This is the third post in a series of posts on making your own mini UPS for DSL modem. Be sure to read the first two parts before you read below.
A battery can be a primary battery or a secondary battery. A primary battery is a non-rechargeable battery while you can recharge a secondary battery.
|Non-rechargeable R6 AA Zinc-Carbon (left side) and Rechargeable HR6 AA Nickel-Metal Hydride by Camelion Battery Co., Ltd, China|
Classifying secondary batteries by chemistry gives us lead, nickel and lithium based batteries. Lithium based batteries are expensive and have tighter tolerances. Lead based batteries are inexpensive, more rugged and can tolerate abuse.
A battery can be high power or high energy. Your car needs a high power battery because it needs higher levels of current for a short duration to crank its engine. Whereas your mobile phone needs a high energy battery because it continues consuming low power for a longer duration.
The capacity of a battery is measured in ampere-hours (Ah). A battery with a capacity of 40Ah can theoretically provide a steady current of 4 A for 10 hours, 2 A for 20 hours, 1 A for 40 hours and so on. Thus the capacity is the product of the discharge current and the discharge time. The capacity of the battery is not a fixed quantity. The capacity decreases as the battery ages. Various other factors also affect the capacity of a battery.
What battery capacity do we need?
The battery capacity in our case depends on the length of power outages. Here in Pakistan a single power outage occurs for a duration of one hour. The one hour duration is typical. In some areas this duration can even stretch to three hours. Whether a single power outage is of one hour or three the total duration of scheduled power outages doesn’t exceeds eight hours. The eight hours is again typical with some areas facing 12 to 14 hours of load shedding. Reading on the internet I found load shedding to be a common problem in neighbouring India too.
What battery chemistry should we select?
Before we find the battery capacity let us first decide about what battery chemistry to use. Every battery chemistry has its own pros and cons. As written earlier, the rechargeable batteries come in lithium, lead and nickel based varieties.
Lithium based batteries are batteries of choice for today’s consumer electronics, aircrafts, electric vehicles. It would not be out of place to mention that the lithium based batteries would be replacing all other battery types soon. These batteries get fully charged in one to four hours. Lithium based batteries have higher specific energy and specific power. These batteries have high cycle (a cycle comprises of a full charge AND discharge) life and are maintenance free. All these factors make this battery type expensive. This battery should not be our choice for this project for its need of protection circuitry and risk of catching fire and exploding when misused.
Nickel based batteries are mostly used in toys, digital cameras, power tools requiring high current and even in aircrafts and electric vehicles. Like lithium based batteries these have high cycle life but have moderate specific energy (lower than lithium based batteries). These batteries are thermally stable, require low maintenance and get fully charged in one to four hours.
Lead based batteries are the one of the oldest batteries. These are even older than lithium or nickel based batteries. Lead acid battery is the most economical than the other two battery types. These have low specific energy, low cycle life, and get fully charged in 8 to 16 hours. All these factors make this battery type inexpensive and easy to use. Lead acid batteries are thermally stable and need some maintenance. Lead acid is a proven technology and should be our choice for this project.
Lead acid batteries come in two varieties.
Flooded Lead Acid
A 12 V flooded lead acid battery is shown in the Figure 1.
|Figure 1: Flooded Lead Acid Battery 12 V|
In this type the electrolyte is in liquid form inside the battery. Also called wet batteries these have removable caps which allow you to add water and check the state of charge using a hygrometer thus making them high maintenance batteries. Flooded Lead Acid batteries are traditionally used as starting, lighting and ignition (SLI) batteries in automobiles. Also these batteries are being used excellently in solar power applications.
Valve Regulated Lead Acid (VRLA)
These are maintenance free batteries as watering is not needed. The oxygen and hydrogen produced are recombined internally with very little loss of water. Also called Sealed Lead Acid (SLA) batteries they are used in places where the use of flooded lead acid batteries can be inconvenient. These are not completely sealed as there are valves that open and release the gas into the atmosphere when the internal pressure exceeds a predetermined value. Figure 2 shows several sealed lead acid batteries of different capacities.
|Figure 2: Sealed Lead Acid Batteries|
The VRLA batteries are further divided into two types. These two are:
In Absorbed Glass Mat (AGM) type the electrolyte takes the form of a fiberglass mat.
In Gel type the electrolyte is gelled.
This gives another name to VRLA batteries, non-spillable battery. Due to their non-spillable nature these batteries can be used in any orientation. The upside down orientation, however, is not recommended.
One important point to remember, especially for my fellow countrymen, is that the automotive battery should not be used with the UPS as is current practice in Pakistan. These batteries are designed to be used in cars. These are high power batteries that can deliver sudden bursts of high currents. Having thinner plates in them, these batteries should not be discharged too deeply. Being inexpensive these have low cycle life as well.
The batteries which are designed to be used with UPSs are called deep cycle batteries. These have high cycle life, have thicker plates and can tolerate deep discharge. The deep cycle batteries can be discharged to 100%. However, any lead acid battery should not be discharged more than 50%. This can drastically reduce the service life of the battery.
Let us return to the issue of finding the required battery capacity. Assuming that the power goes out for a total of eight hours a day, the basic calculations to obtain the minimum capacity of the battery to run our modem for these eight hours are then
Current Consumption of DSL modem = 230mA
Capacity needed for powering up the modem for one hour = 230 mAh
Capacity needed for powering up the modem for 8 hours = 230 mAh × 8 = 1.84 Ah
Since a lead acid battery should not discharged more than 50%, thus capacity needed = 1.84 / 0.5 Ah = 3.68 Ah
Any lead acid battery with the capacity greater than 3.68 Ah can be used. The more capacity, the better.
Since our application is simple we don’t need complex, rigorous calculations and formulas to come up with minimum capacity needed. If we were designing a mission-critical equipment like that which will be used in saving a human life in hospital we would really use those complex calculations.
In the part-4 of this series we will discuss about the possible designs of the battery charger for our mini UPS and some technical aspects of the lead acid batteries that should be considered during the design of its charging mechanism.