'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 archive.
'Li-Poly' batteries are a rechargeable lithium-ion batteries in a 'soft' polymer pouch caseing. They have a very high energy density: a lot of power stored in a small and light package. Li-ion cells in general retain their power capacity over hundreds of charge cycles and are capable of high current discharge rates.
Li-Poly batteries require careful attention to safety. Overcharging, overheating, or short-circuiting a charged Li-Poly can result in fire or explosion.
Confused by Li-Poly Descriptions
Q. I'm reading descriptions for Li-Poly batteries that say things like "2600mAh 6S 60C Li-Poly Pack". What does that all mean?
A: One by one:
'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.
'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.
'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
Q: I am having a hard time deciding which battery to purchase. I'm building a 3lb beetle with 2 BaneBots 28mm 16:1 brushed motors. The Team Tentacle Torque Calculator says:
Total Peak Amps: 4.56 Amps
Amp Hours Required - 5 Min: 0.266 AH
For the weapon I'm using an Axi 2808/24 motor at 7.4 volts rotating a 10" x 1" x 1/4" steel bar. With 20 spin-ups I'm getting 0.16 AH from the Run Amok spinner spreadsheet. So total need 0.266 + 0.16 = 0.426 [AH]
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:
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?
Rounding up, the total continuous current requirement is 5 amps for the drive and 15 amps for the weapon, totaling 20 amps.
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
Q: Are A123 cells the same as Li-Poly?
A: No. A123 Systems developed and manufactures a different type of lithium ion battery using:
"...a proprietary form of lithium iron phosphate (LiFePO4) which delivers high power and energy density, extended life cycling, and excellent safety performance."
Lithium ferrophosphate (LFP) cells are much less likely to ignite due to abuse or damage, and they require less complex protection from overcharging. LFP cells deliver 3.2 volts per cell versus 3.7 for Li-Poly, and have slightly different charging requirements, but some lithium chargers have setting to accomodate both types of cells.
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
Q: Do Li-Poly's come charged fresh out of the box? The other day I was fitting some connectors to an unused, uncharged battery, when I produced a spark and a tiny wisp of smoke.
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.
Q: What's the difference between Li-Polys and the lithium ion battery in my computer/phone/drill?
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
Q: Can I use my old NiCad battery charger to charge my new Li-Poly battery if I'm really careful?
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!
Q: What's a 'Balance Charger'? Do I need one?
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.
Q: I bought a fancy Lithium charger, but I can't make sense of it. What are all these buttons and plugs?
Q: Do Li-Poly batteries need to be charged periodically to maintain capacity? I haven't touched my pack for about a month -- do you think the capacity dropped a significant amount?
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
Q: My Li-Poly battery is bulging with air under the shrink wrap. Will that affect the performance of the battery? What should I do to maintain my Li-Poly battery?
A: IT IS NOT SAFE TO CONTINUE TO USE THIS BATTERY! Do not attempt to charge or discharge the battery. Dispose of it immediately.
How to dispose of a damaged lithium battery
In a plastic container, dissolve 1/2 cup of salt in 1 gallon of cold water.
Drop the battery into the salt water and leave it there for at least two weeks.
Remove the battery, wrap it in paper, and throw it in the trash.
Your Li-Poly battery has 'outgassed' from damage caused by excessive heat. The damage cannot be repaired, and further use of the battery may cause it to burn violently. The usual causes of this type of failure are:
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.
It's also possible that your charger is providing the wrong charge rate. Consult the battery documentation and NEVER charge Lithium cells with a charger not designed for the purpose.
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.
Q: My team travels with a steel pot with lid, welding gloves (to the elbow style) & a small (3 lb) ABC fire extinguisher. Assuming safe handling and charging practices, are we correctly equipped in case of a Li-Poly fire?
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.
Do not use water on a Li-Poly fire!
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.
Mounting Li-Poly Packs
Q: I'm concerned that my Li-Poly charger is too slow to keep my battery charged at the tournament, so I plan to have multiple battery packs that I can swap out and keep a couple on the charger at all times. It's always good to have a spare, right?
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.
Running Li-Poly Low-Voltage 'Cut-Off' Hardware
Q: My motor controller doesn't have a low-voltage cut-off to protect my Li-Poly battery. The few lithium cutoff devices that I find out on the 'net are designed for non-reversible throttle ESCs. Will these be usable in a robot with channel mixing? If not, how can I protect my battery from over-discharge?
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!
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Copyright 2016 by Mark Joerger -- all rights reserved.