Batteries
From HBRC WIki
Contents |
Burke's battery laws.
by Dennis Burke
- Buy lots of batteries, they take longer to charge than discharge.
- Consider Li-ion or LiPo, they pack more punch per unit weight and hold their charge longer.
- Give yourself enough headroom in your battery voltage to keep your CPU running till the bitter end.
What is a Amp-Hour (AH)?
by Chuck McManis
An Amp-Hour is the measure of storage capacity for a battery much like a gallon is the measure of the storage capacity for a jug. Milliamp Hours are thousandth's of an Amp Hour. The actual storage capacity of the battery is measured in Joules, but no one knows how to convert Joules to anything useful in their head so the less accurate amp-hour measure is used.
In an ideal world an Amp Hour would represent the ability of a battery to supply one amp of current for one hour. Conversely, it would supply 1/2 amp for two hours. However, because we're really talking about energy, and not water, what happens is this:
- The battery has a finite internal resistance which can be fairly low (NiCd batteries have internal resistances that are less the .01 ohm), and so the battery is represented in real life as an ideal battery in series with a resistor. When you draw current from this arrangement the resistor dissipates i2 * r watts in heat. That dissipated energy comes out of the energy storage capacity of the battery. Thus some of the 'amps' being requested, never get to the circuit. They heat up the battery instead.
Battery manufacturers know this too so what they do is pick the "ideal" current drain of the battery (based on ways the battery may be used in a circuit) and they characterize their battery and then use the number they get empirically as the amp-hour rating. For example, Alkaline batteries are tested with a 10mA power load.
Chuck's school of hard knocks generally derates the batteries Amp-Hour rating 75% for "high current" operation. So a 4 Amp-hour battery I'll treat as a 3 Amp-Hour battery (4 * .75) if I'm going to be using it for motors and such.
by Curt Meyers
One last detail about amp-hour rates. The battery spec. should contain a designator "c/xx". Where "xx" will be a number, typically xx=20. This designates the time used to measure the discharge. So, "c/20" indicates the discharge occurred over a 20 hour period, other batteries might list "c/2", a 2 hour period. Or maybe even a battery power capacity usually given in watts or kW.
If you go to a battery dealer they can provide these specs and usually show you graphs of the batteries voltage and current versus time.
A nice retail place to go is Batteries Plus at 903 East El Camino Real in Mountain View. They have all this information available.
Battery types
| Type | Advantages | Disadvantages |
|---|---|---|
| Liquid Lead Acid | ? | Does not like to be completely drained Can spill Battery Acid |
| Sealed Lead Acid Gel Cells Wikipedia: Lead-acid battery | No leakage problems High current Good value for money. | Does not like to be completely drained Long charge times. |
| NiCd Wikipedia: NiCd | No leakage problems inexpensive can take more abuse -- agressive discharge recharge cycles. | Likes to be fully drained between discharges |
| NiMh Wikipedia: Nickel-metal hydride battery | No Leaks Last about twice as long as NiCd. | |
| Li-ion Li-poly Wikipedia: Lithium-ion battery | Low weight, twice the voltage per cell | Potential to explode or catch fire if carelessly charged/ discahrged. Requires additional circuitry, potection More expensive. Requires a special charger, and battery balancer. |
Please note that this table is incomplete. If you have any input, it would be gratefully appreciated.
Liquid Lead Acid
=== Sealed Lead Acid Gel Cells (SLA) === Heavy weight batteries for larger robots.
== NiCd == More information than I care to summerise is available at:
NiMh
by Mark Huetsch
I was recently in need of batteries with a large capacity and a high discharge rate and I discovered these F-size NiMH batteries 14Ahs with with a capability of discharging at a rate of 60A. For the money, these seemed to be a nice middle ground between NiCd and Li batteries: more expensive and with less punch than the NiCDs, but with better recharge characteristics, and less expensive than the Li options but with worse recharge characteristics. Lead acids , though cheap, kept throwing my design process into a bit of a feedback loop: to make up for the extra weight, I needed more powerful motors, but more powerful motors demanded more current, which in turn meant heavier batteries. For my situation, in which low weight was important, the leads seemed like a bad deal.
Li-ion Li-poly
by Terry Slocum
I forgot an important safety tip. If you decide to use LiPO batteries, use a protection circuit. As mentioned by Camp LiPOs catch fire. One of the thing that can cause this is discharging them below 2.4V per cell. The protection circuit shuts down the robot down if the voltage is too low. Saving you from ruining an expensive battery, and possibly creating a bonfire bot. Most of the protection circuits also keep you from over charging or discharging the battery too quickly. Here is a page with some examples:
Charging
A word about fuses
by Chuck Rice
Generally small batteries (AA, C, and D cell batteries) do not need fuses, but when you get into the area of motor driver batteries at 2, 5, 10 and greater amp-hour levels, be aware that these batteries can carry quite a wallop. It is a good idea to include a fuse near the battery for short circuit protection. Automotive fuses are good for this and with the blade type fuses, you can get cheap clip-on fuse holders that are easy to install. Choose a fuse that is rated a bit higher than your combined motor stall current. Remember that it is a lot easier to replace a fuse than to rewire your motors, and you reduce the risk of having to chase a flaming robot!
Power Connectors
There is a good writeup on standard power connectors at PowerStream
Sources
| Tenergy Corporation | http://www.all-battery.com/index.asp |
| Batteries Plus | 903 East El Camino Real in Mountain View |
