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06-23-2010

Lithium Ion Batteries for Robotics. Part 2

LiIon Batteries

Talk to your Battery

Many of the newer battery packs are "Smart". There is an Intel-Duracell spec on this that many battery makers follow. It uses the SMBus (System Management Bus, an I2C derivative) to provide communications between the battery, the charger and a host computer. The battery acts an SMBus master by telling the charge controller what settings to use for charging. The host computer is not required but it can ask the battery for information and be informed of an event, such as charging voltage applied or high temperature. My charger talks to SMBus batteries and allows the battery to set the charge parameters. When I refer to "Smart Battery" from here on I mean the SMBus system, "smart" refers to smart batteries in general.

Unfortunately, not all batteries speak the same language. Most of the modern batteries are smart but few advertise what protocol they use. Some batteries will only charge if they are talking to a charger speaking the right dialect. I was lucky enough to get one that said "Smart Battery" right on it. My second battery (Gateway 11.1V 6400 mAH) uses a protocol I never did figure out. I found a Gateway charger for a similar battery but it would not converse with my battery.

Here is a good site for Smart Battery info.

http://www.sbs-forum.org

LiIon Battery Characteristics

A Li-Ion cell produces a nominal 3.6 or 3.7 volts per cell. Operating voltage is really 4.2V down to 3.7V after which they decline rapidly. Cells are combined to produce higher voltages so common voltage ratings are 3.6, 7.2, 10.8 and 14.4 volts (or 3.7, 7.4, 11.1 and 14.8 volts). Cells are wired in parallel to increase battery capacity. The asking prices are pretty much a function of the popularity of the laptop they are associated with. The first one I bought was for a no-name brand, thus a low price. The battery lists for about $160 new.

The batteries usually include a number of safety devices such as varactors that act as resetable fuses, thermistors that monitor battery temperature and report back to the charger, over voltage monitoring, etc. The minimum features should be a varactor and thermistor. I finally had to strip the smarts out of the Gateway battery to get it operational but I installed a 2A varactor and a 10K nominal thermistor that can be monitored by my charger for protection.

The battery parameters that you need to know to charge safely are:

  • Number of series cells, 1 to 4
  • Maximum Charge voltage, usually 4.2 volts per cell, sometimes 4.1
  • Maximum Charge Current, often expressed as 1 C or .5 C where C is the nominal battery capacity in mAH.
  • Battery pin-out
  • Thermistor nominal resistance at room temperature - 10K is most common
  • Smart protocol (if any)

This information is sometimes written on the battery case but usually is not. Web searches usually don't reveal the information either as batteries are normally tied to specific laptop model numbers and even basic parameters like nominal voltage and capacity are not often not shown. You need to know how to determine these numbers yourself.

Numbers of cells is determined by dividing the battery nominal voltage by 3.7. Maximum charge voltage is usually 4.2 volts per cell. If you are unsure use the safer 4.1 volts. Maximum charge capacity is normally close to 1C but can be as low as 0.5C. Use 0.5C if you have no other information. Battery pin-out can be tricky. My Gateway has no pin markings at all. Battery plus and minus were not even obvious as it required that two pins be shorted before it would even work. The thermistor can be found as a resistance to ground. The Gateway battery is 10K, the other one is 300 ohms. The remaining pins are likely used for comm but be careful. Test the pins for activity after it is power up. You could have 12 volts appear on a pin and damage your uC input. I have added an input resistor and 5.1V zener to my two comm inputs for protection against this.

Check this article out for more info on the batteries themselves.
Here's a picture of my first battery.

My robot operates at just under 1 amp when it's running around. Theoretically it should run for over four hours between charges - and it does. In addition, the Li-Ion battery is 10% smaller in volume than and about 3/4 the weight of my old 12V 2300 mAh sealed lead acid battery.

Charging

Charging the batteries takes some control or the batteries will be damaged or could even explode. I was not able to find the specifications for my battery (a major plus for the smart battery concept), therefore I had to make educated guesses based on units with similar model numbers and parameters.

The most common specifications call for charging currents between 0.5C and 1C where C is the nominal battery capacity expressed in mAh's. Some specs indicated charging rates of 2C would be acceptable. Higher charge rates resulted in somewhat reduced capacity (15% for 2C vs. 1C). Overcharging can hurt the battery and could cause it to blow up. On the larger batteries such as the 6400 mAH the charger capacity normally will limit this anyway. Barring any other information I would say 0.5C would be a safe value.

Charging generally follows these steps:

  • Current control at the Max Charge Current is used until the battery voltage reaches the voltage threshold (normally 4.1 or 4.2 volts per series cell). The battery is about 70% charged at this point.
  • The voltage is now controlled very accurately (this is very important) at the threshold voltage while the current drops off naturally. Once the current reaches the lower cutoff ( about 100 mA per parallel cell typically), charging is stopped after a top-off time delay.

Using this program and charging at 1C should fully charge a battery in 2-1/2 hours.

To be continued

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