FRC Battery Guide: Charging, Care, Testing, and Safety

An FRC robot runs on a single 12-volt, roughly 18-amp-hour sealed lead-acid (SLA) battery, and looking after it is one of the cheapest ways to make your robot more reliable. A battery that's fully charged, rested, and low on internal resistance is the difference between a robot that drives strong for a full match and one that browns out with thirty seconds left on the clock.

Batteries are the single most neglected part of most rookie robots. Teams pour hours into a shooter mechanism and then run every match on the same tired battery straight off the charger. This guide covers what the battery is, how to charge and store it, how to test it with a Battery Beak, how the SB50 connector fits into the power path, and how to handle all of it safely.

What an FRC battery actually is

Every legal FRC robot uses one 12V SLA (sealed lead-acid) battery, also called an AGM battery. Nominal capacity is around 18 Ah, and a full one weighs about 12.6 lb (5.7 kg). It is a genuinely heavy, genuinely hazardous chunk of lead and sulfuric acid, not a hobby-grade LiPo.

The one number worth memorizing is the resting voltage: a fully charged battery reads roughly 12.7 to 13.0 V at rest (no load). If a "charged" battery is sitting at 12.3 V, it isn't actually full. Under the load of a match, voltage sags, and a weak or half-charged battery sags further and sooner — far enough to trip the roboRIO's brownout protection, which sheds motor output and makes the robot feel sluggish or cut out late in a match. Our lesson on why the robot goes limp walks through the staged brownout thresholds and the systematic fixes.

The rest is rules and specs the game manual pins down: you may run only one battery at a time, it has to come from the manual's approved COTS list (check the current manual — the list changes), and the main power path uses 6 AWG wire behind a 120 A main breaker. Our full lesson on the 12V SLA battery and Anderson connectors covers the specs, the legal rules, and battery care in depth, so this guide stays focused on keeping the battery healthy.

Charging: do it right, every time

Charge with a smart SLA/AGM charger, and don't rush it. FRC rules cap how fast you're allowed to charge — recently the limit has been 6 A — so a legal charger won't blast current into the battery. Check the current manual for the exact figure, but the practical takeaway is the same: slow, complete charges are healthier than fast ones anyway.

A few habits separate reliable teams from the rest:

• Charge fully, then let it rest. A battery straight off the charger is warm and its surface voltage is artificially high. Let it sit 20 to 30 minutes before a match so the voltage settles and you get an honest reading.
• Never deep-discharge. SLA batteries hate being run flat. Every time you drain one down near or below its cutoff, you shave permanent capacity off it. Pull a battery before it's dead, not after.
• Charge off the robot. At competition you charge batteries at your pit charging station and swap fresh ones in — never on a powered robot.

Beyond charging technique, the batteries themselves need to be managed as a fleet: numbered, cycle-tracked, and rotated so a tired pack never sneaks into an elimination match. That whole discipline — labeling, load-testing, rotating, and retiring packs — is a topic of its own, and our lesson on battery management like a pro team lays it out.

How many batteries you need

Enough that you always have a fully charged, fully rested one ready to go. A single battery can't cool down, rest, and recharge in the ~8 to 12 minutes you sometimes get between matches. Most competitive teams own at least four batteries, and six or more is common. Rookie teams should budget for a minimum of three. The math is simple: one on the robot, one resting after charging, one on the charger, and spares so an elimination run never forces you onto a hot or half-charged pack. Multi-bank chargers that top off several batteries at once are worth the money for any team running a full event schedule.

The SB50 connector and the power path

The battery connects to the robot through an Anderson SB50 connector — the chunky two-contact plug you unplug every time you swap batteries. From there, power runs to the 120 A main breaker, then into your power distribution board (a CTRE PDP or a REV PDH), and out to everything else. If wiring that path from scratch is new to you, our companion guide on how to wire an FRC robot walks the whole battery-to-motors chain.

For battery care, the SB50 matters because it's the connector you handle constantly — and it's a very common source of mysterious brownouts, so treat it as a maintenance item, not a fit-and-forget part:

• It's genderless, so standardize on one color. The two halves only mate if they share the same color housing, so teams pick one (red and gray are both common) and stick with it, so any battery plugs into any robot.
• Crimp the 6 AWG contacts properly. A cold or loose crimp adds resistance, and resistance in the main power path becomes voltage sag under load. Doing this connection — and every other one — right is its own skill: see crimping and connectors done right.
• Keep contacts clean, tight, and restrained. Corroded or pitted contacts heat up and rob voltage, and vibration slowly works a dangling plug loose over a match. If a connector looks burnt, discolored, or feels loose when mated, replace it.

A robot that browns out only sometimes, seemingly at random, very often has a bad connector or a worn crimp somewhere in this chain, not a bad battery. When you're chasing that gremlin, the SB50 and its contacts are one of the first things to check.

Testing: the Battery Beak and what the numbers mean

The Battery Beak (from CTR Electronics) is a small handheld tester that plugs onto the SB50 and, in a couple of seconds, tells you three things: resting voltage, internal resistance, and an estimated cranking-amps figure. Of these, internal resistance is the one that actually predicts whether a battery is worth using.

Internal resistance, measured in milliohms (mΩ), is basically how hard the battery fights back when you draw current. Low is good. A fresh, healthy FRC battery typically reads in the low double digits — roughly 10 to 13 mΩ. As a battery ages and gets abused, that number climbs. Once a battery drifts up toward the high teens or into the 20s (mΩ), its voltage will sag badly under match load, and it's time to retire it for competition use.

Exact numbers vary by battery model and by tester, so don't obsess over a single reading. The real move is to Beak your whole fleet and compare. Rank them by internal resistance, keep your lowest-resistance batteries for elimination matches, and pull the outliers — a battery that reads 8 mΩ higher than the rest of your fleet is your weak link, whatever the absolute number says. Turning that into a repeatable, logged process is a project in itself: our battery management and logging system mini-project builds the numbered, tested fleet around exactly this workflow.

You can also watch battery voltage live during a match: the PDP/PDH reports it over CAN, the Driver Station displays it, and you can log it. If you want to pull and record that telemetry so you can correlate voltage sag with what the robot was doing, our live power-monitoring dashboard mini-project streams voltage, current, and energy off the power distribution board.

Care and storage between events and in the off-season

Lead-acid chemistry has one big enemy in storage: sulfation. Leave an SLA battery sitting discharged and lead sulfate crystals harden on the plates, permanently killing capacity. So the rule for storage is simple:

• Store batteries fully charged, never discharged.
• Top them off every one to three months during the off-season — a battery on the shelf slowly self-discharges even with nothing connected.
• Keep them somewhere cool and dry. Heat accelerates aging.

Between competition events, treat storage the same way: charge everything up before it goes on the shelf. Come back a couple of weeks later, Beak the fleet, and you'll know exactly what you're working with.

Batteries don't last forever. Under heavy FRC use, a battery is often good for something like one to three seasons before its internal resistance climbs enough to retire it. When you do retire one, recycle it — SLA batteries are nearly 100% recyclable, and any auto-parts store or battery recycler will take them. Never throw one in the trash.

Safety: lead-acid deserves respect

A charged FRC battery can dump hundreds of amps into a dead short almost instantly. That's enough to weld a dropped wrench to the terminals, cause serious burns, melt tools, and start a fire. This is not a theoretical risk — it's the most common way people get hurt around the electrical system. The non-negotiables:

• Never let anything metal bridge the two terminals. Keep the SB50 connected or the terminals covered whenever a battery is charged. No loose tools, jewelry, or wristbands near exposed terminals.
• Carry batteries with two hands, and never by the leads. The case is plastic and it holds sulfuric acid. A cracked case leaks acid, which will ruin the battery and can burn skin and eyes.
• Retire damaged batteries immediately. A swollen, cracked, leaking, or physically dented battery is done. Bag it, keep it away from metal, and recycle it. Don't try to squeeze one more match out of it.

There's more to it — ventilated charging, hydrogen venting, spill response — and it's a team-culture habit, not a one-time lesson. Our lesson on sealed lead-acid battery safety covers handling, charging, inspecting, and disposing of the 12V pack without acid burns or fires, in the context of the rest of your shop and pit practices.

The short version

If you remember nothing else: charge fully and let it rest, keep your SB50 connectors clean and tight, Beak your fleet and race your best batteries, store everything charged, and never short the terminals. None of it costs much, and all of it makes your robot noticeably more reliable when it counts.

New to the electrical side of a team? Start with Getting Started for the big picture, then work through the learning paths to build up the rest of your control-system knowledge alongside battery care.