Robot Battery Care, Charging, and Fuses

Started in the 2023-2024 season, Tech Tips are segments released in the FIRST Tech Challenge Team E-mail Blast. Sometimes the Tech Tips are included in whole in the email blast, but sometimes there is more content than is reasonable in the email blast so partial content is included in the blast with the rest of the content here.

Battery Charging

There are three robot main batteries that are legal to use in FIRST Tech Challenge, and they are all 3000mAh NiMH batteries with an attached 20A fuse. However, the manufacturers of the batteries have different battery chargers and different recommended charging settings for the batteries. When charging the TETRIX MAX 12-Volt battery, on the battery the manufacturer recommends charging at the 0.9A charge rate (the lowest setting on most selectable battery chargers) using the Global NiMH battery pack charger. The Matrix 12-Volt battery with the same form factor is recommended to be charged with the goBILDA 12V battery charger, which does not have a user-selectable charge rate switch but has a max charge rate of 1.0A. However, the REV 12-Volt Slim Battery is recommended to be charged with the REV Battery Charger using the 1.8A charge rate setting. To ensure safety, proper charging, and a long battery life, make sure you’re charging your batteries at the manufacturer’s recommended charge rates!

Battery Maintenance

Nickel-Metal Hydride (NiMH or Ni-MH) batteries, like those used in FIRST Tech Challenge, do require periodic maintenance to keep them healthy! Every day, NiMH batteries lose on average 1% of their charge capacity at normal room temperature - at colder temperatures this decline slows a bit but does not stop it. This means that every 2-3 months it’s important to recharge your batteries to keep them healthy - there is no off-season for batteries! It’s also recommended to mark your batteries with tape and a sharpie to mark (1) Your team number (never lose a battery at a competition!), (2) What year the battery was purchased, (3) Give your batteries names so you can differentiate batteries easily, and (4) optionally provide a tick mark each time the battery is recharged. NiMH batteries can generally last 200-300 recharge cycles before their internal resistance declines to the point where it’s time to replace them, and keeping track of charge cycles is an easy way to track how “used” the battery is before needing to have its internal resistance checked.

Battery Fuses

Every legal Main Robot Battery in FIRST Tech Challenge is required to have an in-line replaceable fuse on the battery, you’ll find the fuse housing on the red (positive) cable on your battery between the battery and the connector (the top lifts off, exposing the fuse). This fuse helps protect your battery and your electronics from prolonged or excessive over-current. The fuse used with all legal batteries is a 20A Automotive-Mini (ATM) blade-style fuse, and can be found in virtually every auto parts store. It has a yellow-colored housing which easily identifies it as a 20A fuse. If you find that your battery’s voltage suddenly drops to zero (when tested using a battery tester or multimeter) it’s probably because you’ve blown your battery’s in-line fuse.

A fuse is a short span of specially-designed electrical wire intended to carry electrical loads up to a very specific amount of current. When the current loads exceed the rating, the wire within the fuse begins heating up - the more the load exceeds the rating, the hotter the wire will get. Eventually the wire will heat up so much it self-destructs and melts or burns up, breaking the circuit. This fuse-melting condition is often called “Blowing a Fuse”; the fuse is thus destroyed and is no longer usable, but it protected the electronics in the circuit as its last selfless act.

How does a battery fuse get blown? These are two of the most common reasons why a fuse can be blown:

  • Overcurrent Conditions - The Robot has components (generally actuators, like servos and motors) that can pull a combined current that is more than the robot’s electrical circuit can safely carry. The main electrical power wires on a robot are required to be a minimum 18AWG, which can easily continuously carry up to 16A of current. When components pull a combined current far exceeding this limit, generating unsafe heat in excess of what the wires can tolerate (risking melting the wire insulation which could lead to short circuits and fire), the fuse blows to protect the circuit. The wire size and fuse limit has been carefully selected for the safety of the robot’s electrical system.

  • Short Circuits - Usually this happens if unshielded wires of opposite polarity touch each other in the robot’s electrical system, like when performing electrical maintenance on switches or wires (ALWAYS unplug the battery before performing any maintenance on a robot!). Other causes can be failed electronics and damaged components. This causes an extremely high current load to travel through the battery, near-instantly causing the fuse to blow. When replacing the connector on a battery, ALWAYS remove the fuse prior to performing any work - this protects the person doing the maintenance AND protects the fuse!

Always make sure your main battery fuse is replaced with the proper fuse (20A for FIRST Tech Challenge) and make sure you’re always following all safety guidelines when working with your robot’s electrical system!

Got any questions about battery care? Come start or join the conversation on the FTC Community Forums!