This is the fourth post in a series of posts on making your own mini UPS for DSL modem. Be sure to read the first three parts before you read below.
We have now all information about our load (the DSL modem) and the battery and its time we look into the battery charger. The design of battery charger depends on our selection of the battery. Is our battery lithium based? Is it lead acid or nickel based? The battery which I am using is (see Figure 1).
|Figure 1: 12 V 7.5Ah Sealed Lead Acid Battery by Leoch Battery Inc.|
This is a sealed lead acid battery from Leoch Battery Inc. China. Its rated capacity as seen printed on the side of the battery is 7.5Ah. This capacity is rated at 100 hours discharge rate. It means that this battery will deliver 75mA for 100 hours at a temperature of 25°C before the terminal voltage of the battery reaches a final voltage of 1.80 V per cell that is 10.8 V for this 6-cell battery. And the rated capacity at 20 hours discharge rate is 6.84Ah for this battery. All of this information is available in the datasheet of this battery. Click here to download the datasheet of this battery.
When buying a lead acid battery for either cyclic use or standby use it is best to look at its capacity rated at 20-hr discharge rate.
The cycle use is the use when you repeatedly cycle the battery. This means you charge a battery then use it for, say 5 hours continuously and then put to it recharge. In our case we need battery to power our DSL modem for eight hours while for remaining time the battery will remain on charge. Thus our use will be cycle use.
In standby use the battery remains mostly on standby. The battery will be used very scarcely such as one or two times per month.
What is the best way to charge a lead acid battery?
The proper way to charge a lead acid battery is through 3-stage charging. The three stage charging fully charges the battery in shortest possible time while maintaining the health of the battery.
|Figure 2: 3-stage charging graph for lead acid batteries|
Image by Hankwang/Wikimedia Commons
Also called the bulk stage shown as I-phase in the Figure 2, this stage restores most of the capacity of the battery. At the end of this stage the battery will be approximately 80% charged. In this stage the charger delivers a constant current. The terminal voltage of the battery gradually rises while current going into the battery remains constant. This constant current is usually 10% of the capacity of the battery. The maximum value of this constant current should not be greater than 25% of the capacity of the battery.
For example, if your battery capacity is 100Ah then 10% of 100Ah = 10A (the ‘h’ is simply dropped). This is also written as 0.10C where C is the capacity of the battery. The rising terminal voltage and constant current during this stage are also shown in the Figure 2.
Also called the absorption stage shown as Uo-phase in the Figure 2, the charger enters this stage when the terminal voltage of the battery reaches the maximum voltage, Umax (see Figure 2). The maximum voltage, Umax is also known by “charging voltage.” The voltage is held at this maximum voltage while the current going into the battery is allowed to decrease. When a battery is near full charge it accepts less and less current. This situation continues until the current going into the battery becomes less than 1% (Imin in the Figure 2) of the capacity of the battery, where the charger enters the stage 3. This maximum voltage is around 2.4 V per cell. For a 6-cell lead acid battery this translates to 14.4 V.
However, one should always consult the documentation provided by the battery manufacturer rather than the guess work. For my battery this required information is printed on the battery itself. In Figure 1, it is specified to be between 14.4 V and 15.0 V. In the datasheet of this battery it is further specified that this voltage range is applicable when the ambient temperature is 25°C (77°F) (see Figure 3)
|Figure 3: Recommended voltages at a reference temperature for absorption charge stage for a sealed lead acid battery|
For temperatures other than 25°C (77°F) we should calculate the new voltage range for Umax. This is done by taking into consideration the temperature coefficient. As seen the temperature coefficient is negative which means we should decrease the Umax at higher temperatures while increase the Umax at lower temperatures. For example, if the ambient temperature is 5°C then
|Figure 4: Calculations to finding temperature-compensated absorption charge stage voltages for a sealed lead acid battery|
Thus at 5°C (41°F) the voltage range for maximum voltage, Umax during stage 2 will be 15.0 V – 15.6 V.
Also called the float stage shown as U-phase in the Figure 2. The charger enters this stage when the current going into the battery is less than 1% of the rated capacity of the battery. The 1% figure is typical with some chargers entering this stage at 3% of the rated capacity of the battery.
At this stage the battery is fully charged and the charger could be left connected indefinitely. The terminal voltage of the battery is held at a constant “float voltage” of around 2.25 V per cell that is 13.5 V for a 6-cell battery. Again the documentation from the battery manufacturer should be consulted rather than relying on guess work. Temperature compensation should also be applied for finding the correct float voltage if the ambient temperature is other than that specified in the datasheet of the battery. The float stage maintains the battery at 100% state of charge. During float stage there is still little current going into the battery which is just to compensate for the self-discharge of the battery.
We now know the charging algorithm so naturally the next task is to design the charging circuitry. But before we do that let us first search on Google. It is possible that there might be an existing solution somewhere out on the web. Luckily there is not one but many solutions since charging a battery is very a common thing. Some solutions are:
- Charging circuits based on a microcontroller
- Charging circuits based on dedicated charging ICs such as UC3906 from Texas Instruments
- Other charging circuits based on analog electronics
The best solution is option 1. After that the best solution should be option 2. I decided to go for option 3 as the components needed are easily and commonly available and everyone can make such a charger using little effort.
It's all about a circuit diagram now. You can find the circuit diagram of mini UPS for DSL modem in the next post i.e., part 5 of this series.