Lead batteries AGM (Absorbent Glass Mat - “absorbent fiberglass”) is a further improvement in battery technology, classic sulfuric acid batteries. Due to their advantages, they have taken a strong place among the already known types of batteries.
Monobloc sealed AGM battery
What is an AGM Lead Battery
In traditional batteries, the electrolyte in the battery cell (bank) is contained in a liquid state. Instead of bulk liquid, AGM batteries use special fiberglass material with high porosity. The pore sizes in it are of the order of tenths of a micron. Such a liquid does not splash inside the jars during shocks and does not flow out when tilted. At the same time, ionic conductivity is ensured with a low internal resistance, as in conventional batteries. Moreover, the internal resistance of AGM batteries is lower than that of conventional batteries.
AGM batteries are used to start automobile engines, uninterruptible power supplies, communication equipment, emergency lighting, etc. Until recently, they were used in electric vehicles.
Sectional view of an element with filler
The glass fiber packing of the cells is not completely filled with electrolyte, there is some space for the recombination of electrolysis gases. If the charge current slightly exceeds the certified values, these gases are released (hydrogen and oxygen during recharging) in an insignificant amount. However, this is enough to swell and break the body. Therefore, a safety valve is installed in the battery.
Such batteries are of the VRLA type (Valve-Regulated Lead-Acid - lead-acid with a safety valve). The valve releases excess gas and closes. Depending on the design of the battery case, the valve may be common to all compartments, or each element has its own.
Valve system for venting
The location of the electrodes in the battery is usually flat. This is a stack of parallel plates separated by non-conductive separators. AGM battery can be made with a cylindrical, more precisely, a spiral arrangement of electrodes. This doubles the highest working pressure inside the element, which can, in this case, reach 2.7 kg / cm2. The spiral arrangement was patented in the United States and until now the owner has not sold the license to Europe or Asia, so such batteries can hardly be found on the Russian market.
Despite this, batteries with a flat electrode arrangement have their advantages. They have a longer service life. The current given by flat banks is lower than that of cylindrical ones, about one and a half times. However, in this case, such a battery is capable of producing a current of 500-900 amperes. (Compare with 200-300 A for a starter starter battery.) The increased current delivered to the load is explained by the lower electrodes than polarization cans.
The polarization phenomenon is that the electrodes are covered with tiny gas bubbles, because of this the contact area with the electrolyte decreases and, of course, the current decreases. Usually, polarization does not begin to appear immediately, but after a few seconds after applying a large load. So, if the engine is “not picked up”, then you have to wait a while.
In AGM technology batteries, the fiber pad immediately absorbs gas bubbles. This occurs due to capillary forces, it can be said that the filler “wipes” the wafers from the bubbles.Therefore, the high current efficiency is significantly extended in time, compared with a purely liquid version of the element. This is the main feature of AGM batteries. It is hardly worth specifying how this affects when starting a car.
Nevertheless, you should not believe the claims regarding the wonderful properties of AGM in winter. Low temperature acts on such batteries according to all laws of physical chemistry and does not improve their properties in any way in comparison with classical banks.
Pros and cons
The benefits are as follows:
- 1.5 times more current;
- work in an inclined position;
- no acid leaks;
- no electrolyte needed.
The disadvantages are the same as with all lead batteries: high weight (about 30% heavier than usual), relatively low capacity (compared to lithium), low environmental friendliness, like all lead batteries.
Batteries of various designs
How to choose?
When choosing an AGM battery, you need to start from the starting current that the starter requires from the customer. Since an AGM battery is more expensive than usual, its purchase should be justified. If the car has a diesel engine and it is stored in the garage at a temperature not lower than -20 ° C then it is worth buying. Such a battery, of course, also does not hurt the inhabitants of warm regions, but you can do without it.
When starting from AGM, on average, no more than 1% of the charge is taken, so this battery will be suitable for those who are often forced to turn off and start the engine.
AGM Battery Operating Modes
For proper operation, you must understand how to charge the battery. The battery can be in one of three states:
- storage;
- charge;
- discharge.
The state of charge is also divided into several stages, which is caused by the chemical characteristics of the charge process:
- high current charge;
- normal current charge;
- recharge;
- storage (with connected switchgear).
When discharged by AGM cells are able to give noticeably more ampere-hours than conventional batteries.
Important! AGM batteries must be charged with constant voltage, the upper limit of which is limited. Otherwise, at least the battery life will be drastically reduced. Charger voltage must be stabilized. During long-term storage, it is useful to constantly recharge the AGM battery with a special charger.
The reason for such precautions is that in the electrochemical series of voltages, lead is placed next to hydrogen, and a +0.12 V excess of the voltage above that indicated is enough, and hydrogen will begin to be released. This is especially evident at the end of the charge. If you just “tumble it down” as the men in garages do, then excess hydrogen will simply break the microporous packing. And the quality of the battery will plummet. Electrolyte boiling does not threaten a liquid battery, but AGM is a completely different matter.
A conventional charger charges about 70% of the battery. For a full charge close to 90% of the nominal capacity, a 3-stage charge mode is used using specialized switchgear (float charger).
Graph of current and voltage when charging AGM
The figure shows the processes of charge (red line of the graph) and discharge (blue line). The symbol C20 means the battery discharge mode in hours: 20 hours. We can get the corresponding current, knowing the battery capacity. Let for example, the battery capacity written on the label: 80 A / h. At C20, the current will be 80/20 = 4 A.
The charge in 3 stages begins with a stabilized current, a quarter of the rated capacity. For a battery of 80 A / h, this is 20 A (not more than 28 A). The temperature should not exceed 35 ° C, otherwise the battery life is reduced. The device monitors the voltage, and when it reaches 2.45 V per can, it switches to voltage stabilization mode. The charge current gradually decreases. When it is installed at about 0.5 A, the device switches to storage mode (self-discharge compensation, float) and stabilizes the voltage at 2.3 V per bank.
Typically, motorists neglect the "subtleties", and lose money on this, often buying new batteries.
AGM Battery Care
It is not entirely true that such batteries are maintenance free because they are maintenance free. From time to time, you need to remove the battery, recharge it if necessary, and carefully wipe the case, especially in those places where the terminals come out. Dirt, and the obligatory presence of moisture and salt there leads to significant self-discharge of the battery. In addition, when cleaning the body, you can notice a crack in the body in time and avoid getting acid on the car body parts.
