So I was recently asked for a good link to a break down about batteries. Unfortunately, I didn’t have a good link to point him to, so I wrote my own. I decided to share it and build on top of it for the community. I will try to remove my opinions as much as possible, so sorry if I seem biased at times.
A FOR WARNING: Take all manufacturer ratings with a grain of salt. A guy on reddit named mooch315 has tested a lot of cells of all different formats, but especially 18650s, and you can find those test results here. His tests show some cells rated to 15a are actually higher discharge cells then some that are rated to 30a. Manufacturers do their own testing, so mooch315’s goal is to give the world an objective rating and view of the cells he tests.
Formulas you need to know:
mAh = 1000*Ah Wh = Ah * max voltage W = Amps now * voltage now
Lipo vs Li-ion
Li-ion is great for:
- Flat ground riding
- Riding slowly
- Light riders
Unless you do a really large pack (6p or higher at 10s or 12s). They sag, which means the voltage drops quickly for a short period of time under heavy discharge, to go back up to slightly less than where is was before the heavy discharge after the heavy discharge is over. If your using current control (as most are with the VESC), this means you lose power (since you have less watts, reference formula #3 above). They are incredibly safe. The tests they do which are listed in the spec sheet are incredibly abusive, yet none caused a fire or explosion. Likely most of those same tests on a lipo would cause a fire or explosion. Larger packs (10s7p or 12s6p or larger) can overcome the sagging issue, as does setting your amp limits lower. Most cells say you can discharge up to 20 amps continuous. However, you will lose quite a bit of range at that number, and you want to aim for more like 10 amps continuous per cell in parallel.
Lipos give better performance if using a smaller pack, but they can’t take abuse well (discharging too low or over charging too high), and can become volatile. The discharge curve is different. Li-ion drop off a lot at the beginning, and then discharge at a steady decline. Lipos drop off a lot at the beginning, and then slowly decrease in voltage, and around 3/4ths of the way through the discharge, they drop off a cliff. The result is lipos give a better performance in current control mode than li-ion batteries of the same size (i.e voltage and Ah).
There is a newer firmware mod which was created by @Ackmaniac that allows for watt control mode, which should get ride of this problem. But that’s another whole discussion in it’s own.
Here’s the is discharge curves of an average lipo and li-ion cell. Look at how quickly the line going from right to left goes down.
NOTE: Not all lipos or li-ion cells discharge exactly like this. This is a generality.
Li-ion cells come in a few varieties. The most common is the 18650. The 18 refers to the diameter of the cell in mm, and the 65 refers to the length of the cell in mm. There’s also the 26650. The most common 26650 cell is the A123 cell.
Lipo cells come in all manners of shapes an sizes, expect cylindrical cells (funny enough). They can be very flat, or very thick, in any of the x, y or z directions. Instead of a storing energy in this cylindrical shell, they energy in a pouch. This pouch is easy to puncture, unlike the li-ion cylindrical shell, which is one of their weaknesses. I don’t think I need to tell you want happens when you puncture a lipo battery.
Another thing. Lipos are rated to around 1/3rd of the life cycle of li-ion packs. So while lipos look cheaper up front, li-ion can be cheaper in the long run if you take good care of the cells. Obviously, the higher rate of discharge per cell in li-ion packs, the less life they will have. This is not a linear thing, so a 12s3p will not have half of the life cycle of a 12s6p. At 20a con, the cell only gets 400 full cycles. at 10a max, you get 1200 full cycles. so the 12s6p should in theory, if they are both ran at the same max amps, get 3 times the life, not double the life, of the 12s3p.
So to sum it up, heres the cost/benefit analysis.
- Safer (by a lot)
- Longer life cycle
- Sags less (thus when you draw high amps in current control mode, which most use with the VESC, you don’t lose power (and in this case, power refers to speed and/or acceleration))
- Can be put into shapes not possible with a li-ion cell (great for boards that need to be thinner than 18mm)
I should mention now, that there is another technology called LiFePO4 or Lithium Iron. These give you the same shape as a lipo, same high discharge rates of a lipo, but are safer and have a longer life cycle than lipos. Seems like a win-win, but they are very expensive compared to both li-ion and lipo, and theres not a lot of sources out there for Lithium Iron batteries, so they are not very commonly used.
Parallel vs Series
Theres 2 configurations of cells. Parallel and series.
Parallel means negative to negative and positive to positive. The result is two cells become essentially one. They will automatically balance each other out to the same voltage. Parallel config doubles the Ah, meaning you’ll get more range.
Series means the voltage is doubled and the Ah stays the same. To do this config, hook one positive to one negative, and use the open positive on the one cell and open negative on the other cell to discharge.
Higher voltage means more power and less amps need to be drawn. Higher Ah means longer range.
However, Wh or Watt Hours is the best measurement of the size of a battery.
Stay you have 12 li-ion cells. The typical li-ion cell is 2500 mAh in size, and is a 1s (since it’s 1 cell at a 3.7 nominal voltage).
You could do a a 12s1p (12 cells in series, 1 cell in parallel) or 1s12p (1 cell in series, 12 cells in parallel) or, since nobody would typically do either of those con figs (unless using the A123 cells, which are the cells boosted uses, in a 12s1p), a 6s2p would be usable, but still weak. The point though, is going between series and parallel, they all have the same Wh, since we have the same amount of electricity being stored in all of those configs.
Most people aim for a 10s or 12s, because you can get a lot of power and draw less amps. The more amps you draw from your battery, the hotter your battery and esc will get.
Many still use a 8s or 6s battery, but it’s more of a budget board thing. Boards at those voltages are weak when compared to the same board at 10s or 12s.
To get more range though, since you can’t really go higher than 12s (very few esc’s can handle higher than 12s), you put more cells in parallel to get more range.
Back to our example, a typical li-ion battery in:
12s1p will have 2.5 Ah total at 50.4 volts. 2.5 x 50.4 = 126 Wh 6s2p will have 5 Ah total at 25.2 volts. 5 x 25.2 = 126 Wh 1s12p will have 30 Ah total at 4.2 volts. 30 x 4.2 = 126 Wh
No matter the config, all of these batteries will have around the same range, since you have the same amount of cells (thus same amount of electricity) in all of these configs.
This is why when comparing peoples batteries sizes and ranges on here, we usually reference the Wh of our packs, not just the S and P of the pack.
IMPORTANT NOTE: The legal limit to fly on an airplane in the US with a battery is 99 Wh, so if that’s important to you, keep this in mind. Europe has similar rules also.
Sag is directly correlated between max amps you will draw to max amps you can draw. if you plan to draw no more than 60 amps, and have 240 amps, your result is little sag. If both of those numbers are 60, expect a large amount of sag, which means decreased performance and less range.
Sag is completely normal, in both lipo and li-ion cells. They will both sag, but to different degrees.
Li-ion cells are pretty easy to charge. You can use a standard “laptop” style charger at the max voltage of your battery pack (for example, a 12s battery needs a 50.4v charger, since 4.2 (max voltage per cell) x 12 (number of cells in series) = 50.4). Most Li-ion packs use a BMS or Battery Management System.
Lipo batteries however are usually charged using a balance charger. These chargers accomplish the same goal of balancing the cells during charging as the BMS, but without the need for a BMS. The downside is you need to plug balance cables into the balance charger, so charging now means more cables to deal with.
Both li-ion and lipos can be charged through balance chargers or laptop style chargers (with or without a BMS). You would not use a BMS and balance charger together however.
Most use balance chargers with lipos because a balance charger is cheaper and easier to use than a BMS, and lipos, according to manufacturer data, last about 1/3 of the life cycle of li-ions. Li-ion batteries are more expensive and last 3x times longer (again, theoretically), so a bit extra for the convince of the BMS makes sense. However, a BMS for a lipo is not a bad idea if you plan on replacing your lipos with the same lipos when they die, as you can use the BMS with the new battery pack, since it’s the same battery.
Battery Management Systems
Most li-ion batteries use BMS’s, but most lipo packs do not. The reason being lipos are usually charged using a balance charger, while li-ion’s use a “laptop” style charger. The “laptop” style charger simple provides a voltage at a constant amperage. The battery charger does exactly that too, but when a cell reaches 4.2 volts, it will start discharging is at the same rate through that cells balance lead, thus leaving that cell at 4.2 while the rest of them catch up. The BMS does exactly the same thing.
The goal of the BMS is simple: make sure the cells are all very close to the same voltage after a full charge, prevent cells from discharging too low, and prevent cells from being charged too high. Since the VESC can prevent cells from discharging too low, most just use the balancing and preventing cells from charging too high features of the BMS.
Again, as stated above, both a BMS and Balance charger accomplish the same goal with respect to charging. The BMS is more convenient though, as you need 1 cable instead of multiple to charge.
My opinion about flying with batteries
If you want hot swappable batteries that are under 99 Wh, Lipos are a better option IMO because 99 wh 18650 li-ion batteries will not output very many amps. This will result is horrible sagging and a bad experience (to me at least, cause I like a lot of power).
The Lipos I recommend are the Zippy Flight Max 8000 mAh series, either 2s, 3s or 6s linked in series. The 30c version at 12s and 8 Ah is capable of 240 amps continuous. The li-ion version of that would be basically a 12s3p, which is capable of 60 amps continuous. Remember, always take advertised numbers with a grain of salt also. Most of us will want 60 amps at least continuous. Though that 12s3p li-ion can do 60 amps continuous, it will sag very badly.
If you do use lipos, get a good balance charger (not a knock off imax, cause those things are dangerous) and set it up so you can charge them quickly (if doing 4x3s, wire it so you can disconnect and charge them as 2x 6s’s, since most lipo chargers are limited to 6s, and they’ll charge faster this way). If you take care of them, lipos are safe. I’ve used over well over $1000 worth of lipos (which is quite a few battery packs) without any incidents, even when I have abused them. All lipo fires (besides when theres a known problem with a cell) start during charging or discharging, and almost always, it’s during charging. That’s why a good balance charger is important. They will almost certainly not explode if they are not charging or discharging, unless you puncture a cell or physically abuse the cell.
If you use li-ion, try to keep it to 10a max per cell in Parallel. This will reduce sag, increase battery life, and increase range. This can be tricky, which is why I recommend a 12s6p (the same pack that @chaka also recommends to eliminate sag) or higher in size. Smaller packs will work, but they will negatively impact performance.
The biggest problem for batteries is finding where to buy them.
Lipos are easy, I’d recommend HobbyKing first and foremost. They have a great battery finder that helps you find a battery that will fit to your specs.
Li-ion however, is hard. There are so many knockoffs out that that even telling the difference between a real and fake cell is almost impossible without testing the cells first for performance. I know some have recommended Battery Bro before. If others have some recommendations, please post below and I will update this. Amazon is hit or miss, and banggood from what I’ve seen is mostly miss.
For BMS’s, Best Tech Power and Su Power Battery are both commonly used sources.
I hope the world of batteries is a bit clearer to you now. Everything in this guide is what I have learned through first hand experience of building and testing electric skateboards for the past 2 years. If anything seems mis-leading or incorrect, as always, please let me know. My aim is to be as factual as possible, while giving new users a good insight into batteries.