Combat Robot Lithium Battery Selection and Care - FAQ

Li-Poly Battery FAQ 

'Ask Aaron' receives many questions about the selection, use, and care of lithium batteries in combat robots. I've re-edited some previously asked questions Q&A from our archives into a FAQ that should answer the most common inquiries on this topic. More questions and answers about combat robot batteries can be found in the in the Radio and Electrical archive.

Confused by Li-Poly Descriptions

A: One by one:

1. 'Milliamp Hours' (mAh) is a measure of how much current a battery can provide for how long a period on a single charge. A battery with a 2600 mAh rating might be expected to provide:
   • 2.6 amps (2600 milliamps) for an hour; or
   • 0.26 amps for 10 hours; or
   • 26 amps for a tenth of an hour.
   In real world performance, a higher current demand will decrease the available current by a bit, so the 6 minutes at 26 amps may be optimistic.

2. 'S' refers to the number of cells in the battery pack. A 6S pack has six cells. Each cell in a Li-Poly pack delivers a nominal 3.7 volts, therefore a 6S pack delivers 6 times 3.7volts = 22.2 volts.

3. 'C' is a measure of the maximum safe continuous current the pack can provide without causing damage to the Li-Poly cells, in multiples of the battery mAh capacity (C). The capacity of this pack is 2.6Ah, so the maximum safe continuous current draw is 60 times 2.6 = 156 amps. Some manufacturers will also provide a 'burst' current that the battery can safely provide for a very short (~1 second) period. Drawing current greater than the cell rating will cause damage and may result in a battery fire.

Selecting the Right Li-Poly Battery

Now I'm entirely clueless on how 'Continuous Discharge' relates to 'Max Continuous Current'. I thought the E-Flite 1500mAh 7.4V Double Cell 2S Li-Poly Pack, 13g would cover my beetles needs but it's only a guess. Do you have any ideas on which battery would efficiently handle the robots power needs, and more importantly how you calculated which one was effective?

A:You've done a great job using the appropriate tools:

• The Team Tentacle Torque & Amp-Hour Calculator; and
• The Team Run Amok Spinner Excel spreadsheet

First, let's clear up the 'Continuous Discharge' and 'Max Continuous Current' confusion using that 1500 mAh E-Flite Li-Poly pack you mentioned. Continuous discharge for that pack is '20C'. The 'C' relates back to the capacity of the pack -- 1500 mAh. That pack can discharge at a maximum continuous rate of twenty times 1500 milliamps = 30 amps = maximum continuous current.

There are two main considerations in selecting a Li-Poly battery pack of a given voltage:

• Capacity: if your calculations are correct you only need 426 mAh to power your 'bot for a 5 minute match. Adding a little 'buffer' of capacity for unexpected events, anything over 600 mAh capacity wil be fine.

• Discharge rate: tougher to estimate. A short burst of current up to 1.5 times maximum continuous current won't harm to the battery, so you have a little room for error.
  • For the drive: the Tentacle Calculator shows a peak amp requirement of 4.56 amps at maximum 'push'.

  • For the weapon: I estimate the required continuous current by dividing the maximum continuous amperage rating of the weapon motor by 1.5. Your Axi has a continuous amp rating of 22 amps divided by 1.5 = 15 amps. The Axi can theoretically draw over 60 amps at stall, but your brushless ESC will limit stall current, and your weapon drive should prevent the motor from stalling, right?

Something like the The 800 mAh E-Flight 2S 30C pack should cover your power needs nicely. It has almost twice he power capacity you need, and the 24 amp continuous current rating exceeds your estimated requirement of 20 amps. Just don't bog down the weapon motor!

A123 Lithium Ferrophosphate Batteries

A: No. A123 Systems developed and manufactures a different type of lithium ion battery using:

LFP batteries are available from multiple sources but are more difficult to find than Li-Polys, are more expensive, and come in a smaller range of sizes and capacities. Some robot events that prohibit the use of other lithium batteries for safety reasons may allow use of LFP batteres.

Rookie Mistakes with Li-Poly Batteries

A: Li-Poly batteries are permanently and irreparably damaged if discharged below about 2.8 volts per cell, so they are stored and shipped with a partial charge -- typically about half of their rated capacity.

Watch that battery carefully for early signs of damage - like 'puffy' swelling of the wrapping. Discard it if you see problems.

A: LiPoly batteries sold on the hobby market are lithium ion batteries, but there are some differences in the packaging:

• Li-Polys are packaged in a lighter, flexible polymer casing rather than the rigid case used in conventional lithium ion batteries.
• Lithium ion batteries in commercial products have protection circuitry built-in to prevent overcharging, current surges, dangerously high temperature, and over-discharge. The Li-Ion battery in your computer or phone that says it's 'completely discharged' has been shut down by the protection circuitry while the cells are still at a safe voltage.
• LiPoly batteries sold in the hobby market are raw Li-Ion cells directly wired together without protection circuits. They are succeptable to being over-discharged if the circuit they are powering does not incorporate undervoltage protection.

Charging LiPoly Batteries

A: NO! Lithium batteries have much different charging requirements than other battery types. Incorrect charging can damage the battery and may cause it to burst into flames. Never use anything but a charger designed to accomodate Lithium Polymer batteries to charge your Li-Poly!

A: Yes, you need one. Overcharging a lithium cell can result in catastrophic failure with flames and great gouts of smoke. A balance charger monitors and seperately charges each individual cell in a multi-cell battery to assure that cells with slightly different capacities will not be overchaged. This prolongs the life of your battery, assures that full battery capacity is reached, and minimizes the risk of fire.

A: It is best to charge your LiPo battery out of the 'bot and in a charging bag, but it is possible to charge via the main power wires while it is in the 'bot. Charging this way may allow the charge state of individual battery cells to drift from optimum over the course of multiple charge cycles, but for a few charge cycles during a tournament this is OK. Run a balance plug discharge-recharge cycle to re-balance the individual cells when you get back home.

If it is difficult to get to your battery connector to unplug it for charging, you can buy a small charging jack suitable for small 'bots that can be mounted thru an external panel. See the wiring diagram at right.

Some event organizers prohibit in-bot charging for safety reasons. Check with your events before showing up with a 'bot that will not be allowed to compete.

A: You bought a 'bargain' chinese charger with a poorly translated manual that makes no sense. It's your lucky day: How to Charge your LiPoly Battery (video).

Shelf Life

A: Li-Poly batteries have excellent charge retention and capacity recovery after storage. A Li-Poly battery stored for six months at room temperature will recover about 95% of its capacity on the first charge cycle. It's still a good idea to discharge/charge cycle your rechargeable battery (Li-Poly, NiCad, NiMHd) a couple of times before a competition to assure full capacity. Always follow the manufacturer's procedure for cycling.

Recognizing a Damaged Li-Poly Battery

A: IT IS NOT SAFE TO USE THIS BATTERY! Do not attempt to charge or discharge the battery. Dispose of it immediately.

• Drawing more current from the battery than it can safely provide. Its replacement needs to have a greater 'C' rating to assure that it can deliver more current without failing.
• Discharging the battery below its safe minimum voltage level (about 2.8 volts per cell). You may need a battery with a greater mAh capacity rating to avoid draining the battery so low.

Why be so cautious about damaged lithium batteries? Here's what happens when a Li-Poly ignites. A good tech inspection at any combat event should always examine lithium batteries for signs of damage and should immediately refuse entry of a 'bot using such a battery -- no exceptions.

Electrifly.com has a good article on Li-Poly care.

Safety Precautions

A: You're correct to treat Li-Poly batteries with respect. An improperly charged, damaged, or too-rapidly discharged (shorted) Li-Poly can burst into flame. YouTube has plenty of videos of this happening. Typically there is a ball or jet of smoke and flame from the ruptured pack which may propel the battery some distance, so just placing the battery on a fireproof surface during charging won't do -- it needs to be contained.

You're much better equipped to handle a Li-Poly battery fire than most teams. Manufacturers recommend charging Li-Poly batteries in a fireproof container with an ABC dry chemical fire extinguisher handy. You have that covered, and the welding gloves could certainly be useful. I'd be just a little worried about the lid blowing off your steel pot -- Li-Poly ignition can be fairly violent.

If you do get a fire there may be multiple flame-ups as individual cells ignite, so it's best to stay back and deal with it from a distance. Use that dry chemical extinguisher or just back away if it can burn safely. Sand will smother the fire effectively if a dry extinguisher isn't available.

If you're tired of hauling around that steel pot, there are specially designed bags for charging and storage of Li-Poly batteries. They're easy to pack, are less likely to short out your charger if something goes 'poof', and the lid won't blow off.

Always charge your Lithium battery with a correct lithium battery charger, never use a 'puffy' battery (indicates damage and gas release), and do observe safety protocols. Read thru the Great Planes Li-Poly manual for safe handling practices.

A: You probably have a dozen lithium ion batteries in your home: laptops, tablets, phones, power tools, and 'bots. They all have the same base chemistry. You hear about lithium ion batteries in polymer cases (LiPos) blowing up because they often suffer from great abuse in hobby service. Promptly dispose of any damaged, swollen, or 'puffy' LiPos.

Spontaneous ignition of properly stored LiPos is quite rare. The smoke from a LiPo fire is considered toxic and should be avoided, but the quantity produced by a couple insect-sized batteries 'going off' in your home is much more "bad smog day in SoCal" than "kill your familly in their sleep".

Mounting Li-Poly Packs

A: Mounting Li-Poly batteries properly is tricky -- they're a little 'squishy'. If you just strap the pack to the chassis with a couple narrow zip-ties, a hard impact will cause a lot of localized pressure that can crush the battery enough to cause an internal short. Shorted Li-Polys burn! Ideally, the battery pack should be padded and securely enclosed within a rigid container.

Good Li-Poly mounting does not make for quick and easy battery swaps. If you're planning on swapping packs between matches you'd better place a lot of thought into making those swaps quick and foolproof. If your robot took a lot of damage and you need to scramble to get it ready for the next match you don't need any extra trouble when swapping out the battery.

Li-Poly Low-Voltage 'Cut-Off' Hardware

A: Picture this: you're in the middle of a robot fight. Do you really want circuitry that shuts down your robot to 'protect' your battery?

Unless strictly required by the event rules, disable ANYTHING that can decide to shut down your robot: low-voltage protection, fuses, circuit breakers -- all of it! You do NOT want your ESC shutting your robot down when you're winning with 30 seconds left in the match.

Size your battery to provide ample power for a 'worst case' match. If you're worried, you can use an audible warning module to let you know when your battery is getting low and give you the option of shutting down.

Burn the battery if you need to, just win the fight!

Flying with Li-Poly Batteries

A: Yes, you can fly with LiPos, with certain restrictions. Here is the official guidance from the Federal Aviation Administration: PackSafe - Lithium Batteries.

Quick summary:

• Lithium batteries not installed in your robot must be packed in your carry-on baggage.
• Any number of Lithium packs for personal use may be carried, but each battery pack is limited to a capacity of 100 watt hours (amp hours × voltage).
• Discharge the packs to storage voltage and tape over the connectors. The batteries will be removed from your bag for inspection, so pack them on top and be ready to tell the TSA agent their watt-hour capacity.
• Your specific airline MAY allow you to carry-on up to two larger capacity packpacks not to exceed 160 watt hours each -- but do not count on it!