What is the difference between lead-acid and lithium battery technologies?
The primary difference is the chemical composition of the battery used to store and discharge energy. Lead-acid is a well-established technology that is inexpensive and due to the intrinsic self-regulating resistance is easy to maintain. This means that when lead-acid batteries are connected in a combination of series and parallel connections to reach the desired voltage and capacity, respectively, no additional battery management is needed. These two factors make lead-acid the leading solution for industrial UPS installations.
Lithium batteries are common in consumer products and are growing strongly in the industrial market, as they offer smaller dimensions, hold their output voltage longer, and have much longer lifetimes - especially in warmer temperatures. The trade-off at the moment is the comparatively higher price than lead-acid batteries. The material cost of lithium batteries plays a strong role in the price, but also the requirement for a battery management system. Lithium batteries differ from lead-acid, in that the internal resistance is not intrinsically regulated. If left unregulated, the lithium battery will continue to dump energy until failure. For this reason, safety control circuitry is added to the UPS1100 to actively prevent such situations, disconnecting the battery before a dangerous situation can arise. It is worth noting, that this safety circuitry has successfully made lithium battery technology an attractive and safe alternative to lead-acid batteries.
What is important to know about shipping lithium batteries?
All lithium battery variants are grouped together and listed as “Dangerous Good” under guideline UN38.3. This imparts important requirements before such devices can be shipped – offering the maximum safety for the people charged with transporting these devices. Within Europe, such “dangerous goods” must be reviewed by a certified agent to ensure the classification and packing requirements are met before shipping.
These requirements are important, as they specify how you are to ship such devices. For instance, when shipping the UPS1100 5Ah LiFePO4, one must isolate the 2 battery modules within the pack. This information is available in the operator’s instructions within the packaging. Furthermore, when shipping a UPS1100 5Ah LiFePO4 per Air, the classification UN 3480 – PI965 stipulates that the battery state of charge (SOC) cannot exceed 30%. Please refer to the NEC ALM V7s technical data, see the link below, to determine what terminal voltage to which it correlates.
Fig. 1 shows instructions how to connect or isolate the battery modules within the UPS1100 5Ah LiFePO4.
How do I store lithium batteries over long periods of time?
Firstly, when operative, the UPS1100 installed together with the UPS1600, will maintain and regularly verify the battery functionality through a number of tests (see “R Test” in the user manual). Should the application have long periods on inactivity and in turn no access to the mains, the batteries will experience self-discharge, a natural phenomenon of all batteries. The self-discharge rate of the UPS1100 batteries is approx. 3%/month for a temperature of 20° C. This value is temperature dependent and becomes more unfavorable for an increasing temperature, and more favorable for a decreasing temperature. As a consequence, the rechargeable battery should be recharged every 6 months. This recommendation assumes the battery is isolated (fuse removed). When the battery is not isolated from the application (UPS1100 fuse installed), the self-discharge rate increases between 6-9%/month at 20°C.
What is deep discharge?
The chemical reaction responsible for providing electrical energy works most efficiently within a range of voltage levels on the battery. If the battery voltage should fall too low, a state referred to as deep-discharge, the ability to effectively store energy during charging becomes diminished. If left long enough, this chemical imbalance can cause lead-acid batteries to leak. The exact voltage where this occurs depends on the chemical composition of the battery and is supplied by the battery manufacturer.
This is a known property of batteries and is taken into account within the UPS1600. The typical operation range for the UPS1100 batteries is between 18,5-27,3V (lead acid) and 21,5-28,8V (LiFePO4). Should the battery fall below these typical operation range, the UPS1600 will signal the alarm "Battery deep-discharge, no buffering possible" and switch to a specific charging algorithm to carefully charge the battery. A process that does require lower charging currents and in turn longer charging times. If the battery has fallen below 6V (for lead-acid) and 12V (for LiFePO4), the battery can no longer be reliably recharged and has suffered a significant loss to capacity. This triggers alarm "Battery deep-discharged, no charging possible" on the UPS1600, notifying engineers the batteries should be replaced.