LiTech pack teardown

[RagingGrandpa]

The Data

This is a LiTech pack with P42A cells, from a Gotway RS.
It had water damage, so I tested and then disposed of it.

The LiTech model number is LP 24S2P 10A 30A L002
24s2p is the cell configuration.
I presume 10A and 30A are the continuous current ratings for charging and discharging. 
Pulse currents can be much higher, as I discovered.

Focusing on the safety behaviors, the relevant circuits are shown below: 
 

 

 

Safety devices include:
U1, U2, U3: Shutoff FETs
S1, S2, S3: Thermal switches 
NTC1, NTC2: Temperature sensors
F1: Melting fuse
And, an alarm buzzer.

The cell voltages, balancing resistors, and NTC thermistors are monitored by cell management ASICs. I couldn't read their chip labels on this BMS due to lacquer, but they closely resemble the IC's LiTech used in the Sherman Max BMS which were Cellwise CW1274. 

There is no programmable memory nor CPU, and there is no communication or signaling to other devices outside the pack.

From bench testing, I confirmed the following behaviors: 

 

* Scenario 2 charging current: I did not test charging above 18A, which was the maximum of my equipment.
* Scenario 10 melting fuse: I was unable to sustain a medium-level current suitable for melting the fuse, using my crude dummy loads. So the "60amps for 1 minute" rating is an estimate based on typical 50A-rated ATC fuses.
* Scenario 11 overcurrent trip: I used a cheap DC clamp ammeter which wasn't fast enough to reliably measure the peak current, and with it I detected various currents from 80 to 115 amps, before the trip. Also note: I shorted the pack thru a 250V 12A-rating melting fuse, and the pulse was brief enough that my fuse did not melt. It trips fast. 

More multimedia here.

Comments appreciated!
 

Edited December 11, 2023 by RagingGrandpa

[RagingGrandpa]

Q&A

Q1: Can I really charge a Molicell RS at 18A x 2 ??
A1: No. It's a 2C charge rate, and these are sealed packs with no cooling. The cells will get hot and trigger a shutoff. 

Q2: Can this pack be charged through its output?
A2: Yes! (That's exactly what regenerative braking does.) Additional packs of this design, connected at their outputs, would also be safely re-charged, if only one pack had its charging connector energized. 

Q3: How does cell balancing work?
A3: It's traditional resistive top-balancing, managed by CW1274.

Q4: Since the FETs stop overcurrent, what is the Fuse for?
A4: We need to allow high pulse currents, but the pack is not built to sustain them. As I demonstrated previously, the weld strips in this form-factor of pack will melt under sustained loads in the 75-amp range. The 50A fuse is nice because it's external to the pack: it blows before the pack gets damaged, and you can replace it easily. 

Q5: If I have two packs in parallel (like RS) and one of them shuts off during charging for cell-overvoltage, won't the second pack continue charging? (Even if the pack with the charging input shuts off?)
A5: Right! Because of the U3 body diode, the BMS can never stop the charging current from flowing through the power output, and recharging the other parallel packs. So, it will lead to imbalance between packs.
Eventually, the balancing resistors would bleed away the high cell... but in the meantime, the other pack will be recharged, and then when removing the charger, the disabled pack will try to activate again. If the voltage mismatch is high, it may trip or blow its fuse. A hopefully-rare vulnerability.

(more to come)
 

My Conclusions

This pack was well built. 
Every weld was great, nice soldering, everything securely mounted, double-insulation, armored cover for the IC's, all cell ends protected... nice. 
Perhaps the only gripe was that a few drops of water did manage to get past the shrink and glue. 

Holy crap, water damage is sneaky! 
It was only after ripping the weld strips off the cells that I appreciated the damage. 
I was wondering if disposing of this pack was a mistake... it wasn't!
https://forum.electricunicycle.org/topic/34849-if-your-euc-battery-gets-just-a-little-wet-and-still-works-why-replace-it/

It's annoying that cell over-voltage shuts off the output. 
It leaves open the possibility that a fast sudden braking maneuver could momentarily drive a fully-charged pack above the 4.25V/cell threshold, resulting in a cutout crash, and likely a damaged controller due to regen braking continuing and its voltage having nowhere to go. 
But I think it's a compromise that was necessary, because of the possibility of multiple packs connected in parallel and being recharged by only one of them. We need this 4.25V charge-stop safety feature, and if the pack is being recharged -through its output- by other parallel packs, full shutoff is the only way. 
p.s.: Gotway classic BMS handled this more gracefully, but with more complexity: by using additional signaling wires to the other parallel packs. 

Mixed feelings that cell under-voltage shuts off the output. 
A normal EUC would be beeping and tilting long before the 2.8V/cell threshold; and this built-in undervoltage protection makes these packs appealing for repurposing in other DIY projects (like AC inverters). 
But if there is cell imbalance and one cell sags lower than the others, this shutoff could occur without warning. 
And what if a cell monitoring connection comes loose?! (Something to test next time.) 

(I stopped manually discharging the cell group at 1.95V in my testing and did not drive it all the way to zero. It always accepted charging in that test, so I suspect there is no dead-cell 0V charge-stop either.) 

It's annoying that the alarm beeper does not activate for cell under-voltage. 
In the relatively-common situation of cell imbalance, there could be one cell at 2.9V while the rest are 3.9V, and the EUC will let you ride blissfully unaware. 
We need the alarm to notify us of this problem, before the 2.8V shutoff crash happens. 
And, if the voltage decay happened during long-term storage, we need the alarm to notify us before we attempt re-charging the pack. 

We can't closely monitor the temperature of every cell. 
There are 48 cells in the pack, but just 3 cells with temperature sensors. It's a pragmatic choice, and reasonable... but I wouldn't rely on the pack's shutoff mechanisms to regulate fast-charging, for example. Emergencies only please. 
In the nightmare "slow-cooking-then-fire during storage" situation that some unlucky Li-Ion pack owners have suffered, the hope is that inside this sealed and insulated pack, the heat would conduct around enough to reach one of the thermal switches and sound the alarm. But there is a really long distance between B13 and S2... 
Of course, if internal discharge "slow-cooking" is happening, there is nothing the BMS can do to stop it- so if your EUC starts beeping while in storage, jump into action! Move it outside! 

What about inrush?
I've seen 160A inrush pulses when connecting an EUC controller and its large capacitors... I suspect it could cause a nuisance trip of the overcurrent protection. 
Perhaps these BMS were intended to be used only with the XT90S "anti-spark" controller connectors, which contain built-in precharge resistors. 
If using this pack in an EUC without an anti-spark connector, know that if the controller will not turn on after a first connection, unplug and replug a few times before trying again. 

Respect the beeps!
For cell over-temperature during riding, the alarm will start sounding at 70°C. STOP RIDING. Let it cool until the alarm stops, and then ride more gently. No harm. 
If you ignore it and keep going aggressively, the EUC will cutoff and drop you once it reaches 85°C. It's a generous gap: at least 10°C of advance warning. Heed it. 

Cheers
 

Edited December 12, 2023 by RagingGrandpa

[alcatraz]

First thing that came to mind when seeing the pic was that output shutoff. 

I suppose a failure there would be covered by the dual pack redundancy but still. 

If they had to design a single pack or a Master v1 type pack (all series) would they still do it this way? I'm guessing no.

It keeps us coming back to the discussion: minimize rider injury vs prevent self destruction. Which is the right priority? Would you ride a 10year old Litech pack?

[alcatraz]

I'm curious what the braking currents do to those output mosfet. Can current flow freely without long term harm to the component?

I'm watching a lot of laptop repairs on youtube and one thing that happens sometimes is that the mosfet gate voltage drops (it gets pulled down for various reasons, contamination, wear, etc) causing the mosfets to not fully open (act as resistive load) so they burn out.

Maybe they're the kind of mosfet that is open by default. That would be good.