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How To Charge AGM Battery
AGM battery, also known as VRLA battery, is a sealed valve-regulated lead-acid battery with AGM material as the separator. There are mainly three types. One is used as a starter battery for automotive due to its high current performance. One is focused on deep cycle performance, used in solar & renewable energy, UPS backup power, telecom base stations, CCTV, and other BESS(Battery Energy Storage System). The 3rd type is focused on higher current discharge as for motive power.
Deep cycle AGM battery is a type of VRLA battery focused on deep cycle performance. Sealed, maintenance-free, the biggest advantage, excellent deep cycle life, achieve more than 1000 cycles at 50% DOD.
SYNERTAC Deep Cycle AGM Battery
AGM battery is very sensitive to overcharge and over-discharge, improper charging and discharging will lead to the reduction of battery life. So the proper charging process is very important.
The Charger
If you are using a VRLA battery smart charger, it is certainly the best choice. Can we use a regular charger?
Well, it depends. You have to be very professional and careful, as well as follow the principle of three-stage charging.
The whole process could be very complicated, so in general, it is still recommended to have a dedicated smart charger.
CTEK Smart Charger
Three Stages Charging
The THREE STAGES CHARGING are Constant Current charge (bulk charging), Constant Voltage charge (absorption charge), and Supplemental charge (float charging).
The 1st stage is constant current charging, CC for short name. We set the charger voltage to the battery (pack) cycle charge voltage, and the current to the cycle charge current. Please note that too high voltage or too high current will overcharge the battery and damage the battery. The specific voltage and current will be explained in detail in the next 2 parts. During this phase, the battery voltage rises to a constant number.
The 2nd stage is Constant Voltage, where the battery voltage remains at a constant value and the charge current decreases. After the current is reduced to a very small value, stage 3 begins, reducing the voltage to the float voltage.
The 3rd stage is supplemental or trickles charge. The battery current is shallow, and this phase is critical to activate the whole materials inside the battery, which helps to maintain a good health condition of and extend the lifetime.
Charge Curve
For more detailed information, please visit this link:
Charge Voltage
The charge voltage includes cycle charge voltage and float charge voltage, fast charge voltage. In general, the float charge voltage is used for daily floating charges, the lowest. Cycle charge voltage is used for recharging after discharged to a certain depth, it is higher. Fast charge is the voltage of rapid full charge under an emergency condition, the highest, not commonly used.
The nominal voltage of a single AGM battery cell is 2V. In different combinations, the popular voltages are 2V, 6V, 12V. 24V and 48V are also found in battery banks. Specific cycle charge voltage and float voltage refer to the following table. All parameters are generally based on 25°C. Please note that the appropriate voltage is different for flooded or gel batteries.
On regular basis at 25°C for 12V deep cycle AGM batteries, float charge voltage is 13.5V
13.8V, cycle charging voltage is 14.4V
Some batteries are marked with charge voltages on the label or print, including float charge voltage and cycle charge voltage. For example, as shown in the above photo, the float voltage is 13.5V
13.8V, and the cycle charge voltage is 14.4V
14.9V. You can use the charge voltage directly. Or you can also choose the safer intermediate value or minimum value. The highest value of the cycle charge voltage (15.0V) could be considered as the fast charge voltage, which is always considered to cause damage to the battery.
Temperature Compensation
Temperature compensation is -30mV/°C for cycle charge voltage, -20mV/°C for float charge voltage
At low temperatures, the charge voltage may be slightly higher. When the temperature is high, it is extremely important to be careful of overcharging due to high voltage.
Conditions without Print
If the charge voltage is not indicated on the battery, the reasonable cycle charge voltage can be estimated according to the open-circuit voltage (OCV) when the battery is fully charged. Generally, the open-circuit voltage of the 12V battery when fully charged plus 1
1.5V is a reasonable cycle charge voltage.
If you do not know the open-circuit voltage when the battery is fully charged, you can occasionally charge with 2.4V per cell blindly, but it is more difficult to judge when the battery is full. There may be overcharge, causing certain damage to the battery. Note that it’s 2.4V per cell, NOT 2.4V per unit.
If you only need to charge the battery once in the way described in this article, you can skip this step. If you want to use it for a long time, it is recommended to perform this step carefully to determine the open circuit voltage when the battery is 100% fully charged. We call it as OCV (open circuit voltage) at 100% SOC (stage of charge).
There are several ways to know this value:
First, use a DC voltmeter to measure the voltage of the battery that remains more than one hour after fully charged, and the value is directly obtained.
Test Battery Voltage
Second, get from the documents. The value will not be printed on the battery, but usually we can get it from the manufacturer’s operating instructions or MSDS.
Third, for a flooded battery, it can also be obtained by testing the density of the electrolyte of the battery when fully charged. The method is OCV=SG+0.84, for example, the density of the electrolyte is 1.35 g/mL, and the open circuit voltage of the battery is OCV=1.35+0.84=2.19V.
It is important to get the voltage when the battery is fully charged. It can be used to verify that if the battery is fully charged, to determine the battery charge and the depth of discharge.
Charge Current
The charge current is related to the rated capacity of the battery. It is generally 0.1C
0.4C, which is 1/10 to 4/10 of the rated capacity.
If it is a 100Ah battery, it is 10A
40A. It can be measured with a DC ammeter, a clamp meter.
Well, some of the most advanced manufacturers on the market today use thin plate technology. They increased the charging current to 1C, which is equal to the rated capacity. High-current charging will greatly shorten the charging time, but these high-end batteries are not popular enough.
0.4CA are considered in most of the applications.
Charge Temperature
The charge temperature of AGM battery is between 0℃ and 40℃, please keep the charging temperature within this range, otherwise, it is harmful to the battery.
Battery Capacity
Battery Capacity can also be found from the print o label.
In general, the print will indicate the rated capacity of the battery, as shown in the photo, the battery is marked as 200Ah, but the UPS battery is sometimes represented by W. For this kind of battery, the simple calculation method is the wattage divided by four.
Charge Connection
There is not much to say, the positive pole of the charger is connected to the positive pole of the battery, the negative pole of the charger is connected to the negative pole of the battery, then the charger and the mains are turned on, then the battery is charging.
It is worth mentioning that for multi-cell battery charging, I recommend parallel charging. Because parallel charging can improve the consistency of the battery, and low-voltage charging is also safer.
It should be noted that when charging in parallel, the cable should adopt the diagonal method to improve the consistency of the current shunt. The so-called diagonal connection method is as follows.
12V Batteries in Parallel
Determine the charging time, or when to stop charging
If it is not blind charging or charging with an excessive voltage, the charging current is reduced as charging progress.
If the battery is charged by the manufacturer’s recommended charge voltage, in general, the charging current can be considered to be fully charged when it is reduced to 0.03C. For example, a 100Ah battery, if using the voltage recommended by the manufacturer, is considered to be substantially fully charged when the charging current is less than 3A.
Alternatively, the time required to charge 1.2 times the required amount of power is the charging time. For example, a 100Ah battery, fully discharged, needs to be filled with 100Ah to be full; 1.2 times 120Ah; if the charging current is 0.15C or 15A, then 120÷15=8 hours can be almost fully charged.
Some cases during charging
The battery is warm.
During any stage of charging, if you feel the battery is warm to your touch, it is normal, no need to worry.
But if you feel it is very hot, even like burning, you have to stop charging immediately or reduce the charging voltage or current, or both.
It is not right if the battery is “HOT”
A sound like bubbling.
Energy is released in the form of heat when the current passes through the electrolyte causing the water particles to break down. H2 is released, O2 is released, and steam is released. There are other gases like hydrogen sulfide that smells like rotten eggs, it is toxic (especially in a small room). That’s why it is important to keep the battery room ventilated.
If the VRLA batteries are making a bubbling sound but are still within safe charging range, they should be able to recombine the evaporated electrolyte internally. If there is a hissing sound, then you know that it’s gone too far and have to lower the voltage or current or both of them. Or stop for a while and then re-try.
Constant Current Stage only allows the cells to reach 80% SOC. The Constant Voltage Phase is to get the cells to 99%, where some slight bubbling is acceptable. Be sure to monitor the battery voltage and temperature and keep it below 35℃ and ensure that all battery cells have plenty of electrolytes.
Charging in Systems
Solar & Renewable Industry
Victron Charge Controller
Victron Charge Controller
For Solar Charge Controller, it will perform 3 functions:
1. Regulate the output charge voltage, converting the unstable voltage generated by the solar panel into a stable charging voltage out. It also follows the three-stage principle.
2. Monitor the battery voltage. If the battery voltage is too low, cut off the load.
3. Cut off if there is reverse current at night.
UPS chargers and other energy storage system charging equipment, also have similar functions of regulating voltage, protecting batteries, and cutting off the reverse current.
Important Notes
Watch your charging voltage and charging current
Pay attention to special events, stop charging or lower the voltage or reduce the current in time.
Summary
If you are in AGM / VRLA battery business, it is quite important for you to be experienced in the charge & discharge, it will help to extend your battery life and gain more trust during the sales.
Another useful kit to extend the life of batteries in series is the balancer, for more details please check the below link:
Battery Glossary – Charge Voltage
Charge Voltage – the amount of battery voltage when the battery is fully charged or the voltage available at any given point during the charging process.
Various sources describe charge voltage in two different ways, so we’ll cover both here.
Voltage when the battery is fully charged
For this explanation its important to know:
- The voltage of a battery gradually decreases as it discharges
- Most batteries actually have a voltage slightly higher than what you see on the label. 12 volt sealed lead acid batteries usually, in reality, are 12.6 or 12.7 volt batteries when fully charged. Manufacturers make batteries this way because producing units of exactly X volts is actually surprisingly hard. Voltages vary even between batches coming off the same production line. As such, it is safer to have a margin of error so they cannot be accused of marketing a 12 volt battery while selling an 11.7 volt unit. The charge voltage refers to this ‘real’ voltage when the battery is fully charged.
Voltage then is a measure we can use to see if a battery is fully charged, but only if we know what the real voltage should be, not what is on the label. To stop charging a 12 volt battery when it reaches 12 volts would be to undercharge it and doing this regularly will reduce its life span. As such, the term charge voltage refers to the ‘real’ voltage of the fully charged battery in order to know when it is really truly charged.
Voltage available at any given point during the charging process.
The graph below shows how the voltage drops in a particular 12 volt 12 Ah battery as it is discharged. The left most blue line shows what happens if the battery is connected to a 36 Amp Hour appliance – the voltage will drop below 11 in just over 5 minutes. However if the battery is connected to a 0.6 Amp Hour appliance the voltage will still be above 11 after twenty hours.
This graph for a sample 12v 12Ah Sealed Lead Acid battery shows how long it can power devices of various amperage.
During the recharging process the battery gradually recovers its full voltage. Some refer to the charge voltage as how much voltage the battery has at any given point during the recharging.
Guide: Maximum Charging Current & Voltage For 12v Battery
We use batteries in our daily life, so recharging the battery is what makes most of us confused.
Charging your battery on a higher voltage or current can increase the battery's plates temperature which as result will decrease the battery life cycles
So in this guide, I'll explain about maximum & minimum charging current and voltage for a 12v battery
Before we calculate the charging current and voltage first of all let's discuss the difference between current or amps and voltage
Voltage is the amount of pressure that is needed to let the electrons flow & Current or amp is the number of electrons
What is the maximum charging voltage for a 12-volt battery?
The charging voltage of the battery will depend on a few factors, its state of charge or discharge & weather temperature.
And of course, every battery will also have a different voltage limit while recharging
There are mainly 3 stages of charging the battery, Bulk, absorption, & Float stage
Bulk Stage: when the depth of charge of the 12v battery is 80%, the bulk stage means your battery is 80% discharged. So in this stage, the battery will use the maximum voltage input voltage
Advertisements So a 12v lead-acid or AGM battery will use 2.4-2.45v per cell (Read the values on your battery). So 12v battery contains 6 cells so it'll be 14.4-14.7 voltage
Absorption Stage: When the battery is 80% charged is known as the absorption stage. So, in this case, the battery will maintain a lower voltage and the amps will decrease as the battery state of charge will increase
Float stage: This is the stage in which your battery is fully charged or in the maintenance stage. So, in this case, the voltage will decrease, and also the amps will be 0 or 1
If the battery isn't completely charged you can use higher voltage without causing any damage to the battery because the charging response takes priority over any over-charge chemical responses until the battery is completely charged. This is why a battery charger can operate at 14-15 volts during the bulk-charge phase of the charge cycle
When your battery is below 80% charged it will safely accept the higher voltage (read the spec of your battery to figure out the maximum voltage) and maximum current (Which should not be 20% of the total capacity of your battery)
When the battery is 80% charged then the voltage will stay stable 12-12.7 volts (Check the spec of your battery for accurate value) but the current or amps will start to decrease as the battery charge will get closer to 100%
if you're using a charge controller it will automatically do the calculations or you can manually put these values in case to increase the lifespan of your battery.
Chris Tsitouris is a renewable energy professional with 10+ years of experience as Director of Engineering at Solar Spectrum, previously working as Project Manager at SunPower and Energy Analyst at the National Renewable Energy Laboratory. As a thought leader, Chris has authored numerous articles and research papers.