Bike options for a fat bloke?

[trex]

lipo is considered safe between 3.7V and 2.9V for electric vehicles against 4.2V to 2.5V for normal e-bike batteries. You pay for 10Ah and can only use 6-7Ah out of the pack and still have to alarm it. Another problem is voltage sag. If you draw a lot, then you have to up the voltage of the alarm.

 

[Geebee]

lipo is considered safe between 3.7V and 2.9V for electric vehicles against 4.2V to 2.5V for normal e-bike batteries. You pay for 10Ah and can only use 6-7Ah out of the pack and still have to alarm it. Another problem is voltage sag. If you draw a lot, then you have to up the voltage of the alarm.

Have you used high capacity hobby LiPo's? voltage sag is not an issue, my packs are the cheapo low current ones ONLY rated for 200 amp continuous output and 300 peak (36v 10 Ah pack), any voltage drop at ebike current level is negligible.
I used to run my 5Ah pack down 4 to 4.5 Ah with zero issues and never hit low voltage alarm levels, I dont recommend going below 80% capacity regularly on any Lithium but it hasnt hurt my batteries capacity doing it occasionally.

Can you provide a reference for your statement about capacity at E-bike current levels with R/C batteries as I have never come across that idea before.
LiPo is normally used between 4.2v and 3.2v as an absolute minimum, I always goto 4.2v and have never heard an alarm as mine are set to 3.3v under load.

 

[trex]

this is a typical 6C discharge curve, I suppose it is an approximate use of a 36V 5AH lipo pack on an BPM e-bike at full throttle climbing a hill:

As you shouldn't charge it at over 3.7V nor discharge it below 3V, the useable portion on the horizontal axis is between 1 and 9.
that graph shows 1 minute discharge per unit.
When you approach 9, how close do you think is safe?
I reckon if people can see how little time they have from the alarm to overheating, they'd think twice about using RC lipo on e-bikes.

 

[Geebee]

this is a typical 6C discharge curve, I suppose it is an approximate use of a 36V 5AH lipo pack on an BPM e-bike at full throttle climbing a hill:

As you shouldn't charge it at over 3.7V nor discharge it below 3V, the useable portion on the horizontal axis is between 1 and 9.
that graph shows 1 minute discharge per unit.
When you approach 9, how close do you think is safe?
I reckon if people can see how little time they have from the alarm to overheating, they'd think twice about using RC lipo on e-bikes.

The low voltage alarm is just that, it is an alarm ie. the last line of defence, its not a case of ride until the alarm goes off and then keep riding.
Mine are set at that level for safety, I have never had an alarm go off, the same as I don't ride until the battery cuts off on a normal pack.
If either event occurred I would not use the battery to get home.

That chart if using a 5Ah battery as stated would be 30A continuous discharge, what is the tested batteries continuous discharge rating, is 30 Ah within its accepted range?
It would not effect my usage anyway as an alarm is the last line of defence, but you have not provided the data to know if the curve is relevant not mentioning the fact that most people would not consider a 5Ah battery the needed size for 30 A continuous discharge.

You never gave the reference for your previous statement?

 

[Deleted member 4366]

That discharge curve isn't correct, neither is your statement about 3.7v. A lipo cell is nearly empty at 3.7v. Lipos have about 80% of their charge between 4.1v and 3.6v. The normal charge voltage is 4.2v. Once they drain down to 3.6v, they start to accelerate downwards so you have to be careful once they reach that voltage.

All the lipos that I've had gave the nominal capacity when used with an ebike, i.e. two 5ah packs would give 10ah of real use, as measued with a wattmeter.

 

[trex]

if lipo is safe for e-bikes, why aren't e-bike suppliers falling over themselves flogging them? there is a place for lipo, that's toys, not transport.

 

[Deleted member 4366]

There's a risk to using lipos. It's a bit like going out to sea in a small boat. When you go to the coast, you can see thousands of people enjoying the sea in their small craft. It's quite safe if you've obtained the necessary knowledge. If not, it can be very dangerous. There's thousands of people using lipos in their model aircraft and helicopters without any incidents, but, inevitably, there are people that don't obtain the nesessary knowledge first. Presumably some of them suffer the consequences. If they were that bad, we'd be seeing warnings on the TV every night and sensationalist newspaper articles like when that free non-lipo pack thing went up.

In summary, lipos are not for regular dunderheads. Electric bike dealers can take that risk; however, they're perfectly safe for normal hobbyists that care and understand what they're doing.

 

[Lardo]

I think it is largely irrelevant as I'm going to get a full bike in all likelihood.

I've been to SYEBC and recommended to me were :
Batribike Granite Pro - http://www.syebc.co.uk/bikes/granite-pro
Batribike Granite LCD - http://www.syebc.co.uk/bikes/granite-lcd
Free Go Hawk - http://www.syebc.co.uk/bikes/freego-hawk

I've not been able to ride one yet as it has been tipping it down today.

I've also been looking at the Woosh Big Bear and the Kreiger.

Any other options?

 

[STEVEMANFA]

The oxygen 13AH MTB is a nice bike

 

[trex]

...
You never gave the reference for your previous statement?

I have never been for scaremongering, there are plenty of youtube videos for that. The point I am making is, lipo cells do not have adequate protection against thermal runaway. To compensate, you will have to cut short the useful voltage range that your lipo is capable of.

For e-bikes, the battery must last between 2 to 6 years. During this time, the failure rate will go up from around 3% for the first year to about 40% in year 6. Without protection, when your cell pack fails, it will fail spectacularly.

 

[Lardo]

It is, but is 50% above budget.

 

[STEVEMANFA]

If your budget is about £800 to £1000 I think I'd buy a kit and pick up a good secondhand bike and fit the kit on that.
That way you will have a nice bike

 

[Nealh]

Lardo already has a very nice donor bike why spend more then he needs to.
Kit it up with a BBS or a Oxy drive kit, if you want cheaper buy Cwah's 750w bbs that is getting cheaper every day.

 

[soundwave]

http://www.e45.de/

bit over budget tho

 

[Lardo]

I've just ordered a BBS02 500w CD kit from Woosh.
Just hunting for an appropriate battery now.

 

[Lardo]

AAARRRRGGGHHHHHH!!!!! It is just out of stock and they have refunded the payment :-(

 

[BAH48]

Try eclipsebikes.com

 

[Lardo]

Got one ordered, thanks. Now the wait...

 

[BAH48]

Seems to be some confusion and misinformation about Li-Ion batteries.
I find the following table helpful.

4.2 volts 100%

4.1 about 90%

4.0 about 80%

3.9 about 60%

3.8 about 40%

3.7 about 20%

3.6 empty for practical purposes

<3.5 = over-discharged

 

[Deleted member 4366]

That table doesn't look right either. I've done lts fully discharge tests on batterries. Non LiFePO4 ones are all similar, regardless of chemistry. 4.2v is fully charged. 4.1v is about 95%. They then go down linearly to about 3.5v for lipos and 3.3v for anything else. After that, you have somewhere around 5% to 10% left and the voltage dops rapidly. How far you can go at that point sepends on how much power you use because the sag increases too, so you'll trip the low voltage control if you take too much power.

Lifepo4 behaves diffferently. Nearly all the charge is between 3.3v and 3.0v, which would be between 39.6v and 36v. You charge them o 3.65v and can go down to 2.0v, but there is virtually no charge between 3.65v and 3.3v, and 3.0v and 2.0v.

Do not listen to those that say that you can make your battery last longer by charging it yo 80% and discharging to 20%. That only works on certain batteries, but not on normal ebike batteries with a normal BMS.