Help! Bought a used ebike with battery/BMS issues..

[WheatyPop]

Recently I bought a used Velowave Ranger ebike, and I have spent quite a bit of time fixing it up. I finally had it up and running, but I discovered a pretty severe issue with the battery.

The battery works fine during pedal assist 1 and 2, but if I go any higher, the battery will shut off and not turn back on. Full throttle has the same effect. After I plug the battery into the wall, it will turn on again. This seems to be a problem with a weak or unbalanced battery pack - I just don't know how to fix it.

I'm currently trying a long top-balancing session to balance the cells in the battery. I'm not much of a tech guy so any help or assistance is appreciated. thank you

 

[spokewrench]

That happened to me because a bike had sat too many weeks or months. I plugged a watt meter into the wall, a timer into the watt meter, and the charger into the timer. (Watt meters and timers can be under $10 these days.) The watt meter gave me an idea of how much charging was going on at the moment. The timer was to be sure it didn't continue to charge when I was away. I'd charge it in increments of an hour or two. It worked out well.

 

[Chazmo]

Just to add to what @spokewrench said, please be careful if you do this to try to revive and balance the cells. Do this in a place that’s away from combustible materials. If a battery has a weak cell, there can be several reasons for that, some of which may not be repairable by recharging.

If a lot of heat is being generated with your charger on you might want to be circumspect. Do you know anything about the history of the bike? Good luck!

 

[WheatyPop]

@spokewrench My bike has also been sitting for quite a few months, and this could be a similar issue. Do you think you could elaborate on what I need to do and look for when using the watt meter and timer? The battery is sitting at 100% currently also. Thanks for the help

 

[Gionnirocket]

If it's at 100% then it has no problems. How are you determining this?
I would read the voltage at the output terminals to better understand it's condition.
If it does have storage related issues you may be able to coax it back to reasonable health by slow charging. I doubt very much that your BMS has the ability to balance charge.
Also don't leave it at 100% as that is harmful to battery health as well. Store your battery at approximately 50% and charge it to the desired level before riding.

 

[spokewrench]

@spokewrench My bike has also been sitting for quite a few months, and this could be a similar issue. Do you think you could elaborate on what I need to do and look for when using the watt meter and timer? The battery is sitting at 100% currently also. Thanks for the help

I'd see what the watt meter reads with the timer on but the charger disconnected from the battery, probably under 2.0 watts. With the battery connected, that's what the watt meter should read if the BMS has really switched off; sometimes I've seen a green light on the charger, but the wattage showed that the battery was drawing a little.

Typically, a charger will draw more than 100 watts and taper to almost nothing. In this case, the taper was slow. I believe battery fires usually start during charging, and it takes time for a bad cell to reach a critical temperature. When the taper took too long, I didn't feel any hot spots, but I shut the timer off for two reasons: so I could leave the area without worrying, and to let things cool in case a cell was dangerous.

Then, when the charger would show a green light, I'd shut the timer off and turn it on later to check. The first time I checked, the green light didn't come on and the charger wanted something like 10 watts. I'd charge until the BMS switched off, switch off the timer, and come back later to check. Something that might be called balancing was happening when the charger was off. On each cycle, it asked for less power until it didn't ask for any. It probably would have balanced if I'd simply left the charger on, but I would have been uneasy.

I had no more trouble with that battery.

 

[harryS]

If it's badly unbalanced, as you probably know, just one cell group may be stuck a little above the 3 Volt minimum, and a heavy load like full throttle causes that group to sag below the minimum and shut the battery off.

Looking at the overall battery voltage probably won't show that unbalance, but it would be good to know what you are getting after the charger turns green, Got a meter and care to say what you measure?

I doubt that many inexpensive batteries put in balance BMS. It might actually be safer to retire the battery if some of the cells are badly unbalanced. That's a sign of deteriorating chemistry when the cells can't hold charge like their neighbors. If balancing forces them above what they can sustain, in a worst case scenario, they might overheat.

 

[spokewrench]

I doubt that many inexpensive batteries put in balance BMS. It might actually be safer to retire the battery if some of the cells are badly unbalanced. That's a sign of deteriorating chemistry when the cells can't hold charge like their neighbors. If balancing forces them above what they can sustain, in a worst case scenario, they might overheat.

Probably most e-bike batteries lack active balancing, but if they didn't have passive balancing, their service life would be short. In passive balancing, each cell is protected by a shunt which at a certain voltage begins bypassing current. That way, if there are 13 cells in series, you can shut off a charged cell while others continue to charge.

The problem is that at the shunt voltage, the charged cell will continue to charge, albeit very slowly. Suppose of 13 cells in series, 12 are slowly overcharging while shunt current charges the undercharged one. If the 12 each reach .01 volt above fully charged and the low cell is still .12 low, the sum of their voltages will indicate that they're all fully charged, and the BMS will shut off charging. That's why some say passive balancing is inaccurate as well as slow.

Even a new battery can arrive unbalanced, due to cells with different self-discharge rates over a period of weeks or months. As of 2020, Radpower's instruction for a new battery was to leave it on the charger 12 hours, take a ride whether 20 miles or around the block, charge it another 12 hours, take it for another ride, and again leave it on the charger 12 hours.

A lead-acid car battery may be fully charged at 12.6 volts but come off the charger at 13 more. With car batteries, that extra voltage is called surface charge. In a car, you can drain the surface charge by running headlights a few seconds or letting the battery sit a few hours. With an e-bike battery, you need to drain off possible "surface charge" in some cells for the BMS to tell if other cells are undercharged. Radpower recommended taking a bike for a ride to drain surface charge, but I just let a battery sit an hour or two.

 

[Gionnirocket]

Probably most e-bike batteries lack active balancing, but if they didn't have passive balancing, their service life would be short. In passive balancing, each cell is protected by a shunt which at a certain voltage begins bypassing current. That way, if there are 13 cells in series, you can shut off a charged cell while others continue to charge.

The problem is that at the shunt voltage, the charged cell will continue to charge, albeit very slowly. Suppose of 13 cells in series, 12 are slowly overcharging while shunt current charges the undercharged one. If the 12 each reach .01 volt above fully charged and the low cell is still .12 low, the sum of their voltages will indicate that they're all fully charged, and the BMS will shut off charging. That's why some say passive balancing is inaccurate as well as slow.

Even a new battery can arrive unbalanced, due to cells with different self-discharge rates over a period of weeks or months. As of 2020, Radpower's instruction for a new battery was to leave it on the charger 12 hours, take a ride whether 20 miles or around the block, charge it another 12 hours, take it for another ride, and again leave it on the charger 12 hours.

A lead-acid car battery may be fully charged at 12.6 volts but come off the charger at 13 more. With car batteries, that extra voltage is called surface charge. In a car, you can drain the surface charge by running headlights a few seconds or letting the battery sit a few hours. With an e-bike battery, you need to drain off possible "surface charge" in some cells for the BMS to tell if other cells are undercharged. Radpower recommended taking a bike for a ride to drain surface charge, but I just let a battery sit an hour or two.

Can you provide a source for this information as it's contradictory to all I've read. Are you saying this information is provided by Radpower? If so, I'd be very interested in seeing it.
Most low cost batteries lack any balancing ability... But if quality matched cells are used, it's not an issue.
Passive balancing is done with a bleed resister that absorbs the excess energy and prevents full cells from charging further.
And if your new battery arrives unbalanced... It's garbage. A new well matched pack will stay that way until used or abused.

BU-803a: Cell Matching and Balancing

BU meta description needed...

batteryuniversity.com

Learn About Batteries

Battery University™ is a free educational website offering hands-on battery information.

batteryuniversity.com

 

[spokewrench]

Can you provide a source for this information as it's contradictory to all I've read. Are you saying this information is provided by Radpower? If so, I'd be very interested in seeing it.
Most low cost batteries lack any balancing ability... But if quality matched cells are used, it's not an issue.
Passive balancing is done with a bleed resister that absorbs the excess energy and prevents full cells from charging further.
And if your new battery arrives unbalanced... It's garbage. A new well matched pack will stay that way until used or abused.

BU-803a: Cell Matching and Balancing

BU meta description needed...

batteryuniversity.com

Learn About Batteries

Battery University™ is a free educational website offering hands-on battery information.

batteryuniversity.com

In 2020 Radpower said to leave a new battery on the charger 12 hours 3 times. I didn't like that advice, and they now say balancing isn't necessary. I don't think that's exactly true.

I like Battery University. They say cells should be matched for voltage and capacity, but for packs of more than 24 V, balancing is necessary, and most balancing is passive. It says the bleeder resistor in passive balancing starts to draw current at 70 to 80% of full-charge voltage. That's information I was looking for!

If 4.3 volts is full charge, a semiconductor in series with the bleeder resistor would begin to conduct around 3.2. Above that point, more and more current would bypass the cell through the bleeder resistor. That would help less-charged cells catch up without overheating the more-charged cell. That's balancing.

Myth or Fact: Lithium-ion Batteries Self-Discharge After Being Fully Charged - Lithium ion Battery Manufacturer and Supplier in China-DNK Power

Uncover the secrets of lithium-ion battery discharge: Why does it happen, how fast, and what practical tips ensure optimal performance?

www.dnkpower.com

This source says an 18650's self discharge is 0.5 to 3% per month. It says that below 5 C and above 45 C ( 41 and 113 F), self-discharge can be much faster. It says that during the time a cell sits between 80 and 100% charged, self-discharge can be up to 10%. What's more, Battery University says that during a cell's service life, self-discharge increases, and not all cells in a pack increase the same.

The bike in question was more than 2 years old. The voltage on the display suggested that it was about half charged, but it would cut out when I opened the throttle. If most cells were about half charged but some were nearly empty, that would be a difference in self-discharge of about 2% per month. That's how I learned what to do with passive balancing and infrequent charging. A timer and a watt meter are nice, but the important thing is to shut off the charger when the light turns green, then turn it on later to see if the BMS wants more charging. I may have had that bike another year, and the battery performed like new.

 

[harryS]

Self-discharge on healthy cells is largest at full charge, and drops significantly as the charge decreases. The UB3480 shipping regulations call for shipping lithium-ion at 30% SOC. about 3.43 volt per cell. At this level, they can sit a long time.

I've got bare Samsung 30Q cells, bought new in 2021, that I never used, and they are still close to the as-delivered 3.43 volts.

In a battery with a BMS, the latter circuit does drain a small amount of current, but I think in most cases that a battery at 1/2 charge with healthy cells will last a year before it goes so low that `the BMS has to shut down.

 

[spokewrench]

I discovered that the KT controllers on two bikes would draw a little current unless the battery was switched off. If I switched a battery off, I could find the same voltage after it sat a month or more. The battery on my Abound switches itself off when I shut off the controller.

I'd had my Radrunner more than 2 years when I discovered that an imbalance could be detected and corrected by trying the charger again after charging and letting it sit. After that, I liked to check all three batteries. I never again found an imbalance.

Why did it happen once but not again? When a lithium ion cell reaches 80% charge, you need to cut the charging current way back to prevent damage and fire. That's when balancing starts because the less charged batteries are still getting full current. I mostly pedal, so I may charge less than once a month. Less charging would mean less balancing. The Radrunner had a 14 Ah battery and a 2 amp charger. That would mean 500 ma per cell for less charged ones to catch up when more charged cells reached 80%.

The Radmission has a 2 amp charger for a 10 Ah battery, meaning cells could catch up at 667 ma, more efficiently bringing them into balance. The Abound has a 3-amp charger, meaning 750 ma per cell, bring less-charged cells into balance faster still. In some battery packs, the bleeder circuits turn on when a cell reaches 70%, giving less-charged cells more time to catch up. Passive balancing would work better with some batteries than with others.

 

[spokewrench]

i’ve analyzed the last 14 times I charged my Abound battery, rated at 47.1 volts and 678.2 watt hours. My display tells me what percent of the charge remains. If I subtract that from 1, I know what percent of the charge I’ve used. Multiplying that by 678.2 would tell me how many watt hours I needed to replace.

Three of those times, my battery was at 48 percent, meaning I needed 52 percent, or 353 watt hours. If losses in the charger, BMS, and cells amounted to 15%, I might need 406 watt hours from a wall outlet to charge the battery. Instead, my charger used a different amount each time: 465, 447, and 506 watt hours.

Varying inefficiency pointed to passive balancing. I had thought imbalance came primarily from differing self-discharge rates. The figures proved me wrong. With self-discharge, the imbalance would increase daily. I’d needed 500 extra watt-hours to charge 11 days after the last recharge, while I’d needed only 467 extra watt-hours to charge 92 days after the last recharge.

The charging efficiency didn’t correspond to any of the other 7 columns of figures on my spreadsheet. That’s when I remembered Wilhelm Peukert’s law from 1897. Due to a kind of chemical short circuiting, the capacity of a battery can decrease exponentially with an increasing load.

In 2020, the Journal of the Electrochemical Society published a study of the Panasonic CG 18650 E. Panasonic says it can produce 5 amps, but engineers found that drawing 3 amps had the effect of draining the battery at 5 amps. Drawing 4 amps had the effect of draining it at 23 amps.

On a stretch where I may ride 20 times a month, I can ascend a quarter-mile hill on the sidewalk or the highway. The sidewalk is rough, but the road is narrow and crooked enough that traffic might be unable to pass me safely. I can pedal up the sidewalk, but if I choose the road, I’ll keep the throttle wide open for a minute to go as fast as possible against gravity and the wind load. Maybe that draws 6 amps per cell, but the battery drain may be several times higher.

I’d wondered why sometimes I rode more than 400 miles on half a charge and other times less than 40. It came down to how often I chose to throttle up that hill. If the Peukert effect doesn’t affect cells equally, that would also explain the need for balancing, depending on how many times I’d throttled up that hill.

My Radrunner was a single-speed, so I was more likely to throttle up that hill instead of pedaling. I’d replaced the 16-amp controller with a 35-amp one, which would probably unbalance cells worse. Its 2-amp charger would balance cells more slowly than the 3-amp Abound charger. That would explain why at one point the Radrunner battery was seriously out of balance, but it hasn’t been a problem with the Abound (or the single-speed Radmission with its 25-amp controller).

 

[Gionnirocket]

Wow.... You're the king of assumptive science/knowledge.
I may go into my workshop and mash every spokewrench I own with a 5lb sledge