Battery life

[John Roach]

Hi having just built my first bike by fitting a Bafang 500w mid drive kit to a 1994 Moulton APB which turned out to be somewhat less than straight forward. The kit uses a 36v motor and a 36v battery of 14.5 amps.
Battery life is a little disappointing. After about 30 miles the battery is still showing around 30% but even the slightest gradient will force the motor to stall. Is this normal ?
I also note that some info on the web sees higher voltage batteries fitted, not being electrically minded it seem a bad idea to fit a 48v battery to a 36v bike.
Ant moments please
John

 

[Fordulike]

Battery life is a little disappointing. After about 30 miles the battery is still showing around 30% but even the slightest gradient will force the motor to stall. Is this normal ?

When you say stall, do you mean you feel a pulsing through the pedals/motor, which is the Bafang hitting its LVC (Low Voltage Cut-off)?

 

[wheeliepete]

Sounds like you are suffering voltage sag in the battery when high current is required to climb a gradient, and as said, the motor is cutting out. Your controller will be demanding 25 amps on full power, and depending on the quality of the cells in your battery, this may be too much for it at a lower voltage. Not all vendors take the care to match the battery with the motor they supply. Did you buy them together and from where? Some motors can be run at a higher voltage, but your Bafang is not one of them.

 

[John Roach]

When you say stall, do you mean you feel a pulsing through the pedals/motor, which is the Bafang hitting its LVC (Low Voltage Cut-off)?

I have not noticed that it just provides no power

 

[John Roach]

Sounds like you are suffering voltage sag in the battery when high current is required to climb a gradient, and as said, the motor is cutting out. Your controller will be demanding 25 amps on full power, and depending on the quality of the cells in your battery, this may be too much for it at a lower voltage. Not all vendors take the care to match the battery with the motor they supply. Did you buy them together and from where? Some motors can be run at a higher voltage, but your Bafang is not one of them.

Thanks I bought it all from Eclipse, the battery is a Dolphin 36v 14.5 using samsung cells. It is the one I chose so can't blame them it just seemed the obvious choice.

 

[anotherkiwi]

Your battery has 2900 mAh cells which means if you bought a Samsung celled battery they will be 29E cells. These are rated for 8.25 Amps discharge but that is their peak discharge and they deliver much less during continuous discharge.

In conclusion your are experiencing battery sag, that motor really requires Samsung 30Q cells to give it's best.

 

[Fordulike]

I have not noticed that it just provides no power

Sounds like it could be down to what wheeliepete has highlighted in post #3. A solution would be to buy a programming cable, as mentioned in this thread: http://pedelecs.co.uk/forum/threads/after-market-bbsxx-crank-drive-gear.32691/#post-462858

Then program the controller to limit the amps to a lower value. This way you should be able to eek some more miles out of the battery, whilst also extending the life of the battery. You will notice a drop in performance of the bike, but the whole distance/performance/battery life thing is always a compromise to find the right balance.

 

[anotherkiwi]

Or you could just stop using the higher assistance levels. Faced with a similar problem I down-tuned my controller to 11 Amps max. I could still climb hills, just slower.

With the GSM when I get down on battery volts I use lower gears.

 

[John Roach]

Your battery has 2900 mAh cells which means if you bought a Samsung celled battery they will be 29E cells. These are rated for 8.25 Amps discharge but that is their peak discharge and they deliver much less during continuous discharge.

In conclusion your are experiencing battery sag, that motor really requires Samsung 30Q cells to give it's best.

Thanks I think I get that There is that option, here is the detail "commonly referred to as the 'Dolphin' battery pack, this downtube mounted battery consists of 50 x Samsung INR18650 30Q (3.7V 3Ah) connected 10 series 5 parallel.
I chose the 14.5 amp battery as the cost hike did not seem to be good value, looks like my mistake, would this upgrade be enough or should I look for 25amps please excuse my ignorance on this

 

[anotherkiwi]

Thanks I think I get that There is that option, here is the detail "commonly referred to as the 'Dolphin' battery pack, this downtube mounted battery consists of 50 x Samsung INR18650 30Q (3.7V 3Ah) connected 10 series 5 parallel.
I chose the 14.5 amp battery as the cost hike did not seem to be good value, looks like my mistake, would this upgrade be enough or should I look for 25amps please excuse my ignorance on this

30Q is a 15 Amp discharge cell or about twice what you have now. 5 x 15 A = 75 A peak discharge so at 25 Amp controller pull you will be using 1/3 of that capacity or .33C. It should be a night and day difference.

Cost wise there is a 1 € difference in price between the two cells so 50 € when bought in bulk. Just checked they have a £55 difference in price which seems honest to me.

 

[Fordulike]

30Q is a 15 Amp discharge cell or about twice what you have now. 5 x 15 A = 75 A peak discharge so at 25 Amp controller pull you will be using 1/3 of that capacity or .33C. It should be a night and day difference.

Agree, I have used both the Samsung 29E cells and currently the Samsung 30Q. IMHO, the Samsung 30Q cells are far superior to the 29E, when used in a high drain situation.

 

[wheeliepete]

I chose the 14.5 amp battery as the cost hike did not seem to be good value, looks like my mistake, would this upgrade be enough or should I look for 25amps please excuse my ignorance on this

This is a very easy mistake to make, so don't be too hard on yourself. Most of us have only gained the knowledge we have by doing exactly the same thing. The 14.5 Ah quoted on the battery is the capacity of the pack, how much power it contains, not it's ability to release it, which is more important when you are running a higher powered motor. AS AK said, the 30Q cells will release almost twice the power as the 29E's, but have only a very small capacity difference.

 

[John Roach]

Thanks for that.
Looks like its spend time. At least it fits the same holder which took a lot of fitting to a Moulton. I am not looking for speed I just thought a bigger motor may get my 18 stones up the hills better. I didn't fit the throttle to my bike just using the assist mode usually on level 2 or 3 getting quite addicted
John

 

[John Roach]

Or you could just stop using the higher assistance levels. Faced with a similar problem I down-tuned my controller to 11 Amps max. I could still climb hills, just slower.

With the GSM when I get down on battery volts I use lower gears.

The power loss is total on all assist settings. the lower ones working up to a point on the flat.

 

[awol]

The battery spec says
Continuous Discharge Current: 20-25A and the motor could be pulling 25a so is that not within the spec of the battery?
With a max discharge of 42.5A available.
What discharge C rating would you look for as a good guide for a suitable cell for a specific power need?

 

[anotherkiwi]

You want about twice the power of the controller or more so in 5P that means 10 Amp continuous discharge. Real tested discharge, not always the one on the spec sheet... I https://lygte-info.dk/review/batteries2012/Samsung INR18650-29E 2900mAh (Blue) UK.html

I quote:
Max. Discharge Current: 2,750mAh (continuous discharge), 8250mAh (not for continuous discharge)
and:
This cell has a rather low maximum current, to get the maximum life of the battery, but it is possible to draw high current peaks.

High constant discharge, large capacity, low cost - pick two

 

[John Roach]

Hi All
thanks for all the input but I have made a mistake in there battery I am using
I do have the Q30 cells
This is the spec
Nominal Voltage: 36V
Norimal Capacity: 15Ah
Cells: Samsung INR18650 30Q
Configuration: 10S5P
Pack dimensions (mm): 360 L x 82 W x 130/110 H
Weight: 3.46KG
Protection System: Built-in BMS
Continuous Discharge Current: 25-30A
Maximum Charge Voltage: 42V
Discharge Cut-off Voltage: 28V
Standard Charge Current: 2A (3A maximum)
Charge Method: CC/CV
Charge Temperature: 0°C--45°C
Discharge Temperature: -10°C--60°C
Recommended Motor Application: 36V 250W - 36V 750W Motor
Estimated Battery Life: Life cycle of 700+ charges (based on recommended discharge rates)

this appears to be the most powerful 36v I can get. so at least I am glad I made the last minute change of mind. Problem still exists so it looks like I need to look at programming which is a bit scary
John

 

[John Roach]

Or maybe the battery indicator is less than accurate ? and 30 miles is about it ?

 

[anotherkiwi]

OK! So you have a BBS02 36v 500W version (25 Amp controller) and a 15 Ah 30Q celled battery, a marriage made in heaven!

Option one:

De-tune the motor controller to 18 Amps - your motor becomes the 350W version. Hot off the charger that is still 750 controller Watts and 648 W at nominal voltage or about 500 W at the rear wheel.

Option two:

Find someone who has tuned a BBS02 with success for longer range and gentler battery use and copy their settings.

Option one and two can be combined of course.

You are using about 18 Wh/mile, that is the high end of battery use, I average 5.8 Wh/km = 9.33 Wh/m or about half. I am using a 250W 36v GSM mid drive with a 15 Amp controller, I weigh 80 kg (working on that...) and my bike is a 28" wheel trapeze frame weighing about 23-25 kg.

Or maybe the battery indicator is less than accurate ? and 30 miles is about it ?

You need a Watt meter between the battery and the controller, it will give you all the information you need about your battery usage. That is how I know I average 5.8 Wh/km - return trip to my GF's house is exactly 9 km and the watt meter reading is 53 Wh used. You also get peak Amps and peak Watts as well as battery voltage (real) and mAh taken from the battery.

 

[awol]

Maybe your using too high gears too often?
These mid motors like to spin fast, at their efficient rpm which also uses less battery.