Electric Bikes - What's better: more V or more Ah?
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12-28-08, 10:45 AM
Got a WattWorld kit on my mountain bike. It is a Tonxin hub motor, 250W with 24V 10 A/h LiIOn battery. Problem is that is cuts off almost immediately on a steep hill. Vendor offers me to switch for a 24V 16 A/h battery. Will that do it? Would I be better with a 36V battery to tackle our predominently hilly terrain?
i would say more amphours. overvolting a 24volt motor to 36volts for hills is not a good idea. you risk cooking the motor.
12-28-08, 07:29 PM
Well that would depend if you hill is at the end of your ride or at the begining.
If it is at the end it could be AH. However if it is at the begining, it is definitely not AH that you need. That means your battery doesn't have the power to carry you up hill and you need more V.
Be careful not to exceed the power dissipation of your motor as it was stated. You will overheat it and burn in up if you run it beyond its rating for very long.
A few seconds at a time is ok.
12-29-08, 07:34 PM
If your batteries have Battery Management (monitoring?) System, BMS, it could be that circuit is killing the power to protect the batteries from an excessively high discharge current. If that's the case then, strange as it seems, a higher AH pack may solve the problem as it's BMS will allow proportionally more discharge current before cutting off power, maybe enough to get you up and over the hill.
16AH should allow for a 60% higher current than 10Ah.
Let me cover my you-know-what by saying while I have a decent grasp of electricity, I know enough about batteries to be dangerous :o
250w motors don't do steep hills well period, that is about 1/4 hp. You provide more power than it does pedaling potentially.
12-30-08, 08:05 AM
You are correct Mabman, I test rode a Giant with 250W hub motor and found it was of little use on the hills around Ballard (a Seattle district). Since it was a pedalec I can't really say what the hill would have felt like without the 250W, I just know I was whupped when I got to the top.
Maybe a geared 250W hub would do better.
12-31-08, 06:55 AM
It is possible that you are lowering the voltage enough under load that the motor controller is cutting out, this is a value set by the manufacturer. I would go with a higher voltage system if the motor and controller can handle it.
For uphills, what you want is torque, rather than speed.
Any given motor will give you more torque if you add amps, and more top-speed if you add volts.
Adding amp hours and adding amps is not the same thing, BUT if you use the same type of battery and add more amp hours then your battery's maximum amps will also go up.
So it sounds like what you want is more amp hours.
Note that your total range (assuming similar riding conditions) is mostly determined by your watt-hours, which is to say amp hours times volts.
So 36v 10Ah has the same number of watt hours as 24v 15Ah, but the 24v battery would be a better choice for steep up-hills and the 36v battery would be a better choice if you wanted to increase your top speed.
12-31-08, 10:45 AM
This has been a rich & informative thread. On another french thread, people very much agreeing with your point but advised to control the demand of the engine with Cycle Analyst. As they stated, on a steep hill, the Tongxin geared hub moter is likely to pull all the energy it can and its controller will not limit it. Hence the fast drainage of the battery.
Question: can the stand alone model work with any controller & how do you manage to control the throttle given that I have a pedelec system without throttle.
12-31-08, 07:58 PM
If you go to http://www.ebikes.ca/simulator/ (http://www.ebikes.ca/simulator/)you can compare a given motor with a varity of battery types and different voltages, this may be useful to you.
01-01-09, 01:50 AM
a stalled DC motor is essentially a DC short. It will draw a tremendous amount of current if the battery can provide it.
It is possible that the hill is too steep. That upgrading your battery won't significantly improve anything.
01-01-09, 07:17 AM
This is the motor diagram published by the manufacturer
01-01-09, 08:49 AM
sacrekin, going back to your original post, you say the motor "cuts off almost immediately on a steep hill", I'm curious if this is because the battery becomes drained or because something shuts off power to the system? In other words, say it were to cut off and you then tried to ride on a level road, would the system come back to life or is the battery dead?
01-02-09, 02:04 PM
Misslexi, I do believe the cut off originates from the battery as 1 or 2 mins later it recovers & I am able to proceed with the riding. It actually recovers faster if I remove the battery & plug it back in when its charge monitors become active again. This phenomenon only happens on a steep hill.
To clarify on several comments, the Tongxin hub motor (also called Nano in the UK), is capable to handle either 24 or 36V. However, my controler is a 24V. I have checked it several time and it never gets hot, even after riding the hill.
Last, this phenomenon seems worse than 4 months ago when I got the bike. Now it is colder (5-10 deg Celsius). Does outside temparature makes a difference on the performance of the battery? I thought Lithium batteries were not sensitive to temparature?
01-02-09, 04:25 PM
On a 500w motor I've tried three different 36v lifepo4 batteries with different capacities: Rated at 10ah, 12ah and 15ah. I have very steep hills around some areas and the 10ah battery will start to cut off if I am riding up a hill for a long time or if it gets VERY steep. With the 12ah battery, this happened occasionally and wasn't as much of a nuisance anymore. And now, with the 15ah, the battery never cuts out. Also, every time I've upgraded I have noticed the battery pulls with greater power, a 36v/15ah pack is impressive to say the least. I believe for the hills, the larger battery pack has the benefit of a higher current-discharge-rate.
01-02-09, 04:57 PM
This sounds more like the BMS protecting the battery pack; the fact that you can reset it or allow for a brief recovery period is the clue.
Speaking about LiFePO4 batteries, I do believe they are sensitive to ambient temperature. It is not recommended to store nor charge them in a cold environment. I'm not sure if that's true of Li batteries in general.
Antranik's observations should be helpful to you, someone who's been there, done that.
So, if you decide to go the route of upgrading batteries, I would also move up to 36V if possible. I know this requires a different controller but it may be well worth it. You could technically keep your AH the same because for any given motor output power, you will need less current at the higher voltage. In general all components should run cooler for the same power output at the higher voltage, heat loss being proportional to current squared. It would sure be nice if you could try some of this out without having to buy it first.
01-14-09, 02:11 PM
Ok, after this I decide to return the Tongxin kit and add some $ to get a far more relaible Panasonic based bike. All the problems have vanished & yes with a 250 Watt engine I could run 35kms mostty uphill (+500m) and have 2/5 of the 10 Ah battery still available.
Bottom line: All these kits have cheap electronics and cannot work without massive batteries.
01-14-09, 07:11 PM
Great news that you can ride your eBike instead of think about it :thumb:
Some of the Chinese vendors are new to the game I think, they pull parts together without realizing the importance of the system as a whole. My experience with them is it won't take long to figure this out and get with the program. In the meanwhile, we are their "field" testers :(
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