Wattage conundrum

[Emanresu]

.. or where did those watts go.

Purchased an additional battery to extend the riding range and ran into a problem. Wonder if the sparks wizards can explain my conundrum.

The bike uses a 500W /36v / 14ah system. I purchased an additional [compatible] battery 420W /36v / 11.6 ah. From the first battery (500W) I regularly get 40 miles on an 80% assist over the rolling countryside. The 420W battery only generated 12 miles when the expectation would be 33 miles (420/500 * 40). The "remaining range" indicated when the second battery was use at first said 25 though but only got 12.

Now my simple logic - as shown above - is that the watts are stored power and the amp hours only the rate the watts are delivered.

Am I missing something?

 

[vfr400]

Amp-hours are the capacity of the battery when discharged at something like 0.2C (0.2 x no. of AH), so an 11AH one should have 78% of the capacity of a 14A one, or if the 14AH goes 40 miles, the 11Ah goes 31miles; however, it's not as simple as that. Cell capacity goes down as the current goes up, and some cells have difficulty dealing with high currents, which makes their capacity go down even further.

Another important factor is how many cells are in the battery. A typical Hailong type downtube battery has 50 or 60 cells in it. Some only have 40 cells (all assuming 36v). Now say you have a 20A controller. The 60 cells (10S6P) would be giving 20/6 = 3.3A each, while as the 40 cells (10S4P) would be giving 5 amps each (50% more), which is a lot. Cheap cells are happy at 1A, but can't keep up with 5A, so the voltage drops and the battery cuts off.

Generally, cheap batteries have cheap cells in them that are unable to deal with high currents. You should never buy a battery without knowing which cells are in it unless you ride with low current.

If you fit a wattmeter to your bike, you could see how many AH you get from the battery when you use low power and how many when using high power. The difference can be a lot.

If you want specific advice/info, you need to tell us specifically what you have, otherwise we can only give general info, which might not apply to your conundrum.

 

[Emanresu]

Batteries are from the same manufacturer and AFAIK contain the same cells.

So the calculation from what I understand you've highlighted (11.6ah/14ah)*(420w/500w)* 40miles range = 27 miles. Not far short of the 25 "remaining miles" flagged up when the second battery was first used. The drop to 12 miles may be partly accounted for by the terrain/climbs but it is a big drop.

Will need another few runs to see if I can get it close to the possible 27 miles

 

[Nealh]

We ned to know more about the new battery, a link to the product preferable.
Critically as vfr has said it depends on the cells used in the battery and your controller max current rating.

With top quality cells one can get abut 25/30miles in low power. My lightweight small 24 cell 44v battery (<264wh) only uses 2 cells in parallel and 12 in series, importantly it is the 20a rating of one of the very best 18650 cells that one can buy that makes this achievable.

 

[Nealh]

A 11.6ah & 14ah battery can't use the same size mah rated cells, the 14ah would have to be 14.5ah then both would use 2900mah cells.
4 cells in parallel for 11.6ah is 2900mah cells, the 14ah battery will either use 2800mah or 3500mah cells depending on how many in parallel. The cells may be the same manufacturer but the cell mah rating and designation will mean current discharge characteristics are different as is their real time usage.

Unless one knows exactly the cell type used i.e. Samsung 29e/35e or Lg MG1, MH1 M29/M36 or some other designation and brand then one can't assume battery B will be xx% of battery A.
If one knows the average discharge current used by the bikes controller, one can then look at the known discharge graphs for the cells in use to make a good comparison and also make the comparison on how many cells in parallel are used.

4 or 5 cells in parallel can make a huge difference as the former may be 20% minimum less capable of the latter if the same cells are used.

 

[vfr400]

So the calculation from what I understand you've highlighted (11.6ah/14ah)*(420w/500w)*

Don't confuse watts with watt-hours. An 11.6Ah 36v battery holds a charge of 36 x 11.6 = 417.6 watt-hours.

If your controllerwas delivering a continuous 10 amps to the motor, that would be 10 x 36 = 360 wattts and it would last for 417.3/360 = 1.16 hours.

 

[Emanresu]

Thanks for the advice and the insight into batteries. I can't tell you which cells are being used as there is nothing in their literature. Nor anything about the controller. They are - literally - black boxes.

But when I come round to replacing the cells, I'll take advice.

 

[sjpt]

Thanks for the advice and the insight into batteries. I can't tell you which cells are being used as there is nothing in their literature. Nor anything about the controller. They are - literally - black boxes.

But when I come round to replacing the cells, I'll take advice.

There might be indication of which cells are being used on a label the battery case. If there, it may or may not be true.

 

[Emanresu]

There might be indication of which cells are being used on a label the battery case. If there, it may or may not be true.

Page 11. I have the 417W and the 504W. Interesting to see there is a 630W which is not yet available.

https://mediap.industry.panasonic.eu/assets/custom-upload/Devices/E-Bike%20systems/PCT_catalogue_EN_0720.pdf