4@ matched(all the same) 4-to-6 cell RC Li-Poly packs of 3,700-to-5,000mAh capacity wired in a 2s2p configuration with one 4-6 cell four slot charger should do the trick.
Just a simple wire harness in a bag with four plugs for the four little RC packs and plug them in and go and then when you get home unplug them from the wire harness and plug them into the four slots on the charger.
That's my first though. "C" rating on those packs will be more then enough, I don't think they even sell them anymore with less then a 15C rating. Light weight and powerful and should give him just about the duration he is looking for.
I'm using a very similar set-up only in a 1s3p configuration with HobbyKing Zippy house brand name 6-cell 3,700mAh packs on one of my hill climber only builds that uses a motor that runs at lower voltage levels then the hub motors he is looking to use. With a hub-motor built for 36-48V he is probably going to need at least a 2s configuration with at least 4-cell packs to give him at least 30-volts or so, don't think you would want to run on much lower voltage then that.
Well, doing some quick shopping for a good deal, if you went with 4 of these:
They weigh about 2/3 of a kilogram or 1-1/2 pounds each so that is about 2.7 kilograms or 6 pounds for four of them.
They are basically shaped like little bricks that are 2 inches square (5cm) square by 6 inches long (15cm) and you would have four of them that size. You don't need that big of a frame bag or rack to hold them as a result.
They are $40 each and you would need four of them so that would be $160 but you also need some plugs for them to build your wire harness (adds a few dollars each depending on what plugs you use) and then you need a charger for them which if you buy 4 individual chargers for those type of packs (you want to be able to charge them all at once with a charger or charge slot for each of the four) then the cheapest decent quality single pack chargers capable of chargin a 5-cell pack I found on the same web-site were running about $20 or so with cheaper ones on back-order so that would be four of those at that price each for another $80 or so for a charging deck. I did mention though that they do make four slot chargers that it is one charger that can do all four packs at the same time which the HobbyKing house brand one that can do up to 6-cell packs (including the 5-cell packs I linked too) is currently $69.99 also known as about $70 which is slightly cheaper and its all there in one:
That charger (as are most chargers designed to charge the RC Li-Poly packs) is designed to run off of 12V car battery power. You can buy a power supply for them to plug into your wall socket or you can get an old car battery and car battery charger and plug in the car battery charger to the wall outlet and hook it up to the car battery and then hook the charger up to the car battery and presto you have your own power supply set-up and it will even continue to charge your bike batteries even if the power goes off at least until the big car battery is drained down. Obviously for a city slicker appartment dweller that probably isn't best and you would have to spend a little more money for the 12V power supply to go with the charger for bench charging (also sold by RC hobby shops including the online one I've been linking too) but for a rural farming/ranching country guy like me I've got a big shop full of all kinds of junk and finding a car battery or two and a car battery charger and setting up a spot on the shop bench for them is the better option because its stuff I already have and I've got the shop space.
Long story short going with RC Li-Poly packs can give you a lightweight 6-lb (2.7-kg) battery set-up that can fit in a variety of bike bags or racks considering the batteries consist of four packs where each pack is a little brick that is 2-inches (5cm) square by 6-inches long (15cm) and you have four of those little bricks and the wire harness with four plugs to fit in a bike bag or rack or something. Nothing like trying to find a place to mount the bulk of a great big cinder block size and weight e-bike pack. Not as much duration of course but for a low end torque build intended to be used just as a boost for hills or pulling away from a stop can certainly be sufficient. It all depends on what your want.
if you have no prior experience with batteries, and no knowledge of volts, amps, series , parallel, etc.. , i strongly recommend you NOT start with RC lipo.... seriously.. read thru this and if it's at all over your current level of understanding.. get a ready made pack to start with.
take note of the pictures at the very bottom of that page...
The "problem" with having just a helper for the hills and if it's not really useable at speed is you are then luging it around and not getting as much out of it as you could, like when going into the wind, going up 1 or 2 % hills... JMO Also depending on type of motor and set up, there could be some resistance/drag when not used and you are pedaling harder...
This stuff is fascinating.
When you folks say a battery capacity is 125 watt hrs-do you literally mean if drained to zero SOC it can deliver 125 watt hrs-or that 125 watt hrs can be delivered 500-1000 times-which means it must have maybe double that capacity?
Toyota- normal Prius-has a 1500-1800 watt hr battery pack-but Toyota is very conservative-only allowing it to drain about 500 watt hrs or so-70% state of charge-It is nickel metal hydride-not Lithium. At 30mph 500 watt hrs would be a bit under 2 miles-level highway for a prius.
How aggressively do you guys drain these battery packs?
personally, i never use more than 75% of my available capacity.
my trike, 100v 13.5ah = 1,350 wh
Norco Chaos 100v 10ah = 1000 wh
Beach Cruiser 36v 10ah SLA = 5ah usable = 180 wh
BMX 48v 10ah = 480 wh
Ladies comfort bike 36v 14ah = 504 wh
I use power 100% of the time when ever i ride any of my ebikes, and pedal as much as i feel like pedaling.. if you wanted to pedal without the motor most of the time and only use it occasionally, you want the smallest, least heavy setup possible... if you opt for a hub motor, forget 24v, start with at least 36v.
80% drain is the number I try not to violate with lithium batteries, leaving 20% of the capacity still in the pack. For lithium batteries they can take that level of drain fairly well and still have a decent life span especially if they are the heavier more stable LiFePO4 chemistry.
NiMH is actually one of the battery chemistry that can take the highest level of drain will little to no damage. 95% drain for them works fine only leaving just 5% or so of their capacity to facilitate re-charge. I've got a old 1-Kwh NiMH pack one one of my old e-bikes and I've even drained it totally dead 100% discharge a few times and its still works pretty good after over three years of service and still holds well over half its original capacity. Crazy heavy and like lugging around a cinder block (NiMH packs are way heavier for the same capacity as the new Lithium chemistry packs) but its still doing its job and if I'm just going to buzz around locally and don't want to pedal hardly at all but ride like its an electric motor-cycle I use that bike with its massive direct drive rear hub motor thats the maximum legal power allowed in my state and can do a standing wheel-y on take off from a dead stop if I ram the throttle down. A real PITA to just pedal without the motor though because of the weight of that big motor and battery pack and the cogging drag of the direct drive hub motor.
If your still using lead-acid (AKA= scraping the bottom of the barrel) then 50% is as deep as you want to cycle them.
Although not normally used on e-bikes Ni-Fe and four way nickel/iron/zinc/oxygen batteries allow 100% cycle depth with almost unlimited life span. Ni-Fe are very heavy (just as bad as lead acid) and have been around for over half a century and are the very best option for stationary power storage, I have a huge bank of military-surplus ones that provide the backbone of my home power system that were built during the cold war and installed in almost every subterranean defense structure and fallout bunker and they still work great to this day (I've got a huge number of Kw of storage capacity with them in my home and shop). The four way nickel/iron/zinc/oxygen batteries were invented in my own home state by a local company were I have met people that work there and they offer the potential of nearly twice the capacity of current lithium chemistry batteries in the same physical size weight with 100% duty cycle and longer life. Since its a semi-liquid "quicksand like goo" that the battery is filled with that makes it work it is also possible to drain/pump out the electrically neutralized goo into a different battery for charging and fill the battery of the vehicle with fresh goo. Basically with that kind of battery it is not only possible to just r-charge the battery but also to charge in a separate large reservoir at home or in a gas station like facility and just drain and tank up the battery on the vehicle in a continuous recycle cycle and its all the same goo for all batteries of this type regardless of size. Unfortunately, that technology is just barely now making it to market and its first application is massive stand by power applications like for hospitals and such, it's going to be a while before it works its way all the way down to off the shelf e-bike and other electric vehicle applications (which I am greatly anticipating).
Ypedal does have a good point about it being simpler for a newbie to just buy a ready made pack. 36V, 10Ah or so would be equivalent to the RC pack set-up I suggested.
I've built my own packs soldering tabs together on individual cells and wiring in protection circuits and such so sometimes I forget not everyone has that level of technical know-how and can just jump in head first and swim like a fish in these waters.
As I understand it the Prius is not an electric but an electric/IC-gas hybrid and the battery pack and electric motor just act as a storage reservoir using excess IC power output to charge the batteries and then pulling power from the battery for extra boosts of power when needed. You don't need a very big battery pack when using that kind of set-up and you don't really need to consider using all of its capacity. An all electric (or human/electric hybrid) on the other hand you can't recharge the battery until your done with the trip so battery capacity is much more critical and fully using as much of the capacity as you can without hurting the battery is also critical to keep the weight down. Only using 60% of the capacity instead of 80% means you need a noticeably larger and heavier battery to do the same job so generally you use as much of the capacity as you can "get away with".
@ In General Response to thread and some of things Ypedal mentioned:
As to how much of the motor power that is used and how often during riding an e-bike there are basically three categories of e-bikes in my mind:
----- Hill Climbing Helper ONLY E-Bikes = Most of the time you pedal, motor is only used at lower speeds for climbing and maybe to increase acceleration from a dead stop. Minimal weight and no noticeable drag when pedaling only is very important in these builds.
----- True Human/Electric Hybrid E-Bikes = Both power sources work together, pedaling almost all the time and motor being used almost all the time through the entire power range. Carefully matching the human and electric motors to each other so they work together in tune with each other is very important in these builds, usually are mid-drive set-ups where the electric and human power are combined together and run through the bikes full gear range together.
----- Electric Mopeds = e-bikes that have pedals on them mainly just for show and legal requirements. Pedaling along with the motor doesn't do a whole lot and you really don't want to have to pedal them without the motor.
Most of my builds are in the middle category with the first category not far behind. Only one build in the last category (which I mentioned above).
As far as the power of the motors, I've got a couple builds where the total power is just barely under the legal limit in my state (2-hp / 1.49-Kw) and I've also got one build where the motor has a maximum power of only 36-watts (that's not typo) but that one is only to take advantage of a legal loop hole in MT state law that says that bicycles with helper motors are not subject to the state FRAP law and may always "take the lane" on any road and do not need to be under one of the safety exceptions to the FRAP law to do so. Most of the time I actually use that tiny motor as a generator to keep the small battery pack which also powers my lights charged up but it will work as a motor too (state law only gives a maximum power limit, don't say anything about a minimum for the loop hole to apply). Other then that on my next smallest motor build is only a 150-watt motor.
As far as the size of the battery pack, my smallest pack only holds about 100-watt-hours of capacity. My largest pack configuration holds over 2-Kw-hours of power (which considering that's on a cargo bike that has a 360-watt mid-drive can provide some pretty incredible range numbers with the pack lasting for hours and hours).
I will consider carefully my options and make a decision when I get the bike to near completion. This should be sometime end January.
@ turbo and others, Many thanks for the help. I greatly appreciate it.
I will update this thread when I have decided which way to go. In the mean time, have a happy Christmas and new year!
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