What's the max capabilities of my battery with 25a cont. and 50a peak burst?

Hey, I finished my first eBike project and have done a fair few miles on it so far. Im hooked! The commute to work is much nicer than a car.

My bike is a road bike with a Q128c rear hub, 20a sine controller and 48v 18ah battery. However, I recently bent one of the connectors on the motor and have issues with it cutting out frequently. I've also learnt why most people don't do road bike conversions (no suspension, punctures etc). Im taking the oppertunity to do another project but buying a hybrid bike.

I spent alot on my battery, but its from a respected UK builder. The aim is to keep the battery and just buy a new motor and controller.

I do however want to up the speed and power from the 500w of the q128c but a bit confused as to what my battery can safely handle. The builder said it has a Smartec 25a continuous BMS that can handle 50a peak bursts.

I won't be running the bike at full power that often as just commuting, but would like to have a bit more fun so looking to the 30mph mark.

My maths says 25 * 48 gives me continuous 1200w but the peak would be 2400w at 50a.

How do I handle this from a controller point of view to not cause any issues with the battery? And also what sort of watt rating motor could I be looking at? 1500w?

Thanks!

The builder of the battery should be able to tell you how much max amp you can take from the battery.
If not, what cells is the battery built of, and how many in paralell?

elmannen said:

The builder of the battery should be able to tell you how much max amp you can take from the battery.
If not, what cells is the battery built of, and how many in paralell?

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Looking back through my email chain with them I can see he mentions Panasonic GA cells and 17.5aH capacity and as mentioned, a Smartec 25a continuous BMS with 50a peak short bursts.

The cells are good to provide 25A, so if the BMS can also do 25A continuously, that's a safe limit on how much power you can draw from your battery. It's not exactly gentle on it, but I have had good results from my 13.5Ah battery composed of Panasonic GA cells while running it at 25A. In your case, the C rate of the discharge would be substantially lower.

Note that the cutting-out issue with your current motor might be the BMS intervening to protect your battery. In that case, you've probably already reached the limit of what your battery will do.

Chalo said:

The cells are good to provide 25A, so if the BMS can also do 25A continuously, that's a safe limit on how much power you can draw from your battery. It's not exactly gentle on it, but I have had good results from my 13.5Ah battery composed of Panasonic GA cells while running it at 25A. In your case, the C rate of the discharge would be substantially lower.

Note that the cutting-out issue with your current motor might be the BMS intervening to protect your battery. In that case, you've probably already reached the limit of what your battery will do.

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Interesting, so I should look for a 25a controller at the most really then? The cutting out is defo something I have done, only started doing it when I bent some of the connections and didn't do it for 100 miles before that. Now it does it every 30 seconds!

The current controller is also only 20a. I'll be looking at a 48v 1000w motor then really tops.

I wonder if you have abnormal resistance in your damaged connectors. That might cause a fault condition in the controller or BMS, maybe?

You don't want a controller that demands more from the battery than its BMS can supply continuously. But your battery is big enough that you could transplant a more powerful BMS, like 30-35A. (Only if the cell interconnects and battery plug can tolerate the added current.)

If you buy a Kunteng (KT) or other programmanbe controller, you can reprogram the controller to limit how much power it will draw and thus protect your battery. Grin's Cycle Analyst will performa a similar function without reprogramming the controller.

So, you could choose to err on the side of a buying a more powerful controller and dial it back if it is too hard on the battery - if you shop with having that option in mind. Of course, more powerful controllers tend to be bigger and heavier. So you should consider that as well.

Chalo said:

I wonder if you have abnormal resistance in your damaged connectors. That might cause a fault condition in the controller or BMS, maybe?

You don't want a controller that demands more from the battery than its BMS can supply continuously. But your battery is big enough that you could transplant a more powerful BMS, like 30-35A. (Only if the cell interconnects and battery plug can tolerate the added current.)

Click to expand...

Thanks for this and the other replies.

Just spoke to the battery builder and he said using a 30a controller be fine and again, it can handle 50a bursts. He said los of people have been running the BBSHD from the same battery which pulls 30a.

Time to figoure out which 1000w motor and controller to go with!