All About Battery Usage!

duracell_9vWhen we designed the ChapR, battery life was a major concern; but so was portability.  So we struck what we think is a good balance between the two by using a 9-volt battery in the ChapR.  Here is a discussion of battery life that should help you understand not only how long the ChapR battery will last, but also how you calculate battery life in general. A battery is an energy storage device.  It has a certain amount of energy stored within it that we extract as electricity.  Different batteries have different amounts of electricity stored within them.  The size, chemistry, and voltage of the battery all contribute to how much electricity is stored in a battery.

For example, a typical 9-volt alkaline battery has 500 mAh of electricity stored within it.  The unit “mAh” is short for “milli-Amp Hours”.  A milli-Amp (mA) is 1/1,000 of an Amp.  So 500 mAh indicates that the battery can supply 500 mA of power for one hour.  But it also means that the battery can supply 250 mA for two hours, or even 100 mA for five hours (note the simple math there).

mAH_equation

In a perfect world, this would mean that a device that uses a 9-volt battery and uses 100 mA of current could run for five hours before the battery is exhausted.  Practically, however, batteries aren’t that efficient.  As they discharge, they start to lose their ability to “keep up” with the voltage they are designed to supply.  So as a battery gets low on energy, its voltage starts to drop.  For a 9-volt battery, its voltage can easily drop to 5 volts when its energy gets low.  And the problem is, devices that use 9-volts are really not happy when the battery is only supplying 5 volts.  Some devices using 100 mA may only last for 3 hours with a 500 mAh battery because they can’t operate on the lower voltage that is supplied when the battery discharges.

lithium_9v

Fortunately, there are better batteries.  A typical 9-volt lithium battery has 1200 mh of energy stored in it.  Using the simple math above, this could last for 12 hours in a device that uses 100 mA.  But lithium batteries aren’t perfect either, their voltage lowers as they discharge as well.  Unlike alkaline batteries, though, the lithium battery voltage doesn’t lower as much, or as quickly as alkaline batteries.So how long will a 9-volt battery last in a ChapR?  The ChapR uses an average of 100 mA when in use.  This is only an average because the energy used by the ChapR is related to how many gamepads are plugged into the USB ports and how much you are using the gamepads (moving the sticks or pressing buttons).  When connected to your robot with two gamepads and you frantically pressing buttons and moving joysticks, the ChapR will use 110 mA.  If you stop frantically moving the joysticks, that will go down to 90 mA.  If you are only using one gamepad, that number will be closer to 80 mA. Unfortunately, the ChapR is one of those devices that hates 9-volt batteries when they stop supplying the correct voltage.  If the voltage falls below 7 volts, the ChapR will shut-down.  For a 9-volt lithium battery, this means that you will get approximately 8 hours of continuous (somewhat frantic) use of the ChapR with two joysticks. Most of us, though, don’t drive our robots frantically for 8 hours straight.  Instead, we’ll drive for a while, tinker for a while, and then drive again.  The ChapR was designed with this usage pattern in mind.  It will automatically shut itself off after 10 minutes (this is configurable) to save battery.  And then when you turn it back on, it will connect to your robot normally within 3 seconds.  So in practice, you will get far more than 8 hours of “drive time” from your ChapR with a lithium battery. We highly recommend lithium batteries for the ChapR.  Even though they are more expensive, they are far more economical because of their lower voltage drop than alkaline batteries.  You will get up to 4 times more life from a lithium battery at roughly 2.5 times the cost of an alkaline battery.  But if you can get a good deal on alkalines, then go for it!

hitech_lithium_9v
You can also use rechargeable 9-volt batteries in the ChapR.  But keep in mind that normal rechargeables only have around 175 mAh of energy, and have a lower voltage that alkaline or lithium batteries, so you will be recharging a lot.  There are lithium-ion rechargables with up to 700 mAh, so they would be a far better choice if you want to use rechargables.

The ChapR is quite new, so we don’t have conclusive data as to the actual time that a battery will last.  But here is a table of our best guesses for different battery types.  For the “normal use” data, we made a guess at normal usage patterns, and multiplied the “frantic” use case by 1.5 and rounded.
NOTE that the ChapR consumes no power when it is turned off.  Sure this is really what you would expect, but we had to work hard to make sure that was the case!  You can leave the ChapR in the drawer for months and it won’t drain the battery…but we highly recommend drive practice.  So don’t leave your ChapR in the drawer, drive!
Standard Disclaimer: We worked very hard to design the ChapR to get great battery life.  We also did our research to come up with the “average” amount of time you’ll get out of a battery.  But we really can’t guarantee these numbers.  Your mileage will vary with everything from your usage patterns, to your particular gamepad controllers, to the brand of battery you use.  We’ll continue to update this table as we collect actual data.