Sealed lead acid (SLA), nickel cadmium (NiCad), and nickel metal hydride (NiMH) batteries can be successfully used for competition. Two other battery types worth mentioning are the Lithium Ion and the Alkaline types. Although not recommended, these two battery types are common enough that some people might consider using them in their robots.
In most competition robot contests, the regular lead acid batteries that are used on automobiles, boats, and motorcycles are prohibited because these batteries al-low access to the internal liquids, and they can leak acid if they are turned upside down or if they become damaged—which can also damage the arena and pose a safety hazard. The lead acid batteries that are allowed in these events are called sealedlead acid batteries, because they have no ports for checking the internal fluids and they can be operated in any orientation (see Figure 5-5). These batteries are often called Gel-Cells, immobilized lead acid batteries, or glass-mat lead acid batteries.
FIGURE 5-5
A sealed lead acid battery.(courtesy of Hawker batteries)
Sealed Lead Acid
The rugged construction of SLA batteries is well suited for combat robot use. SLA batteries do not leak and they are a mature battery technology. Figure 5-6 shows various SLA batteries.
In general, the Ahr rating of the SLA is specified at the 20-hour rate. Multiply by 0.33 (see Table 5-1 for the 0.33 conversion factor) to convert this 20-hour rate to the 6-minute rate. For example, an SLA battery with a capacity of 12Ahr has a usable 6-minute capacity of 4.1Ahr (4.1Ahr = 0.33 × 12Ahr) and will provide an average current of 41 amps (41 = 10 × 4.1Ahr) for the 6-minute duration. Typical SLA bat-teries have a peak current delivery capacity of 10 times its 20-hour capacity. In this example, the battery can supply a peak current of 120 amps (120 = 10 × 12Ahr).
Sizing the Battery
If your robot draws an average current of less than 40 amps and has a peak current less than 100 amps, you can select from SLA, NiCad, or NiMH batteries with ease. Just size your battery to make sure that the 6-minute rating and peak current rating is higher than your robot requires.
If your robot will draw an average of more than 40 amps or more than 100 amps peak, use SLA batteries or parallel packs of NiCad or NiMH batteries. The SLA is easier, but not necessarily better. Remember, do not mix different types and sizes of batteries together.
FIGURE 5-6
Various sealed lead acid batteries.
(courtesy of Hawker batteries)
Hawker brand SLA batteries (www.hepi.com) have peak current delivery up to 40 times the 20-hour capacity. For example, the Hawker 16Ahr battery can source 680 amps for 5 seconds—or about 42 times the 20-hour capacity.
Charging is accomplished by applying 2.45 volts per cell and limiting the current to the battery manufacturer’s recommended charging current. The exceptions to this rule are the Hawker batteries, which do not require current limiting. A 12-volt SLA battery has six cells, so the charging voltage is 14.7 volts (14.7 = 6 × 2.45 volts).
If you are leaving the battery on the charger for an extended period of time, the charging voltage should be reduced to 2.27 volts to 2.35 volts per cell. When storing SLA batteries, you should be sure to charge them fully every six months. A good automatic automotive charger will work well for fast charging; however, it is im-portant to use a battery that can handle fast charging and to use a charger that does not force charge into the battery after it is fully charged.
Following are some of the advantages of using SLA batteries:
■ It’s the least expensive rechargeable battery type, so it’s easier to purchase more than one battery at a time.
■ Up to 300 charge/discharge cycles to 80-percent capacity are possible.
■ When stored at 25oC, it loses less than 1 percent of its charge per day.
■ It can supply the highest current of any battery type.
■ The wide range of battery capacities makes it easy to size the battery to the job.
■ It gives some advance warning before going dead. For a 12-volt battery, the voltage gradually lowers from 13.2 volts (full charge) to 10 volts (empty), making it relatively easy to tell how much charge is left in the battery.
■ It handles fast deep-discharge better than other battery types. This is true as long as the battery is placed on a charger quickly after the discharge.
■ The Hawker brand Cyclon and Genesis and Odyssey SLA batteries can be charged in about 30 minutes to about 1.5 hours depending on how large the charger is.
Following are some of the disadvantages of using SLA batteries:
■ It has the highest weight of any recommended battery type.
■ The 6-minute rating drops the effective Ahr rating more than any rechargeable battery type.
■ Because of gas venting problems, most SLA batteries cannot be fast charged.
■ Because the acid in the SLA battery will attack the plates of the battery when discharged, it must always be stored in a charged state and must be periodically recharged when in storage. If stored uncharged for an extended period of time, the battery will die.
Which SLA Manufacturer Is the Best?
Most SLA batteries have similar capacity performance. Even so, the Hawker brand (formerly Gates) stands out as the best SLA battery manufacturer. Cyclon, Genesis, and Odyssey batteries can be 1.5-hour fast charged (or faster), can be re-peatedly fully drained with little battery degradation (down to 9 volts), have the lowest shelf leakage of the SLA lineup, can supply three times more peak current than other batteries with similar Ahr ratings, and have good shelf life.
The SLA battery manufacturer to avoid is Panasonic. Many of the Panasonic brand SLA batteries have built-in thermal cutoff switches (a safety feature), making fast, high-current discharge impossible. The Power Sonic brand seems to have a good price/performance value. For the largest robots, the Optima battery brand is great. Optima is a good battery, but the 12-volt Optima weighs almost 40 pounds.
Are SLA batteries too heavy to have a competitive advantage? Not at all. Electric wheelchairs, golf carts, even electric racing go-karts and boats use SLA batteries. If your robot requires high sustain currents or high peak currents, the SLA battery may have the best performance.
Nickel Cadmium (NiCad)
The rugged construction of NiCad batteries is well suited for combat robot use.
Though NiCads are a mature battery technology, they are still seeing incremental improvements in price and performance. Fast-charge/fast-discharge NiCads are required for competition applications.
The Ahr rating for this battery type is specified at the 1-hour discharge rate. To de-termine the 6-minute, run-time capacity, multiply the 1-hour capacity rating by 0.9 (see Table 5-1). Sometimes, even with a fast-discharge NiCad, this 6-minute dis-charge rate is higher than a NiCad’s datasheets will allow. For example, a D-cell NiCad battery pack with a capacity of 5Ahr has a usable 6-minute capacity of 4.5Ahr (4.5 = 0.9 × 5Ahr) and will provide an average current of 45 amps (45 = 10 × 4.5A) for the 6-minute duration. Even so, a typical fast-charge/fast-discharge C-cell or D-cell NiCad datasheet will show only an average drain of 35 to 40 amps, with short duration (less than 100 milliseconds) peak currents of 100 to 130 amps. For higher current draw, you need to parallel multiple battery packs together or run out-side the manufacturer’s recommendations.
Fast charging is accomplished by applying the current equal to the Ahr rating of the battery for about 1.5 hours. Charge must be terminated when the battery starts to heat up, when the battery voltage begins to decline, or some combination
of the two. Generally, a charger designed for this purpose must be used. Excellent fast chargers for NiCads are readily available.
Slow charging can be accomplished by sending a current equal to 1/10thof the Ahr rating of the battery for 15 hours. It is important that you not allow the bat-tery to remain on this type of charger for long periods (longer than 24 to 48 hours) or else the NiCad will suffer from voltage depression (about .1 to .2 volts per cell).
Figure 5-7 shows various NiCad batteries.
Following are the advantages of NiCad:
■ It has an excellent cost verses performance ranking.
■ For long-term use and with proper care, the NiCad can be less expensive in the end—even less than the SLA.
■ With proper care and storage, NiCads can last through more than 1,000 charge cycles—though a chance to run this many charge cycles is not likely to happen in the harsh world of a combat robot.
■ NiCad packs are small, so they can be stored in your refrigerator for long periods of time.
■ The NiCad battery is moderately priced, so you can purchase more than one battery pack.
■ The energy density is good—three times that of SLA—and in this application surpassed only by NiMH.
FIGURE 5-7 Various NiCad Batteries(courtesy of Panasonic)
■ NiCads can be stored with or without a charge, without damaging effects.
However, it is usually safe to store the batteries in the discharged state.
■ NiCads have no memory effects when used for this application. Because they are fully discharged during a combat match, this avoids memory effects.
Following are some disadvantages of NiCad
■ When stored at 25° C, the NiCad battery loses 1 percent of its charge per day.
■ When fully charged, the NiCad will self-discharge to an 80-percent charge in about three weeks.
■ Occasional cycling to 80-percent voltage is required to keep the internal resistance of the battery low. If the robot is noticeably slower, you know the battery has reached this 80-percent level. It is best to do this every 20 charge cycles or so. During the testing phase, usually the batteries are repeatedly drained.
■ NiCads are high-maintenance batteries, requiring careful monitoring, charging, cycling, and low temperature storage to yield long life.
■ NiCads have cadmium; and although safely housed in the battery, cadmium is a toxic element and must be disposed of properly.
Nickel Metal Hydride (NiMH)
The rugged construction of NiMH batteries is well suited for combat robot use.
This is an emerging battery technology that is still seeing constant improvement.
Fast-charge/fast-discharge NiMH packs are required.
The Ahr rating of this battery type is also specified at a 1-hour rate. Multiply by 0.92(see Table 5-1) to convert this 1-hour rate to the 6-minute Ahr rate. For example, a D-cell NiMH battery pack with a capacity of 6.5Ahr, has a usable 6-minute ca-pacity of 6Ahr (6 = 0.92 × 6.5Ahr) and can provide a calculated average current of 60 amps (60 = 10 × 6Ahr) for the 6-minute duration. Even so, the specification data sheets show that for the fast-charge/fast-discharge C-cell or D-cell batteries, the maximum average current is only about 40 amps, with the peak current limit of about 100 amps. For higher current draw requirements, it is necessary to parallel the batteries.
For fast charging, use only a charger designed for NiMH. Using a charger designed only for NiCads, for example, will usually destroy NiMH batteries. Because this technology is relatively new, chargers for this type of battery are harder to come by than NiCad or SLA chargers.
Following are some advantages of NiMH:
■ The NiMH energy density is the best of all the usable battery types currently available.
■ With proper care and storage, NiMHs will last through more than 300 charge cycles.
■ Because NiMH packs are small, it is easy to keep them in the refrigerator for long-term storage.
■ The voltage output remains constant until almost fully discharged.
This provides full power to your robot for the duration of the match.
■ They can be stored without a charge without damaging effects.
■ They have no memory effects when used for this application.
■ They have no cadmium, so they don’t have the related health problems.
The emerging NiMH battery technology will see improvements. In time, expect a lower cost, a higher number of charge cycles, lower internal resistance resulting in a higher maximum current rating, and lower self-discharge rates.
Following are some disadvantages of NiMH:
■ It is the most expensive rechargeable battery technology.
■ It has the lowest life of the rechargeable battery technologies. After 300 charge/discharge cycles, the battery capacity measurably degrades while the internal resistance increases.
■ When stored at 25oC, the NiMH battery can lose up to 5 percent of its charge per day. When fully charged, the NiMH can self-discharge to an 80-percent charge within five days!
■ Occasional cycling to 80-percent voltage is required to keep the internal resistance of the battery low. If the robot is noticeably slower, you know the battery has reached this level. It is best to do this every 20 charge cycles or so.
■ NiMH are high-maintenance batteries, requiring careful monitoring, charging, cycling, and low-temperature storage to yield long life.
Alkaline
The alkaline battery is the most common primary battery in America. It is used to power most products from radios to flashlights. Small robot kits often will use them as the power source. Alkaline batteries cannot handle a high rate discharge, so they don’t work well for combat robots.
The alkaline battery works best when powering low-current devices. When used to power high-current devices, the performance is dismal. Even so, many robot kits use AA alkaline batteries to drive servos and the onboard electronics. When stalled, these servos can try to draw 1 ampere, bringing short order to the AA alka-line batteries. Usually, these robots will see a performance increase if the alkaalka-line batteries are changed over to standard NiMH or NiCad cells.
Following are some advantages of alkaline:
■ They are readily available.
■ They have the least expensive startup cost.
■ It is easy to replace the battery with a known fresh battery.
■ They are low maintenance—you can throw away the old ones.
Here are the disadvantages of alkaline:
■ In the long run, they are the most expensive battery type.
■ They have the poorest 6-minute energy density of all the batteries.
Lithium Ion
Lithium ion batteries are common rechargeable batteries used in computing appli-cations. They have high-energy density when current is pulled out at a moderate rate. However, the voltage drops when pulling current out at a high rate. In addi-tion, the battery can fail when pulling out current at a higher than moderate rate.
Therefore, lithium ion batteries do not work well for combat robots. Another neg-ative factor is that the typical shelf life of the lithium ion rechargeable battery is only two years if stored at 25° C.
Combining Drill Motors, Batteries, and Battery Chargers
Many cordless power drills come with rechargeable batteries and fast chargers.
Many competition winners have used these drill/battery/charger combinations to have a complete solution to the problems of supplying motors for the robot, getting good batteries, and getting fast chargers. In addition, spare batteries for these motors are readily available. Four-wheel-drive robots using four power drill motors have had good success in the combat arena. If you go this route, use good-quality cordless power drills with NiCad or NiMH battery types.