'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.
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:
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:
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:
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:
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:
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 have seen people just plug their bot in on a charger between fights without removing the battery or using the little white balance plug. How are they doing that?
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.
Q: I bought a fancy Lithium charger, but I can't make sense of it. What are all these buttons and plugs?
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).
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 USE THIS BATTERY! Do not attempt to charge or discharge the battery. Dispose of it immediately.
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:
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.
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.
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.
Q: I finished my first antweight and I'm working on a beetleweight. I've researched how to properly store the Lipos, so I keep them at proper storage charge in a ventilated heavy steel box in the all-concrete basement of my house. I don't really know how much toxic smoke they would output of they "went off". Is it enough to go through my home ventilation system and kill everybody or what?
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
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.
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!
Copyright 2016 by Mark Joerger -- all rights reserved.