Power drop as eBike battery runs down

Zedredcycler

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Nov 6, 2017
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I really don't understand how voltage limiting, current handling, and current limiting work within an electric bike controller - I'm hoping someone on here can enlighten me!

A little while ago I bought a standard 1000w 48v electric bike conversion kit on eBay, and a separate 10.4ah 48v battery. When I'd connected it all up and started riding it I noticed that the full range of my setup was about 13 miles, but that after riding it for about 10 miles the power started to significantly drop. For the last mile, you could only get a slow crawl out of the bike, with power dropping to about 5% of full charge power.

I figured this was because of the voltage drop of the battery, which over the course of a discharge changed from around 51v to 42v, and so I messed around with the configuration settings on the LCD display on my bike and changed the expected voltage to 36v. This completely fixed the problem bringing the power level back up to about 80%. In fact on the 36v setting, the bike seemed to be locked at 80% max power through full to almost all of the discharge of the battery, and the battery would die quite suddenly when it was completely empty (I know this isn't good for the life of your battery, I don't do it often).

My questions are about the mechanism behind this, and whether they are automated systems that achieve the same effect. I want to buy a custom controller from China, and I don't want the power to drop significantly as the battery discharges.

Is there a special component I can ask them for to emulate the effect above?

Do I need to get a controller with both 48 volt and 36 volt settings?

Is running a partially discharged 48 volt battery (outputting 46v to 42v) through a controller expecting 36 volts bad for the battery or controller?

Many thanks in advance x
 
You've bypassed the controller's safety cut-off, so now you're relying on the BMS cut-off, which is much lower. That's really dodgy because you're taking the cells to their minimum voltage, which could be as low as 2.5v each. That will quickly send your battery out of ballance and can even lock the BMS from allowing charging.

Normally, you'd have 75% power at low voltage cut-off compared with a fully charged battery. That's because the controller controls amps rather than power. I've experienced the same power ramp-down as you on some OEM bikes, but I didn't get a chance to figure out hw or why they did it. I've never had that on any of my own bikes or ones that I've done/fixed for other people. Which controller are you using? Maybe there's a better choice.
 
FWIW, the main reason controllers that decrease power as battery drops in capacity/voltage do this is to be easier on the battery.

The harder you push the battery when it is already at a low state of charge, the more it heats up inside and the more it unbalances it, and the harder it is on the cells.

If you have a good battery that's large enough to handle the power draw even at a low SoC, this controller behavior isn't needed.

But for very small or low-quality batteries that can't easily handle the power draw at low SoC, it might prevent problems with the battery, or at least delay them.


For now, unless you're sure of the quality/capabilities of teh battey you have, I'd recommend putting your controller's settings back where they were, for battery longevity, and simply getting a second battery to parallel with the first if you need more range and/or better performance. Or replacing the orignal battery with a larger better performing one.
 
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