I’m building a robot with my kids. I’m faced with a battery dilemma and would certainly appreciate some advice.
The power train of the robot consumes about 2-2.5A and operates in the 6-9V range. As I recently acquired a few good protected 18650s (XTar 18700 and Panasonic 3400mah), my initial thought was to use a couple of them in series, or four of them in a 2S2P config. I don’t really need more than 45-60 minutes of runtime.
The benefit of using the 18650s is that I can just shuffle the batteries between the robot and my flashlights whenever needed.
The alternative is to use a cheap Lipo pack for $10-$15, and a Lipo charger. I have neither. So I have to buy them.
I have read quite a bit about Li-ion and been using these 18650s for about a month and would prefer to just stick with that to avoid yet another battery format that I have to deal with if I buy Lipo. On the other hand, the Lipos appear cheap. They are flat and seem to have a pretty good discharge rate for robot / RC type of applications.
What do you suggest I do? Stick with the 18650s, or get a cheap Lipo battery pack?
I’d go lipo’s but I have a charger + cells already and I can justify their use/expense for a variety of reasons. If your not going to be pulling more than 5amps (peak) then you should be fine w/lion’s just beware that there is a reason that Lion’s are not commonly used in RC anymore; the cells often blew up under high load (Since they couldn’t take the current) making their use prohibitive until Lipo’s came along and provided a much better, higher draw power supply (That didn’t blow up) ;).
Make sure you run matched cells that are also balanced! Don’t want to start any fires when running multiple cell packs. Lipo’s often have “taps” or balance plugs that allow for a monitor and balance charger (or port on a charger) to be hooked up in order to monitor cell voltages and keep cells balanced.
I wouldn’t say that lipos are much more dangerous, it’s a similar principal that’s used on modern lions but at larger and more convent scale than can be found in lions. Cost wise they usually are better due to the lack of housing (Most lipos are wrapped in foil and sealed with shrink wrap) however modern cells are still quite safe. Barring a puncture of the cell they don’t vent often unless severely imbalanced, heck I saw one that had puffed to twice it’s size and kept working (Not in active use anymore mind you, but it was amazing it didn’t combust).
Take in mind that you should treat them with the same regard that a series lion light should be treated and you’ll be fine. In most cases Lipo’s are made to vent without building up pressure and you avoid most of the damages that are found in flashlight explosions. Fire yes, but it’s unlikely you’ll have chunks of aluminum flying around (Unless the cell’s sealed).
If your only going to be doing this once or twice then the lions will probably suffice, else I’d recommend moving into the Lipo world for applications like this. Smaller form factor, higher energy density and lower weight make lipo’s more appealing for robotics than lions have been traditionally.
I’d just be wary of mixing lion cells, that can end pretty badly if you run em too low since capacity’s, discharge rates, etc will vary and could result in damaged of venting cells. You’ve really got to keep an eye on them if you go that route.
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If your only going to be doing this once or twice then the lions will probably suffice, else I’d recommend moving into the Lipo world for applications like this. Smaller form factor, higher energy density and lower weight make lipo’s more appealing for robotics than lions have been traditionally.
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Thanks, Dobanodnao. Do have any recommendation for manufacturers, models, and retailers for Lipo batteries?
Check out HobbyKing, they sell a bunch of diffrent sizes and brands that can fit a varity of ueses. I’d probably grab a turnergy Lipo if you were using it on a ground based object, I hear some of the more air focused ones have slightly ligher HS and such (it’ll wear a bit faster, really no big deal).
I’ve had good luck with their nanotech ones but I haven’t tried any of the newer “A-spec” ones. Looks like a much higher constant C rate with really high peak discharge rates up to and over 100C; I’m thinking I’ll be placing an order before too long ;). The standard tunergy packs work well too, I have a few around that still work great. I’d avoid rhino and while they work fine the one I have here (I got it cheap w/no connectors) just doesn’t seem to be built as well.
Most of the Hobbyking battery’s use XT60’s (Smallish housed bullet connectors) which can be either replaced or adapted to other types of connectors if needed. Larger cells usually come with bullet connectors. It’s advisable to check before ordering, they have most kinds of connectors too in-case you need to wire up adapters.
Shipping’s kind of expensive but I haven’t had an issue with them and their definitely legit, good quality cells. There are some cheaper places as well but I trust HK in general. Long as you aren’t exceeding the cells constant discharge rate you should be fine, peak bursts around 30c and above are quite common and you shouldn’t have an issue finding a good cell of at least 20c constant.
Higher cost “premium” cells really do not seem to be worth it, epically since they haven’t proven any significant advantages over regular mass produced cells. You might get a slightly better binning but at sometimes 5x the cost it’s just worth it to order more v order “Premium”. Most good lipos sold at hoppy shops and though Hobbyking won’t have any issues if properly taken care of. Unlike cheap xxxfire batteries most of the Chinese/hk ones seem to be quality and it’s pretty obvious if you have a cell that you should not trust (Cheap HS, lack of balancing wires, cheap wires, etc).
You don’t get fancy protection circuits on lipos like lions however some common sense and a little balance checker should keep you in pretty good shape; it’s worse for an RC airplane user to lose his plane than the battery, sudden cutout protection circuits aren’t generally welcome (And sap power/cause parasitic drain, limit outputs).
The circuit/power-train should drain only as much power as it needs so higher C cells are generally better, best to stress the circuit rather than the battery.
For you it’s not good though unless you want to spend $ and get a decent charger and power source for it and a battery monitor unless you have a good monitor in your toy that checks cells individually.
Personally I would get a Lipo, lighter, more power, better sizes.
At 2-2.5A you will be fine with the 18650s, get a holder for them or make it in your toy and you’re good to go. Keep in mind to protect the batteries if they are unprotected.
The cheap holders (black ones, go for $0.99) for 18650s, that is 65mm and protected batteries might not fit or fit very hardly because they are longer. I’m going to cut one of mine even though mine are unprotected. It’s just pain to get them out as it nicks the battery which I don’t like.
I’ve had one of these little solar-cell battery power pack things around for a couple of years as a cell phone backup power supply for the earthquake kit
This isn’t the seller but it’s the widely sold one that looks like this:
Looked into the gear bag today and the case was popped open and the LiPo cell had ballooned to 3x its normal thickness, breaking the little tabs around the edge of the case that held it together.
I’m glad there were no sharp edges inside there next to it. Spooky.
I’m in the battery industry and I can assure you stick with top brand only 18650’s i.e. Sanyo/Panasonic (same company) Samsung, Sony or LG. All the rest are cheapies & low cycle life.
Do not use Lipo’s They are for the hobby nutters who don’t mind risking blowing their fingers off (sorry guys but it’s absolutely true). Sure lipos can offer thin & unique sizes and very good high current output (the only reason they are used in high current RC usage instead of LiIon) but they are unstable, unprotected, offer no internal cell protection and for lifetime use they offer lousy cycle life compared to LiIon. Energy density in LiIon is also higher than Lipo. Whilst this contradicts what other posters here are saying, reality is no lipo touches a 3400mAh Panasonic for energy density especially after a couple of hundred recharges. If you really need high current cells (at 2-3Amps draw you don’t need these) there are quite a few available also from the same manufacturers stated above. These are normally used for making powertool battery packs. Cell capacities are lower than standard rate cells though.
Other than low rate LiPo’s made by some of the Japanese companies for ultraslim or tiny mobile devices you dont see Lipo used inside the devices of the major device manufacturers who instead favour LiIon cells. That’s got to tell you something!
Protected 18650’s are the safest. Aside from the external protection circuit the premium brand LiIon cells also feature inbuilt CID’s (Current Interupt Devices) & PTC’s (thermal cutout) as an added safety feature so if things go really bad like an internal short circuit they go ‘pop’ internally, not explode as some might say incorrectly. The XTAR protected models you’ve chosen are a very wise choice.
