BionX 26v 9.6Ah Teardown & 18Ah rebuild

It’s been a while since I’ve done a pack teardown and rebuild of a new pack.  The most recent one was the Trek Valencia Ride+ pack teardown and rebuild back in January (Part 1, Part 2, Rebuild).  That one was a bit of a mess - corroded cells, held together with some sort of adhesive foam, and generally not at all fun.

This is a companion discussion topic for the original entry at

(Comments from Blogger)

2016-08-15 by Unknown

Hi, thanks for this very interesting article. I have an older 26V 250W system. I took the pack apart so I could mask it off for painting (hated the white on my black bike) and I think it looked the same inside as the one you worked on. As an older big guy, 275lbs (when I bought the bike/system) I was worried that it wouldn’t be powerful enough to haul me around, but am very happy with it. I think it should be noted that quite a few of the cheaper systems list peak Watts for their motors and if Bionx did the same the 250 would be listed as a 350 so I think that is why the little motor does better than I expected. My rides were a mix of flats and small hills and, yes, I did have to build up my legs and stamina to handle some of the steeper grades, but it wasn’t long before my little mule was able to take me where ever I wanted to go. I am interested in a pack rebuild/upgrade after I decided how to transfer my 250 to my cruiser so will bookmark this page.

2016-08-15 by Nick Laing

Any info on the BMS and balancing circuit here? how do you balance the 7s? I dont see any rainbow cable coming from the BMS?

2016-08-15 by Russell Graves

Nick -

The BMS only does low voltage cutoff. You’re correct in that there’s no balancing circuit/ribbon, though the cells remain very well balanced.

There will be a future post on this, but the older BionX packs are using a Spinel LiMn chemistry (Sony US18650V cells) that have enough of a self balancing behavior at high state of charge that the packs remain balanced if used normally. I’ve tested some of the different packs that come in for rebuilds still holding a charge (many of my rebuilds are stone dead), and the cells are balanced to within 0.01v (the tolerance of my current voltmeters). It’s an interesting way to build a pack, and it works fairly well as long as the cells remain in good shape, but the highest capacity one can get on those cells right now is 2250mAh - so about 50% lower than the best energy density out there. You can’t use the higher energy density cells due to the lack of self balancing.

I have some ideas on ways to work around this, but I’ve not built any proof of concepts yet. I need to reverse engineer the I2C and CanBus protocols used first…

2016-08-18 by Richard

I found your blog last month and have been reading them all. Thanks
I can’t believe that you still use lead solder, well i agree with you that lead solder works the best. I still have what is left of two 5 pound spools .032 and .050. The fact that they were given to me in the late 70’s is a blessing. When my battery pack dies i’m sure to hope that you still will be rebuilding them. Again thanks for sharing your ideas and work.

2016-08-18 by Russell Graves

It’s still quite trivial to buy leaded solder for repair use and the like. I’m not bound by RoHS. My primary spool right now, purchased a few years ago, is 60/40 Sn/Pb.

2016-08-28 by Brian in Edmonton, AB.

Hi Russell, really like what you are doing with the Bionx battery rebuilds. I have a used Bionx system that was given to me but it needs a battery pack rebuild. I have done a teardown on the 36V 9.6Ah pack, it is the same as your posts of Aug 2015. This pack was previously rebuilt and the temperature sensor thermistor is busted, the bead is broken and the wire for connection to the neg side of battery pack is gone. By chance have you ever measured the resistance of one of these thermistors? Don’t want to proceed with a rebuild until the thermisor is replaced. Brian in Edmonton, AB.

2016-08-28 by Russell Graves

Brian -

To the best of my knowledge, the BionX packs use a standard “10k at 25C” thermistor. That’s what I use if I have to replace a thermistor on a rebuild. I seem to recall testing one at some point and concluding that’s what it was.

2016-08-31 by Brian in Edmonton, AB.

Thanks for the info about the thermistor. I did some more circuit tracing for the thermistor circuit and found that it goes to the charging connector. However, the charger has no connection to the thermistor, only two wires go to the 42V 2A charger. The charger would need to provide current to the thermistor since on the battery board there is only a connection through a 510K ohm resistor to one of the analog inputs of the PIC18F252 processor. Looks like the thermistor is only there for temperature monitoring in case a high current charger is used. I put the battery pack back together for now with a 10K resistor soldered on the board and dispensed with the wires for the thermistor. Did a test ride and it works OK.

2017-01-23 by Unknown

Great article! I can see why you are such a fan of RoHS labels :slight_smile:
I find your blog very useful as I am currently in the process of rebuiling a bionx battery.
Just wondering if there are any new spinel cells out there beside the US18650V3.
~3500mAh is becomming mainstream these days - would you consider using inr/nca for your future rebuilds (+bms) ?

2017-01-23 by Russell Graves

Alex -

I haven’t found anything compatible except the US18650V3s, as a direct drop in replacement. You could, potentially, build a higher capacity pack if you added an external balancing BMS, but there’s not really space for that in the current packs. You might be able to fit a very small one in this pack as there’s some extra space, but it would be tight.

I’m working on some paths to get higher capacity cells in the packs, but it requires a non-trivial amount of reverse engineering and design work - so nothing in the immediate future, unfortunately.

2017-01-24 by Unknown

10S5P will leave enough space for a BMS while almost doubling the original capacity.
The only problem is that a BMS will not balance anything under 4.2V.
The Bionx charger stops at 41.5V and the benefit of lower voltage will depend on the quality of cells.

How the US18650V3 is handling the current compared to the old US18650V ?
The old cell is rated 20A and despite the good remaining capacity of my 10S6P pack (about 8.8Ah), my battery sags a lot under load.

The battery gauge is depleted in 15 minutes but the bike usually runs another 20-30 Km until the battery becomes flat.

2017-01-24 by Russell Graves

As far as I can tell, the US18650V3s are holding up to the current just fine - the BionX systems don’t pull very many amps out.

I have seen a number of the older BionX systems reporting an empty battery when things will continue many miles past empty. I think there may be some sort of aging built into the BMS that assumes batteries degrade at a certain rate, and this is wrong over the 8+ years a lot of the packs are working. Plenty of packs I’ve seen come in are in the 8+Ah range out of a 9.6Ah pack, yet report empty very, very quickly while the pack still has plenty of voltage left.

Check the voltage under load - you can use pins 2 and 4 on the charge port to read the direct pack voltage. I don’t think you’ll see it sagging that far.

2017-01-25 by Unknown

Interesting - I am using the console to read the voltage. Usually, after about 10 km, the max assistance level is decreasing (<64), the error 25 is flashing and the voltage drops from 39/38V to 34/33V under max load. I’ve tested how much current is flowing during acceleration and is was in the 20-30A range which is almost 1kW. The battery has a 30A fuse and I assume that the max continuous current is limited to 30A.

One more thing, during the rebuild, I am supposed to keep the BMS connected to a power supply to avoid resetting the kit ? What happens if I disconnect the BMS ? I am supposed to enter an activation code ? Thanks!

2017-01-25 by Russell Graves

Sagging under high load is fine, though I’m surprised the system is pulling >20A - this is a 26V system?

On the older systems, I’ve never had to keep the BMS powered - I don’t know about the newer systems, though.

2017-01-26 by Unknown

36V/350W system - I have another 36V kit which is limited to 500W and despite being a mid drive, it has noticeably less power. I am currently measuring the efficiency of both systems. Until now, it looks like the mid drive is using about 1-2W less power per Km than BionX - with constant gear changing.

As for the rebuild, I might experiment either a 11S5P arrangement or perhaps a mix of 2S VTC6 with 4S V3 - in this way perhaps high current and self-balance could be achieved.

2017-01-26 by Russell Graves

Ah, ok. I’ve wondered about how much BionX was playing fast and loose with power ratings. The 350 I rode felt like it had a lot more than 350W behind it when climbing hills…

I would really caution against doing mixed battery chemistry/model builds. That’s a good way to get some insane equalization currents flowing around the pack, which won’t do anything good. Getting 30A out of a 6P pack is only 5A per cell, which is well within the limits of anything modern.

2017-01-27 by Unknown

You are right - With 11S I will no longer be able to regen and if I mix low/high resistance in parallel I will probably stress the cells. I would probably stick to the original configuration hoping that the voltage drop will be less important. Thanks a lot for your advice ! I will post some pictures once I complete my rebuild.

2017-01-29 by atomek1000

Omg, you are so bad at welding batteries. Only 2 strips per series connection? Are you kidding me? It is insane if you make packs like this and charge money for it… Learn first!!! BMZ pack was also shit design but they couldnt probably care less. You should if you instruct people and offer services.

2017-01-29 by Russell Graves

I’m building a replacement pack for a reasonably low amperage system - the fuse is generally 20A on the older packs.

At the current draws involved, two strips are just fine - the resistance on such a short interconnect is quite low, so the heating and power loss are minimal.

What, in particular, would you suggest I change on this particular build to meet your standards? Additional strips on the series interconnects? Cooper wire soldered on top of the strip? And, given whatever changes you suggest, what would the additional power delivered to the motor be with those?

I’m not building 50-100A packs here. BionX doesn’t draw that many amps.

2017-03-01 by Crossman

I have a BIONX 26V 7S6P battery with BMS Smart Connect 5.2.
I would like to replace the battery with a 37V (10S4P) battery
Do you the think Smart Connect Board could manage this voltage ? is it compatible with any voltage 26/37v or must I reprogram something ?
Best regards

2017-03-01 by Russell Graves

Crossman - I don’t believe that you can change voltages on the boards. At least, it’s not something I’ve been willing to try with how rare the original BMS boards are.

However, I do have a substantial capacity boost on the 26V battery available as a rebuild (nearly twice the stock capacity). Get in touch with me via the contact form in the right column if you’re interested in that.

2017-03-01 by Crossman

Ok Russell, I thought that the Smart Connect Board could manage different voltage. Is it programmed inside ? because when looking to a BMS 26v, and another one BMS 37v , I really don’t see the difference ( I have the two batteries 26v and 37v ).
What are the best cells at the moment ? NCR18650PF ?
Best Regards

2017-03-01 by Russell Graves

I believe the same board can deal with different voltages. I don’t have enough information to understand how to program it, and BionX doesn’t exactly publish that stuff. If you can get the information out of them, I could certainly make the changes…

I use Sony US18650V3s for my rebuilds as they’re the same spinel LiMn chemistry with the self balancing behavior the LiMn cells exhibit. Since the packs don’t have balancing wires (until you get to the 48V packs), you need self balancing cells or things will drift over time and become dangerous.

2018-06-28 by Unknown

I did a rebuild on an 8 year old battery pack.
10s6p, 42V, Sony Ericsson US18650V1, March 24/2008 is the original cells.
All is good but for 1 issue: When starting the dash, the battery symbol appears, then disappears.
Everything still works fine but no battery indicator so we do not know what is left in the pack.
Charges fine, discharges fine.
All the case to bike connections are connected to the board, no broken or loose wires.
Battery pack only has 2 wires.
Is this a reprogramming issue as the battery was disconnected from the BMS/Controller in the case?
Any ideas?

2019-05-28 by bonny

Could you possibly tell me if the yellow wire is earth ?
Thank you in advance

2019-06-01 by Russell Graves

Red is pack positive, black is pack negative, yellow is a 10k NTC thermistor connected to the negative terminal.

2019-07-22 by Unknown

le transistor Q12 quel référance?