A Bit About Batteries
First of all I wish to credit the illustration. It is not my drawing but was 'borrowed' from http://www.latestcarblog.com/2011/07/lithium-ion-car-batteries-made-powerful.html
Please let me know if there is any reason I should not use it.
There are several types of batteries available for our robots and it could be confusing to many people as to which ones to buy. I will give a brief description of the types you may come across.
I will cover standard torch/flashlight batteries, alkaline, NiCd, NiMH, Li-Ion and lead-acid (motorcycle batteries).
General tips for any type battery.
•Never leave batteries in the charger for too long and certainly not past a day.
•When you are charging batteries they may get warm. Allow them to cool to room temperature before using them.
•Never freeze a battery as can cause it to rupture.
•If possible, always use "smart chargers"; the type the senses when the battery is fully charged and switches off or to a low-current trickle charge to avoid damage to the batteries.
•Manufacturers discovered that charging rechargeable batteries to only about 40%, gives them the longest shelf life for sitting in a store before they are sold, so always charge rechargeable batteries before using them. This first charge is typically for two hours or more. After that rechargeables will usually come to full charge in only about a half hour.
STANDARD FLASHLIGHT (or TORCH) BATTERIES
These consist (generally) of a carbon rod in the center which forms the positive anode terminal, surrounded by electrolyte and cased in a zinc casing, which forms the negative cathode terminal. These are generally not useful for robots and should only be used if you have nothing else available. Output voltage is ~1.5 volts per cell which gradually drops as the cell is exhausted.
These batteries marked an improvement over the carbon/zinc and copper/zinc batteries giving a longer lifetime. They are generally made with manganese dioxide and zinc electrodes in an electrolyte containing hydrogen peroxide. Output voltage is ~1.5 volts per cell with an energy density of around 110 Wh/kg. Like standard batteries the terminal voltage gradually drops as the charge is exhausted.
The type called "rechargeable alkalines" requires a constant voltage source of between 1.62 and 1.68 volts to charge.
NICKEL-CADMIUM (NiCd or NiCad)
As the name implies, these batteries use electrodes of nickel and cadmium in an electrolyte of potassium hydroxide. The cadmium is considered a hazardous material so these should not be thrown into normal garbage to end up in a landfill. In fact in 2008 the EU effectively banned the sale of batteries containing cadmium (with a few exceptions) If you are interested, see the directive here: http://europa.eu/legislation_summaries/environment/waste_management/l21202_en.htm
Most people think of these as being 1.2 volts per cell, but they will actually charge to about 1.3 volts with 1.25 volts being the nominal voltage, so 4 of them gives very close to 5 volts. These batteries tend to hold a constant terminal voltage until the charge is nearly used up and then the terminal voltage will drop off suddenly.
Because of crystal growth inside the cells, they exhibit what most refer to as "memory effect". These batteries will give the longest life (over 1000 charges) if they are "fully" discharged before recharging. The effects of crystal growth can be reversed by several cycles of fully discharging the batteries before recharging. Crystal growth does not generally become a problem when recharging a partially charged battery as long as the charge is kept at a very low value (less than 1/10 their available discharge rate), and called trickle-charging.
Setting on a shelf unused, NiCd loses about 10% of its capacity within the first 24 hours, after which the self-discharge drops off to about 10% per month. This is called self-discharge.
NICKEL-METAL HYDRIDE (NiMH)
Made similar to nickel-cadmium cells, except the cadmium is replaced by rare-earths or nickel alloys with several other metals. The same electrolyte, potassium hydroxide, is used and as before the nominal terminal voltage is 1.25 volts and like NiCd batteries, the terminal voltage stays up and drops off suddenly as the charge runs out.
There are many types of NiMH batteries, but in general they do not suffer from memory effect. They do have a high self discharge rate, –from one and a half to two times the rate for NiCd cells. To combat this problem a new version of NiMH cells have come out called LSD type. Now before the drug addicts out there get all excited, I should hasten to say this stands for Low Self Discharge. Manufacturers claim the discharge rate on LSD cells doesn't exceed 30% per year if fully charged and stored at no more than 20°C (68°F).
The current capacity for NiMH cells has increased in the last few years. Originally they had capacities in the 700 mAh range, but newer ones reach 2700 mAh [mAh stands for milliampere-hours.
There are many, many types of lithium batteries and I will not go into them other than to direct you to http://en.wikipedia.org/wiki/Lithium_battery for more information. Instead I will mention only the most common you might use. TIP: Unlike Nickel batteries, avoid repeated deep discharges of Lithium batteries.
This is the most common type of lithium battery, but is not generally rechargeable. They are nominally 3.0 volts per cell and may be found on motherboards of computers keeping the bios memory intact. I have a couple that have been in use powering bios chips since 1992 (& laying in a drawer since 1998), that still read 3.0 volts and are still hooked up and doing their jobs for twenty years so far.
These rechargeables have a rigid case and are found primarily in cameras or as the main power batteries for laptop computers. They have an energy density of ~140 Whr/kg and a nominal terminal voltage of 3.6 volts. Some say these batteries lose recharge capacity due to aging in 2 to 3 years whether in use or not.
Polymer in the name refers to the electrolyte in the cell being held in a polymer material. These also use a lighter polyester laminate foil case which is great in applications (like model airplanes or helicopters) where weight is important. Their terminal voltage is a nominal 3.7 volts but can be charged up to around 4.2 volts.
Charging these batteries can be a problem. For one thing, the terminal voltage should never be allowed to drop below 3.0 volts or it will permanently damage the cells capacity. (The actual limit is closer to 2.75 volts before damage occurs, but manufacturers say 3.0 volts to be sure you have a buffer.) Recharge rate must be carefully controlled as well. There have been incidents involving Lithium ion laptop computer batteries self-combusting and one major manufacturer has recalled a large number of such batteries.
Do NOT try charging LiPo batteries with just any charger. Get a charger specifically designed for lithium polymer. They will thank you for it by not exploding. (!)
If you notice the schematic for my robot, Shrödinger (http://letsmakerobots.com/node/25953), you will see I charge his NiCd and NiMH batteries from his six solar cells, but when the LiPo batteries need recharging, I use a special LiPo charger purchased specifically for them.
[You may wonder, why I have three different types of batteries on Shrödinger? It is simple really. I had NiCd batteries on hand, but needed more power. I bought NiMH batteries. Then in use and testing I decided he still had a problem that the batteries would not give enough juice instantaneously, so I opted to get a couple dual Lithium Polymer batteries which worked out great. Overall he works fine now, and both types of nickel batteries have similar charge characteristics and may be charged from his solar panels. You will note that I put a separate schottky diode in series with each type, but that was because some are used for motors and the others are tied to the electronics boards. Using separate batteries prevents motor noise from getting into the electronics.]
These are the normal batteries you will find in a car or motorcycle. They consist of lead and lead dioxide plates in sulfuric acid. Batteries for a car or motorcycle contain liquid sulfuric acid which can spill if tipped or can bubble out of the battery if overcharging occurs. While this older type is much cheaper, the only kind we really should use in a robot are the ones that come sealed, referred to as SLA or sealed-lead-acid, and also called gel-cells. These may be tipped over and no acid pours out. Most people think the output voltage is 2 volts per cell, but if you read a lead acid battery you will find they charge to 2.2 to 2.3 volts per cell (so a 12 volt car battery will typically read around 13.2 volts) The voltage on a lead-acid battery drops gradually as the charge is exhausted. If left stored in a discharged state, the cells deteriorate to the point where they cannot be recharged. You should fully charge a lead-acid battery before long storage.
Some terms you might need to know:
Ampere-hour (Ah): This measures the total amount of energy available from a battery before its charge is used up. In small batteries this is generally given in thousandths of an amp. (i.e. milliampere-hours or mAh)
Anode: This refers to the positive terminal of a battery.
Cathode: This refers to the negative terminal of a battery.
Electrolyte: This is a material that separates the anode and cathode in a battery and allows
the transfer of ions from one to the other.
Energy Density: This measures the amount of energy available per unit mass (or volume). It is normally measured in watt-hours per kilogram and is written as Wh/kg.