Battery Capacity - why so much?

[Kevin]

While reading these threads, a lot of people seem dissatisfied with 340Wh or lower batteries. At a range of 35-40km (based on figures from a recent review of LHOTZ), that's 1.5 hours of riding at top speed, or 2-3 hours at a more leisurely pace. Personally, I'm rarely even willing to drive a car for 30 minutes unless it's a road trip, and EUC riding seems like it would be more strenuous (although more interesting) than driving.

So I am curious about why people want/need the larger 500+Wh batteries - is it for crazy long commutes? Peace of mind? Or do people just go out riding recreationally for that long? Initially I was going to get something just 170-260Wh for 20-30km range, as 7km (14 roundtrip) is about as far as I could see myself going without just driving a car, but I feel like I'm missing something as 340Wh seems to be on the low end of what people are looking for.

[Daan]

My wheel only goes about 22km on flat terrain (a little more probably). When I am going just to ride on my wheel (usually forest/nature roads) I often almost deplete the battery so I in those times I wish for a little more battery; but I agree, that is usually after about 2 hours which is normally enough for me   If I would have 340Wh that would give a super nice safety margin. 

Of course, it does seem that on many wheels, a bigger battery means more stable power and less chance of a battery cut-off! Good reasons too to get  a big battery.

(but personally, for me, being able to take it on a plane is a high priority so max 160Wh ... or easily swappable batteries )

[dpong]

Daan makes a great point when he mentions "safety margin".  The last gallon of gas in your car is every bit as viable as the first gallon.  Not so with batteries.  The last 20% of battery life does not provide you the same assurance as does the first 20%.   Others can explain it better than I, I'm sure.   I believe my wheel to be safest when it is still above 50% battery.  Then it is still pretty safe while it is above 30% battery.  After that its safety features start kicking in (and rightly so) to limit my ability to over-ride its now somewhat limited capabilities.  

[SirGeraint]

On my longest trip I went 10.63 miles in 1:10hr/min. and my 320Wh battery was at 24% but it was starting to tilt-back at slower speeds and even more so when I went over small bumps.  So, I would say I would like to ride longer than my battery can go.  Plus I would like to go for an hour, stop and have lunch or dinner and then return (without having to charge).  So there's 2 reasons longer distances and safer at the end of the trip.

 

[Cloud]

First of all you dont get 30-40 km trip out of a 320 battery. You will get maybe15-25 depending on your weight , ambient temperature, riding style, terrain, hills etc

second, i dont understand the argument : try standing still for 90 minutes, you cant stand that long, why do you need to ride longer? Well, the thing is you dont have to stand still..you can ride for half hour. Stop for 5 minutes, ride again, stop, etc...when i ride for fun i feel that my 680 battery is not enoigh for me and i want to ride longer

Third : distance will depend of weight and heavier people like myself ( 100kg) wont get the same range

fourth: in the colder month the travel distance can reduce down to 50%

Fifth: what if you want to ride in the afternoon and then again in the evening? You may not have time to charge your battery between rides. 

6th battery safety margin like poeple above mentioned

 

right now i am finding that my 680 wh battery is by far not enough capacity for me. I think it would be more or less ok with double the capacity.

 

 

[Kevin]

I see. Thanks for all the feedback!

[esaj]

While reading these threads, a lot of people seem dissatisfied with 340Wh or lower batteries. At a range of 35-40km (based on figures from a recent review of LHOTZ), that's 1.5 hours of riding at top speed, or 2-3 hours at a more leisurely pace. Personally, I'm rarely even willing to drive a car for 30 minutes unless it's a road trip, and EUC riding seems like it would be more strenuous (although more interesting) than driving.

So I am curious about why people want/need the larger 500+Wh batteries - is it for crazy long commutes? Peace of mind? Or do people just go out riding recreationally for that long? Initially I was going to get something just 170-260Wh for 20-30km range, as 7km (14 roundtrip) is about as far as I could see myself going without just driving a car, but I feel like I'm missing something as 340Wh seems to be on the low end of what people are looking for.

My reasons for getting larger packs are 

Range

I ride only recreationally, and the (up to) 26km range per charge of my 264Wh Firewheel severely limits my route options. I've ridden about 70km on single days at best (in three trips), and I wish I could do more in one go, as the charging takes time, and then I could also try routes I haven't been able to ride (because the battery wouldn't last to take me back). "Range anxiety" also sucks, sometimes I get worried that I'm going to run out of battery even on my longer 20+ km routes (as the range varies according to head-winds, temperature, riding style etc), and it has happened on occasion, but usually only at the last uphill climb before my house.

Oh, and I've had no trouble riding 1-1.5 hours straight without stopping on the Firewheel with good shoes... the pedals are comfortable once you get your foot positioning right and have the "correct" shoes (hiking boots in my case), with sneakers I sometimes get numb/aching feet if my foot positionings not "perfect", with the boots it doesn't matter that much if they're a bit "off".

Power

The amount of parallel battery packs affects a lot on how much power the batteries can give. Doesn't matter if the packs are low (132Wh) or high (210Wh) capacity, as long as the cells are good quality. Consider that most good cells usually have around 10A (amperes) maximum continuous discharge (there are cells that have even higher continuous discharge rates), which when drawn from single pack causes high voltage sag (meaning the voltage goes down during higher current draw, and which can lead to a BMS-cutout with the low voltage-protection triggering), and thus somewhat lower power. The more current you draw per pack, the more the voltage drops (temporarily, it will bounce back up once the current draw goes lower). With four parallel packs, with the wheel pulling that same 10A, it's divided among all the packs, and only 2.5A per pack is drawn, which causes less or no voltage sag at all (depending how "full" the packs are and how high discharge currents the cells can handle without the voltage starting to sag more or less). Now, assuming that the voltage of a pack would be around 60V (that's the nominal voltage, more fully charged battery stays above 60V even with voltage sag), 10A * 60V = 600W. That's pretty much the maximum power a single battery pack can give continuously, during temporary peaks it can go much higher (but the voltage will start to drop). With four packs that would be 4 * 600W = 2400W continuous. In real life, it's not that simple due to the voltage going up and down, these are based just on nominal values.

Cells also have what is called pulse current or pulse discharge, which is usually around twice the maximum discharge, that means they can go over the continuous max discharge for short periods, but that usually causes a lot of voltage sag. So, more packs = less stress per pack, which means less voltage sag, which means more power, not just range. You could also argue it's a sort of a safety issue, as the BMS cut-out won't occur as easily with more packs (unless the temperature is really low or really high, or you've ridden them to almost empty). Of course it can still occur, so the safest option is to shunt the BMS in the battery (ie. bypass the low voltage- and high current-protections which cut off power), at the downside of increased risk of cell damage or even fire, if the packs where shorted for example, as the BMS will not cut the power and the cells will overload, or if the mainboard does not tell you to stop riding when the voltage is closing the critical level, which is around 2.5V per cell (16 cells * 2.5V = 40V)  for most Li-ion -chemistries.

Even if you have 350W-rated motor, the actual temporary spikes during riding will be far higher than 350W (that's just the continuous power the motor was designed for, temporary maximum power can go far beyond that). Consider for example my tests with vee73's 14" Gotway MCM2s: http://forum.electricunicycle.org/topic/889-more-data-ks-peak-power-hit-2400w/?do=findComment&comment=10224 . That is a 500W rated-motor, but during my test rides the peaks went to 1.5 - 2.5kW (1500-2500W), and could be around 700W for longer periods riding in the hiking paths.

 

Daan makes a great point when he mentions "safety margin".  The last gallon of gas in your car is every bit as viable as the first gallon.  Not so with batteries.  The last 20% of battery life does not provide you the same assurance as does the first 20%.   Others can explain it better than I, I'm sure.   I believe my wheel to be safest when it is still above 50% battery.  Then it is still pretty safe while it is above 30% battery.  After that its safety features start kicking in (and rightly so) to limit my ability to over-ride its now somewhat limited capabilities.  

That's because the battery voltage goes down the lower the remaining charge inside it is. The battery meters in the wheels actually measure the voltage from the pack, not really how much watthours (Wh) / milliamperehours (mAh) are remaining. That's also why you can see the battery meter dipping during acceleration or hill climb, and then go back up, because the higher current required for acceleration/climbing causes voltage sag.

For example, the 16S-packs are usually around 67.2V when fully charged. When my Firewheel tells me to stop (ie. the battery is "empty"), the voltage at rest is usually 56V (56V / 16 cells = 3.5V per cell), but I believe it can and will go temporarily lower than that during riding (fast acceleration & hill climbing on more empty battery). Gotways let you ride it even lower, I think 51V (51V / 16 cells = 3,1875V per cell) or something. Not taking the voltage sag into account (which will drop the voltage even further with low battery than more full battery), 51V * 10A = 510W per pack vs over 600W (and beyond when going for pulse currents) per pack on a more full battery, but actually lower as the voltage will drop with that high current. So no wheel will actually let you (or at least never should let you) ride the battery to totally "empty", and riding with low voltage is downright dangerous (risk of BMS cutout, or overpowering the batteries with fast acceleration or hill climbing).

[EUC Extreme]

1. There is no need to download as frequently.
2. Performance is reduced by just under battery power.
3. Even with a big battery on a level reached 80 km. Off-road it can be reached in less than 25 kilometers.
4. The small, lightweight battery is of no use.
5. A large and heavy battery there is no harm

[Kevin]

-snip-

Thanks for the detailed explanation @esaj! Did a little more reading. A quick google turned up the following info:

• Individual cells seem to be available in capacities ranging from 900-4000mAh with no particular pattern to the 'steps' of capacity. 900 was an outlier though, with most starting at 2000mAh and 4000mAh seeming uncommon. So we can probably make an educated guess that most packs will be in the 120-230Wh range.
• Current ratings ranged from 7A to 30A for individual cells, with 20A being a very common figure (no indication of peak vs. sustained). However these were listed under a product category for 'high drain', so this may not be typical?

Based on this we can probably make a stab at guessing the # of packs of some capacity figures:

130-230Wh - 1 pack

230-300 - 1 or 2 packs

anything in the 300's - 2 packs

King Song 174, 340, 520, 680Wh - 1,2,3,4 packs respectively

 

But with the current capacity per pack varying wildly, this actually gives us very little information on the power output  It seems entirely possible for a 2-pack design utilizing very high current cells to be on equal footing to a design using 4 packs of lower current cells.

 

P.S. Oddly enough, I've designed around LiIon stuff in the past, but always for digital low-power electronics. So while I'm familiar with cell voltage ranges/curves and BMS cutoff, I've never needed more than say 0.02A of current. Forget this 'green' thing, EUC's are power gobbling monsters  

[esaj]

Thanks for the detailed explanation @esaj! Did a little more reading. A quick google turned up the following info:

• Individual cells seem to be available in capacities ranging from 900-4000mAh with no particular pattern to the 'steps' of capacity. 900 was an outlier though, with most starting at 2000mAh and 4000mAh seeming uncommon. So we can probably make an educated guess that most packs will be in the 120-230Wh range.

Based on my findings, the 3500mAh cells are the largest the "high quality" manufacturers (like Samsung, Sony, LG, Panasonic) produce. They could have come up with a 4000mAh cells also lately, not sure about that. However, there are some cheap chinese manufacturers making cells that claim to be 4000mAh (or even 4500mAh), but in reality, tests have indicated they usually give out about 1500-2000mAh. Sometimes they have actually been re-wrapped USED cells, that can give even less... 

 

• Current ratings ranged from 7A to 30A for individual cells, with 20A being a very common figure (no indication of peak vs. sustained). However these were listed under a product category for 'high drain', so this may not be typical?

I think (but am not sure) that most "high drain" -cells also typically tend to be of the lower capacity ones. Maybe it's a tradeoff between high capacity and high maximum current? Although, it seems the manufacturers are constantly researching minor changes in the formulas and additives, and the capacities and maximum currents go up (slowly) all the time.

Based on this we can probably make a stab at guessing the # of packs of some capacity figures:

130-230Wh - 1 pack

230-300 - 1 or 2 packs

anything in the 300's - 2 packs

King Song 174, 340, 520, 680Wh - 1,2,3,4 packs respectively

Sounds about right, except I think the around 520Wh (528Wh usually) are actually 4 packs (4 * 132Wh). From another post of mine in the IPS Zero -topic:

I think at least earlier IPS-models used lower voltage (52-55V / 14 or 15S?) -packs, but it could be 16S-pack(s) in the Zeros. The 130Wh-version is likely single 16S-pack, typical cell capacities seem to be 2200mAh (2.2Ah), 2900mAh (2.9Ah), 3200mAh (3.2Ah) and 3.5mAh (3.5Ah). Typical nominal voltage for most wheels is 60V (= 16S). From these you can gather the typical battery sizes wheels have (per pack):

60V * 2.2Ah = 132Wh

60V * 2.9Ah = 174Wh

60V * 3.2Ah = 192Wh

60V * 3.5Ah = 210Wh

So, likely 130Wh Zero has one 132Wh pack, 260Wh has two, and 340Wh-version has two with larger capacity cells (2.9Ah), as 60V * 2.9Ah * 2 = 348Wh. But this is just guessing based on specs and the fact that the weight of the 260Wh and 340Wh -versions is the same (so same amount of packs, as there isn't any weight difference between different capacity cells, or if there is, it's a gram or two per cell at best).

At least from what I've seen, those four pack sizes (132, 174, 192, 210) are the most common.

 

 

But with the current capacity per pack varying wildly, this actually gives us very little information on the power output  It seems entirely possible for a 2-pack design utilizing very high current cells to be on equal footing to a design using 4 packs of lower current cells.

Maybe, but there's always probably at least some voltage sag going around, so I'd expect 4 packs to be better than 2 (at least when their combined max current is as high as the two packs, maybe even if it's slightly lower than 2 packs, as the voltage sag per pack might still be lower).

P.S. Oddly enough, I've designed around LiIon stuff in the past, but always for digital low-power electronics. So while I'm familiar with cell voltage ranges/curves and BMS cutoff, I've never needed more than say 0.02A of current. Forget this 'green' thing, EUC's are power gobbling monsters  

Yes and no, unfortunately we can't break the laws of physics... moving around mass of tens, if not over a hundred kilograms does require a lot of power. Wheels using hundreds and up to a thousand or two watts (1-2kW peaks) isn't really that much, if you think about motorcycles (tens of kilowatts) or cars (hundreds of kilowatts). A single liter of gasoline contains about 8.83 kWh (8830Wh), or equivalently, a gallon of gasoline has 33.41kWh (33410Wh)! Now, how many kilometers do you get per liter (or per gallon) on a car or motorcycle? Considering I get almost 10km per 100Wh on the Firewheel...

We did some calculations in another topic, and the power needed especially on hill climbs is substantial:

If using the equation from here: http://www.dummies.com/how-to/content/calculating-the-force-needed-to-move-an-object-up-.html
it is
  Fpush = m*g*sin(theta)+µs*m*g*cos(theta)

Where Fpush is the force (in newtons, not watts), m is the mass to move, g is the gravitational constant (9,80665 m/s2), sin(theta) is the sine function of the uphill angle, the part after the sum (+) is the static force of friction (which I'll leave out, as I don't know the coefficients, and probably it should be rolling resistance and airdrag instead of static friction to get the refrigerator moving in the example?)...

Plugging in the values of m = 90kg and 10 degree slope, we get:

90kg * 9,80665m/s2 * sin(10 degrees) = 153,2616... N

1 Newton (N) = 1 (kg * m) / s2

Now we add the speed, 10km/h, which needs to be changed to basic units (meters per second), to get the watts (W, power), 1 watt = 1 J/s, 1 J = joule = 1 N*m (newton meters), so

N*m/s  =  J/s = W (watt)

10km/h is 10000 meters / 3600 seconds = 2,77777..., round it to 2,778 meters per second (m/s)

So:

153,2612... N * 2,778 m/s = around 425,7W to move 90kg uphill at constant speed of 10km/h without taking the frictions, rolling resistance etc. into account.

I was wondering how the Firewheel can then easily take me up such slopes with even higher speeds (and friction/rolling resistance/whatever!). But then again, I'm lighter so let's say Firewheel (about 13kg) + me (57k) + safety gear and other stuff (3? kg) comes in around 73kg:

73kg * 9,80665m/s^2 * sin(10 degrees) = 124,312... N
124,312... N * 2,778 m/s = around 345,3W

So the mass matters a lot here. The real power needed will be even higher due to losses between battery, wiring, mainboard, motor etc + rolling resistance, air drag etc.

Those aren't that exact calculations, as there's no resistances taken into account, power needed for balancing etc. With higher speed, steeper hills and higher mass, the values go up fast... air resistance (especially against head wind) also plays a big role.