Sealed acid batteries - properties and applications.

Lead-acid batteries are still present in many devices, despite the availability of modern lithium polymer, lithium-ion or supercapacitors. The affordable price of lead-acid batteries and the simple design of the charger make them a popular source of energy for both buffer and cyclic operation. Sealed lead-acid batteries do not have to work only in the vertical position (they can work in any position except for the position with the pole terminals down). Maintenance-free batteries VRLA (Valve Regulated Lead Acid - i.e. with self-regulating safety valves and internal gas recombination) occur in the technology AGM (Absorbent Glass Mat - with electrolyte trapped in a glass mat separator) and GEL (with electrolyte in the form of a thick gel). Batteries in GEL technology can withstand more cycles than AGM and have greater resistance to deep discharge, while AGM batteries can deliver higher powers and have a lower internal resistance. GEL batteries dissipate the heat generated inside them well, they will work well, for example, in renewable energy installations. AGM batteries work better at higher currents, delivering higher power, they will work well, for example, in UPS devices. VRLA batteries do not require maintenance during operation, so there is no need to add fluids to them, the risk of electrolyte leakage has been reduced and the resistance to environmental conditions has been increased. Despite their tightness, VRLA batteries must not be sealed in a sealed casing, as overcharging and improper use may emit flammable gases that may explode without proper ventilation.

Regardless of the technology used, you can meet the colloquial term "gel" in relation to AGM and GEL batteries. Batteries VRLA / SLA they come in different sizes (and capacities), the voltage usually specified as 6V (three cells) or 12V (six cells). There are also batteries with a voltage of 2V and a capacity of several hundred Ah (or even 1000Ah), intended for series connection into larger units. The stated battery capacity relates to specific operating conditions (e.g. discharge time, temperature), usually the given capacity relates to a discharge within 20 or 10 hours. Battery capacity will be different from the nominal for faster discharge (it will be lower for higher currents). Capacity is usually marked with the letter C and expressed in Ah.

Operation - cyclical and buffer operation.
The cyclical operation of the battery consists in its cyclical charging and discharging, e.g. in vehicle drives or energy storage systems obtained from renewable sources.
In the case of buffer operation, fully charged batteries wait for a power failure, allowing for continuity of power supply to the loads (UPS, buffer power supplies, emergency lighting).
VRLA batteries can be charged with an appropriately adapted charger, taking into account the charging current (usually 0.1C20HR, i.e. 10% of the (twenty-hour) capacity), limiting the charging current to 0.1C should ensure good battery life. For example, a 45Ah battery should be charged with max. 4.5A to keep long service life. VRLA batteries should not be charged with a current greater than 0.3C, the charging process will be faster, but the battery will wear out faster.
In more advanced charging systems, the temperature of the batteries is taken into account, and techniques are used to increase the battery life (e.g. discontinuous charging in the case of buffer operation).
Some manufacturers have products dedicated to buffer or cyclic operation, others provide universal batteries that should be charged to voltage for buffer operation. 13.5-13.8V (2.25-2.3V / cell), and for cyclic operation 14.4-15V (2.4-2.5V / cell).

The conserving voltage for a single cell depends on the ambient temperature:

Voltage V / cell	temperature ° C	battery voltage "12V"	battery voltage "6V"
2.35	0	14.1	7.05
2.33	10	13.98	6.99
2.27	twenty	13.62	6.81
2.25	25	13.5	6.75
2.23	35	13.38	6.69

Cyclic charger should stop charging after reaching the assumed voltage threshold. You can see a drop in battery voltage when charging is interrupted. After 24 hours of resting the battery, you can determine the degree of its charge by measuring the voltage at the terminals.

State of charge	voltage V / cell	voltage on the battery "12V"	battery voltage "6V"
100%	2.15	12.90	6.45
80%	2.09	12.54	6.27
60%	2.06	12.36	6.18
40%	2.02	12.12	6.06
twenty%	1.97	11.82	5.91

For buffer operation, manufacturers offer battery models with a lifetime of ~ 5l, 10-12l, as well as specialized 15-20l. Recommended operating range for buffer operation is 20-25 ° C.
For cyclical operation, some manufacturers provide an estimated number of cycles that the battery should withstand, but the actual number of cycles depends on the depth of discharge in the cycle (the shallower the more cycles can be achieved) and the temperature (the higher the battery, the shorter its lifetime).
The battery can be discharged in cyclic operation in the temperature -20-50 ° C. Both low and high temperatures have a negative impact on the lifetime and parameters of the battery, one should take into account the capacity (low temperature) and lifetime (high temperature) limitation.

For unused batteries, it is recommended to charge them once every 6 months, which allows you to compensate for self-discharge. The mentioned 6 months is the minimum time of keeping 50% of the stored energy at 20 ° C, for 30 ° C the time to the next charging is reduced to 4 months, and at 40 ° C 2 months.

Storage time	temperature
6 months	20 ° C
Four months	30 ° C
2 months	40 ° C

Batteries operating in series connection should be "equalized", that is, come from the same source and production period, have the same service history, and thus have similar internal resistance. The entire set of batteries in series should be replaced.

Depending on the capacity, the batteries can be equipped with terminals in the form of "slides" or screws. When fastening with a thread, pay attention to the tightening force, indicative

Diameter	Thread pitch (mm)	Screw tightening torque (Nm)
M5	0.8	2.0-2.9
M6	1	4.1-5.2
M8	1.25	8.2-9.9
M10	1.5	14.7-19.2

During assembly, be careful not to lead to a short circuit. Resistanceinternal of the battery is very low (several to several dozen m?), the value of short-circuit current can reach several hundred A or even several kA (with higher battery capacity). In the case of series connected battery packs, the total voltage can be life-threatening.

Final discharge voltage.
The VRLA battery cannot be discharged "to zero". Often you can come across a voltage cut-off value of ~ 10.5V for a "12V" battery (1.75V / cell). Such a voltage value is usually set by the systems protecting the battery against excessive discharge. A discharged battery should be charged immediately so that it is not stored in a discharged state.
In the event of a complete discharge, the sulfation of the plates is significant, which significantly increases the internal resistance of the battery. Fully discharged battery should be charged with a voltage of 2.27 V / cell with a current not greater than 0.1C20 in order to avoid excessive heating. The minimum charging time should be 96 hours.

Cut-off voltage 10.5-10.8V for the "12V" battery it is a good generalization, allowing for example to build a module protecting the battery against excessive discharge. If we have control over the discharge rate of the battery, we can precisely control the cut-off voltage.

Cut-off voltage V / cell	Discharge time	battery voltage "12V"	battery voltage "6V"
1.60	up to 15 minutes	9.6	4.8
1.65	up to 1h	9.9	4.95
1.70	up to 5h	10.2	5.1
1.75	up to 8h	10.5	5.25
1.80	up to 10-20h	10.8	5.4

It is worth looking at the documentation of your VRLA battery, it will allow you to select the optimal operating conditions for the device.

In which projects do you use VRLA batteries, what undisturbed parameters are important to you?

Material prepared in cooperation with the company volty.pl