. Around 2000mAh capacity one is used one cell. Larger capacity one has assembled batteries with a balanced circuit though, It's an actual situation that there are many power supplies made by unknown manufactures.

How could we manage to use assembled NiMH batteries?
As for a radio control toy, there is a device which could charge and discharge each cell in particularly of assembled batteries package. It can't check variability though, as for the usage it's enough cause overdischarge is rare.
And, users who has a radio control toy for its race makes a group with batteries closed capacity due to recombinant.
That's called matching. Variability is just a natural presupposition and re-assembled batteries without variability would be used due to recombinant.

For the matching, severalfold cells and an instrument for determining cell capacity would be needed. Accumulated used time of each cell is not question. Due to grouping by capacity with same brand cells, as for radio control users, large capacity group would be a set for race, and small capacity group would be a set for practice.
i.e. such a matching could never be commonly usage *g*

As for a commonly usage, some people might have a few set of batteries for rotation, and if he/she had an instrument for determining cell capacity, matching with the few set could extend cells life exponentially. Then, how could we do withough the instrument?
Of course we could do only climb down. That's none other than exchanging earlier, and discharging till proper final voltage by a discharger. With that, the negative cycle could be avoided.
At first, variability could be noticed due to discharge, cause a cell deterionated would be ended to discharge earlier, or it could not discharge by a discharger in case of overdischarge. This cell would be ended to full charge earlier.

In any case, it's better to use high performance and multi functionally devices.



In Europe there are chargers and dischargers controled by a personal computer, and which are used by ordinary people. It means that proper usage of a secondary battery is widely known.
Recently there are chargers and dischargers controled by a smart phone.

POWEREX MH-C9000
-- www.mahaenergy.com/mh-c9000/
This is a regular multi functionally charger and discharger for four AA or AAA batteries controled in particular.
Set current, verify capacity and voltage, current, time both charge and discharge, five modes as charge, refresh analysis, breakin, discharge, cycle charge.

HiTEC X4 ADVANCED
-- www.hitecrcd.co.jp/products/hitec/charger/X4_advanced/
This is a regular multi functionally charger and discharger for four AA or AAA batteries controled in particular, and be controled by smart phone.
Set current, verify capacity and voltage, current, time both charge and discharge, inner resistance, graph on a smart phone.

It's easy to do breakin, cycle charge and so on with such a multi functionally device.
As for the breakin, it does few cycle charge with slight current and makes batteries in best condition, cause a new battery would be deterionated by large current.
As for the cycle charge, it does few cycle both discharge and charge, and its purpose is same as breakin, for not only a new battery but also a battery left derelict and a battery deterionated.

Most tasks could be done with decent charger and discharger.

What device is decent? At first, it should control cells in particular, next, could select currents especially low current. Quick charge is often used as sales appeal though, it has no worth. It's better to be able to determine depleted, to have thermistor, to have a fuction refresh which could discharge.

By the way, XTAR is good manufacture for charge a Li-ion battery.
For an example, recently priced down as follow
XTAR VP2, two cells controled in particular, express voltage, for rating voltages as 3.2/3.6/3.8V, charge current option as 0.25/0.5/1A

Around that time, I didn't have the POWEREX MH-C9000 but SANYO MR-58, Panasonic BQ-CC21 and a discharger made by myself, I wished to do the breakin and the cycle charge though that's very bother. As was expected, those all processes could be done by one device. With this reason, I made a charge and discharge board. However it's not automatical like the MH-C9000.
Specifications as follows


-- It's too hard to hand-build that, and I tried to invert a charger named BQ-CC21 as right one on a right picture to the charger circuit.
BQ-CC21 has fuctions of detection battery depleted and 'smart charge' which are in high repute. I don't know how does 'smart charge' work which might be just a replenish charge in guessing.



This circuit picked up below is an autocut self discharger for one cell, with a LED as a monitor lamp, which colored blue for discharge. And red for charge, used clear bombshell 3mm LEDs both of them.

For one cell. Resistor 1.5ohm should be used 2W above.



As for a right pictures, it's not better to use such a battery box for a charger or a discharger.
It disturbs correct charge and discharge cause it has a contact resistance. That might be detected as a wrong battery which could not be charge cause inner resistance is large.

Of course I searched various battery boxes though, could not find good one. And I fixed the wrong terminals to could be in good condition, due to solder between a terminal and a wire directly. With that, the contact resistance would be almost disappeared. A space length would be short slightly, which should be adjusted with a longer AA cell (size is kinda different by brand) and it's best to be compressed slightly.

As for a picture, grommets were used for +terminals though which was exchanged to terminal products for a battery cause the grommet seemed to be brass which would be easily under oxide layer.

That is applicable to lines of circuit with charge current and discharge current. e.g. a switch could not used for such a part.




Transplanted charger board to the shell.
Added a jack and a main switch with a monitor lamp.

A battery box and a controled panel are located upper side of the shell.


Felt fear to treat the high voltage at home used.

Taked care never to mistake.

Added covers to the plugs for prevention tracking, which was diverted shrink tubes. It might be better to bond at the base.


The control unit would be fixed by screw bolts at surface of the shell. Its screw nuts would be fixed due to be filled with something putty or resin.
With that, the shell could be opened with keeping the control unit and the battery box still fixed to the main frame.





There are four tact switches and four slide switches, four holes at upper side of the battery box. Four holes are used to turn hemi fixed resistors for adjusting the final voltages.
That's the control unit.





Tried to open.






on the desk.
became to be messy.





discharger.
nothing around center, that space is for thermistors, and
lots of holes around.




Resistors as load for discharge.

Those would be high heat and should be attached a cooling body.




Discharge circuits are located backside of the battery box.

I had come to be worry whether space is enough or not.




Messy cables






It seemed to might be worked out.






A cooling body product is rugged,
hand-build by aluminum plate.





Finished






Recycle charge old batteries

At first charge




And discharge

It's a self discharger that's no need power supply.




As for AAA cells, used adaptors to AA.
exchange charge current to 275/550/1100mA by switching.


printed ratings table to a seal.









Tried to open the shell.

hope no need to do that ever again.






It's better to buy the MH-C9000 *g*

In actually, its frequency of use is higher than the MH-C9000 which is used for matching only. It's almost enough to be able to discharge and charge by low current. It's not used to charge by 1100mA even if AA battery.

It's complete as functions except to measure the capacity of a cell.

As I mentioned at top of this page, this circuit was found on the internet here the page. This circuit is same as the page, and no explanation about it at here.
I felt it's great as follows


This circuit is used for one cell. Resistor 1.5ohm should be used 2W above. Monitor lamp is a fairly lamp.

I had a discharger made by mayself which detected the voltage of discharge by the comparator and could set the voltage exactly. But it needed an external power supply. And it converted electric energy to heat by a power transistor which was felt kinda illegal method.
After this construction, I had come to use this discharger cause it's convenience to use as no need an external power supply.

It's better to use a LED instead of a fairly lamp.
ICs named HT7750, 7730 could be used as a dcdc for lighting a LED even if white or blue LED as a monitor lamp cause the ICs could be worked by 0.7V above.




As for a right picture, it's a cheap charger made in China which is primitive circuit named tapered charge method. It would condemn to get out cells after full charge which time should be calculated by a user. That would bring overcharge easily.
i.e. That has a high risk.

Of course I had no intention to charge by it.

However the shell was cute and its holders were in good shape, I tried to use the shell of a discharger.

It could be in common used for both AA and AAA cell.







There was a plug at backside which space might be air-intake for exhaust heat.

A space for a trans was kinda wide which could be used for resistors for converting electric energy to heat.
However a space for basements is very small.




This is used for two circuits.

Two units located left side of a left picture are control units would be mounted to the original basement.

A power transistors and resistors located right side of a left picture would be mounted to relatively wide space in which original trans was located. That unit would be high heat and need air flow.
In shade though, Copper plates was affixed on the basement with resistors which prevented to conduct heat to side of circuits.







Tact switches and VRs for adjusting the final voltage.


Holes for exhaust heat. It's around 50 celsius at upper side.

Fairly lamps were used as pilot lamps.