Voltage and speed drop-off AND hill climbing drop-off

[KirstinS]

I have the 250w 36v 8fun kit comes with a 9ah battery

Two things I've noticed are

1) I'm getting quite a bit more mileage than I expected or I actually need for that matter (25 to 30 miles).

2) Speed on flat unassisted is 14mph (GPS) BUT drops quickly as does climbing ability.

I think I should expect the speed to drop as voltage drops on the battery as it drains……. but I'm doing 11pmh by time battery is 1/2 drained (according to indicator on battery itself). Does this sound right to you ?

More oddly I am losing hill climbing ability as the battery drains - its very noticeable after just one segment of battery light goes out (1 of 6). I thought torque/hill climbing ability was to do with amps ? So I'm confused why a small amount of battery drain/voltage drop seems to require more legwork on the same hill - the amps are still there to be taken from the battery, right ?

Thoughts much appreciated - I'm sure view is too simplistic (voltage =speed, amps=torque)

 

[Deleted member 4366]

That's all normal. Those 9aH bottle batteries are not particularly strong. If you can afford it and have room(probably longer), there's a Panasonic-celled one of 11.5aH that can give a lot more current, so speed will be maintained and hill-climbing will be better.
E-bike 36V11.6AH Panasonic bottle battery-EcoPedelec.com

 

[mountainsport]

Good morning KirstinS

In my own opinion this seems to be normal and nothing major to worry about,and also not forgetting that your battery pack amperage is only set at 9.

Also the LED lights on the battery will not give you a true 100% accurate reading,not implying that you should rule this out all together it's just to give you a guide line to what is basically happening.
But then again this could be a safety measure to warn the rider letting them know that there is a certain amount of juice left in the battery for instance 75% but truely 85% is remaining,so that 10% of juice can get you safely around the corner to your door step.

Mountainsport

 

[KirstinS]

Fair enough - thanks

Still don't understand the electriconics/physics behind this

Or why a 11ah battery will help. Thought this just gives more range which is not really the issue

 

[KirstinS]

I emailed for a price on that battey d8veh, thanks

Let's see - guess this is the same one cyclezee will be using

 

[themutiny]

I'm sure someone more knowledgeable can answer, but I thought it was to do with the C rating of the Cells?

 

[Deleted member 4366]

Fair enough - thanks

Still don't understand the electriconics/physics behind this

Or why a 11ah battery will help. Thought this just gives more range which is not really the issue

Different batteries have different chemistries that affect how much current they can give and for how long. This is given either as a C-rating (1C =one times the aH of the battery) or a maximum discharge rate. Whenever you buy a battery, you should check this because it's important.

Your battery is rated at 1C, so can give 9 amps continuous. It can give more, but then the voltage starts to fall off while it's doing it. You'll see this when you go up a hill and your green and amber lights go out leaving only red, then when you get back on the level, the green one comes back on again. Your controller is probably set to 12 amps, which is generally quite low, but more than what your battery can give for a prolonged period. The other battery can give 20 amps continuously and 40 amps for short periods, which is more than twice your one, so drawing 12 amps up a hill won't bother it at all. The end result is that with your battery, you'll have about 30 -33v going up a hill, while the other one will stay at about 38v, so about 20% difference in power. Actually it's even more difference than 20% because as NRG points out below, you'll be going slower, which pushes your motor into the zone where it runs inefficiently, so half of your battery power will be completely wasted making heat instead of motion.

With the Panasonic battery, you could even increase the current from your controller up to say 16 amps, which would give much better hill-climbing plus the ability to maintain a higher speed.

 

[NRG]

The voltage is sagging under load as the battery as d8veh pointed out is quite weak hence the speed is dropping and the motor producing less power.

With the throttle wide open WOT the motor is in full commutation mode IE: the voltage at the motor is equal to (more or less) the battery voltage, if the battery is weak it drops and the motor produces less power, also as it slows it becomes less efficient so it turns more of the available power into heat rather than forward motion. You need a stronger battery!

 

[KirstinS]

Ahh thanks, the penny drops !

Well it was a cheap kit so what can you expect

Sounds like a shunt mod would be really bad idea ! Btw controller is 14amp ( or so says the label)

 

[jackhandy]

I also have the8fun kit (bought in july) fitted to my bike.

I have the same controller as kirstinS & a bottle battery which is advertised as having panasonic cells: The battery is labelled as 15 amp continuous; 18 amp pulse (10 minutes).

Is this a more robust battery than kirstin's? & if so, is it capable of providing more power than the controller is allowing?

I've opened-up the controller & can't see a shunt - photo attached - can this controller's output be increased?

 

[Deleted member 4366]

I also have the8fun kit (bought in july) fitted to my bike.

I have the same controller as kirstinS & a bottle battery which is advertised as having panasonic cells: The battery is labelled as 15 amp continuous; 18 amp pulse (10 minutes).

Is this a more robust battery than kirstin's? & if so, is it capable of providing more power than the controller is allowing?

I've opened-up the controller & can't see a shunt - photo attached - can this controller's output be increased?View attachment 4196

Possibly the batteries have changed. your one should be OK for a bit more power.

Is that the shunt hiding under the thick red wire? You normally find it next to where the thick black wire from the battery is fixed.

 

[NRG]

Yes, looks like it to me...

 

[KirstinS]

Now I'm confused as mine is the same battery(or is labelled and advertised as such anyway)

 

[Deleted member 4366]

Do a Google search for "CGR18650CG" and then click on the pdf, third one down, which is for Panasonic CGR18650CG cells, which is probably what you've got.

There's a graph of discharge characteristics. They show a line for .2It (1amp), 1It (9amps) and 2It (18amps). The voltage scale is for one cell. You can see how the voltage goes down as the battery discharges. The lower the volts, the lower the power you get. You can also see that there's about 0.4v difference between 1 amp and 18 amps because the chemistry can't work fast enough when you draw a lot of amps so the voltage drops a bit (sag). As you have 10 cells in series, there will be about 4V difference under high power, so you've lost 10% of your volts when climbing steep hills. If your battery was already half way discharged, it would be at about 3.75 v per cell or 37.5v overall, which is about 4v down from the starting point of about 41.5v, so now you lose 20% of your volts when you go up a steep hill. All this is for a perfectly balanced battery in good condition. The LEDs on your display are directly related to voltage. Top speed is also directly related to voltage, so as your LEDs go out, your speed will go down. The graph also shows that the voltage drops off rapidly as your battery approaches empty, so so will your speed and power going down as well.

So, the sag shows on your LEDs. A strong battery sags less. If your battery already sags a lot, it's not such a good idea to increase current by soldering the shunt. To make a good judgement, you really need to know exactly which cells you have and the limit on your BMS. A battery that says 18amps max has proobablly got the BMS set to 18amps. If you go higher than that, the BMS will cut off the power unless you solder the shunt in the BMS too, but then you have to check that the fuse is set high enough.

 

[KirstinS]

As ever d8veh brilliant info, thanks very much