Vorpal Combat Hexapod Battery Recommendations

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There are several battery types that will successfully power your VH12 Hexapod. The batteries you choose will affect how long the robot will run, recharge time, etc.

This article explains your options.

Contents

Battery Safety Tips

Rechargeable batteries are by far the most cost effective and environmentally friendly kind of batteries, but some simple safety rules are recommended:

  • Never, ever put non-recharegable batteries in a battery charger!
  • Do not mix different types of battery, do not mix old and new batteries.
  • If you are using rechargeable batteries, make sure you are using the right kind of charger for the batteries you are using. Follow all manufacturer recommendations.
  • Never leave batteries charging unattended. Never charge batteries "overnight". It is safest to charge batteries in a place that will not be damaged by heat, just in case the rare instance of a damaged battery overheating during charging occurs.
  • Investing in a "smart" charger is well worth the extra cost, as they will generally keep your batteries in much better shape than "dumb" chargers. You can buy a dumb charger for under $10, but if it damages your batteries so they can't hold a full charge, did you really save any money? A good smart charger may be a few dollars more, but will keep your batteries running for many hundreds of charges. Don't be penny wise and pound foolish!
  • If you are charging batteries, it is normal for them to get warm, but if they feel hot, if smoke is coming off them, if you smell a "burning plastic" type odor, if you see sparks, or if they appear to be swelling or discoloring, disconnect the charger immediately from the wall socket and wait for things to cool down before touching. Exercise caution, remove smoking batteries to the outdoors or a ventilated area.
  • If there is any visible damage to a battery or if it appears to be swelling, stop using it immediately and do not attempt to recharge it.
  • Some kinds of rechargeable battery cannot be disposed in the trash but must be recycled, follow all instructions and markings on the battery and follow your local community recycling practices.

Gamepad: Any kind of 9v Transistor Battery

The gamepad has minimal power requirements and can be powered by either non-rechargeable or rechargeable 9v transistor batteries. (These are the common rectangular batteries often used in smoke detectors, small radios, clocks, etc.) Alkaline, NIMH, NICAD, or LION batteries in the 9v rectangular format will all work fine for the gamepad. The gamepad was designed to fit a standard 9v battery in its battery compartment. You can expect several hours of usage. Do be sure to turn the gamepad off when not in use.

Technical info: The gamepad typically draws about 50 to 100 mA, which is mostly consumed by the Bluetooth module. Thus a 500 mAh alkaline 9v battery will last about 5 to 10 hours, while a 250 mAh NIMH rechargeable battery will last about 2.5 to 5 hours.

Hexapod: Only Use Rechargeable Batteries

Non-rechargeable batteries, such as alkaline batteries, cannot produce enough current (amperes) to drive the robot reliably.

The major types of rechargeable battery used today are Nickle Metal Hydride (NIMH), Nickle Cadmium (NICAD), and Lithium Ion (LI-ON).

Below we discuss the different options for the robot.

NOTE: Our kit comes with the holder for the two 18650 batteries discussed first below. The other options are for Makers who are building using their own parts or who wish to experiment with different battery options. Switching the battery holder will require soldering, but if you're a Maker you should be ok with that!

Two 18650 Size LI-ON Batteries

This is the type of battery holder we ship standard with our kits, because these are great batteries. You may have never heard of them before, but don't be put off by that, they are by far the best we've found. They balance run-time, cost, and weight very well for this project.

These batteries can be found on Amazon.com and camping stores. While a little less common than AA or AAA, these are actually ideal batteries for this robot, giving you long run times at a reasonable weight and a bargain price. You can find them for about the same cost as the more common batteries (two 18650 cost about the same as six AAA but provide several times as much run time! The chargers are also about the same price as AA or AAA chargers.)

The "18650" battery is commonly used in flashlights and many of these are made to output high currents which easily are high enough to power the hexapod. Two in series will provide 7.4 volts nominal (which is enough for the hexapod's needs). These batteries can provide very long battery life for the robot. For example using two 3000 mAh 18650 cells you can expect about one full hour of continuous motion. (However note: it is best to let the servos "rest" after several minutes of vigorous activity, you shouldn't run the robot full-out for an hour!)

Generally speaking, you should choose an 1865 with at least 2500 mAh capacity to ensure you are not overtaxing the batteries. Often manufacturers will publish the number of "Amps" the battery can safely output. The robot only needs at most about 3 Amps during short surges but it typically draws about 2 to 2.5 Amps.

Note that these batteries require smart chargers made specifically for LI-ON. Do not use a charger that is marked for NICAD or NIMH unless it also explicitly says it is safe for LI-ON.

We have done tests on the following brands and models:

  • EBL brand 3000 mAh 18650 batteries (they're red and have a white ring around the + terminal)
    • 60 minutes to first "cut out" while running "demo mode".
    • Usable up to 80 minutes if you're willing to reboot the robot after cut outs every now and then
    • Currently cost about $3 per cell from Amazon in a 2 or 4 pack.
  • EBL brand 3000 mAh "Protected" 18650 batteries (red with yellow ring around the + terminal)
    • 25 minutes to first "cut out" while running in demo mode.
    • Usable up to 60 minutes if you're willing to reboot the robot every now and then.
    • Currently cost about $4 per cell from Amazon in a 2 or 4 pack.

Just be careful, you will sometimes see 18650 (and other types of battery) advertised with outrageously high mAh ratings, like 6,000, 8,000 or even 10,000. Check customer ratings and make sure other people are confirming they are for real.

Protected vs. Unprotected 18650 Batteries

Some 18650 batteries are marked "protected" while some are not. The "protected" batteries have a circuit in them that will automatically stop the battery from being shorted out or draining too low. LI-ON batteries can be permanently damaged if they are over-discharged.

You can use either kind of battery with Vorpal as long as you follow common everyday battery safety procedures. For example, never leave the robot turned on when you're done using it. It would be best if you always remove the batteries after you're done for the day, to ensure the robot doesn't get turned on (which could drain too much power, over time, from an unprotected battery).

The advantages of using protected cells are:

  • They automatically cut off when they drain below a preset low voltage.
  • Therefore, you don't have to worry as much about damaging them by accidentally leaving the robot on at the end of the day.

The disadvantages of using protected cells are:

  • They are slightly larger, so it will be slightly harder to get the cells in and out of the battery holder. We suggest putting a string, ribbon, or piece of tape down on the battery holder before inserting the battery, allowing you to get them out easier later.
  • Because of the protection circuit, these batteries will have a shorter run-time even though they have the same mAh rating.
    • What happens is, very short term surges in the servo motor power usage will fool the protection circuit into triggering, cutting off power to the robot. You will know this has happened because the robot simply turns off, all lights go out, and it sags to the floor.
    • Turning the robot off then back on again clears the protection circuit and allows operation again.
    • This "cut off" effect will happen after long periods of intense use (such as demo mode or full-out competition).
    • As the battery drains, these random cut offs will get more and more frequent.

Example:

  • An unprotected EBL 3000 mAh battery will last about 60 minutes under continuous motion like "demo mode", then will start experiencing random "cut off" situations (requiring a restart) more and more frequently until it won't start again at approximately the 80 minute mark.
  • In contrast, a protected EBL 3000 mAh battery (they have a yellow band around the + side) will work for about 20 minutes without any rest, but then will start cutting out every 5 to 10 minutes, and will stop working around the 60 minute mark.

The conclusion is: unprotected cells actually work a little better in this robot. Just be sure you do not leave them in the robot after use, to avoid the possibility the robot will be left on and drain the batteries to the point of damaging them.

For Makers Only: Alternative Batteries

6xAAA Rechargeable NIMH or NICAD Batteries

You can use a 6xAAA battery holder to power the robot using NIMH or NICAD batteries. Typical battery life for continuous use after a full charge would be 15 to 20 minutes of continuous motion using batteries with 1000 mAh capacity. Of course, if the hexapod is moving sometimes and motionless other times, your actual time may be far longer.

6xAA Rechargeable NIMH or NICAD Batteries

You can use a 6xAA battery holder to power the robot using NIMH or NICAD batteries. If the batteries are rated 2000 mAh (typical for this size) you can expect 20 to 25 minutes of continuous motion (but different brands may vary significantly from this rough estimate). However, because AA batteries are about twice as heavy as AAA batteries, they do put a little more stress on the servo motors.

Other Options for Advanced Users

If you know about batteries and wish to design your own system for the robot, here are some factors to bear in mind:

  • The robot requires 2 amps during typical use but may spike up to 2.5 to 3 amps for short periods.
  • The voltage regulator requires 6.5 volts input to keep the servos powered at 5.0 volts nominal, so do not design a battery system with less than about 6.5 volts output at 2 amps. Running the servos with too low voltage may shorter their life.
  • The maximum recommended battery voltage is 12.0 volts, beyond that the 5v regulator used for the servos might be damaged and the Arduino Nano onboard regulator (which is fed directly off the battery, not the 5v regulator used for the servos) might overheat.
  • The weight of the batteries is also critical. The heavier the batteries, the more stress is put on the servos. The maximum weight of batteries should be no more than 200 to 250 grams. Note that the 2x18650 setup that comes with our kits weigh about 110 grams.

Example 1: the robot will run quite well on two AA LIFEPO4 batteries (which have 3.2v nominal cell voltage and when charged will meet the 6.5 volt minimum, and can output easily 3 amps). These are also very light at about 80 grams.

Changing the Battery Connector

This is a build-it-yourself robot kit, so if you wish to use RC battery packs with battery connectors such as Tamiya, PowerPole, JST, or XT60, go right ahead and make the change. A few minutes with a soldering iron is all it takes to make the battery pack swappable using a connector.