Power Supply For Robots Part 2 - Online Article

Batteries

Batteries are by far the most commonly used electric power supply for robotics. Batteries are so commonplace that it's easy to take them for granted. An understanding of batteries will help you choose batteries that will optimize your robot's design. The rest of this article will examine batteries. There are hundreds of different kinds of batteries. We will look at the most common batteries employed for hobbyist use: carbon- zinc, alkaline, nickel-cadmium, lead-acid, and lithium.

Battery Power

Regardless of battery type, battery power is measured in amp-hours, that is, the current (measured in amps or milliamps) multiplied by the time (hours) that current is flowing from the battery. What does that mean to us? Well, it's pretty straightforward. Suppose a battery is rated at 2 amp-hours (Ah). This means the battery can supply 2 A of current for 1 h. If we reduce the current drawn from the battery to only 1 A, the battery will then last 2 h. If the current is further reduced to 500 milliamps (mA), the battery will last for 4 h. If you do the calculations for the three different scenarios, you will see battery life (time) is in direct proportion to the current draw:

Current Time Battery Rating
2 A 1 Hr. 2 Ah.
1 A 2 Hr. 2 Ah.
0.5 A 4 Hr. 2 Ah.

It becomes an easy matter to rearrange the equation to tell you how long a battery will last given a particular current draw. For instance, suppose your robot draws 0.35 A (350 mA). If you are using the same battery (2 Ah) as just discussed, then divide the battery rating (2 Ah) by the current draw (0.35 A) to find the time the battery will last (5.7 h). Mileage will vary. Batteries provide more access to their electric power rating when used intermittently, which allows time for the battery to recuperate. Continuous duty is efficient if the load is light. For robotics, especially when powering drive motors and components, we often don't have this option. In this case one tries to provide greater battery capacity.

Battery Voltage

Battery voltage varies throughout the life of a battery. If you measure the voltage on a fresh D-sized 1.5V alkaline battery, it will read approximately 1.65 V. As the battery discharges, its voltage drops. The battery is considered "dead" when the voltage drops to 1.0 V. Typical discharge curves for carbon-zinc, alkaline, and nickel- cadmium (NiCd) batteries.

Notice: A fresh NiCd 1.5V battery actually delivers about 1.35 V. While its initial voltage is lower, its discharge curve is fairly flat compared to that of carbon-zinc and alkaline batteries delivering a constant 1.2 V.

Primary Batteries

Primary batteries are one-time-use batteries. The batteries we will look at in this class deliver 1.5 V per cell. They are designed to deliver their rated electrical capacity and then be discarded. When building robotic systems, discarding depleted primary batteries can become expensive. However, one advantage to using primary batteries is that they typically have a greater electrical capacity than rechargeables. If one is engaged in a function (i.e., a robotic war) that requires the highest power density available for one-shot use, primary batteries may be the way to go.

Rating Primary Batteries

There are a number of primary batteries available. The differences in batteries relate to the chemistry used in the battery to produce electricity. The choice of a primary battery is a tradeoff of price versus energy density, shelf life, temperature range, discharge slope, and peak current capacity.

  • Carbon-zinc At the low end of primary batteries is the carbon-zinc battery. This battery hasn't changed much since 1868 when it was developed by George Leclanche. The carbon battery has a low energy density [1 to 2 watthours per cubic inch (Wh/in3 )], poor high-current performance, sloping discharge curve, and bad low-temperature performance. It is inexpensive but obsolete.
  • Alkaline- manganese This is simply an alkaline battery. It has an en- ergy density of 2 to 3 Wh/in3, improved low-temperature performance, and a sloping discharge curve, not as severe as carbon-zinc batteries. Its cost is moderate.
  • Lithium- This is a premium battery with a high energy density (8 Wh/in3), excellent low-temperature and high-temperature perfor- mance, and a long shelf life (15 years). It is also lightweight, but expensive.

Secondary Batteries

Secondary batteries are rechargeable. The most common rechargeable batteries are NiCds and lead-acid. We will start with NiCd batteries.

One disadvantage to NiCd batteries is that they have a lower voltage, 1.2 V per cell. So a C cell battery will deliver about 1.2 V instead of 1.5 V. The effect becomes more pronounced when using multiple cells. For instance, a "9V" NiCd battery made from six NiCd batteries will deliver approximately 7.2 V.

Automotive lead-acid batteries are rechargeable but are not suitable for robotics. The reason is that automotive batteries are not designed to be completely discharged (run down) before being recharged. These batteries can supply high currents for short periods of time (car starting) and need to be recharged almost immediately.

Completely discharging the electric power a rechargeable battery contains before recharging the battery is called deep cycle. There are deep-cycle lead-acid batteries available, mostly because of the solar power industry, but you will find these batteries carry a higher price tag. When building robotic systems, you should use deep-cycle rechargeable batteries.

Secondary batteries, while initially more expensive, are cheaper in the long run. Typically secondary batteries can be recharged 200 to 1000 times. In many cases a simple recharging circuit can be built into the robot so that it becomes unnecessary to remove batteries for charging.

Rating Secondary Batteries

  • NiCd: NiCd batteries and sealed lead-acid batteries are the most common rechargeables, with NiCd batteries being more popular. Both types of batteries have lower energy densities than primary batteries. NiCd batteries only provide 1.2V per cell, in comparison to primary batteries which provide 1.5V per cell. Manufacturers claim that NiCd batteries are good for 200 to 1000 charge-recharge cycles. However, NiCd batteries will die fast if they aren't recharged properly. The life expectancy of NiCd batteries is 2 to 4 years. Without use, a fully charged NiCd battery will lose its charge in 30 to 60 days. NiCd batteries are designed to be recharged at 10 percent of their rated capacity. This means that if a particular NiCd battery is rated at 1 Ah, it is safe to recharge the battery at 100 mA (1 A/10 100 mA). The terminology used to describe the above recommended recharge rate is "C/10." NiCd batteries are designed to be charged using a constant current at the C/10 rates. Because of inefficiencies, it is necessary to charge the battery for 14 h to get a full charge. While manufacturers claim that it is OK to overcharge NiCd batteries at the C/10 rate, most engineers recommend switching over to a trickle charge after the initial 14 h at C/10. A trickle charge is usually rated at C/30, or 1/30 of the battery's capacity. A trickle charge for our 1-Ah battery would be around 33 mA (1 A/30 33.3 mA).
    Memory effect: A disadvantage to NiCd batteries is the memory effect. If one repeatedly recharges a NiCd battery before it has completely discharged, the battery forms a memory at that recharge level. It then becomes difficult to discharge the battery past that remembered level. Obviously this can severely limit the battery's capacity. To correct that problem the battery must be completely discharged, by leaving a load connected to the battery for several hours. Once the battery is complete discharged, it can be charged normally and will function properly.
  • Lead-acid: Gelled-electrolyte battery cells (gel-cells) are similar to automotive batteries. They are sealed, maintenance-free, lead- acid batteries. They don't make gel-cells in the familiar D, C, AA, AAA, or 9V battery cases. Gel-cells are typically larger and may be used in larger robots. Gel-cells are available in numerous voltage ratings, from 2V to 24V, and current capacities. These batteries may be charged with a current-limited constant voltage or a constant current like NiCd batteries. Typically to charge a gel-cell, one applies a fixed 2.3V to 2.6V per cell. Initially the battery will draw a high current that tapers down as it charges. When fully charged, the battery need only draw a trickle charge (approximately C/500) to maintain itself in a fully charged state.

Gel-cell batteries vary from manufacturer to manufacturer. To safely recharge a gel-cell, you should check the manufacturer's recommendation. In general, a simple charging device can be made using an LM317 voltage regulator. A fixed voltage (2.3V per cell), constant current C/10 is applied to the battery. When the battery reaches a full charge, the constant current source is removed and a regulated voltage is applied. Many gel-cell batteries do not like to be deep cycled. Therefore it becomes necessary to monitor battery voltage under load. When the battery voltage drops by a specified amount (check the manufacturer's data sheet), it needs to be charged.

Building a NiCd Battery Charger

NiCd battery chargers are inexpensive. Typically it is not worth the time and effort to build a stand-alone charger for common-size batteries such as AAA, AA, C, D, and 9V. However, if one wishes to incorporate a built-in charger for a robot, then knowing how to build a custom battery charger is important. While most inexpensive chargers will charge batteries only at the C/10 rate, even after the batteries have received a full charge (14 h), the charger we will build will drop the current down to a C/30 rate after the batteries are fully charged. This is the recommended procedure for charging NiCd batteries. This will help ensure a long service life to your rechargeable battery.

The following information will allow you to design a system for charging a custom NiCd battery pack. The prototype charger shown in Fig. below is a stand-alone unit for illustration purposes. The design can easily be placed inside a robot. The robot will need to have a power socket that connects to the power supply. In between the socket and power supply, you should add a double-pole double-throw (DPDT) switch. The DPDT switch connects the power supply to either the robot's circuitry or the charger. This prevents powering the robot, which would reduce the current flow to the batteries, while the batteries are being charged.

Image1

DPDT switch controlling charging to battery pack

Image2

Basic power supply for charger circuit

he power for the charger may be supplied by either a standard transformer or a VDC plug-in wall transformer. Wall transformer will be preferable because it supplies a DC voltage. If you are using a standard transformer, you must build the power supply, using a line cord, switch, fuse, bridge rectifier, and smoothing capacitor. In either case you should match the transformer (or wall trans- former) power output to the battery pack you are charging. Matching the voltage and current to the battery pack reduces the power the LM317 must dissipate; for example, you wouldn't want to use a 12V transformer to charge a 6V battery pack.

Fig. of "Basic power supply for charger circuit" is a basic VDC power supply for the charger. The power supply can be made to provide 6V, 12V, 18V, 24V, or 36V depending upon the transformer, bridge rectifier, and capacitor chosen. The Schematic of charger circuit is illustrated below. It uses an LM317 voltage regulator and a current-limiting resistor. The resistance needed to be provided by the current-limiting resistor depends upon the current needed to charge the battery.

Current-limiting Resistor

Most NiCd battery manufacturers recommend charging the battery at 1/10 of its rated capacity, referred to as C/10. So if an AA battery is rated at 0.850 Ah, it should be charged at 1/10 that capacity, or 85 mA, for 14 h. After the batteries are fully charged, manufacturers recommend dropping the current to around C/30 (1/30 of battery capacity) to keep them fully charged without overcharging or damaging the batteries in any way.

Image3

Schematic of charger Circuit

To calculate the resistance to be provided by the current-limiting resistor, use the formula

R=1.25/Icc where Icc is the desired current.

Plugging in our 200 mA (0.2 A) yields

1.25/0.2 = 6.25 ohms

The resistance of the current-limiting resistor for this charger should be around 6.25 ohms. In the schematic of charger circuit, this resistor is labeled R2. Notice the R2 value listed in the schematic is 5 ohms. You should choose a common resistor value as close as possible to the calculated value.

C/30 Resistor

To drop the current to a C/30 range, we add another resistor whose value is 2R, or about 12.5 ohms. In the schematic this resistor is labeled R3. Again a resistor with the closest value to the calculated value is used. In this case the value is 10 ohms.

How the charger works

The charger uses an LM317 voltage regulator as a constant current source. The C/10 current-limiting resistor is identified as R2 in the schematic of charger circuit.R2 you will notice is only 5 ohms as compared to the calculated 6.25 ohms. This standard value is close enough to the calculated value for proper operation. The C/30 resistor is R3 on the schematic. Again the standard value of 10 ohms is close enough to the calculated value for proper operation. Later on we will see that it's possible to fast-charge the batteries because of the voltage-sensing capacity of the circuit. V1 is a 5K-ohm potentiometer. It is set to trigger the SCR when the NiCd batteries are fully charged. The SCR, once triggered, allows current to flow through a DPDT relay.

When power is applied to the circuit, current flows through the LM317 charging the batteries at a C/10 rate. Resistor R3 is shorted by one-half of the DPDT relay. Current also flows through resistor R1, which is a current-limiting resistor for light-emitting diodes (LEDs) D1 and D2. Upon power-up, the red LED D1 will be lit. The red LED indicates that the circuit is charging.

As the batteries charge, the voltage drop across V1 becomes greater. After about 14 h, the voltage drop across V1 is great enough to trigger the SCR. When the SCR is triggered, current flows through the coil of the DPDT relay. The relay switches, causing the red LED to go out and the green LED to turn on. The green LED signals that the batteries are fully charged. The other half of the relay switches, opening up the short on resistor R3. With R3 now in the current path, the current flowing to the NiCd batteries is cut to a C/30 level. Diode D3 prevents any current from the batteries flowing back into the circuit.

Determining the trigger voltage from V1

For the circuit to function properly, the SCR must turn on when the NiCd batteries are fully charged. The easiest (best) way to do this is to place depleted batteries in the charger, charge the batteries for 14 h, and then adjust V1. When the batteries are fully charged, slowly turn V1 until the relay clicks and the green LED turns on. Design notes When building a charger for your application, keep these points in mind. The main considerations are choosing the C/10 and C/30 current-limiting resistors. Use the given formulas for selecting these values. Current-limiting resistors should be rated around 2 W. If the charging current is high (greater than 250 mA), heat-sink the LM317. If the charger is switched on without the NiCd batter- ies being connected, the relay will switch immediately, turning on the green LED and providing a C/30 current.

When building a charger for higher voltages, increase the value of R1 proportionally to limit the current flowing through the LEDs. For instance, for a 12V unit make R1 680 ohms; for a 24V unit make R1 1.2K ohms. At high voltages you may need a low-ohm-value, current-limiting resistor connected to the DPDT relay. Measure the C/10 and C/30 current flowing to the batteries. These measurements will ensure that the proper current is being supplied to the batteries.

Series and parallel charging

How the batteries are configured determines the voltage and cur- rent of the transformer one should use. If you have eight C battery cells in parallel, you need to multiply the current requirements of each individual cell by 8. If the cell is rated at 1200 mAh, the C/10 requirement per battery is 120 mA. For eight cells in parallel, you need close to 1 A (8 120 mA 960 mA 0.96 A) of current. The voltage required is just 1.5 V. The ideal transformer's output would be 1.5V at 1 A. If the eight cells were held in series, the current requirements would be 120 mA at 12V.

Fast Charger

Many of today's NiCd batteries are capable of accepting a fast charge provided that the circuit can sense when the batteries are fully charged and drop the current to C/30. Typically to fast-charge a battery, you double the current for half the time. So you charge a battery at C/5 for 7 h. You may want to start with a C/10 charging current and adjust V1, and then switch resistor R2 for a resistor with half the value.

Parts list

  1. U1 LM317 voltage regulator
  2. L1 DPDT relay (5V or 12V)
  3. D1 Red LED
  4. D2 Green LED
  5. D3 1N4004
  6. Q1 SCR
  7. V1 5K-ohm PC-mounted potentiometer
  8. R1 330 ohms, 0.25 W
  9. R2 5 ohms, 2 W
  10. R3 10 ohms, 2 W
  11. R4 220 ohms, 0.25W
  12. Wall transformer

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