Kent e-bike - battery has a possible bad cell?

[fedaykinofdune]

So, I bought a cheap Kent e-bike(350w, 36v, 10.4Ah) from Walmart (yes, I know, it'll be my wife's once I build my own ). The bike is actually of fairly good quality, but the battery is questionable. The specs boast a 20-25 mile range, though with the way I ride 13-15 miles might be a more realistic estimate. My usual round trips are only 2-3 miles on average so it took a while to notice the that my battery may not be up to par. The indicator still had 3 of 4 bars but the light on the battery was showing red while under load the other day, and I couldn't have gone more than 5 miles. I pulled the battery and got out my meter, and noticed that one one of the + terminals was normal, but the other + was only around 6.5-7v (pretty much same results with battery powered on or off). Both + terminals read nearly 36 while plugged into charger (after charge cycle finished and with battery powered off), but the side that measured low immediately dropped to 6.5v when I unplugged the charger. Am I correct in assuming that both + terminals are supposed to deliver 36v? Not much info on my battery, just the following listing.

Reention Eel Mini Pro 36v 17ah 48v 13.6ah Hidden Ebike Battery For Electric Bike - Buy 36v Battery For Electric Bike,Electric Bike Battery 48v 14ah,48v Lithium Battery For Electric Bike Product on Alibaba.com

Reention Eel Mini Pro 36v 17ah 48v 13.6ah Hidden Ebike Battery For Electric Bike - Buy 36v Battery For Electric Bike,Electric Bike Battery 48v 14ah,48v Lithium Battery For Electric Bike Product on Alibaba.com

m.alibaba.com

If anyone would be kind enough to offer me a bit of guidance here, I'd appreciate it. I still have a few days to return the thing before the 30 days is up, and just reorder the same bike, but I'd rather fix it if it's not too much of a hassle.

 

[Benjahmin]

A fully charged 36v battery should be 41-42v. The link you've put up is for a 48v battery, this should be 54v fully charged.
So not sure what it is you've actually got there.
Getting a red light during power demand is a symptom of voltage sag. This normally happens when more current is being demanded that the battery is capable of delivering.
Have you checked the actual output of your charger?

 

[vfr400]

Your assumptions and conclusions are wrong, also the way you're measuring it is wrong. Tell us how many pins are there on the charger, then we can explain what's going on.

How heavy are you?

 

[fedaykinofdune]

About the link earlier.. My bad. It's definitely a 36v battery and bike, and the max I've seen during charging is 42v, and fully charged it sits right at 36v... The couple of times I really pushed the battery, by the time I got back, the light on the battery was red, but the meter still had 3 of 4 lights. I wonder if the meter is actually accurate.

And vfr400, I'm about 170-ish. And In the meantime since I posted I've found some more reading materials. The charger itself is a standard 2 pin barrel plug, which can plug directly into the battery or into the frame of the bike. The battery has 4 pins, if I sit it upside down with the terminal facing me, it looks like + + - -
Yeah, my assumptions were definitely wrong. Took a peek under the cover, to see how the terminals are wired, and the first + has a very thick wire, while the other + doesn't. So yeah, please pardon my ignorance, I'm good with pc's & pistons, but don't know much about these eBike batteries.

Anyway, I definitely would like to hear whatever you're willing to share, especially if it'll help dispel some of my pesky assumptions!

Tomorrow I've got several places to go, but I'm going to run on lv2 to give it a fair test.

 

[vfr400]

Some batteries have remote switching, which is what the lower voltage pin is probably for. Other ones, but not yours, have comms between the battery and charger. The only way to figure out exactly what your pins do is to is to folloe the wires from the receiver pins. Batteries with comms to the charger have at least 4 pins on the charger connector. 2 pins means it's dumb.

The charger is 42v, which is what you should see in your battery when it's hot off the charger. It's typical for that to go down to about 41.7v when the bleed resistors in the management system drain down the individual cells a bit to balance them. Anything less than 41.5v hot off the charger indicates a problem of some sort.

The charge level indicator lights on the battery and on the bike are notoriously innacurate and unreliable.

When riding the bike, it's normal to lose a segement (or LED) or two when you take full power from the battery. That's called "voltage sag", which is due to the battery's internal resistance. An old tired battery has high internal resistance, so it sags a lot. When it starts prematurely cutting out when you take full power, it's time to replace the battery. An under-powered battery also sags a lot because it simply can't keep up with the demand for current.

Different types of cells within the case have different performance characteristics. Cheap ones have low power ratings.

If ever you need a new battery, your case is pretty standard, but the wiring to the pins isn't. It's normal to only have the outer two connected, or sometimes they double up each pair. Both those types would be interchangeable, but yours is obviously wired differently. It should be possible to adapt a standard one, but I'd need a lot more info about the wiring to tell you how.