Intro to Batteries - Novice
- Jeffrey Scholz
Last updated - July 2006
DISCLAIMER: Combat robotics is a dangerous sport by it's nature. Extreme caution should be taken when performing any operation explained on this site. Any injury or death resulting from the use of these pages is the sole responsibility of the user and not totalinsanity.net. By undertaking the construction of a combat robot you assume all responsibility for your actions. When building a combat robot, always make sure you are, or are with, a responsible adult.
Build Responsibly.
To understand this page well, you will need some small electrical background. Read these two pages so you get a better understanding of this discussion:
- Lingo (if you haven't read this already, shame on you ;-))
- howstuffworks.com/battery.htm Don't worry about the chemical reactions part, but pay close attention to everything else.Done? Good. Let's get started.
Here are some terms you should be familiar with at this point
-Ah rating: Remember, 1000mA is one Amp
-Cell
-Chemistry
-Cell voltage range: The maximum voltage a cell will attain down to the lowest it can be discharged. Do not drain cells below its voltage range.
-Cell voltage: This is the average voltage of the battery during its cycle, counting the fact that the falling voltage usually isn't linear
-Common pack voltages: The voltage rating of packs manufacturers usually make. For Sealed lead acid batteries, 12 volts is a common pack voltage (6 cells). 52 volts is not a common pack voltage (I'm not even sure if it exists).
-mAh rating
-Pack/ Battery: a combination of cells, usually in series.
Watt-hour – This is how much power a battery provides for one hour. This is sort of like the Ah rating, but also factors in the average voltage during the discharge cycle. For example, a 1 Amp hour battery at 1 volt is one watt/hour. A 1 Amp hour battery at 2 volts is 2 watt hours because it has two times as much voltage.
-Short, short out, or short circuit: This happens when the both terminals are directly connected together without any electronics in between. This state is very destructive and should be avoided.
-True rating: Batteries are rated by how much power they can source for one hour. However, combat robots drain them in three minutes. This will significantly reduce the Ah rating under those conditions. Multiply the manufacturer's number by its true rating percentage. For example, a 10 Ah battery that has a true rating of 90%, you should only count on it having 9 Ahs of power.
-Typical weightclass usage: There is a good reason we don't use sealed lead acid batteries in antweights: they're too heavy. We also don't use lithium polymer batteries in super heavy weights: they're too expensive in that class. If you already know what class you will build in, you will get an idea of what chemistries you should consider.
-Voltage
-Watt-hour/kg: This is how much power a battery has per kilogram of weight. This is used to measure its energy density.
Safety:
The batteries mentioned here are far more powerful than any consumer battery available at supermarkets. As such, you need to follow some extra precautions. Here is the number one rule: NEVER short circuit a battery. It may be entertaining to connect the two terminals of an AA battery with tin foil, or to lick a 9 volt battery – that's fine with those weak power sources, but not with these. Sealed lead acid and lithium polymer batteries can easily explode under those conditions. Nickel cadmium and nickel metal hydride probably won't blow up but any of them can source enough current to melt metal and start a fire or burn you. You should also take extra precautions to ensure the batteries don't short circuit inside the robot or during construction. Soldering a connector to the end can be very precarious, have a wire cutter handy to cut ONE of the wires if you accidentally bridge the connector with errant solder. When trimming the leads of the battery, cut them one at a time. Keep all conductive surfaces (metal carrying electricity) inside the robot insulated that is, covered with a material that doesn't conduct electricity, such as heat shrink tubing or electric tape.
Having said all that, I've never heard of someone getting more than a minor injury while working with these batteries. If you respect the amount of power in these things and use common sense, you'll be fine.Combat Chemistry:
You know some basics about chemistry, but how do you know which is appropriate for combat robots? Where do you get them? Which battery do you use?
Well, I'll put your mind at ease with some good news, there are only four chemistries used in combat robots today, SLA (sealed lead acid), NiCad (nickel cadmium), NiMH (nickel metal hydride), and Lipoly (lithium polymer).
Sealed Lead Acid (SLA)
Energy Density: 40 Watt hours/kg
Cell Voltage: 2 volts
Common pack voltages: 6V, 12V, 24V
Typical weightclass usage: Middleweights and up.
Typical Ah ratings used: 4.6Ah to 28Ah
Cell Voltage Range: 2 volts (fully charge) to 1 volt (fully drained).
Of all the batteries here, this one has the lowest energy density. However, they are the least expensive and can source a tremendous amount of current for short periods, sometimes in excess of 300 Amps. Unfortunately, these batteries aren't designed to put out high amps for a three minute match, so you need to multiply the manufacturer's Ah rating by 30 to 40 percent to get the true Ah rating for our purposes. That is, a 14Ah battery should be rated from 4.2 to 5.6Ah. The SLA battery is well suited for a robot that will need to source a tremendous amount of current for a short time, such as during spinning up a heavy weapon, and then settle down to a lower level, after the blade is up to speed.
When using these batteries, keep in mind that if you drain the cell voltage below 1 volt per cell, you can permanently damage the battery. Usually, when the voltage gets that low the robot will slow down significantly. This is a warning to charge your battery. Do not ever short out these batteries, as they can source enough current to cause the battery to explode. For the same reason, don't overcharge them; use a charger that automatically cuts off charging once the battery is fully charged. Store these batteries charged and in a cool place.Sources:
robotmarketplace.com/Nickel Cadmium (Nicad)
Energy Density: 50 Watt hours/kg
Cell Voltage: 1.2 volts
Common pack voltages: 4.8, 6, 7.2, 8.4, 9.6, 12, 18, 24, 36
Typical weightclass usage: Hobbyweight to Super Heavyweights, usually up to middleweights.
Typical Ah ratings used: 700mAh to 3.6Ah
Cell voltage range: 1.5 volts (full charge) to .9 volts (fully drained)
This chemistry has a greater power density than SLA at the cost of lower peak amperage due to its higher internal resistance. However, it can source more Amps continuously per Ah making the true rating closer to 90%. Nicads do not like being charged when the voltage is above 1 volt, as they will "memorize" that number and cut out at that point during the next use. You can do this two times in a row, but you must fully discharge it and then fully charge it afterwards. This isn't really a problem for combat robots, since the battery is usually fully drained after each match anyway. Store Nicads discharged (.9 volts per cell) in a cool place and charge them when ready.
Sources:
robotmarketplace.com/
battlepack.com/Nickel Metal Hydride (NiMH)
Energy Density: 90 Watt hours/kg
Cell Voltage: 1.2 volts
Common pack voltages: 4.8, 6, 7.2, 8.4, 9.6, 12, 18, 24, 36
Typical weightclass usage: 150g to 60lb. Has been used higher
Typical Ah ratings used: 370mAh to 4.2Ah
Cell voltage range: 1.5 volts (full charge) to .9 volts (fully drained)
NiMH has even greater power density than NiCd. However it can't source as much current. This is why it is generally used 60lbs and below. Aside from the "memory effect", maximum current levels, and power density, these batteries are identical to Nicads.
Sources:
robotmarketplace.com/
battlepack.com/Lithium Polymer (Lipoly)
Energy Density: 150 to 180 Watt hours/kg depending on the battery
Cell Voltage: 3.6 volts
Common pack voltages: 7.4, 11.1, 14.8
Typical weightclass usage: Fairyweight to Hobbyweight. Has been used higher
Typical Ah ratings used: 145mAh to 8Ah
Cell voltage range: 4.2 volts (fully charge) to 2.4 volts (fully drained)
Lithium Polymer batteries, (Lipoly for short) have the greatest power density. At this point, high Amp-hour rated batteries are extremely expensive, and because of this, mostly insect classes use these batteries. While they have the highest energy density, they are also the most volatile; take the same precautions with these batteries like you would with SLAs. Don't drain the battery below 2.4 volts as you will permanently degrade its performance. Store lithium polymer batteries fully charged.
These batteries have a true rating of 100%. The "C" rating signifies its maximum current sourcing capability. That is, a 1Ah battery rating 20C can source 20 Amps. A 2Ah battery rating 20C can source 40 Amps.Sources:
robotmarketplace.com/
hobby-lobby.com/
battlepack.com/Charging your batteries
All the batteries listed here are rechargeable. I'll cover some aspects of chargers here:
Power Supply: Most chargers need a power supply to run off of, usually 12 volts. As such, you will probably need to buy one with your charger.
Chemistry: The charger must be compatible with the chemistry of the battery you are trying to charge. Some chargers can handle only one chemistry, whereas some can handle all four.
Cells and Ah rating: Some chargers can't charge a 36 volt battery. Others can. Make sure the charger you get can handle the battery you plan on using. Likewise, some chargers can't handle a 10Ah battery. Others can. Factor this in too.
All three specifications are always included on the sellers website. What you should not do is turn on the charger and hook up the battery. Read the manual first so you know how to adjust the settings for the particular battery you are charging.
Chargers are available here:robotmarketplace.com/
battlepack.com/Connectors:
Most batteries come with connectors. Sometimes you'll have to solder on your own. Note that you can use connectors on your motors to make removing them easier. Here are some features to consider.Max Amperage: You're probably familiar with this already. If your battery can source 12 amps, don't use a 5 amp connector.
Polarity protection: A polarized connector will not allow you to hook up the battery incorrectly. Most connectors are polarized.
Type: Let's take a look at an outlet and the plug that connects to it. The connector that has two metal prongs sticking out are is the male/header. The outlet is the female/receptacle (it goes by both names). Never use a male connector on a battery as the two exposed terminals can short out on a metal surface. Batteries should always have a female/receptacle connector on them.
Brand: There are far too many brands to list them all here. The most common ones used by combat robot builders are available on the robot marketplace.
Summary:
At this point, I hope you have a basic understanding of batteries. You know what safety precautions to take when handling them. You also have an idea of the voltage and Ah ratings used in combat, as well as which chemistries are used in the different weightclasses. Finally, I have supplied some links to relevant distributors. Believe it or not, we have only scratched the surface. You have noticed that some technologies overlap each other in some weight classes. Also, which Ah should you select for your robot? This will all be covered in a future tutorial.Choosing:
The basic gist is, SLA's are great for robots 120 lbs and up, if you want a cheap battery. NiCads work in all weightclasses, but can be expensive if outfitting a robot 60lbs and up. NiMh are similar to NiCads, but can cost even more, so are generally limited to the sub light classes. LiPoly is exclusive to 12's and under, and even 12lbs is pushing it with this technology.One last Note:
You may have noticed that wet cell car batteries were not included on this list. Wet cell batteries are illegal at all combat robotics competitions. If wet cell batteries are fractured they pour acid all over the arena. Because of this, only dry cell batteries like those listed above are legal. Never design your robot around a car battery, as it will be illegal for competition.
© 2006 by T.i. Combat Robotics unless otherwise noted.
