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SLA Battery Charging
Table of
Contents
- Basics
- Columetric Efficiency
- Minimum voltage
- Cyclic versus Standby charging
- Temperature compensation
- Overnight
Charging
- Unregulated Transformer-based Chargers
- Taper Charging
- Constant Voltage Charging
- Constant Current Charging
- Fast
Charging
- Fast Charging Options
- Three step chargers
Basics
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%.
Minimum 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 undesireable chemical reactions take place. The typical charging voltage is between 2.15 volts per
cell (12.9 volts for a 6 cell battery) and 2.35 volts per cell (14.1 volts for a 6 cell battery). These voltages are
appropriate to apply to a fully charged battery without damage. If the battery is not charged you can use much higher voltages
without damage because the charging reaction occur first.
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 marketing 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.
Cyclic versus Standby 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. Voltage table for cyclic use charging. The higher voltages (above the gassing voltage)
should only be used on flooded batteries that can have the water replaced:
| Battery Temperature |
Charge Voltage per cell |
Charge Voltage for a 12 Volt battery |
Gassing Voltage per cell |
Gassing Voltage for a 12V battery |
| -20 °C * |
2.67 to 2.76 |
16.02 to 16.56 |
2.97 |
17.82 |
| -10 °C * |
2.61 to 2.70 |
15.66 to 16.2 |
2.65 |
15.9 |
| 0 ° C * |
2.55 to 2.65 |
15.3 to 15.9 |
2.54 |
15.24 |
| 10 °C |
2.49 to 2.59 |
14.94 to 15.54 |
2.47 |
14.82 |
| 20 °C |
2.43 to 2.53 |
14.58 to 15.18 |
2.415 |
14.49 |
| 25 °C |
2.40 to 2.50 |
14.40 to 15.00 |
2.39 |
14.34 |
| 30 °C |
2.37 to 2.47 |
14.22 to 14.82 |
2.365 |
14.19 |
| 40 °C |
2.31 to 2.41 |
13.86 to 14.46 |
2.33 |
13.98 |
| 50 °C |
2.25 to 2.35 |
13.5 to 14.10 |
2.30 |
13.8 |
Voltage table for standby use charging:
| Battery Temperature |
Charge Voltage per cell |
Charge Volage for 12V Battery |
Gassing voltage |
| -30 °C * |
2.7 |
16.2 |
|
| -20 °C * |
2.34 to 2.38 |
14.04 to 14.28 |
2.97 |
| -10 °C * |
2.32 to 2.37 |
13.92 to 14.22 |
2.65 |
| 0 °C |
2.30 to 2.35 |
13.8 to 14.1 |
2.54 |
| 10 °C |
2.28 to 2.33 |
13.68 to 13.98 |
2.47 |
| 20 °C |
2.26 to 2.31 |
13.56 to 13.86 |
2.415 |
| 25 °C |
2.25 to 2.30 |
13.5 to 13.8 |
2.39 |
| 30 °C |
2.24 to 2.29 |
13.44 to 13.74 |
2.365 |
| 40 °C |
2.22 to 2.27 |
13.32 to 13.62 |
2.33 |
| 50 °C |
2.20 to 2.25 |
13.2 to 13.5 |
2.30 |
* Note that a fully discharged battery freezes solid at about 0°C, a fully
charged battery
freezes about -40°C.
Overnight Charging
Unregulated Transformer-Based Chargers
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 starts in. These must not
be left to trickle or float charge a battery, they must be disconnected when the battery is fully charged. This is not a
problem with flooded batteries as long as you check the water periodically and refresh it. Sealed lead acid batteries can
recycle the generated gasses as long as they are being overcharged at less than C/3. However, leaving the battery to be
overcharged even at C/10 will corrode the plates if left on for weeks at a time.
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.
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 PowerStream because someone else sold them an
unregulated charger without explaining the tradeoffs, and the end-user complaints forced them to look for a better charger.
Most of the time the complaints come from commercial customers rather than consumer customers. We prefer to offer the inexpensive regulated chargers that use switchmode power conversion.
Taper chargers
Another cheap way to charge a sealed lead acid battery 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.
Regulated taper chargers are very useful when you need a 12V or 24V battery backup. A taper charger
in parallel with the battery, in parallel with the load makes an effective battery back-up. You should take care to ensure that
the taper charger is designed to give continuous current equal to the load plus some left over for battery charging. It is also
important that the current limit of the taper charger is the voltage-cut-back method, and not the hiccough method or other PWM
methods.An example
of suitable switching type regulated taper chargers that can be used in battery back up applications is here
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 suprisingly
cheap, efficient, and small. They are rapidly taking over the overnight charging requirement in consumer equipment. An example
of a switching type taper charger is here.
Constant current chargers
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. This charger will switch to a constant-current mode when desulfating is necessary, and to a multistage precision charger at other times.
Constant Voltage 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 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. The large chargers at An example
of a switching type taper charger is here are constant voltage chargers.
Fast Chargers
Fast chargers are higher power units, designed to charge in less than 4 hours. These chargers require
active charge termination and often have advanced features such as battery test, bad battery recovery, and automatic
maintanence.
Typical Charging curves for PowerStream quick chargers.
This
charger starts at 8 amps and maintains a near-constant current until nearly full.
This is the
fundamental algorithm of the PowerStream quick chargers for lead acid batteries. The curve shown is for a 24 volt (12 cell)
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.
Examples of fast chargers are shown at http://www.powerstream.com/scooter.htm
The exact details of current and time depend on the charger
size and the battery size.
Maintenance, keeper or 'tender'
Chargers
Any multistage charger that has a "float" mode can be used to maintain batteries during the
'off-season.' Particularly useful are the small, inexpensive switchmode chargers that consume very little excess power, or the small low-power chargers that can automatically desulfate lead acid batteries.
Custom design and manufacture of state-of-the-art battery chargers, UPS,
and power supplies for OEMs in a hurry!
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Fax: 801-764-9061 