• Howdy! we're looking for donations to finish custom knowledgebase software for this forum. Please see our Funding drive thread

Dewalt A123 BMS battery interface solution


100 kW
Sep 2, 2008
SE Michigan, USA
I just finished making boards for the dewalt packs so you can use them without taking them apart. I thought I should post a new thread on it here. These packs have a 3 year warranty and as long as you stay under 20amp per pack you should have no problem using them for ebikes, they should last just as long as if they were used on power tools. I recommend using 3 or more packs in parallel so you can share the current load. With 3 packs you have 60amps available. I'm using 6 packs for a total of 120 amps. The best part is I can still use them for my power tools and misc. uses. The BMS automatically shuts down to protect the packs when they go below 27-28volts. Here are pictures of the boards. I will be selling the bare boards for $5 a piece .5" spacer will be included. PM me if interested

Here is the partlist. You can purchase the rest of the parts from Digikey

(1) 7.7k 1/4watt resistor 7.15k can be substituted since digikey has a minimum order of 5 resistors. cat# 7.15KXBK-ND
(1) 8.5k 1/4watt resistor 7.15k can be substituted
(1) 1k 1/4watt resistor 1.00KXBK-ND
(1) V30100S-E3/45 DIODE SCHOTTKY 30A 100V TO-220AB Digi-Key Part Number V30100S-E3/4WGI-ND
(3) Keystone Electronics quickfit male #1265 Digi-Key Part Number 1265K-ND

The rest is just wiring to your motor controller. Just use the NEG FET and POSITIVE output for power and parallel packs for current. 3 packs should cover most ebikes using hard accelleration. That's 60amp * 32Volts = 1920 watts

Update 1/14/2010: I've decided to sell finished boards for $35. This will be the bare board stuffed with parts and the power tabs soldered onto them. The tabs will be milled on my my cnc so no filing will be needed. I will also glue the bottom spacer so it's a snug fit when plugged into the battery pack.

Note: the boards will work with the 28volt packs as well as the 36volt packs. What was interesting is the 28volt packs had a higher current output of 25amps. I'm guessing it's to make up for the lower voltage.

There have been reports of users losing a pack with the interface boards plugged in now that winter has come. This is due to the older bms circuit not going into sleep mode properly. I don't recommend leaving the packs plugged into the interface board for long periods. Especially, after a full discharge. I usually plug them back into the charger after a ride so it's not something I had to be concerned with. When the battery gets plugged into the interface board, it enables the internal BMS of the Dewalt packs. This is a small processor that requires power to run. That power even though small is still enough to drain the pack. The newer ones go into sleep mode using only minimal current. So it's best not to chance it and just get into the habit of charging the packs after a ride so it's ready for use. Unlike other cell technology these can take a charge anytime, they don't need to be fully cycled.

An inductor will still be needed on most setups http://www.endless-sphere.com/forums/viewtopic.php?f=14&t=10986&start=15 Cost $5
A set of 3 milled connectors would be $2 if you don't wish to file them. 3 needed per board.

Shipping cost is $5, international $7. Contact me at kinf@embeddedtronics.com


  • dewalt sideview.jpg
    dewalt sideview.jpg
    45.5 KB · Views: 38,968
  • dewalt topview.jpg
    dewalt topview.jpg
    61.8 KB · Views: 38,926
Here is the BMS monitored with a power analyzer. Data and pics of the setup.


  • cutoff 2.jpg
    cutoff 2.jpg
    27.4 KB · Views: 38,922
  • cutoff expanded.jpg
    cutoff expanded.jpg
    20.6 KB · Views: 38,967
  • test setup.jpg
    test setup.jpg
    93.5 KB · Views: 38,924
  • test setup2.jpg
    test setup2.jpg
    96.9 KB · Views: 38,922
Here are the connectors used for the power connection. They will need to be filed to this profile. The easiest way is to just use a small file. The metal is fairly soft so it files down easily. On the left is the connector purchased from Digikey on the right you can see I needed the connector to sit flush with the board as well as fit just a little lower. I might mill a bunch of them if there is enough interest.
I'm pretty sure the Dewalt BMS has a 15amp surface mount fuse under the potted surface. I've seen the location documented so you can remove the potting to "fix" a pack with a blown fuse. Are you sure you can safely draw 20amps? Would this interface work for the 8 cell version (DC9280)? Can you give a quick explanation of what the onboard electronics provide that just connecting to the pins do not?

We will not be using the fused 15amp output. That just bypasses the BMS circuit. We are using the pack as they were intended to be used, through the BMS and FET controlled output. This will shut the batteries down if it detects low battery voltage saving the batteries, it will also shut the battery temporarily if it exceeds 20amps, much like a resetable fuse. This is why you will need to parallel 3 packs or more depending on your current draw. It works quite well this way. With the lower current draw maintained, cells stay fairly balanced and we stay within the design parameters of the pack. We also get the advantage of a fast 1 hour charge. I use 4 dewalt chargers and can get back on the road within an hour; very convenient, plug and play solution.

The interace boards have a protection diode so you can easily parallel packs, even if they have different voltages. Helps, on those times you forget if the packs were charged or not.

The circuit is based on this design
The power connectors are soldered onto the interface board. They connect directly to the Dewalt battery packs for the high current. The interface board slides into the battery much like the charger does. You only need 2 connectors, but I left the 3rd one on so it's more secure. I will post a partslist and more info soon.
Those connectors look really slick, good job. I wonder if I would be able to use these in series safetlyfor my 72 volt set up?

Last year I tried drawing about 300 amps using them in series without protection and cooked some dewalt packs :shock: , I set them aside for use in my business tools and bought some Thunderskies witch have worked out well.

I may PM you in a couple of days for 40 of your connectors (2S 20P dewalt packs).
Gday that looks like a good solution and cheaper than buying and hacking flash lights. i have a setup where i use 4 flash lights and 4 batteries wired in parrallel use the PWM dewlat port.

Only comments i would make is that repeated physical on/off connections for charging will put a lot of load and wear and tear on the connectors and the dewalt flash light housing would be more durable in this.

Also the other down side as i have a 4 batts in parallel like you mention is that, towards the end of the battery life unless the batteries are exaclty balanced some packs will drop off 1st then you may reach the 20AMP restable fuse inside the batts. To reset them you have to connect and disconnect the batts which is a pain to do on the road.

the other main problem like you mention is you need 4 chargers at every stop , or staged charging which means many times coming back to the cells.

i think the best solution for ebikes is to wire 4-5 packs in parallel directly off the cells for charging and on discharge direct to the cells, the cells will stay more balanced. Their is no physicle connecting/reconnecting many plugs per charge and you only need one charger per stop, however it takes 4-5 hours. In my case it will work out perfect, charge at home 5 hours, ride to work, charge at work for 5 hours in the 8hr day and ride home :) 4 packs in parralel does work but unless you have plenty of spare amp/hours at the end of a ride you get close to tripping the 20Amps limit easily, and also charging 4 packs wit 4 chargers gets annoying after time, 1 pack for 4 hours is easier in the long run .
After a testing the interface, the space for the dewalt connector needed to be taken up. It currently works, but with the vibrations an ebike entails. It's best to make the contact as secure as possible. Each interface board will come with a spacer. You will need to glue this spacer on. Superglue works great as long as you scuff it up a bit since the circuit board has a smooth surface. I used epoxy, on mine but found superglue to be quicker. If you plan to use this as a power source that is vibration free, you can omit the spacer. Here are the pics with the spacers glued.


  • Spacer.jpg
    99.8 KB · Views: 38,968
Yes, you can series them. The original circuit that this is based upon had 2 of them in series and 2 paralleled to get the high voltage and current.
Hi jk1,

This should be just as durable as the dewalt flashlight batteries. Same type of connectors used. I don't see it being a problem with wear. When 20amps is exceeded, the BMS powers down till the condition clears. You shouldn’t need to remove the batteries. I verified this with just one battery on my ebike. It would cut off on quick acceleration since I’m drawing much more than 20amps. I then let up on the throttle, and power is restored. I can ride my bike if I slowly ramp up, staying below 20amps.

If a pack drops off, it should not affect the others. I’ve already proven you can ride on one pack as long as you are careful of your current draw. It makes no difference if the other packs are not working as long as 1 pack still has some capacity.

You shouldn't need to worry about cell balance since we are staying within the parameters that Dewalt designed. They have many more power tools working flawlessly than we have ebikes running. We are using the original BMS designed by Dewalt and the low current draw will keep the batteries in check. Why no one has done this is to me why people have had problems using the Dewalt batteries in the past.

The great thing about this setup is I can charge in one hour. I don't have to wait long just to get a quick ride in or run an errand. If your bike does not need more than 40amps then you can charge just 2 packs, 4pack or 6 packs depending on the distance you need. Most of my errands only require 2 packs. If you see how I have my bike setup. You will find it very convenient to plug and unplug my packs. I only use all 6 if I know I will be doing some long hauls, why carry more weight then needed.
Very awesome boards and a very reasonable price too! I have been using my Dewalt packs for over 2 years now and still love them and wish these boards were around when I was putting my setup together. Do you know if the dewalt BMS does cell level monitoring? IE one cell drops below 2v it shuts down. If I recall people were still discharging batteries and then getting the error lights on the charger.
The people who were discharging them were taking the power from the 15amp fused output. This does away with the internal BMS circuitry and will run the batteries down to zero if you are not careful. I have destroyed a few cells this way. That is why I decided to do it through the dewalt BMS. It did take a bit of planning, measuring and custom boards had to be built. Figuring out the electronics, CNC and board layout.

Based on the data collected, the Dewalt BMS turns off the batteries at 2.7-2.8volts per cell. This is a very safe range and will protect the cells from abuse. This is the recommended range for long life as well.

Having not to worry about such expensive packs is a big plus, and if something does go wrong. That 3 year warranty will look very good. This is one of the safest way to go with the Dewalt packs without tearing them apart.
This is a very neat solution. Battery pack and BMS all combined in a single weatherproof enclosure. Ideal for lightweight, short-range ebikes. Would you consider offering kits that include the boards and all components?

I don't plan to make a kit since the parts are easy to obtain from Digikey. I just wanted to cover part of the cost of my run. The next batch will be priced higher $8-10 if there is still enough people interested for me to do another run.
Here is the partlist.

(1) 7.7k 1/4watt resistor 7.15k can be substituted since digikey has a minimum order of 5 resistors. cat# 7.15KXBK-ND
(1) 8.5k 1/4watt resistor 7.15k can be substituted
(1) 1k 1/4watt resistor 1.00KXBK-ND
(1) V30100S-E3/45 DIODE SCHOTTKY 30A 100V TO-220AB
(3) Keystone Electronics quickfit male #1265 Digi-Key Part Number 1265K-ND

The rest is just wiring to your motor controller. Just use the NEG FET and POSITIVE output for power and parallel packs for current. 3 packs should cover most ebikes using hard accelleration. That's 60amp * 32Volts = 1920 watts


    9.3 KB · Views: 38,549
    10.9 KB · Views: 2,140
Here is how simple the wiring gets. I plan to use this on a scooter. I won't have to worry about the batteries. As you can see it's all connected. I plan to use two packs to get a useful 10mile range
Video of the test.


  • Scooter wiring.jpg
    Scooter wiring.jpg
    86.8 KB · Views: 36,850
How many dewalt packs would I need to get 15 mile range on 5303 hub motor? Assuming 36v? and then also assuming 72v?
I don't have a 5303 motor to be able to tell you that. It really depends on how much the average amp draw of the motor is at the speeds you want. The dewalt batteries are rated at 2.3ahr, you would need to parallel them to get the amp hour needed, but to keep them happy, it's best to run them to 80% of capacity. A rough general rule is an amp hour a mile for bikes. Of course if you pedal assist you can increase you miles as much as 30-40%.

A scooter is much more efficient due to the smaller wheel giving it a better range. Making them a great alternative to bikes.

morph999 said:
How many dewalt packs would I need to get 15 mile range on 5303 hub motor? Assuming 36v? and then also assuming 72v?
I did some more testing with the boards. Looks like capacitive spikes are a problem. It will shut down the BMS, an inline inductor of 300uh and a diode across the batteries will be needed. This is what was in the original circuit. I was hoping to avoid using them. I will have to do more testing.
Update, looking for a suitable inductor. The one I found, I wasn't happy with the wire gauge, so I looked around and found a ferrite toroid, and started wrapping wire around it. I ran out of wire, but felt it would be good enough for the test. Worked better than I expected. It just goes in between the battery and the speed controller. No bms shutting down from the capacitive discharge coming back from the controller. I left out the reverse protection diode, but will probably add that across the inductor as well. Here is a picture of the inductor I made, the ferrite toroid should be easy to find, search on ebay if you can't find one locally. Just find one that is big enough to wind a bunch of wire around it. They work like noise suppressors. Amazing how well it works. I was able to run on two packs without it shutting down and the combined current of 40amps from the two packs was sufficient enough to keep the bms from tripping as well and if the current does trip, it just goes into momentary off state and comes back on again. You get a stuttering effect if you keep on the throttle. Once I have 3 or more packs in parallel, current won't be an issue.


  • inductor.jpg
    81.3 KB · Views: 36,845

Your interface boards are awesome! Using an e-bike as transportation, with 6-8 Dewalt packs with your boards, how could I charge all the packs using only 1 charger? So I wouldn't have to carry multiple chargers, and dismantle the entire pack for charging.

For example; ride to work, charge entire pack with one connection, then ride home :)

A very simple, expandable, plug and play solution using these Dewalt packs would be ideal! I think you have achieved this to 70% by building your interface boards.

Thanks, and keep up the good work!
I provided access to the 15 amp fused connection. This goes directly to the negative side of the battery. The other side of the enable pin goes directly to the positive side of the battery. You could create a charger with protection diodes to charge each pack individually. Since you don't know the state of charge each pack has, you need to keep them all isolated, since one pack could try to charge another and create large currents that could do some damage. One solution to this is to protect the packs with a 5 amp fuse on the charge connects to all the batteries. Then if you want to charge all the packs together, provided they were closed to each packs capacity, meaning you’ve been keeping all packs charged and discharged together. You can parallel them all together at the end of each run. The fuse will blow if one pack is significantly different from another protecting each individual pack. If there is a slight difference, the 5 amp fuse should be easily handled by the batteries. Now you have all packs in parallel with your fuse charge cable and can charge them with one charger.

Here is a schematic of the charge cable,

It might even be possible to leave this cable connected in the bike all the time as long as you keep all the packs together. I'm not sure how the BMS will handle this, but I suspect it will be ok with it. Then all you have is one connector to deal with. I don't advise this since I have not tested this out and don't plan too since I will be using packs in 2, 4 or 6 depending on my ride distance so the packs might not be the same at times. I will be using 2 packs most of the time since everything is so close, so the 1 hour dewalt charger is ideal for me. A drawback to leaving it installed all the time is one bad bms could take out the rest of the packs. Safer just to use an external charge cable and connect them together with some mutipin connnector.

I've been quite happy with the dewalt bms setup. I’m just running 2 packs till I get the bike finally wired. I had a nice late night ride last night to cool off from the day’s 90 deg heat and rode the packs to empty. I was close to home and just pedaled the rest of the way. I don’t have a lighted wattmeter and it’s a distraction to have to worry about the batteries anyways. Great just to enjoy the ride and let the batteries take care of themselves.

Good link, so you don't over do it with wiring size. Just follow chassis wiring guidlines.


  • Dewalt parallel charge cable.pdf
    70.9 KB · Views: 2,311
Here is something I just discovered about the Dewalt packs. Not all of the Dewalt BMS are the same. There must have been design changes. I’ve been using the packs on my scooter. http://www.endless-sphere.com/forums/viewtopic.php?f=12&t=12031 and found I did not need the inductor, but 2 of my packs would shut off every time on this scooter. I then put the inductor inline and they worked fine. I took the 4 packs that did not need the inductor and tried them on my ebike. They worked fine without the inductor on the ebike. Unfortunately there isn’t any easy way to tell which Dewalt pack require the inductors. Digikey sells the inductors as well as the rest of the parts for the boards. I’m currently just selling the bare board and spacers. If you are using the boards, you might check to see if you can omit the inductor.