Ebike power 101 - for dummies?

[jules]

I feel awfully dumb posting this question here because you guys know so much about electrical systems, but I need help. I'm getting ready to add a 450w motor to my eZip and upgrade the controller and likely the battery type. I'm looking at my options for battery power - I want it all - lightweight, long life, many cycles, durable/dependable, powerful - and at the least expense as possible (lol, who doesn't). The question is, though, how do I calculate what my options are?

I've looked at 48v 20ah ping batteries as the most optimum choice (easiest to hook up for this non-electrical engineer, least expensive of the LiFePO4s) but also saw some guys using Milwaukee and DeWalt packs in series/parallel to power their bikes...I became confused. How does it work if they're using 6 - 28v 2.4 ah batteries to power their 48v 700w bikes (as an example from my probably bad memory because I can't find the link now).

I've researched how power works but it's not getting me any closer to the answer to my question - what is my best choice for batteries besides SLA? I'm in Virginia in case that makes a difference.

This price comparison is awesome. There should be a way to use the watt hour calculation to also factor in overall battery life to get a real "cost per watt hour over x years" number for real comparison.

Thanks in advance!

 

[Link]

I've looked at 48v 20ah ping batteries as the most optimum choice (easiest to hook up for this non-electrical engineer, least expensive of the LiFePO4s) but also saw some guys using Milwaukee and DeWalt packs in series/parallel to power their bikes...I became confused. How does it work if they're using 6 - 28v 2.4 ah batteries to power their 48v 700w bikes (as an example from my probably bad memory because I can't find the link now).

They're overvolting a little. Three sets of two packs in series for 56V 7.2Ah total.

And, yeah, Pings are the best if you want cheap.

 

[dogman dan]

It can get confusing. Fer instance, a 36v lifepo4 is obviously 45 volts right? No wonder us non electrical engineers get confused. Just keep reading and reading and it will start to gell for you in awhile. A ping pack like you describe will be very well matched for a 450 watt motor. Of course, a 450 watt motor could easliy be a 1000 watt motor with the right controller and voltage. here we go again.

One thing that has helped me get a handle on it all is the basic formula. Amps x volts = watts.

Another good one to know is a 1c discharge rate on a ping 20 ah battery is 20 amps. So if you have a controller that is rated at 20-30 amps you will be ok on battery lifespan.

So if you have a 20 amp rated controller, x 48v you will have 960 watts of power avaliable, like to climb a hill or start out. The cruising draw on the battery will generally be much less, but you may have a whole lot of power going through a 450 watt motor on a hill. Most motors can handle that, but not continuously for too long a time.

 

[jules]

Thanks - I understand the amps x volts = watts equation - I was guessing amps and amp hours are pretty much the same when figuring battery power (as amp hours indicate the amount of charge transferred by an amp for an hour) but what I don't get is how volts and amp hours work with these motors. I'm adding a 24v 450w motor to an existing ezip, so I'll have 2 motors; I have a 48v 800w controller.

I've heard that an increase in volts will increase top speed, but what does an increase in amp hours do? In other words, how does a 48v 20ah battery differ from a set of Milwaukee or Dewalt batteries? It seems like some guys are using relatively fewer toolpak batts (like 6 28v 2.4ah) to get lots of speed and torque.

 

[Alex Lar]

Jules, buy LiFePO4s and no anymore problems and extra money in future

 

[michaelplogue]

I've heard that an increase in volts will increase top speed, but what does an increase in amp hours do? In other words, how does a 48v 20ah battery differ from a set of Milwaukee or Dewalt batteries? It seems like some guys are using relatively fewer toolpak batts (like 6 28v 2.4ah) to get lots of speed and torque.

More Volts = Higher top speed
More Amps/discharge rate = Greater acceleration
More Amp Hours (for battery) = longer range.

 

[Kurt]

Because I run the exact same dual motor setup as you are about to make I can give you an idea of what kind of battery will work well with it. I am also no electrical engineer I am a mechanical engineer and had to get my head around all the tec talk.

I have been using a 20ah 48v sla battery on this bike. The nominal voltage is 48v or 4x12v battery's but when you take them fresh of the charger they are a little more than this around 54v. It doesn't take much riding before the voltage settles down to around 50v.Like every one has been saying the more voltage the faster you go. At 48v you will go about 43-45kmh this max speed will decrees a bit as the battery becomes flat and the last 1/4 of your battery you will be lucky to do 35kmh as the voltage sags to around 42v under load at this stage.

The capacity of your battery is its AH rating for example pings 20ah 48v battery can deliver 20amps for 1hr until its flat.Or 10 amps for 2hrs and so on.
Your dual motor set-up will draw an average of say 15 35 amps so lets say 20amps full throttle on flat ground so using a 20ah ping pack you will get 1hr of riding at 45kmh so your range will be 45km.

20ah to 24ah is a good size any bigger and it will start to get heavy and you don't need all that range most of the time.

The C rating on a battery its its ability to deliver amps without damage or a negative effect to the battery's life.Usually the specs are given as a max c rating and a constant c rating. So to use a ping pack as an example again a 48v 20ah ping pack has a c2 rating C being its capacity so 20x2 so the 40amps constant .Also the pack could deliver more amps say a small spike of 50amps for a second or so this will be stated as the max rating .Now you can see if you were to buy a 48v 10ah ping pack the max amp draw you can take from it is 20amps before you might start to shorten its life. So this size battery is designed for low wattage ebikes.The more expensive battery's have a higher C rating for like 5c so you could use a smaller battery but still draw big amps a 48v 10ah high quality battery with a 5c rating could deliver 50amps constant without any ill effects.that said its capacity would still only be 10ah so range would remain short.

One thing with Sla battery's is they never deliver there rated AH capacity my 20ah sla lead battery's deliver around 7ah if I push them hard its more like 6ah.The life po4 battery's when used within there designed C rating will deliver close to there rated capacity. I think pings 20ah pack delivers something like 19.5ah. at c1 or 20amp load.So your range will be much longer.

I would recommender a ping pack 20ah or even 24ah size split into to packs for your bike.

Kurt.

 

[jules]

I appreciate the detail - thanks for indulging me.

 

[dogman dan]

One more thing about amp hours and duct tape batteries. While a Ping battery cell is rated at 2c, constant discharge, notice that we have zero test data at that rate. All I have ever seen is data for 1 c, or 20 amps discharge rate from a 20 ah battery. So everybody pulling more than 20 amps is doing an experiment. Some real world feedback is just beginning to filter in. I and others who's average rate of discharge is in the ballpark of 1 c are doing ok, and those averaging 1.5 c seem ok as well. Those pulling 2c or more are beginning to report loss of capacity, and shorter range after a very small number of cycles, way less than 500. I'm not sure if anybody has 500 cycles on a duct tape battery yet. The easy way to get an idea of your average rate of discharge is to ride nonstop and time it. In my case, after subtracting about 15 minuites spent at stop lights, I ride a bit more than 1 hour before the pack is drained so I can't be riding above 1c average rate of discharge since the pack lasts one hour.

Summarising, for duct tape lithium, or any lifepo4 with a 2c rating, the sise in amp hours is not just important for range, but also for cycle life. Too small will cost you cycles, and maybe a lot of cycles if you are really too small. two 450 watt motors run simultaneously may be in that category so you may need to get lifebatts, PSI, or A123 cells to have a big enough discharge rate. If you punch it with two 450 watt motors, you could easily be pulling 2000 watts, or about 37 amps out of your duct tape cells. Fine for a few moments, but if you ride long hills like I do, you could kill a ping that is under 30 ah. What you really need is some real world data about how many amps you will pull for most of your ride. I think anything over 25 amps is danger zone for a 20 ah ping.

 

[pwbset]

It seems like some guys are using relatively fewer toolpak batts (like 6 28v 2.4ah) to get lots of speed and torque.

I use toolpacks and for me it was that I kept reading about higher voltages would use available amperage more efficiently. It's much more complicated than that simple statement (and I don't even begin to understand it), but in essence you don't have to use extra volts for speed, but they can be used for more efficient power all the time. In other words just 'cause I run 84v nominal doesn't mean I'm going 30mph all the time. It just energizes the system more... if that makes any sense. If you play around with the ebike.ca simulator you can see this. If you have a 36v system and increase the amps a lot you won't typically see the chart changing a ton, but if you keep the amps the same and add voltage the N.M. and speed and all of it goes up, up, up. In the end it all boils down to watt/hrs and efficiency. There are very smart people here that can elaborate on these effects. Just sharing what I remembered reading when I started last spring... none of which may actually be right. :wink:

Plus adding voltage is cheaper than adding amperage for components. High amp controllers get expensive, but you can find a controller capable of 100v and 20a for cheap (and can then solder the shunt for "free" additional amps :wink: ).

 

[Kurt]

I wasn't aware of anyone having longevity issues with ping packs running at c2 if this is true then they shouldn't really advertise a 2c rating its a bit misleading. With the dual motor and same controller as you plan on using have seen 2000w for a fraction of a sec with a fresh battery also while the battery is fresh you can pull 1500 1700 what's sustained if you go up long steep hills. Most of the time its more like 800 to 1000w at full throttle-speed on the flats.

There are options if you find you don't want to spend big on high quality hi c rated battery's and are concerned about pulling to high amps from them. You could buy a cycle analyst this gives you the ability to dial in the amps you want to draw with the push of a button.I did this with my dual motor and found the setting the amps at 25max was more than enough to get me up any hill. That way your batteries will enjoy a easy life.

kurt

 

[dogman dan]

Yup, good advice. The info on Ping packs and how they respond is just beginning to filter in, and only from those who bother to document and track what is going on. So that is going to be owners of a cycleanalyst. I doubt all ping packs running at 2c are dying fast, but those that run 2c exept for when they run 4c seem to be.

A few things I see people saying about thier gear keeps bothering me. One is the idea that the battery likes being used at it's max capacity. I doubt it, why should lifepo4 be any different than other chemistries that all last longer if used moderately. Sure A123's can do big c rates, but how did this Idea that duct tape cells can do that get going? The other one is the idea that if I use say, 400 watt hours to go to and from work, that is all I'll ever need. What about wind, cold, an accidental undercharge? I say you need a big cushion there so when the pack starts losing some capacity you still can make it. And having the capacity means you do a shallower discharge and the pack should last longer. It all seems pretty common sense to me, but I keep seeing posts about lifepo4 is too expensive, so how do I use the least possible sise battery. With the duct tape cells at least, big is cheap, since small is gonna die too quick.

If Jules were to dial in her cycleanylst at 25 amps, she may not get the performance she wants, I don't know, but it would be worth it to make the 20 ah lifepo4 last! It may be just as cheap, though to just buy a bigger pack. Especailly if there are a lot of hills, or towing going on.

 

[Kurt]

As for the performance of the dual motor at 48v 25a . If you have been riding around with 25a 24volt and one motor you will be blown away with how much more power you have .The dual geared motors have that much torque that you don't need big amps to get you up the hills like a hub motor and to reach your top speed you just need the voltage.I backed my amps off to 25a because I was getting the impression that anything more than this was going to wast.

Buying the battery is the hardest thing when building ebikes.I ran with my sla packs for 6 months while trying to find out what voltage and capacity I needed.I have only just now ordered my first life po4 pack last week .I went with headways new 10ah cells in a 36v,20ah pack. My other equal 2nd choice was a ping pack but the exchange rate and shipping costs killed the good deal for me.I got the headways locally so they were a better deal .I will let you know how I get on with the headways.

Kurt.

 

[jules]

Actually the SLA we get is 24v 10ah with a 450w motor. I just ordered my 48v 20ah Ping pack today. I would like to be able to treat the battery well so am interested in a cycle analyst - is this the only good option? http://www.thunderstruck-ev.com/drainbrain.htm It's pricey.

 

[Kurt]

The cycle analyst from ebikes ca at first sounds like a lot of money but .You soon forget about that when you own one and see how usful they are . Not only are they a digital fuel gage for your battery's but a bike computer as well with all the usual features.They a back lit and have a large screen that is easy to read.The best part about them is they give you the ability to control the bikes max speed,amp draw,min voltage. The only way I can put it is its like the differance between driving your car with just the steering wheel,brake and accelerater and then driving it with all the other controls, and everything on the dash.

The support and service of the CA is some of the best I have seen!
Kurt.

 

[jules]

They are also on ebay BIN - I might be able to save 30% again! :wink:

I guess I'll get the standalone version because I don't have a modified Crystalyte controller, correct?

 

[Kurt]

Standalone version is correct.I wish that 30% discount offer was available in Australia.
Kurt.

 

[jules]

They don't take Paypal, and for the promo paypal has to be used. I sent an email inquiring.

 

[jules]

comcycleusa does take paypal but uses a third party checkout system. I sent them screenshots http://flickr.com/photos/mundobizarro/tags/cycleanalystebayerror/ and asked them if it was possible to set up an auction where paypal is the checkout from ebay and they suggested I pay directly through paypal (ie, no).

 

[jules]

They were able to make an auction so I could buy the cycle analyst and get the cashback discount. Just email them through ebay.

 

[julesa]

This price comparison is awesome. There should be a way to use the watt hour calculation to also factor in overall battery life to get a real "cost per watt hour over x years" number for real comparison.

Thanks for the compliment! I wish I could put something like what you suggested together, but unfortunately there are too many variables to consider. For one thing, there are a lot of differences between different battery brands and even different models with the same brand. Are you riding every day, or just on weekends? Shelf life (how long the battery will last when only used very lightly or not at all) is different from cycle life (how many times you can charge the battery before it loses too much capacity to be useful.) Are you going to drain 90% of your battery's energy between every charge, or are you going to use 50% or less? Are you draining it right at the limit of its rated current output, or are you draining it more slowly? Temperature extremes? Are you riding on rough terrain?

Some SLAs (for example some of the ones made for computer UPS applications) are delicate and will die very quickly on a vehicle -- you can lose a cell or two every time you hit a pothole. Some UPS batteries are multi-purpose and will withstand some vibration, so your mileage may vary there. Look for SLA models that are sold for powering wheelchairs for best results -- even then expect 300 charge cycles, 500 at most.

I've read that some NiMH and NiCDs are actually happy to be discharged near the top of their rated current - it breaks up crystal formations in the cell or something.

People here on ES have written that most Lithium chemistries will have a much longer cycle life if you normally only discharge 50% or less before you charge again. Discharging more than 80% will significantly reduce cycle life. Discharging above the rated current will also reduce cycle life. Cheap LiFePO4 can't handle as much current as the high quality (and high-$$$) LiFePO4. If you buy PSI or LiFeBATT or A123 batteries, you would probably get more cycles from them than from the cheap LiFePO4, if you take good care of them. But then again, you might hit the end of your battery's shelf life before you have even come close to it's cycle life. There are a lot of variables. There's really no way to make a real detailed value comparison between a lot of different battery options without buying them all, setting up a lab, and testing them all under the same conditions. Even then, the results wouldn't really be useful to someone whose usage patterns are different from the chosen conditions....

 

[dogman dan]

It's pretty hard to figure out what riding an ebike really costs, especially when you start in with batteries that may go 25000 miles. That figure comes from 25 miles per cycle x 1000 cycles. On my bike and a 20 ah 36v ping I get 20 miles per cycle, but in practice, each cycle is 15 miles. The variables stack up fast! Anyway at one point I tried to calculate what my cost per mile would be for 1000 cycles, including several bike and motor replacements, and came up with around $3.00 per 30 mile trip to work and back. The battery was the smallest cost since I don't expect hub kits to go more than around 5000 miles. The bus is $1.50 but it only takes you 2 miles from my destination, the Subaru is $15, and the 150cc ICE scooter is around $5. Occasionally I need to haul a ton or two of landscape rock to work in my one ton truck, I don't even think about what that trip costs.