Mains charging
HOW BEST TO CHARGE WITH A MAINS BATTERY CHARGER
- Coulumetric Efficiency
- Minimum voltage
- Cyclic versus Standby charging
- Temperature compensation
- Unregulated Transformer-based Chargers
- Taper Charging
- Constant Voltage Charging
- Constant Current Charging
- Fast Charging Options
- Three step chargers
Basics
Coulometric efficiency
The coulometric charging efficiency of sealed lead acid batteries is typically 70%, meaning that you must put 142 amp hours into the battery for every 100 amp hours you get out. This varies somewhat depending on the temperature, speed of charge, and battery type.
Minimum battery charging voltage
Anything above 2.15 volts per cell will charge a lead acid battery; this is the voltage of the basic chemistry. However, most of the time a higher voltage is used because it forces the charging reaction at a higher rate. The voltage to avoid is the gassing voltage, which limits how high the voltage can go before undesirable chemical reactions take place.
The basic lead acid battery is ancient and a lot of different battery charge methods have been used. The lead acid chemistry is fairly tolerant of overcharging, which allows marketing organizations to get to extremely cheap battery chargers. We offer a range of battery chargers from inexpensive to very sophisticated, depending on the requirements of the customer.
Cyclic battery charging versus Standby battery charging.
Some lead acid batteries are used in a standby condition in which they are
rarely cycled, but kept constantly on charge. These batteries can be very long
lived if they are charged at a float voltage of 2.25 to 2.3 volts/cell (at 25 degrees C).
This low voltage is to prevent the battery from losing water during long float charging.
Those batteries that are used in deep discharge cycling mode can be charged up to 2.45 volts/cell
to get the highest capacity.
Temperature compensation
| Battery Temperature |
Charge Voltage per cell |
| -20 °C |
2.67 to 2.76 |
| -10 °C |
2.61 to 2.70 |
| 0 ° C |
2.55 to 2.65 |
| 10 °C |
2.49 to 2.59 |
| 20 °C |
2.43 to 2.53 |
| 25 °C |
2.40 to 2.50 |
| 30 °C |
2.37 to 2.47 |
| 40 °C |
2.31 to 2.41 |
| 50 °C |
2.25 to 2.35 |
|
| Battery Temperature |
Charge Voltage per cell |
| -20 °C |
2.34 to 2.38 |
| -10 °C |
2.32 to 2.37 |
| 0 ° C |
2.30 to 2.35 |
| 10 °C |
2.28 to 2.33 |
| 20 °C |
2.26 to 2.31 |
| 25 °C |
2.25 to 2.30 |
| 30 °C |
2.24 to 2.29 |
| 40 °C |
2.22 to 2.27 |
| 50 °C |
2.20 to 2.25 |
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Voltage table for cyclic use charging |
Voltage table for standby use charging
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Overnight Charging
Unregulated Transformer-Based Chargers
These are the absolute cheapest battery chargers around. They consist of a wall mount transformer and a diode. The transformer is designed to deliver 13 to 14 volts over a reasonable current range. The biggest problem with this approach is that when the current tapers off, the voltage raises to 15, 16, 17, even 18 volts. At these high voltages electrolysis of the water in the battery starts in. These must not be left to trickle or float charge a battery, they must be disconnected when the battery is fully charged.
The transformer is so designed as to limit the current while the battery is in absorption mode. As the battery voltage rises, the current decreases to top off the battery. Because the transformer is used to control the current and voltage these chargers are typically heavy and get hot. For best results the battery should be disconnected from the battery charger within 12-24 hours.
Taper chargers
Another cheap way to charge a sealed lead acid battery is called a taper charge. Either constant voltage battery charger or constant current battery charger is applied to the battery through a combination of transformer, diode, and resistance. The unregulated chargers mentioned above are taper chargers. A better, and not very expensive, alternative is a regulated taper battery charger. These don't let the voltage climb higher than the trickle charge voltage, so they can be also be used to maintain a battery. They won't damage the battery if left on charge too long, and they don't change their charging characteristics if the line voltage should change.
There are two ways to make a regulated charger.
The first is to use a transformer and a simple voltage regulation circuit.
This has the disadvantages of weight and heat, but it is still inexpensive.
The second uses a modern switching power supply in a wall mount or desk mount package.
These low power high frequency battery chargers
are surprisingly cheap, efficient, and small. They are rapidly taking over the
overnight charging requirement in consumer equipment.
Constant current battery chargers
A more sophisticated and not much more expensive battery charger
uses an electric circuit to control the charging current. This method is useful for
recovering batteries that have suffered from extensive storage without charging,
but is capable of overcharging a battery if there is not some voltage limiting function,
usually from the transformer. For this reason these chargers are limited to slow charging.
Constant Voltage battery Chargers (Taper plus current limit)
A circuit that is set for the maximum allowable charge voltage, but has a current limit to control the initial absorption current can produce a very nice charger. This type of battery charger can both charge at a reasonable rate and maintain the battery at full charge without damage. Not all constant voltage chargers are made equal, however, because the maximum voltage is a function of temperature. A temperature compensated charger is a little more expensive, and should be used where the temperature varies significantly from room temperature.
Fast battery Chargers
Fast chargers are higher power units, designed to charge in less than 4 hours. These battery chargers require active charge termination and often have advanced features such as battery test, bad battery recovery, and automatic maintenance.
Typical Charging curves for quick chargers:
This charger starts at 8 amps and maintains a near-constant current until nearly full.
This is the fundamental algorithm of the power master quick chargers for lead acid batteries. The curve shown is for a 24 volt (12 cells) battery charger, but the curve is similar at other voltages. The timing depends on the size of the battery you are using. At point #1 the battery is tested. If the battery is bad a rejuvenation algorithm is started. If the battery is good the charger goes into constant current mode until the voltage reaches 2.3 volts/cell. Then at point #2 the charger goes into constant voltage mode until the current drops to about 10% of the initial value, indicating a nominally full charge. Then at point #3 the charger goes into float charging mode at about 2.3 volts /cell to complete the fill and to maintain the battery. At this voltage the battery is safe from overcharge.
High frequency battery charger
The range of high frequency battery chargers represent 20 years of experience in the field of battery charger technologies. The battery charger range is designed for a wide range of inputvoltage 100V to 230V AC. All output voltages is designed for optimum battery charging. The KOP series battery charger allow for 5 level pre-program ability which allow specific battery charging patterns for any kind of battery.
All the battery chargers offer, boost, float and trickle-charge facility.
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