SLA Battery Charging
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Table of ContentsLead acid batteries have come a long way. They have an incredible number of man-hours in research, science, and manufacturing technology. The high voltage, robustness, infrastructure and low cost will make sure they stick around for a long time.
Weight We have visited at least 10 factories in China. One interesting thing that I learned is that you can judge a sealed lead acid battery by its weight. They said "If you want a cheaper battery, no problem, we will just use thinner plates and less lead." Of course the thinner plates will fail faster and give less lifetime. That is the trade-off. All the battery factories in China run off the same basic profit margin, so if the battery is significantly cheaper, now you know why. You can judge the quality of a sealed lead acid battery by its weight.
Coulometric Efficiency. This is the efficiency of battery charging based solely on how many electrons you push in. If you compare watts in to watts out you have to take into account that the battery charging voltage is higher than the battery discharging voltage. 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. This also means than nothing below 2.15 volts per cell will do any charging (12.9V for a "12V" battery) However, most of the time a higher voltage than this is used because the battery will accept higher currents, enabling the charging reaction to proceed at a higher rate. Charging at the minimum voltage will take a long long time--over 200 hours. At 2.25V per cell (13.5) it would take 85-120 hours to fully charge. As you increase the voltage to get faster charging, the voltage to avoid is the gassing voltage, which limits how high the voltage can go before undesirable chemical reactions take place. Charging voltages range between 2.15V per cell (12.9V for a "12V" 6 cell battery) and 2.35V per cell (14.1V for a "12V" 6 cell battery). These voltages can be applied to a fully charged battery without overcharging or damage, since they are below the "gassing" voltage, and cannot break down the electrolyte. If the battery is not yet fully charged you can use much higher voltages without damage because the charging reaction takes precedence over any over-charge chemical reactions until the battery is fully charged. This is why a battery charger can operate at 14.4 to 15 volts during the bulk-charge phase of the charge cycle. Using modern precision chargers allows both a fast charge and safe floating voltages, allowing them to be left on the battery continuously.
6V batteries need to stay below 7.1V to avoid gassing, and typical charge voltages are 6.9V (float) to 7.5V (bulk charge).
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) (13.5V to 13.8V for a 12V battery). 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 (14.7V for a 12V battery) to get the highest charge rate, as long as the voltage is dropped to the float voltage when the charge is complete.
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 * 2.60 16.02 to 16.56 2.97 17.82 -10 °C * 25.1 15.66 to 16.2 2.65 15.9 0 ° C * 2.45 15.3 to 15.9 2.54 15.24 10 °C 2.41 14.94 to 15.54 2.47 14.82 20 °C 2.37 14.58 to 15.18 2.415 14.49 25 °C 2.35 14.40 to 15.00 2.39 14.34 30 °C 2.33 14.22 to 14.82 2.365 14.19 40 °C 2.30 13.86 to 14.46 2.33 13.98 50 °C 2.26 13.5 to 14.10 2.30 13.8These are the absolute cheapest chargers around. They used to be very common when semiconductors were expensive and regulation was complicated. 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. These high voltages can force electrolysis of the water in the battery's electrolyte. These unregulated chargers must not be left to trickle or float-charge a battery, they should 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, PowerStream's testing has shown that leaving the battery to be overcharged even at C/10 (a 10 hour charge rate) 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 trade-offs, 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, precise, regulated chargers that use switchmode power conversion.
Another inexpensive 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 (even when left on the battery permanently), 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 linear 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. An example of a switching-type taper charger is here .
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.
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 and the battery is on float permanently. The large chargers at An example of a switching type taper charger is here are constant voltage chargers.
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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. I use these to keep my motorcycle and lawn mower charged in the winter, and my car charged in the summer.
Big battery applications such as fork lifts, floats, and golf carts have traditionally used what is called rectifiers to charge their batteries because of the relatively low price for large power levels. The "rectifier" consists of a transformer and diode bridge array and possibly some control or readout electronics. These work well, but the voltage might not be well regulated, which is made up for by using flooded batteries where the water can be topped off. These chargers are not appropriate for sealed lead acid batteries because their water cannot be replaced. And modern switchmode technology has made it possible to make inexpensive well regulated lead acid battery chargers such as this watt 48V charger.
Custom design and manufacture of state-of-the-art battery chargers, UPS, and power supplies for OEMs in a hurry!
With the installation of a new set of VRLA Lead-acid batteries (or any brand or type of battery) it is very important to program the correct charging parameters of the equipment that will be connected to the batteries. Solar equipment such as Solar MPPT Charge Controllers, Chargers and Inverter/Chargers setpoints are critical to ensuring the battery bank will perform to its optimal level and prolong their useable life.
What are setpoints? Battery Charge setpoints are voltage, amperage and time settings programmed into the battery charging equipment connected to the battery bank. Battery charging equipment can consist of one or more of the following:
Why is this important: If battery banks are not charged to specific voltage levels or are overcharged, not held at a voltage level for sufficient length of time the batteries are likely not going to be properly charged. Over time, incorrect or poor charging as a result of incorrect settings (not an equipment failure) will result in poor battery performance (because they are not fully charged), damage to the battery bank (If too high voltage) or battery sulfation (the result of prolonged undercharging).
How are setpoints adjusted: This depends on your battery charging equipment as the process is different across equipment brands. However, below are a few programming options that may exist on your equipment:
When: At the commissioning of a new battery system or after the replacement of an existing battery bank. Charge settings can always be adjusted after commissioning if they are not optimised however, it is best that the charge settings are programmed correctly from the very beginning of a new installation.
Suggested BAE 2V VRLA PVV model voltage charge setpoints are listed below:
It is important to note that equipment brands use different charge terminology and charge algorithms. The below charge stage names and setpoints are intended to be suitable for the majority of equipment.
2v 12v 24v 48v Bulk 2.35 14.1 28.2 56.4 Absorption / Full 2.40 14.4 28.8 57.6 Max Absorption Time (minutes) 180 180 180 180 Float 2.25 13.5 27 54 Equalisation 2.5 15 30 60 Max Equalisation time (minutes) 120 120 120 120 Equalisation Period (days) 90 90 90 90For more information about brand-specific charge set points and information, visit the respective battery model page and the datasheet specifications listed.
BAE PVV 2V VRLA Range of Lead-Acid Batteries
Sonnenschein A602 2V VRLA Range of Lead-Acid Batteries
Battery Energy SunGel VRLA Lead-Acid Batteries
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