My Gtech eBike Sport review!.

[Deleted member 4366]

Ebike motors have a maximum efficiency of around 80%, but that only happens at one particular RPM. A figure of around 70% would be more realistic considering the the range of RPM actually used. There's another loss of 5% for the controller/wiring, and say 5% transmission lost by the time the power reaches the back wheel, so something 60% of the battery energy gets converted into motive power. Add that to your equations and then you can see why it doesn't make much sense for a 100kg guy to use a GTech in a hilly area unless he wants to do a lot of work.

I have a turbo trainer with a wattmeter on it. 100w at the back wheel, where it's measured, is equivalent to steady pedalling that most reasonably fit people can sustain for an hour - maybe 60w if you're not fit.

200w for an hour is a bit of a work-out. I would think that regular cyclists could achieve that if they had the right mind-set.

You need about 250w to sustain 15mph on the flat without any wind on an upright bike (MTB).

That means that unless you want to put in some effort, you'll be draining your battery at about 250watts (150/.60), which would give a range of about 30 miles at 15 mph on the flat (500Wh battery)

If you only went 10 mph, your 100w would be enough to power the bike, so your battery would last forever. In reality, I would say that most people cycle their ebikes in that 10 mph to 15 mph range, so we can say that they would get a battery range between 30 miles and infinity depending on how fast they go.

Now you bring in hills and you could half that range, which means somewhere between 15 miles and half of infinity (which is still infinity).

Next, you can introduce the variable of pedal effort. Someone that likes to pedal hard might average 200w, so at 15 mph his battery would go down at 83W, which would give him a range of 90 miles, or 45 miles with hills. Of course, it depends how many hills. You have to use the figures in the above post to calculated exactly.

A few charges back, I did 190 miles on one charge. I could have gone a lot further. The next charge took me about 15 miles. Same bike, same rider, different speed and different pedal effort.

When buying a bike, the only thing you need to consider regarding the range is the size of the battery. The bigger the battery, the further you'll go. The rest is down to you. There's no magic formula or ingredient X in any bike that will change any of the above except anything that reduces air resistance, rolling resistance or weight.

 

[Tomtomato]

When buying a bike, the only thing you need to consider regarding the range is the size of the battery. The bigger the battery, the further you'll go. The rest is down to you. There's no magic formula or ingredient X in any bike that will change any of the above except anything that reduces air resistance, rolling resistance or weight.

So the things to consider are not just battery size then...

There are some very large differences in weight between models: a Gtech bike weights just 16kg, while many models would weight 25kg or more.

 

[trex]

I would say that the average range you get with your bike depends largely on battery capacity and speed and to a lesser degree, your own weight plus the bike. I always recommend a larger capacity whenever possible because the bike loses speed and acceleration when your battery is less full. A battery with lesser capacity will sag more. The sag is the difference between the voltage at rest and the working voltage as shown on your LCD. The more demand you put on the motor, the bigger the voltage sag. Let's take an example: if you pull 15A from your full battery with a capacity of 15AH, the voltage sag is about 1.3V. Doing the same on a 10AH, it's about 2V. Your 10AH battery loses 42V * 2A = 84W inside, generating heat. It's wasted energy and it's reducing range.

 

[Deleted member 4366]

So the things to consider are not just battery size then...

There are some very large differences in weight between models: a Gtech bike weights just 16kg, while many models would weight 25kg or more.

The weight of bike should be considered along with the weight of the rider. For a 75kg rider, esch 1kg of bike weight means 1% change in effort to ride up a steep hill without assistance, or 1% less power from the battery to get you up the hill. In rality, it's a combination of both - say 50/50. That would mean that the Gtech would get about 4.5% more range than a bike 9kg heavier and with a battery the same size.

A 25kg bike these days is likely to have a battery twice the size of the Gtech, so approximately double the range and probably more power as well because, as Trex says, the voltage would likely always be higher.

Also, you'd have to consider the lower efficiency of the Gtech because of the lack of gears, which would prevent the rider giving normal pedal power at low speed, especially on hills.

Considering everything, I think the watt-hours/mile would be similar.

A light-weight bike would likely have better handling and more convenience than a heavy one. Different people are looking for different things. The Gtech has some advantages for some people, but if you're heavy and you have hilly journeys to contend with, you're unlikely to go far go far on a Gtech.

 

[anotherkiwi]

Let's take an example: if you pull 15A from your full battery with a capacity of 15AH, the voltage sag is about 1.3V. Doing the same on a 10AH, it's about 2V. Your 10AH battery loses 42V * 2A = 84W inside, generating heat. It's wasted energy and it's reducing range.

Lucky you! My 10.4 Ah Li-Ion battery (Samsung 26F) sags the full 3 to 4 Volts and it is a pain. My 10 Ah LiPo (37 V 10S) sags about 0.9 V and the 20 Ah version (2P2S) sags about 0.6 V (0.7 V on my last steep ride with the 46 tooth chainring).

I think the C rate of the battery has a lot more to do with sag than the capacity.

 

[Danidl]

I would say that the average range you get with your bike depends largely on battery capacity and speed and to a lesser degree, your own weight plus the bike. I always recommend a larger capacity whenever possible because the bike loses speed and acceleration when your battery is less full. A battery with lesser capacity will sag more. The sag is the difference between the voltage at rest and the working voltage as shown on your LCD. The more demand you put on the motor, the bigger the voltage sag. Let's take an example: if you pull 15A from your full battery with a capacity of 15AH, the voltage sag is about 1.3V. Doing the same on a 10AH, it's about 2V. Your 10AH battery loses 42V * 2A = 84W inside, generating heat. It's wasted energy and it's reducing range.

Hi sorry but your last calculation is incorrect. The power consumed within the battery is the voltage dropped across the component multiplyed by the current flowing in the component. Assuming a 10a load and a voltage sag of 2 volts that would be 20w. However that is also pessimistic. While the immediate full voltage on the battery pack might be 42v on charge, it will quickly drop down to say 40v off charge and then down to 38v when on load. As the battery discharges on load the output voltage declines a little and the internal resistance increases a little . Basically the ions further away from the electrodes are now being consumed rather than those closer to the electrodes which were consumed earliest. When the voltage drops to go below the lower threshold as set by the BMS typically 32v it disconnects the battery pack from the load.

If the battery is left off load the voltage may recover slightly r ,but this is an illusion and will not be available as power. . When batteries are checked for operation the voltage is measured while on load

 

[trex]

I stand corrected, in the example, it is of course the 2V voltage drop times the current, 15A, 30W or about 5% loss through the internal resistance or internal impedance.
For your information, a typical 36V 15AH battery with Samsung cells (they are specified for 45milli-Ohms per cell) is configured as 10S5P of 29E cells, has 90 milli-Ohms resistance (45 * 10/5) plus a couple of milli-Ohms for the power mosfets and about 10 milliOhms for the wires. The voltage drop at 15A is about 15A x 0.1 Ohm = 1.5V
a 10AH pack is usually built with 10S4P array of 25E, 0.125 Ohm internal resistance for the battery and wires, 1.88V sag at 15A.
Samsung cells are known to have low impedance. Other cells may have slightly higher internal impedance.

specs: https://www.nkon.nl/sk/k/29E.pdf

 

[Deleted member 4366]

Doing the same on a 10AH, it's about 2V. Your 10AH battery loses 42V * 2A = 84W inside, generating heat. It's wasted energy and it's reducing range.

Nearly.

Ohm's law: the voltage drop = resistance x current
i.e 2 = R x 15
Therefore internal resistance is 2/15 ohms
Power lost = current squared x resistance
P =15 x 15 x 2/15 =30W
Power lost with 1.5v drop (15Ah battery) would be 22.5w.

 

[trex]

it was just a typo, picked up already by Danidl and I corrected in post #87.
I tried to convey that the loss through internal resistance means that one have to ride with little assistance on the last 15% -20% remaining charge. A 15AH weighs 500gr more but loses 1/3rd less on internal resistance, is a much better choice.

 

[Deleted member 4366]

The losses due to internal resistance are relatively low because you wouldn't be spending much time at maximum current, particularly on a hub-motored bike like a Gtech because you only draw maximum current at low speed. It's a bit different on a crank-drive bike, where it's pedal speed that counts. The power loss difference between a 5Ah battery and a 10Ah one sagging at 2v instead of 1.5v would be 0.5v, which is about 1.5% difference.

The main reason for lower power on a bike with a smaller battery is simply the lower average voltage.

If you have a 10Ah battery and use 5 Ah for your journey. Average battery voltage will be about 39v, because the battery will go down half way from 42v to 30v, which is 36v. Average from 42v to 36v is 39v

For the same journey on the same bike, A 5Ah battery would empty, so the average voltage would be (42 - 30)/2 =36v

Power is directly proportional to voltage so the average power for the 5Ah battery compared with the 10Ah one would be 36/39 = 92%.

That's one reason to keep your battery topped up. You can only get 75% of the power when it's near empty.

Total difference in average power between the two batteries would be: 8% due to average voltage and 1.5% due to sag - say 10% less power from the smaller one.

 

[trex]

I rode the Gtech a few months back. The battery weighs about 1.5kgs, assuming that it's built with 10S3P, the internal resistance would be in the region of 50 mOhms * 10/3 = 167 mOhms, add to it a few mOhms for the wiring, you get to about 175 milli Ohms. The controller is rated 15A. You can ride it up hill pulling hard on the battery. My estimate for voltage drop is 2V to 2.6V on hills. The reason that bikes like the Gtech and the Karoo don't use much battery is because they are mainly sold to customers who are strong pedallers. Most of Karoo customers reported 70-80 mile range out of the Karoo 13AH battery while most Big Bear customers report around 40-45 miles out of the 15AH battery. Both bikes have throttle, so speed is not an obstacle.

 

[Danidl]

This thread has deviated significantly from the original question ... the review of the gtech bike. However the insights into battery performance are excellent and should be highlighted somewhere. Whey are of much more general interest than solely the gtech. Could the moderator Helen create a new sticky thread perhaps in the technical section and transfer the battery related topics to it? . It would be a shame if they disappeared from general view.

 

[KeithMac]

I agree, my thread has been derailed slightly..

 

[Acer]

We had a guy turn up with a G-tech bike.
To be fair he liked its simplicity,he used it for a 4 mile journey to work and recharged it at work,it introduced him to e-biking.His journey to work was all on the flat
But he tried a 15 mile trip around London including Richmond Park,he struggled to ride it up the hill and had to use maximum power most of the journey,he ran out of power at about 12 miles.
He tried a KTM fun and absolutely loved its performance,we promised him minimum 30 mile range. He intended to try to sell the G-tech on e-bay then come back for the KTM.
KudosDave

If he struggled to ride a Gtech bike up a hill then all I can say is that he is seriously unfit and/or extremely heavy! I just bought this bike and the best thing about it is how well it performs on hills. Some reviews - probably people who have never even ridden one - have stated that the single gear will make the bike extremely limited on climbs, but I have found the complete opposite. With full assist engaged it absolutely flies up steep inclines and that is why I rarely use it - I want a proper work out.

I ride it without assistance most of the time and just click it on if a steep hill comes into play. It rides really well as a single speed bike without power and the lack of maintenance on the belt drive is a massive plus, as far as I am concerned. Thus far I am very pleased.

 

[Woosh]

I ride it without assistance most of the time and just click it on if a steep hill comes into play.

that's the intended market for this bike.
Our Karoo would suit you.
AFAIK, most e-bikers are a bit older and need a bit more assistance.

 

[Deleted member 4366]

If he struggled to ride a Gtech bike up a hill then all I can say is that he is seriously unfit and/or extremely heavy! I just bought this bike and the best thing about it is how well it performs on hills. Some reviews - probably people who have never even ridden one - have stated that the single gear will make the bike extremely limited on climbs, but I have found the complete opposite. With full assist engaged it absolutely flies up steep inclines and that is why I rarely use it - I want a proper work out.

I ride it without assistance most of the time and just click it on if a steep hill comes into play. It rides really well as a single speed bike without power and the lack of maintenance on the belt drive is a massive plus, as far as I am concerned. Thus far I am very pleased.

You need to qualify that by saying how much you weigh.

 

[Acer]

You need to qualify that by saying how much you weigh.

What I weigh doesn't matter. Point is I can fly up hills on this thing . If he couldn't then my point is that it isn't an issue everyone will experience.

 

[Deleted member 4366]

What I weigh doesn't matter. Point is I can fly up hills on this thing . If he couldn't then my point is that it isn't an issue everyone will experience.

Of course it matters. There's no way it'll fly up hills with some people I know on it. I have an ebike that'll fly up any hill in top gear with the battery completely switched off when Bradley Wiggins borrows it. By not putting such a statement in context, it can be very misleading to some people reading it. To avoid that, you should edit your previous post with your weight and level of fitness/strength.

 

[Acer]

Of course it matters. There's no way it'll fly up hills with some people I know on it. I have an ebike that'll fly up any hill in top gear with the battery completely switched off when Bradley Wiggins borrows it. By not putting such a statement in context, it can be very misleading to some people reading it. To avoid that, you In what way is it misleading? It should be pretty obvious from my assumption that the bloke who struggled must be unfit and or heavy that I myself am not heavy. Point is this bike will work just fine on hills if you have reasonable fitness and don't carry too much weight.

Of course it matters. There's no way it'll fly up hills with some people I know on it. I have an ebike that'll fly up any hill in top gear with the battery completely switched off when Bradley Wiggins borrows it. By not putting such a statement in context, it can be very misleading to some people reading it. To avoid that, you should edit your previous post with your weight and level of fitness/strength.

Ok no problem. I am 70 kilos and reasonably fit. Strong, yes, but not lots of stamina. Certainly no Bradley Wiggins and not even much of a cyclist.

 

[Woosh]

If he struggled to ride a Gtech bike up a hill then all I can say is that he is seriously unfit and/or extremely heavy! I just bought this bike and the best thing about it is how well it performs on hills. Some reviews - probably people who have never even ridden one - have stated that the single gear will make the bike extremely limited on climbs, but I have found the complete opposite. With full assist engaged it absolutely flies up steep inclines and that is why I rarely use it - I want a proper work out.

I ride it without assistance most of the time and just click it on if a steep hill comes into play. It rides really well as a single speed bike without power and the lack of maintenance on the belt drive is a massive plus, as far as I am concerned. Thus far I am very pleased.

The gradient is also important. How steep is your hill?