Help: Dewalt Battery fine - but its BMS Failed??

[ebinary]

I noticed today that one of my DeWalt batteries isn't energizer the BMS when I snap it into its slot (which has the proper resistor network wired into it). All of my other DeWalts power up fine in this slot.

The battery takes a charge fine, and even powers a DeWalt flashlight fine (but I think the flashlight draws from the constantly powered blades, not from the BMS).

Does this mean the BMS is dead. How does one kill this thing? I thought its purpose was to keep things from being killed. Normally I ride with 2p on my downhill/flat commute direction, and with 4P in my uphill commute direction. But even the uphill is doable with 2p, so I don't think I am stressing these guys.

Its probably under waranty, but I already over discharged one (through the dang BMS again - what does this BMS do anyway) and I'm waiting a month now for a waranty exchange. I don't want to overstay my welcome.

 

[ebinary]

Are these BMSes easy to swap out (without obviously voiding my waranty)? I know there are some of you who have piles of useless DeWalt BMSes gathering dust that you might want to sell cheap

 

[steveo]

I would open the pack & check voltages .. if theres anything under 2v that will trigger the bms so it won't charge.. if the batteries are fine then the bms is shot

I have a bunch of dewalt bms on hand! let me know if you need any or a lot !!


-steveo

 

[Doctorbass]

I agree with Steveo.. except for the voltage threshold :wink: .. it's 2.5V and up on EVERY CELL that will enable the charger otherwise the cahrger will blink full 3 leds indicating a problem..

You can open the battery anc charge this cell at 0.5A until it reach 3V.. and put it again on the charger.. for sure it will work!.. but it will need to discharge every other cells to the voltage of the lowest first.. that can take few hours.. so i would recommand you to charge the low cell to a voltage close to all the other cells.. that will shorten the balancing time process..

I just built a Dewalt Battery Doctor yesterday.. it give life to every 36V dewalt pack :wink:



Oh.. and i forgot.. I have a couple.. i mean douzens of BMS for them here.. pm me if you need some!

 

[ebinary]

I agree with Steveo.. except for the voltage threshold :wink: .. it's 2.5V and up on EVERY CELL that will enable the charger otherwise the cahrger will blink full 3 leds indicating a problem..

Actually, the problem is the reverse. The battery works (when getting output from the non BMS blades) - this includes the Dewalt flashight. The battery charges fine (NO three blinky lights - normal charging progression). Its just won't work on my bike, which draws through the BMS.

I will PM you or Steveo for BMSes - can they be replaced without obviously voiding the warranty?

Any idea what would kill the output stage of the BMS? I thought that the BMS energized FET was the most robust way to draw from the pack without modification. There is a slim chance the battery arrived new in this state, because I normally test it by - (1) sticking it in the charger and (2) sticking it on a dewalt flashlight - both of these tests work fine.

 

[ebinary]

I just built a Dewalt Battery Doctor yesterday.. it give life to every 36V dewalt pack :wink:

I may need one of those. DeWalt agreed to replace a battery that failed to charge after a couple of cycles, but I have been waiting almost a month for the replacement to arrive. If I'd have known how long I'd be out, I would have tried to bring some cells up to power.

NOT RELATED TO MY CURRENT PROBLEM, BUT:

Our of curiosity - is this a safe, simple way to bring a DeWalt pack back up to chargeable charge?
(0) Say I arrive home from a ride and one of the four batteries is flashing 3 lights (won't charge). If I...
(1) take one of the other packs that is OK but is in a similar state of discharge
(2) stick the good, but discharged pack on a charger for 5 minutes to ensure that it isn't on the verge of over-discharge itself, then
(3) hook the good and bad pack up again in parrallel (no diodes), so that the good pack will slightly charge the bad pack.
(4) try the bad pack in the charger again.

My understanding is the FETs are bidirectional, so that if one battery has a little more charge than the other, the higher charged battery will charge the lower charged battery.

 

[Doctorbass]

I just built a Dewalt Battery Doctor yesterday.. it give life to every 36V dewalt pack :wink:

I may need one of those. DeWalt agreed to replace a battery that failed to charge after a couple of cycles, but I have been waiting almost a month for the replacement to arrive. If I'd have known how long I'd be out, I would have tried to bring some cells up to power.

NOT RELATED TO MY CURRENT PROBLEM, BUT:

Our of curiosity - is this a safe, simple way to bring a DeWalt pack back up to chargeable charge?
(0) Say I arrive home from a ride and one of the four batteries is flashing 3 lights (won't charge). If I...
(1) take one of the other packs that is OK but is in a similar state of discharge
(2) stick the good, but discharged pack on a charger for 5 minutes to ensure that it isn't on the verge of over-discharge itself, then
(3) hook the good and bad pack up again in parrallel (no diodes), so that the good pack will slightly charge the bad pack.
(4) try the bad pack in the charger again.

My understanding is the FETs are bidirectional, so that if one battery has a little more charge than the other, the higher charged battery will charge the lower charged battery.

Dont forget.. these battery are made of serie cells that need to be balanced.. usually when a dewalt 36V battery wont be charged by the charger(3 led blink) it's because that ONE or MORE of the 10 cell inside are lower than 2.5V.. putting this battery in parallel to another battery will not solve the problem..

When you discharge them, if you bypass the BMS circuit and use the blade connector directly, your battery will not be protect against over discahrge of cell inside... THAT'S the thing. All these cells dont discharge exactly at the same time.. they are not perfect.. so if you fully draw your pack until the voltage drop very low like 25V, you have more chance that a cell goes overdischarged.. and wont be charged by the charger...

A good way to be able to use the BMS and profit of all protection it have is to use this circuit that neodymic built to enable mosfet the output connector : http://www.neodymics.com/Images/V24ProtoSwitch070818E.pdf

i've tested it quickly to see what is the low voltage that desactivate the output and posted some vid on youtube:

[youtube]vituFUmEnU8[/youtube]

Doc

 

[ebinary]

A good way to be able to use the BMS and profit of all protection it have is to use this circuit that neodymic built to enable mosfet the output connector : http://www.neodymics.com/Images/V24ProtoSwitch070818E.pdf
Doc

I do use the neodymics method of drawing from the BMS. I am doing 1s4p as opposed to 2s2p, so I don't use their technique for running series packs. But I do use the resistor network to energize the BMS and I draw my power from the second (BMS) blade, not the raw output (fused) blade.

Thats why I am so frustrated. I shouldn't really be able to over-discharge (which I did once). And I shouldn't be doing anything that would blow the BMS (which I've apparently done once). Each battery just slides into a factoy DeWalt battery receiver that has the neodymics resistor network on the appropriate pins. Then the BMS output of each of the packs is just paralleleds without diodes.

I have a 35A cryslalyte a 40Amp fuse on the whole shebang, and 4p Dewalts should be able to deliver 35A in its sleep - fuse has never blown.

 

[ebinary]

Dont forget.. these battery are made of serie cells that need to be balanced.. usually when a dewalt 36V battery wont be charged by the charger(3 led blink) it's because that ONE or MORE of the 10 cell inside are lower than 2.5V.. putting this battery in parallel to another battery will not solve the problem..

I understand you wouldn't want to charge the string the whole way like this, but I guess I don't understand why it won't work. Except I may need to use the direct (fused) blades rather than the BMS blades (because the low cell may deactivate the BMS). For simplicity, lets say this is a 4 cell pack:

[2.6V][2.8V][2.8V][2.8V] - this pack will charge ok in series from a voltage of about 4 x cell voltage, right?

[2.4V][2.6V][2.6V][2.6V] - this pack won't charge in the dewalt charger, but its not fundamentally different from the 1st case except one cell has dropped too low - according to the conservative BMS. If we series charge it just to this state: [2.5][2.7][2.7][2.7], it will now charge in the standard DeWalt charger. Right?

And of course, this probably wouldn't be a good choice if the packs were: [1.9V][2.7][2.7][2.7], but that really shouldn't happen when I draw through the BMS and balance every day. The time it happened, 2 of the 4 packs wouldn't take a charge, but after 30 minutes of rest, one of them DID take a charge. So, the cells are probably just right on the cusp of working.

 

[steveo]

Dont forget.. these battery are made of serie cells that need to be balanced.. usually when a dewalt 36V battery wont be charged by the charger(3 led blink) it's because that ONE or MORE of the 10 cell inside are lower than 2.5V.. putting this battery in parallel to another battery will not solve the problem..

I understand you wouldn't want to charge the string the whole way like this, but I guess I don't understand why it won't work. Except I may need to use the direct (fused) blades rather than the BMS blades (because the low cell may deactivate the BMS). For simplicity, lets say this is a 4 cell pack:

[2.6V][2.8V][2.8V][2.8V] - this pack will charge ok in series from a voltage of about 4 x cell voltage, right?

[2.4V][2.6V][2.6V][2.6V] - this pack won't charge in the dewalt charger, but its not fundamentally different from the 1st case except one cell has dropped too low - according to the conservative BMS. If we series charge it just to this state: [2.5][2.7][2.7][2.7], it will now charge in the standard DeWalt charger. Right?

And of course, this probably wouldn't be a good choice if the packs were: [1.9V][2.7][2.7][2.7], but that really shouldn't happen when I draw through the BMS and balance every day. The time it happened, 2 of the 4 packs wouldn't take a charge, but after 30 minutes of rest, one of them DID take a charge. So, the cells are probably just right on the cusp of working.

You could charge them in series and essecially bring the pack voltage up.

I presume your trying to get the voltage where the bms can see it and charge up again! best way thou would be to find the problem cell and bring that voltage in the ball park with the other cells.!

-steveo

 

[ebinary]

I presume your trying to get the voltage where the bms can see it and charge up again! best way thou would be to find the problem cell and bring that voltage in the ball park with the other cells.!

-steveo

That'a all I'm trying to do. I am still using virgin packs that are under warranty, so I have no desire to crack the case unless its my only choice. However, it has taken DeWalt a month and counting to replace my first under warranty pack, so I'd like a non-intrusive way to get a charge-challenged pack back online immediately.

Now am I correct in thinking that: just hooking two similarly charged packs together in parallel is essentially going to series charge the weaker pack from the stronger pack (until the two packs achieve about the same level of charge)?

 

[steveo]

I presume your trying to get the voltage where the bms can see it and charge up again! best way thou would be to find the problem cell and bring that voltage in the ball park with the other cells.!

-steveo

That'a all I'm trying to do. I am still using virgin packs that are under warranty, so I have no desire to crack the case unless its my only choice. However, it has taken DeWalt a month and counting to replace my first under warranty pack, so I'd like a non-intrusive way to get a charge-challenged pack back online immediately.

Now am I correct in thinking that: just hooking two similarly charged packs together in parallel is essentially going to series charge the weaker pack from the stronger pack (until the two packs achieve about the same level of charge)?

I wouldn't recommend that .. it will work .. but the higher voltage pack could charge higher then 10amps making it higher then what the cells are rated for charging and possible shorting the life of the cells.

-steveo

 

[ebinary]

I wouldn't recommend that .. it will work .. but the higher voltage pack could charge higher then 10amps making it higher then what the cells are rated for charging and possible shorting the life of the cells.

-steveo

Isn't that essentially what you are doing whenever you hook N packs together in parallel (without shotkys)? They are never in exactly the same state of charge, but as long as they are close its safe, right?

Or does everyone Shotky their dewalt packs together... maybe thats what I am doing wrong.

 

[ebinary]

Closing the loop on this thread:

The BMS was NOT blown. Some recent change has made the BMS much more sensitive to the type of current draw required when initially charge the capacitors on the controller. This draw shuts down the new packs, but not the old packs.

If I start a bike up on one old pack, then replace it with two new packs, everything is fine.

It just won't start up on a new pack without shutting down the BMS

 

[ejonesss]

i dont know if the mbs modules have an electronic serial number that dewalt can plug into some diagnostic machine and read.

if the electronic serial number does not match the printed serial number then suspicion of tampering.

Are these BMSes easy to swap out (without obviously voiding my waranty)? I know there are some of you who have piles of useless DeWalt BMSes gathering dust that you might want to sell cheap

 

[dnmun]

Closing the loop on this thread:

The BMS was NOT blown. Some recent change has made the BMS much more sensitive to the type of current draw required when initially charge the capacitors on the controller. This draw shuts down the new packs, but not the old packs.

If I start a bike up on one old pack, then replace it with two new packs, everything is fine.

It just won't start up on a new pack without shutting down the BMS

you could try using the precharge button solution so that the caps would charge up through a resistor before you made the final connection on your powered mains.

also you could take the controller current off the battery through a switch and shut just the controller current off so that you don't have to remove the battery from the controller so the caps never need to be charged up again.

 

[kfong]

You need an inductor as well as doides on each pack.

http://endless-sphere.com/forums/viewtopic.php?f=14&t=10986