Batteries power all sorts of things in the world, so it may not be a surprise to you that bots are among the things which use batteries. But to choose the right amount of batteries and what kind to use, you ought to know a bit about how they are classified.
Batteries are rated by voltage, for one thing. While this concept will be covered more in detail in a future page, voltage is a measure of the eagerness of the electrons in the negative pole of a battery to travel to the positive pole of the battery. The higher the voltage, the higher the eagerness, and the more work the electrons can do. At least this is how I visualize this; for a more accurate depiction (assuming I'm wrong), check HowStuffWorks.com under Batteries. Anyway, voltage is measured in volts, and could be compared to the pressure of water in a hose. The higher the voltage, the higher the pressure of the water. Batteries are also rated by current they can provide, measured in amperes (amps). Current is just the amount of electricity at a single point in time that is travelling through a point in a circuit. So, current is like the actual water in the hose. But there can be no current without voltage, as the "water" would be stagnant. Finally, batteries are measured by capacity: measured in amp-hours. 2 amp-hours means a battery could, in theory, produce 2 amps for one hour, one amp for 2 hours, one-half an amp for 4 hours and 4 amps for half an hour. This is based on ideals, though; there are losses and limits to account for, but this provides a good estimate, at least.
How does this apply to BattleBots? Well, one needs a certain voltage to power electronics in the bot (this depends on the electronics, though), and the voltage delivered to the motors determines the power of the motors. The current the batteries can provide must meet the current the motors demand, or you will not have max efficiency (and your motors will not have as strong a turning force as if they had the current they needed). This is all covered in greater detail in upcoming motor sections. As for capacity, you must determine the average current draw in amperes of your robot and make sure you have enough capacity to last the entire fight. For a 3-minute fight with an average of 60 amps drawn, for example:
(3min)/(60min/hr)*(60 amps)=3 amp-hour, ideally, with batteries that can supply at least 60 amps. For safety, you might want to add another amp-hour for losses that come from drawing the current so quickly out of the batteries.
That is most of what you need to know. What about types, though? Why can't you just use any batteries you can find? Well, you can, but it depends on what you're using them for.
Alkaline batteries are the kind of batteries you probably know best. They're Duracells and Energizers (yes, even the "titanium" kind, ugh). Alkaline just refers to the chemicals in the batteries. These batteries have a decent shelf life and decent capacity and are cheap to start up with, but are almost never rechargeable. They also can't provide ("to provide current" is sometimes called "to source current") much current. They're best for low-amperage electronic situations. The individual batteries are 1.5 volts each.
Sealed Lead Acid batteries are similar to car or motorcycle batteries, but they're sealed, making them safer (since they don't spill acid when you turn them upside-down). They work with plates in an acidic solution, and are capable of high voltages, usually fast charges, and high current output. They also have high capacity. They're also cheap! They're also bloody heavy and large, usually. It's not great budgeting of weight or internal space, but if you can afford weight and space but not the extra money, these are usable. They use units within themselves as "cells", a battery term that refers to parts of a battery. The more cells a battery has, the higher the voltage it provides. Sealed lead acid cells are 2.0 volts each. Note: I mention sealed lead acid batteries (SLAs) in particular because their car/motorcycle counterparts are illegal due to the spillage problem.
Nickel Cadmium: if you think of standard rechargeable batteries, you're probably thinking of NiCads. Again, their name refers to the chemicals used to get the power from within the battery. These batteries provide 1.2 volts each (again, they're called "cells": a 6-cell NiCad pack provides 7.2 volts, for example). Unlike alkalines, which provide steadily less voltage as they run out, NiCads tend to produce near full voltage until suddenly they die. When dead, though, NiCads can be recharged many times. They are about medium-price range, but with the number of recharges available, they pay for themselves. They also have about medium current output and decent current capacity. The only quirk with these suckers is that, rumor has it, if you recharge them at the same level each time (say after you lose 25% of the capacity), it will gain an effect called the "memory effect": in this case, it will stop working at 25% of the capacity. Even if this is true, though, this can be cancelled by completely discharging the cells and recharging them (called a "cycle"). All in all, a good battery type for BattleBot situations. Did I mention they're semi-low weight?
A newer battery technology is the Nickel Metal Hydride (NiMH) technology. These batteries are also 1.2V each, but they have no memory effect and have larger capacities for their weight than do NiCads. But they are more expensive and have less current output than NiCads as well, and need more accurate chargers. The best battery is in the eye of the beholder, though. Weigh your priorities, and choose whether you want to use NiCad or NiMH (or neither).
Another pair of battery types are also out there: Lithium ion and Lithium-poly cells. However, I don't know a lot about these. I know they are expensive and need incredibly-specialized chargers, and have less current output than some batteries, but are insanely light and have an insane capacity for their weight. In the future I will expand this section on them, but I am sure other sources have more information about them.
Like many things in the design of bots, you can manipulate things around to get what you want. However, everything involves a trade-off. You can get more voltage or capacity (and possibly current? Not sure, I'll check this) out of batteries, but it requires more than one of the battery, which adds cost and weight. Take for example a D battery. D batteries have the highest current capacity among alkaline batteries (since they are bigger, they contain more. This is pretty consistent: the larger the capacity, the larger and heavier the battery, but only within a given type of battery. A lithium ion battery can hold more capacity and weigh less than a D alkaline battery, for example). But a D battery only gives out 1.5V, which is very little; you can't power a chip on that, let alone a high-powered motor. So, you need to increase its voltage, say to a common 12V. 12/1.5=8, so if you take 8 D batteries and connect the positive pole of each battery to the negative pole of the next (stack them, in other words), you are making a series connection. With series connections, where opposite poles are connected, voltage is increased. With two 1.5V batteries in series, 3V are produced, with the capacity being the common capacity. With 8, then, it would be 12V, and then the capacity of one D battery. Suppose, however, you had a couple of 6V lantern batteries (also alkaline), and wanted to run a 6V motor, but it just won't turn since there's not enough current there, or will only turn for 1:30, and you need 3:00. Connect the two in parallel. Parallel connection means positive pole to positive pole, and negative pole to negative pole. This increases the current capacity (and possibly the amount of current that can be given). There is no limit to current capacity or voltage, and you can even do both at once. However, the more batteries you use, the more weight you will have. It's best to do this sparingly.
So there you have it: the basics of batteries. And now, a simple list of common voltages for your reading pleasure.
(VDC: Voltage of Direct Current, as in batteries)
(VAC: Voltage of Alternating Current, which switches polarity, as in household outlets)
1.2 VDC: One NiCad or NiMH cells
1.5 VDC: One Alkaline cell
2 VDC: One SLA cell
2.4 VDC: Two NiCad or NiMH cells (in series)
3 VDC: Two Alkaline cells (in series)
3.6 VDC: Three NiCad or NiMH cells (in series)
4 VDC: Two SLA cells (in series)
4.5 VDC: Three Alkaline cells (in series)
4.8 VDC: Four NiCad or NiMH cells (in series)
6 VDC: Four Alkaline cells (in series) or Five NiCad or NiMH cells (in series) or Three SLA cells (in series)
7.2 VDC: Six NiCad or NiMH cells (in series)
7.5 VDC: Five Alkaline cells (in series)
8 VDC: Four SLA cells (in series)
8.4 VDC: Seven NiCad or NiMH cells (in series)
9 VDC: Six Alkaline cells (in series) or One 9V cell
9.6 VDC: Eight NiCad or NiMH cells (in series)
10 VDC: Five SLA cells (in series)
10.5 VDC: Seven Alkaline cells (in series)
10.8 VDC: Nine NiCad or NiMH cells (in series)
12 VDC: Eight Alkaline cells (in series) or Ten NiCad or NiMH cells (in series) or Six SLA cells (in series)
18 VDC: Twelve Alkaline cells (in series) or Fifteen NiCad or NiMH cells (in series) or Two 9V cells (in series) or Nine SLA cells (in series)
24 VDC: Sixteen Alkaline cells (in series) or Twenty NiCad or NiMH cells (in series) or Twelve SLA cells (in series)
110/120 VAC: Household voltage
240 VAC: Non-American (European) outlet voltage
480 VAC: Voltage of your nearest transformer
Read enough? Now try browsing some of these sites to see if you understand what's been covered. If you run across any other things you have questions about, relay them on to me and I'll add it to this page. The links:
BattlePack (http://www.battlepack.com/)
High quality batteries, specially made to withstand the trials of bot-fighting.
Hawker (http://www.hepi.com/)
Hawker makes Sealed Lead Acid batteries which are generally the most respected SLAs by the bot community, especially for long run times.
Power-Sonic (http://www.power-sonic.com/)
Power-Sonic sells mostly SLAs, but also NiCads and chargers.
Tower Hobbies (http://www.towerhobbies.com/)
Tower Hobbies is a great source for hobby stuff. Here, they sell chargers, batteries (especially for receivers and transmitters, as well as for smaller bots like ants and beetles).
Robot Marketplace (http://www.robotmarketplace.com/)
The Robot Marketplace sells almost everything. While it's really a site that lists products for suppliers, I haven't had problems with it, and look at all the stuff! They have a LOT of batteries listed.
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