There are linear and switched battery chargers. The linear low power chargers are the oldest, and most reliable. The charging current has obviously single range setting. The automatic switch-off was missing. One of the linear charger odds is power dissipation. The device is low noise type. Typical design use diode, resistor, and 60/50 Hz pulsed current from current limited transformer with air gap. Please follow the idea of air gap and transformer. It gives straight forward answer why the switched power supplies are sensitive to 1-2kV lightning pulses. The idea of pulsed current comes from Lead Battery. It is a relief for hydrogen gas release from the electrodes. Few chargers have automatic termination of charging process based on charging time, battery voltage, or ambient temperature. The automated recognition of unknown battery technology (Ni-Cd, NiMH, Lead, Li-Ion) is cumberstone. Automated termination of charging process is easier. When the battery is fully charged, the cell terminal voltage climbs up. The charging energy turns into heat and the electrochemical reaction starts to release oxygen. The charging energy is not stored in the battery any more.
The NiCd batteries have memory effect. To invoke the full battery capacity, the battery cell has to go through controlled discharge cycle. Most of the battery cells shouldn't be discharged under 1V. The switched battery chargers generate less heat and permit fast charging. How Much? That's always the question. The disadvantage of this group is sensitivity to pulses on the AC line, and broadband switching noise. The switched supplies are up to kA of current, the linear are for lower power dissipation. A combination of both works well.

Rechargeable batteries:
Pb-lead : acid, heavy, cheap, reliable, life time up to ten years.
Ni-Fe : alkalic, heavy, reliable, for aerospace applications, life time up to ten years or more.
Ni-Cd : sensitive to overcharging, lower capacity, designed to work in lower temperature range. Up to 1000x discharge cycles. Two or three-year lifetime. Life cycle is terminated by growth of Ni whiskers or separator failure. Complete self-discharge within one or three months. Ni-Cd bat has memory effect.
NiMH : more capacity, more tolerant to over-charging, doesn't like too hot environment above 85F. Self discharge in hot or cold environment. Up to 1000x discharge cycles. Two or three-year life time.
Li_xx : max. capacity per volume, limited 300x discharge cycles per battery lifetime.
Alkaline:The alkaline battery is very good. The alkaline rechargeable battery is very bad.

Charging current for NiMH, Ni-Cd
-Slow charge current is 0.1C. The C stands for capacity in [Ah] Amp hours. That means, a 950mAh Ni-Cd battery will be charged with 95mA for 16 hours. (1.6x)
-Fast charge with 0.315C means, charge the 950mAh battery with 300mA for 5 hours. (1.58x).
-Few companies promote charging current 1C up to 2C. E.g. a 2700mAh battery will be charged by 2.7A up to 5.4A current. (The moron mode). The fast mode actually destroys battery performance, capacity, and life expectancy. Fast charge releases a lot of heat, the battery is damaged, and the high current makes its way through battery by tunneling. The number of charging cycles is reduced to one half or less.

Battery Voltage
-The safe NiMH battery voltage at full charge can climb between 1.42V up to 1.55V. The voltage strongly depends on temperature and charging current. The higher temp, the higher voltage.
The NiMH battery is discharged at 1.10V (few percent of capacity left). Nominal NiMH voltage is close to 1.278V. Depends on the manufacturer.
-The nominal voltage of Ni-Cd battery is 1.265V. It is manufacturer dependent. The charged voltage can go up to 1.53V or 1.6V. The discharge voltage is about 1V.
-There is small voltage differences between similar or same technology. The small AA-NiCd cells can give easily 17A of current.

Chips, special,PWM,etc.
We have evaluated few so-called fast chargers. The results were similar. It is worth to note MAX712 and MAX713 chips from Maxim, the engineering condoms. The chips charged 4000mAh batteries with peak current of few tens of Amps (>10A). The current limiter was not working at all. The Maxim's superstitious charger running overnight ended up with battery explosion. That was an interesting reliability check. Believe it or not, maxim tries to sell this shit for 15 years. Most of MAXIM RF products, oscillators, WiFi chips, and tranceivers work the same way. :))
Another charger used the brand name "Energizer". Made in China. Same sheet. It works with minimum user interaction and significantly destroys the battery performance. There were quite few different chargers tested.
It is worth to note Notebook computers. Many notebooks ended up in flames or literally melted itself, because of missing thermal control. Wow, a friend of mine melted Dell with JPL data. Another notebook had wrong design of switched power supply and cooked itself, another had bad linear charger, another had cooked switcher in epoxy cube..
Few ideas to follow:
1) Beware of chips with single ground pin!
The groung currents from voltage reference and pulsed devices will lift the gnd pin voltage at the die level. The chip design is completely wrong. The reference pin, digital, and high current pins should be separate.
2) Beware of low voltage chips!
The chip should work up to 30 or 60 Volts. An induced pulse from motor or lightning will cook the 10V device. The common divider says, minimum of combo chips with Vref are intact to input pulses within 3-30V input range. Jack up the voltage. Above 12 Volts input the voltage reference starts moving up and down. That is problem of some Motorola, National, ST, and different chips. The Vref design is neglected. Check the chips, sweep the input voltage. The PWM should be running during the test.
3) Check the Reset and POR!
The POR, power on reset always surprizes.

Common mistakes
A common divider of most fast chargers are unreliability and uncontrolled charging process.
During the switching, the current pumped into the storage capacitor is not fully filtered and flat. That all ends up with extremely high pulsed charging current for few microseconds, up to seconds. The pulsed current actually discharges and destroys the battery. There is no hardware over-current fuse. The average Microcontroller response, running e.g. 6MHz clock is within milliseconds. The charge & recharge characteristics have a lot of voltage bumps associated with the battery chemistry, which makes difficult and curly decisions, instead of straightforward approach. Probably the right decision can be done based on statistically collected, filtered, and averaged data. With help of sufficiently fast and reliable hardware, software independent state machine.
Lithium rechargeable batteries are quite different, difficult, and require specialized electronics. The development samples from china were left on the bench. Few weeks later I found it popping-up like popcorn, inflated, self-discharged, and ready to explode. NiCd Saft is good. The NiMH and Li from Saft are piece of junk. The world is changing; more and more junk floods the market. Quite good rechargeable battery products are Swiss, Japanese, and Varta from NY.

The sum
The best solution from analogue world seems to be a resistor and a diode. The next to it, is a digital solution with fast independent hardware regulation, overpowering the software part. (Parsec Consulting: The software is worn-in, not worn-out.)

item	Ni-Cd	NiMH	Alkaline bat.	Pb - lead	Li-xx
always recharge	No	Yes	-	moderate	no
4 year capacity	no	no	yes	no	no
self discharge	fast	1-2% per day	10 - 20% per year	-	fast
high curent [12A]	yes	yes, 60% of Ni-Cd	moderate	yes	moderate
temperature range	wide	less	wide	wide	less
durability	more	less	+more	++ good	less
memory effect	yes	small	no	no	small
charging capacity	1.5xC =150%	1.5xC, temp depend.	-	till it bubbles	very sensitive
0.03C trickle charge	hurts	not required	-	ok	no
discharge down to	1V / 0.6 to recover	1V	-	-	fire danger
charging current	0.1 C	0.1 C	-	-	fire danger
charged 1C	>1.55V	>1.61V	-	-	fire
charged 0.5C	>1.50V	>1.55V	-	-	fire
charged 0.1C,time	1.47V	1.49V	1.55V	-	regulated
full charge,   and rest	1.323-1.346V	1.2948V	-	13.8V	>2.9V/3V/3.3V
start charging	<1.28V	<1.29V	-	<2V	<3V
considered flat	1.278-1.279V	1.179-1.185V	0.5V	-	auto shut off

The Gentle Battery Charger
for aerospace applications
schematic diagram

- update 2007

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