NiMH Cells

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The latest rechargeable cell on the market is Nickel Metal Hydride (NiMH).
It has a cell voltage of 1.2v (called "Terminal Voltage" as it is the voltage across the terminals of the cell) and a capacity higher than previous types of cells. It's a real advancement in technology.

Batteries (cells) have come a long way and to understand this cell we need to see how it came to be.
Battery development is an evolutionary process.
Rechargeable cells started with the "Lead-Acid" cell as found in the "Accumulator" or "Battery" of all cars in the past 100 years.
Each cell of a "Car Battery" has a cell voltage of 2.1v and contains an electrolyte of Sulphuric Acid (H2SO4). (Not concentrated Sulphuric acid and not weak Sulphuric acid but a fairly strong solution.)
The battery was charged via a Generator (A DC generating device) and later by an alternator (An AC generating device with a set of diodes to convert the AC to DC). As the cell became fully charged, the electrolyte became heavier (stronger) and its Specific Gravity (SG) could be tested with a Hydrometer (a device that looks like a large syringe and containing a float). As the battery acid got heavier, the float would rise higher and indicate the state of charge.
This type of cell could not be carried as the acid could easily spill and as it charged, bubbles of Hydrogen gas (and Oxygen gas) would be emitted from the cell through a small hole in the filler hole. This gas is dangerous (explosive) and the release caused the level of the electrolytic to fall in each cell and it had to be topped up with distilled water.
The gas also contained a small amount of water vapour and carried some of the sulphuric acid vapour into the atmosphere. This created corrosion of any nearby metal objects and deposits of chemicals on and around the battery. It was very messy.
The next advancement in battery technology was to gel the electrolyte so it did not spill. The cells were also sealed so the battery could be used in portable applications.
But charging was a problem as the gas built up in the cell and caused a high pressure that could explode the cell. A pressure-release valve was included but the concept was not successful until the battery chemistry was changed by adding different elements (compounds) to the structure of the plates so that gassing did not occur until the cell voltage was over about 2.3v.
By providing a detecting circuit, the battery could be fully charged and the charging current turned OFF before the cell started to "gas."
This solved nearly all the problems and the market produced a wide range of "Gel Cells"  or "Sealed Lead Acid" batteries.
But a problem remained.
If the battery was not recharged when the terminal voltage for a 12v battery dropped to 8v, the cells started to "Sulphate" (sulphate-up). This caused a chemical coating to be deposited on the surface of the plates and it prevented the battery being recharged.
For example, a brand new battery left to discharge, would be completely worthless after 12 months.
The next evolution in battery design was the NiCad, (Nickel Cadmium) cell.
It had considerably more capacity (per unit size) than the lead acid cell and the AA, C and D cell appeared on the market.
These cells gradually improved in performance and the capacity increased, the robustness improved, the life of the cell increased and cost decreased.
But one characteristic of the cell could not be altered.
The cell exhibited an effect called "Memory."
This severely changed its capacity over the life-span if the cell was not charged correctly.
Suppose 50% of the capacity of a new cell was used in an application, and the cell was recharged. The cell would not return to 100% capacity. It may return to 95%. If this procedure was repeated, the capacity of the cell would soon fall to 80%, 50% and less.
To prevent this, the cell had to be discharged to zero and fully recharged.
This could be done on special chargers with a discharge feature.
Now we come to the latest in battery technology - the Nickel Metal Hydride cell.
This cell does not exhibit memory problems and the capacity is considerably higher than NiCad.
But there is one very small problem.
When charging the NiMH cell, the voltage across the terminals does not rise when the cell becomes fully charged. In fact it drops a small amount.
This is very difficult to detect and that's the point of our discussion.
There are three ways to charge a NiMH cell. It can be "Fast Charged," "Slow Charged" or "Trickle Charged."
Fast Charging refers to a charge time of 1 hour to about 4 hours. Slow Charging refers to a charge-time of 10-14 hours and Trickle Charging refers to a charge-time of more than 40 hours.
When ever a cell is being charged, you must deliver 150% of the capacity of the cell, to fully charge it (when it has been completely exhausted).

Fast charging a cell involves a very high current and this causes the cell to heat up.
When the cell becomes fully charged, the charge-current is converted to a "gassing" process. The cell has the capability of re-combining the gasses but not at the rate of production during fast charging.
Re-combination of gasses creates a lot of heat and as soon as the cell becomes fully charged, it heats up considerably.
This characteristic can be detected to turn off the charger.
No cell likes to be fast charged and there is some limitation to the life of a cell that has been repeatedly fast charged.
Fast charging a single cell (of the type: 2400mA-hr AA) requires a current of about 2.4 amp. This equates to a wattage of about 2.8watts.
This means you require a 2.8watt power supply for each cell you want to charge (at the same time).
For a 10 cell battery, this requires a 28 watt power supply (plug pack).  This type of plug pack is not available.
That's why fast chargers only charge 1, 2, 3 or 4 cells at a time.
There are a number of fast chargers on the market. They deliver a high current to the cell and detect the terminal voltage or the rise in cell temperature. And the cells must be taken from the equipment and fitted to the charger. These chargers are also very expensive.
For a 12v battery, 10 cells are needed and this requires a lot of work to recharge them.
Our solution is to use the SLOW CHARGE method and keep the current below the "10-hour rate" so the cells will not be damaged.

If the charge-current is 1/10th the ampHr rating of the cell (this is called c/10), the cells will be charged in approximately 14 hours.  After this time nearly all the gasses generated in the cell will be re-combined but the pressure will not build up very slightly. For this reason, the cells must be removed from the charger after 18 hours.

If the charge current is 1/40th the ampHr rating of the cell (c/40), the gasses generated in the cell will be re-combined and the pressure will not build up.
The cell can be kept in a state of trickle charge for a long period of time without damage.
Normally, TRICKLE CHARGE was referred to as the "14-hour" charge-rate, so that a completely exhausted cell would be fully changed if the current was one-tenth the amp-hour rating of the cell. For instance, if the cell was 1.2 amp-hr, the charge current would be 120mA for 14 hours. This referred to a NiCad cell, but for a NIMH cell, the trickle charge time is "40 hours."

We have three circuits to charge NiMH cells. One circuit for a 12v battery (10 cells),  one for 1-8 cells at slow-charge rate and one for 1-8 cells at Trickle Charge rate.
The first two chargers will take about 18 hours to fully charge a set of cells and this is the maximum time allowed. The charger must be disconnected after this time.
The third circuit will trickle charge any type or combination of cells and the charger can be connected for long periods of time without damage to the cells.
The circuits are very clever. They will not overload a 500mA plug pack and not allow the cells to heat up.

This circuit will charge a 12v battery (10 NiMH cells) @ approx 220mA.
It is a very simple circuit but requires correct setting up.
The circuit is designed around a 12vDC, 500mA plug pack. This type of plug pack delivers about 17.5v to 18.5v DC on no-load and about 13v when the current is 500mA.
Our circuit delivers 220mA to a set of AA cells and the output of the plug pack is approx 16v (when 220mA flows) and the voltage across a set of 10 NiMH AA cells is approx 13.5v during the charging process.
This gives approx 4.5v for the "head voltage," and this creates the situation where the plug pack will deliver a current to the battery. 
Firstly, you must remember, every plug pack has different characteristics.
No two are the same and you cannot believe the specifications printed on the package or the plug pack.
That's why you have to follow our guidelines to determine the suitability of the plug pack you will be using.
Do not use a 300mA plug pack. It will get too hot in this application.
Some 12v DC 500mA plug packs are quite small, others are much larger.
What is the difference?
Plug packs (actually the transformer part of the plug pack) have a hidden rating called REGULATION and for this application you must use the biggest and heaviest plug pack you can buy.
REGULATION is the comparison of the voltage under load with the voltage on no-load.
When the voltage under load is only slightly les than the no-load voltage, the transformer is said to have Good Regulation.
If we take a small 12v DC 500mA plug pack, the no-load voltage may be 17.5v, but the fully loaded voltage may drop to 11.9v.
A larger 12v 500mA plug pack will have a no-load voltage of 17.5v, but the fully loaded voltage will drop to 13.2v.
When a smaller current is drawn from the plug pack, the output voltage is slightly higher. That's why we can use a 12v DC plug pack for this application.
We need about 13.6v to charge 10 NiMH cells and a large 500mA plug pack will deliver more than this voltage when the current is 220mA. The small plug pack will not work at all.
But we can't connect a plug pack to the 12v battery without making an important test.
You must place a multi-meter (set to 500mA or higher) in series with one lead when the plug pack is connected to the 12v battery.
The diagram below shows you how to do this.
If the current flow is more than 260mA, you will need to reduce it by adding a resistor in series with one lead.
In most cases, the lowest value resistor in your "parts bin" will be 10R (brown, black, black, gold).
By placing two 10R in parallel, you will get 5R @ 0.5watt:

If you connect them in one of the leads, the current will drop between 20mA and 40mA, depending on the type of plug pack.
You can add another pair of resistors to drop the current further, as shown in the diagram below:

You have now produced a very simple charger for the 12v battery.
It will take up to 24 hours to charge a set of exhausted 2400mA-hr AA cells and you will not have the worry of overcharging them. 
If you have cells with a lower capacity, you will have to reduce the current accordingly. For 1800mA-hr cells, deliver about 150mA.
For larger cells, such as C and D, you can charge them at 220mA for more than 24 hours.
You must remember, when a plug pack is used in most situations such as a radio etc, the current requirement varies enormously with the volume and type of music etc.
That's why a plug pack will not normally get very hot.
But in our case, a high current is being delivered or long periods of time and that's why you should not exceed 220mA.

This circuit will charge 1 - 8 cells @ approx 220mA. It cleverly keeps the current at a constant 220mA, no matter how many cells are connected and the current will remain constant as the cells charge.

This circuit will TRICKLE CHARGE 1 - 8 cells @ approx 75mA. The circuit is adjustable from 10mA to 120mA so any type of cell can be charged.
Believe it or not, you can mix any size and type of cell with this circuit as each will become fully charged after a period of 2 days or more, without you having to think about the situation.
You can mix any type of cells (NiCad, Li-ION, NiMH) and any size (AA, C or D) and they will gradually charge according their own characteristics and capacity.
You don't have to worry about anything.

We cannot provide accurate current-flow for any of the circuits as this will depend on the type of plug pack.
You must place an ammeter (0 - 500mA range) in one lead as shown, and adjust the current to the desired value. The scale around the 500R pot is not linear, so you cannot guess the current-flow. You must use a meter to read the current.
For a set of 2400mAHr cells in TRICKLE MODE choose a current of 60mA.


The circuit uses an adjustable voltage regulator. The LM 317 range of regulators detects 1.2v between the adjust pin and the OUT pin. In other words, the OUT pin is always 1.2v higher than the adj pin.

Suppose the battery voltage is zero. The voltage on the "adj" pin will be zero.
The LM 317 will produce a voltage of 1.2v on the OUT pin.

Current will flow into the battery and the voltage across the terminals of the battery will increase. This is how the battery will get charged. The value of the charging current will depend on the value of "R."
The value of current can be found via the formula:

Where I is the current flowing in AMPS, V is the voltage across the resistor (in volts) and R is the value of the resistor (in ohms).

We know the voltage is a constant 1.2v, so the formula becomes:

Suppose the value of R is 10R, the current will be 1.2/10 = 0.12A = 120mA

In the circuits above, the value of R is the resistance between collector and emitter of the transistor.  This resistance is modified by controlling the current into the base.

As the voltage of the battery increases, the output of the regulator increases and the current keeps flowing at a constant 120mA.
When the battery voltage reaches 13.5v, the output of the regulator will be: 13.5v + 1.2v = 14.7v. The regulator requires approx 3.5v across it to work correctly. This means the IN voltage must be 18.2v  This is the voltage delivered by the plug pack.

Note: The 12v DC shown on the diagrams above is only a value indicated on the plug pack. When the voltage is measured with a multimeter, you will find it is 17v - 18v under no-load and 13v -14v under full load.