Choosing the Right Power Source
If your robot isn’t designed to operate efficiently on disposable batteries without incurring significant costs, or isn’t eco-conscious and powered by solar panels, rechargeable batteries are likely the ideal energy source.
Challenges with Existing Chargers
While there’s a plethora of chargers on the market, not all of them meet our specific requirements in terms of battery types and quantity they can accommodate. Furthermore, some chargers fail to properly care for the batteries they charge, potentially leading to a shortened lifespan for the batteries.
Customizing Your Battery Charger
In this article, we propose constructing a personalized charger using a reliable yet mature IC, the MAX713 from Maxim. Given the uniqueness of each robot, we won’t provide a fixed circuit but will guide you in adapting it to match the specific characteristics of the batteries you intend to charge.
MAX713 Basic Application Circuit
The fundamental circuit of MAX713 is depicted in the figure, but certain components lack specified values. Additionally, there are several configuration options. Through these elements, MAX713 enables charging from one to 16 cells (where a cell represents a basic 1.2 V unit), allowing you to define the charging current, the end-of-charge float current, and select the end-of-charge detection mode. Regarding the latter, for compatibility with various batteries used in robots, we’ve omitted the temperature detection method, requiring a thermal sensor (NTC or equivalent) within the battery. Resistors R4 and R5, combined with hard-wired links to inputs THI and TLO, program MAX713 to detect end-of-charge through voltage variation.
Determining Customizable Elements
Let’s delve into determining the remaining configurable components. This knowledge will empower you to construct a charger perfectly tailored to your requirements. It’s crucial to note that the configuration links can either be permanently fixed on the PCB you design for your charger or connected to multi-way switches, allowing the creation of a versatile and adaptable charger.
Setting Charging Current and Time
To begin customizing your battery charger, determine the charging current, denoted as I_fast, for your batteries with a capacity represented in ampere-hours (Ah). You can calculate this using the formula: I_fast = C / t, where t signifies the desired charging time in hours. However, be cautious; the MAX713 has a limit of 4 hours for charging times. Ensure that the chosen I_fast value doesn’t exceed 4C, the maximum allowable current for fast-charging NiCad and NiMH batteries. Opting for a lower current is ideal as it prolongs battery life. Set the charging time by configuring pins PGM2 and PGM3 of the MAX713 according to Table 1.
Determining Number of Cells
Next, specify the number of cells to be charged simultaneously. For block batteries, you can ascertain the cell count by dividing the battery’s nominal voltage by 1.2 V. For instance, a 9.6 V battery comprises eight cells. If the cell count exceeds 11, the circuit cannot be utilized as-is. In such cases, it’s advisable to charge the batteries in two separate sessions. Program this number by wiring pins PGMO and PGM1 of the MAX713 according to Table 2.
Selecting DC Supply Voltage
Additionally, choose the unstabilized DC supply voltage (VA in the diagram) for your charger, ensuring it is at least 1.5 V higher than the maximum voltage of the battery being charged. However, if your battery has fewer than four cells, the rule no longer applies, as the MAX713 supply must be a minimum of 6V.
Then determine the maximum power dissipated in T1 using the following equation:
PD = (VA— VBATTmird x /fast
where VBAnmin is the minimum voltage of the battery to be charged. Choose T1 accordingly, if necessary fitting it with an appropriate heatsink.
Then determine the value of resistor R1 so the current drawn by the MAX713 will be 5 to 20 mA, using the equation: R1 = (VA— 5)/ / where / is between 5 and 20 mA.
Lastly, determine the value of resistor R6 by using the equation: R6 = 0.25 / /fan and its power by using PR6 = 0.5/fast (theoretically 0.25/fast in fact, but it’s best to use
a safety factor of 2, hence the modified equation).
Charger Operation and Configuration
Your charger is now fully functional and remarkably easy to operate. However, due to the internal processors within the MAX713, it is crucial to establish connections to PGMO to PGM3 before supplying power to the circuit. Failing to do so might result in incorrect configuration recognition. This issue doesn’t arise in a fixed circuit design. However, if your charger incorporates configuration switches, you must power down and then restart the system to validate any changes made through these switches.
Indicator Light and Charging Modes
A LED indicator illuminates during fast-charge mode, utilizing the current I_fast determined earlier. Once fast-charging concludes and the charger transitions to float charge mode, the LED extinguishes. In this mode, the current generated is minimal, allowing the battery to remain connected to the charger indefinitely, if necessary.
To make sure our explanation is crystal-clear, here by way of example are the calculations for a charger for a pack of four 1.2 V NiMH batteries with a capacity of
1,800 mAh that we want to recharge in two hours.
- Calculate /fast: /fast = C/t,e., 1.8/2 = 0.9 A or 900 mA.
- PGM2 and PGM3 connections:
PGM2 tied to BATT— and PGM3 tied to REF, as we want a charge time of 2 hours, i.e. 120 minutes (in fact, we’ll get a maximum of 132 minutes).
- PGMO and PGM1 connections:
PGMO to V+ and PGM1 to BATT— since our battery comprises four cells.
- Determine VA: VA =3 V minimum.We’ll choose 9 V, to obviate any problems with possible supply voltage variations.
- Power dissipated in Ti: PD = (9 — 4*) .9, i.e. 4.5 W. So we’ll choose, for example, a TIP32A, giving us an excellent safety margin (PD„.„ = 40 W).
* fully discharged battery voltage estimated at 4 V.
- Calculate R1: R1 = (9-5)0.01**, = 400 LI We’ll use the closest preferred value,e. 390 a
**: a current of 10 mA was chosen.
- Calculate R6: R6 =25/0.9, = 0.27 LI.
- Calculate the power in R6: PR6 = 0.5 x 0.9 = 0.45 W. So a 0.50-W type is going to be fine.
As you can see, it’s taken us all of five minutes to produce a charger tailored perfectly to our needs. Now it’s your turn…