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| Specialising in power supplies, battery chargers, laptop adaptors, mobile phone chargers and computer power supplies. |
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| Specialising in power supplies, battery chargers, laptop adaptors, mobile phone chargers and computer power supplies. |
The coulometric charging efficiency of flooded 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.
Sealed lead acid batteries are higher in charge efficiency; depending on the bulk charge voltage it can be higher than 95%.
Anything above 2.15 volts per cell will charge a lead acid battery; this is the voltage of the basic chemistry. Most of the time however, 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 charge methods have been used. The lead acid chemistry is fairly tolerant of overcharging, which allows organizations to get to extremely cheap chargers. We offer a range of chargers from inexpensive to very sophisticated, depending on the requirements of the customer.
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.
Voltage table for cyclic use charging
| 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 |
Voltage table for standby use charging
| 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 |
These are the absolute cheapest 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 sets 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 charger within 12-24 hours.
Note to our OEM customers: even though we support our OEM customers with unregulated transformer chargers to help them stay cost competitive, many of our new customers come to PowerSupplies Online because someone else sold them an unregulated charger without explaining the tradeoffs.
Another cheap way to charge a sealed lead acid battery is called a taper charge. Either constant voltage or constant current 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 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 switchers are surprisingly cheap, efficient, and small. They are rapidly taking over the overnight charging requirement in consumer equipment.
A more sophisticated and not much more expensive 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.
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 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.
Last Amendment: 30/11/2006 23:11:23
