Repairing/troubleshooting a Milwaukee (E-Moli) BMS

[lazarus2405]

So I'm in the process of modifying my Milwaukee packs for ebike use. I'm simply soldering 10AWG to the battery tabs to bypass the BMS for discharging, leaving it intact for easy charging using the stock charger and BMS. Of my six packs, one isn't working. If I can't use the BMS to charge, then I have to find some other solution, which from what I can figure it won't be nearly as simple. So please, help me troubleshoot the problem.

So, my symptoms: the pack won't charge.
The charger recognizes the pack, and reads it as being full. (It has a separate readout for a "faulty" pack for various reasons.) It's reading around 27.4V (nominal is 28V, hot off the charger is ~29.1) at the tabs (before the BMS); its built in "Power Meter" reads ~30%.

All balancing leads are intact; I'm sure of that. All cells are level (~3.92V), with no faulty or low cells.

The voltage across the discharging leads varies very quickly. It starts around 30mv and climbs to ~300mv, then drops again to 30. These are the discharge leads made to connect to the tool, so it should be in the 28V neighborhood.

The BMS sits between the negative discharge lead and the negative battery tab. There is no path between the two (resistance beyond my DVM's range).The connection between the BMS and neg battery tab is secure.

So, the negative discharge lead is the positive lead for charging. This connects to an aluminum plate that acts as a heatsink, which connects to the copper sticking out of the two IRF4104 MOSFETS in parallel (which appear to be completely intact). I cannot tell what else this aluminum bit connects to electrically, but I think that it acts as the third connector of the MOSFET. Right, wrong? The two MOSFETs connect to the BMS's PCB with the two outside legs, not a middle.

So, what's the DVM say? Well, the voltage between the positive battery tab and any of the four MOSFET legs is pack voltage. Voltage between the two legs of each respective MOSFET is 1.6mV, and resistance is 1.2kOhm. Resistance between any MOSFET leg and any part connected to its heatsink is something like 7.4MOhm.

The BMS PCB is covered by a spray-on insulating gel sort of thing, so as far as I know it's undamaged. It also means that I can't trace the circuits, but if you tell me what to look for I can dig around. I'll post some pictures below in a bit. If you have any suggestions, or if you need any more information whatsoever, just speak up. Thanks

 

[fechter]

The discharge leads are ~300mv when they should be around 28v?

BMS tripped or something I'd guess.

Is there a way to disconnect the BMS from the cells completely and then reconnect them? This might reset something.

Unfortuantely, I've never seen the guts of one of those, so perhaps a pic might help.

 

[lazarus2405]

Mkay, here goes:



The tab in the bottom left is the negative terminal, and the black wire connects it to the BMS. The two ribbons on each side are balancing leads and the thermistor connection.



The top tab is the positive, middle is the temperature sensor lead, and the bottom is the negative. It connects to the aforementioned aluminum bit through that copper braid, which in turn connects to the copper bit of the MOSFET.

I could desolder the balancing wires and see if I can reset it and then reconnect it.

The dark stuff is the insulating gel. Tyler has a better pic here: http://endless-sphere.com/forums/viewtopic.php?f=14&t=745&start=6


Edit: Exactly what is the role of these two MOSFETS when charging? If they aren't used to lower the voltage while charging, would it be safe to bypass them to charge it?

 

[TylerDurden]

ES member maytag may have some spare BMSs for sale.

 

[lazarus2405]

I though of that. He got 6 last year, built 2 into his bike, and used 1 to replace one of Jondoh's, right? That's my first backup option, if I can't fix the one I have.

My second backup option is to wire some connectors to the balancing leads on another pack and use some stuff from my RC kit to make nice connector.

 

[fechter]

If you measure the voltage on the left hand leg (gate) of either FET (as shown in the picture) against the battery negative, a zero volt reading will mean the FET is turned off. When on, the gate should be around 10v.

 

[lazarus2405]

It reads 1.5mV. Exactly, unwaveringly.

So, it's off. What does that mean? And... can I bypass or repair it or induce it to turn on?

 

[fechter]

It means the FETs are probably not blown.
It also means the problem is likely underneath the black goo.

Bypassing the FET will disable the high current discharge safety (but your controller / fuses will take care of this).

I don't know if it is used in the charging process. If it is, and you bypass it, you could possibly fry the cells (bad).

You might be able to measure a good pack at the end of charge and see if the FET is on or off.

 

[lazarus2405]

Bypassing the FET will disable the high current discharge safety (but your controller / fuses will take care of this).

I'm bypassing it for discharging anyway. No problems there.

You might be able to measure a good pack at the end of charge and see if the FET is on or off.

On a pack with a functioning BMS at full charge, I read 11.5V across the legs. So, open. Does that mean I can safely bypass the FETs to charge the pack?

 

[TylerDurden]

You could try bypassing the FETs for charging, using minimal current and monitoring the pack voltage and individual cell voltages...

The SoC indicator on the BMS seems to still be working (27.4=30%), but you want to double-check with your meter and stop the charge at 29.4V

It should only take about 30min.

:?

 

[lazarus2405]

Something worked!

So, I believe that I "reset" the BMS. I bypassed the FETs with a quick solder of a single strand of copper from some 10AWG. I put it on the charger and go the "faulty" indication. I disconnected my bypass, put it on the charger, and it started charging! Golly! To be sure, I checked the gate voltage and got ~9.6V. So far so good. I babysat it for half an hour, after which it gave the green "charging finished" indicator. The SoC said it was between fully charged and close to fully charged. And low, pack voltage was at 29 and change, cell voltage at 4.15. Superb!

So thanks you two. I couldn't have bumbled into a solution without your guidance.

So, now I just have to fix a bar that came half off, clean up a repair to one balancing lead (damn flimsy ribbon), solder on my 10-gauge, and put it back in the case. Simple, compared to "mystery BMS problem!". Hmm... I should really eat first. Stupid project, causing me to neglect bodily needs.

So, the next question: Do I commit thread necromancy and post the battery build in the placeholder I made three weeks ago in the photos and video forum, or do I just make a new build thread in the e-bikes general forum?

Sub sandwiches would hit the spot...

 

[TylerDurden]

All's well, that ends well... or something like that. Still a mystery, but hey if it works, rock-on.

If you have a MilW flashlight or such, you could cycle the battery a coupla times to see if it performs properly.

I've heard stories of how folks had to connect/disconnect numerous times for charging to reset, discharging to LVC, bringing low cells up to par before charging... I figured Jon or Maytag would have some tips, but you seem to be back in the game.

 

[ejonesss]

you say that you bypassed the fets.

did you just short the s and d pins?

i got a v18 pack used from a local battery recycling bin and i dwanted to find out what the lvc is so i ran it down until it stopped working it is around 17 volts for the lvc.

also now the battery level indicator does not stay on like it did before and the outputs pulse instead of staying on.

i am able to get the pack to take a charge by applying 24 volts to the + and - pins however i am unable to reset the bms.

update: i think the bms may be blown or self destructs because i tried resetting it by disconnecting all the balancing taps and then reconnected them and now the charge indicators do not work at all.

update 2: it turns out that as soon as you remove the bms board the bms self destructs or loses it's ability to work again.

i found this out because as soon as i removed it from the battery pack and re connected it the battery level indicators that lit up before now does not .

if someone knows of a way to reprogram the boards i then could revive them.

i got 2 packs from a local battery recycling drop off and 1 the fets was blown 1 fet was completely blown and the other one melted but still somewhat functional.

the other board died as soon as i hit the lvc and i thought i killed it by shorting 2 balancer pins accidently while desoldering them so i disconnected the second board and re connected it and the same thing.

Something worked!

So, I believe that I "reset" the BMS. I bypassed the FETs with a quick solder of a single strand of copper from some 10AWG. I put it on the charger and go the "faulty" indication. I disconnected my bypass, put it on the charger, and it started charging! Golly! To be sure, I checked the gate voltage and got ~9.6V. So far so good. I babysat it for half an hour, after which it gave the green "charging finished" indicator. The SoC said it was between fully charged and close to fully charged. And low, pack voltage was at 29 and change, cell voltage at 4.15. Superb!

So thanks you two. I couldn't have bumbled into a solution without your guidance.

So, now I just have to fix a bar that came half off, clean up a repair to one balancing lead (damn flimsy ribbon), solder on my 10-gauge, and put it back in the case. Simple, compared to "mystery BMS problem!". Hmm... I should really eat first. Stupid project, causing me to neglect bodily needs.

So, the next question: Do I commit thread necromancy and post the battery build in the placeholder I made three weeks ago in the photos and video forum, or do I just make a new build thread in the e-bikes general forum?

Sub sandwiches would hit the spot...