Rad City 4 Battery Issue (Maybe BMS?)

[stithjim]

So my wife and I rode the Going-to-the-Sun Road in Glacier National Part this June on our Rad City 4 ebikes. Most of the way up, my wife's bike just up and died. Thinking that she just drained her battery, I gave her my battery and slogged through the last couple miles on my, now pointlessly heavy and dead, ebike. When we got up to the top, I had the great idea of using the regenerative braking to try to recharge my wife's battery. It seemed to come alive on the ride down, but died again when I got to the flats and wanted a little pedal assist.
The next day, after a 12 hour charge, the battery seemed back to it's old self. But as soon as my wife used a higher pedal assist or throttle, the bike died. We got it back home and the battery pack, even when freshly charged, will only cause the display to blink on and then go right back off.
I have contacted Rad Power and, after two months, they are sending me a replacement battery. Since I still have the old battery, I wanted to know if I could repair it.

Here is my situation:
1. When I turn on the key, and push the battery level check button, the lights all come on for about a second and then turn off. If I press the button again, I only get a faint flicker before they go out again.
2. When I attach a multimeter to the positive and negative terminals, the voltage reads about 12 volts. When I turn the key, the voltage momentarily (approx 1 second) goes to 53 volts, and then drops to 23 or so volts.
3. When I open the case, everything about the BMS looks good.
4. When checking the voltage of each battery cell, they all read 4.2. All together, they read 54.5 volts.
5. The BMS is an RP1304AA model

So what is my next step? Since the battery pack is good, I would hate to see it go to waste. Is there something I am missing?

Thank you all!

[DickB]

The BMS uses silicon switches called MOSFETs to switch current to the bike. When off, the MOSFETs will allow a very small amount of current to flow. What you are seeing when you use your multimeter is that very small amount of current. It is so small that the meter itself provides enough load to drop the voltage to 23V as you have observed.

For whatever reason, the BMS is not turning on the MOSFETs as it should.

I repaired a battery with similar symptoms that had a failed connection (separated tab) from the battery cells to the BMS. This made the BMS sense a false undervoltage condition on a group of cells, and not turn the MOSFETs on. I'm not saying that a loose connection is your problem, but rather just providing evidence of what can happen with a BMS failure.

It is not easy to diagnose and repair a faulty BMS if that is the problem. Replacing the BMS is easier and probably less expensive.

[garvie]

I also have a Rad City battery that dies suddenly and completely when I am riding. Yet it does not take as long to fully recharge as it did when it was truly empty (so maybe it is not really dying??). Also it always seems to die after about the same amount of energy has been used... about 2 bars' worth, then POOF my pedal assist is kaput.

I have tried rebalancing the battery, replaced the fuses, checked my connections and talked to rad a few times. If it is the BMS, is replacing it the kind of job I could do by myself?

any other advice on what the problem might be is welcome. bike looks, acts and rides as before until very suddenly and with no warning, all power is switched off and the battery bars drop to empty.

[stithjim]

DickB,
Thank you for your insight. My issue is not a loose tab as you have shown. I don't think the internal resistance of the multimeter is causing the errant readings from 53 to 23 volts. When I do the same operation on my good battery, there is no change.

I am actually wondering what kind of BMS I could use as a replacement. Does the BMS discharge circuit ensure equal discharge of all cells? If not, could I connect my leads to the main positive and main negative on the top of the board and run my bike that way?

Garvie,
How old is your bike? If it is under warranty, I would not crack open that battery and contact RadPower instead. Once you get a replacement, then fiddle around with it

[DickB]

I don't think the internal resistance of the multimeter is causing the errant readings from 53 to 23 volts. When I do the same operation on my good battery, there is no change.

Try measuring your good battery with the key off (and the MOSFETs off).

I am actually wondering what kind of BMS I could use as a replacement. Does the BMS discharge circuit ensure equal discharge of all cells? If not, could I connect my leads to the main positive and main negative on the top of the board and run my bike that way?

The Rad BMS uses passive balancing - balancing is done during the charge cycle only. Here is a good article describing the methods: https://onlinelibrary.wiley.com/doi/10.1002/est2.203

Connecting the leads as you described and bypassing the BMS is not a good idea.

If I was convinced that the BMS were bad in my battery, I would probably replace the BMS with one like this: https://amzn.to/3mlQAkq

[stithjim]

DickB,

Okay, I won't bypass the BMS, guess they put that there for a reason.

I used my multimeter to check the voltage present on the positive/negative contacts on the bottom of the battery (where it interfaces with the bike) on my wife's bad battery, my battery, and then my wife's new battery:

Wife bad battery:
Key off-7.4 volts
Key on-jumps to 53.4 for a second and then drops down to 22.6 volts

My battery:
Key off-7.5 volts
Key on-54.4 volts

Wife's new battery:
Key off- less than .1 volts (this battery has different labels than our original batteries
Key on- 51.4 volts (have not charged it, just pulled it out of the shipping box last night)

I did look at the MOSFETs on the BMS. I checked the voltage on each of the MOSFETs and the front three MOSFETs had a 3.3 voltage between the two leads of each MOSFET. The back three had no voltage across the leads of any of the MOSFETS. I wish there was an obvious component that has failed and I can point to it and fix it.

Do you know if there is a schematic for this BMS?

Thank you for the new BMS. I know it is an eBike BMS, but is there a lot of loss using the thin gauge wires vs the nickel strips? I would like to swap it with the same BMS if possible.

Thanks again, I am going to read about balancing now from your Wiley article!

[garvie]

How old is your bike? If it is under warranty, I would not crack open that battery and contact RadPower instead. Once you get a replacement, then fiddle around with it


my bike is really new, less than 2 months old when the problems started. It seemed so random at first, that not sure I have been able to convince Rad that the problem is the battery. Anyway, was just asking if hypothetically it is something I might repair, since so far, it is not clear whether Rad intends to replace the battery.  my fingers are crossed.

[Ticruiser]

My wife's Rad City 3 worked for about a month and then had the same issue.  It was very easy to replicate as with hers it happened within the first few miles.  From a full charge if you ran throttle max it would go about three miles and then shut down with a low battery alert and then nothing.  If I  took the battery out of the the tray and put it back in,  it would reset and allow the bike to work at low power settings for a while but not past 4 bars indicated, at which point you get nothing.

Took almost 2 months to finally convince Rad yesterday it was the battery.  They are replacing it under warranty which will take a while.

[stithjim]

Ticruiser: I have been waiting over a month and a half for my replacement battery, so I know it can take quite a long time to get those replacements.

Garvie: Got my fingers crossed for ya, bud!

[DickB]

Sorry for the late reply; I have been travelling.

I don't have a complete schematic. My Rad battery uses a Sino Wealth SH367008 Li-Ion Battery Management chip. Sino Wealth is a Chinese integrated circuit manufacturer. The spec sheet for the chip is in Chinese, but Google Translate does a good job. The spec sheet does have a sample schematic, but the Rad battery does have additional components in the charge circuit and other differences from that schematic.

The thin gauge wires in the non-Rad BMS are used only to measure the voltage of individual cells and to perform passive balancing. You do need heavier connections between cells and to the power and charge ports. The Rad BMS is proprietary and Rad does not sell any battery components separately.

[deadbird8]

Ticruiser: I have been waiting over a month and a half for my replacement battery, so I know it can take quite a long time to get those replacements.

Garvie: Got my fingers crossed for ya, bud!

Ya'll are scaring me that my 2 week old  Rad City that abruptly lost power could be down for months?   Yikes! 

Are there any schematics to check continuity/power at the harness connectors?  I had swapped batteries with my wife's bike and it does power hers up. After reading this thread, maybe I should see if my battery runs her bike for any distance.