Dewalt A123 BMS battery interface solution

[PeteCress]

Maybe I'll take the opportunity to try a loose wrapping on the smaller ring - more-or-less helter-skelter with turns on top of turns.

The small ring works - just needed more turns.

This try was about six feet of #14 wire, twenty turns - which is as many as I could cram into the center of the ring.

It's a good thing that neatness doesn't count.... -)
View attachment DSC_3211b.JPG

 

[PeteCress]

http://www.mcmaster.com is where I buy a lot of my raw materials.

Wow! The Hardware Store From Hell...

That one's a keeper.

Thanks.

 

[quindiciottobre]

kfong, this is a great job. I'm an electronic designer and I say that this a nice circuit. The price is really honest.

I can't understand why you put that diode. Could you explain me?
But most of all I would like to say that in my opinion will be useful a circuit that drives the DeWalt BMS and its PWM. I think that we could connect directly the motor to the battery and use the internal driver. Don't you think so?

(Sorry for my English)

 

[kfong]

The diode is needed for battery isolation. It allows you to parallel batteries of different voltages or charge levels. The inductor is needed to prevent the Dewalt BMS from shutting down. On certain motors the inductor will not be needed. I was able to use the boards on a brushed Currie motor without the inductor.

It is possible to use the Dewalt BMS as a motor controller, but you would be limited to using it for brushed motors. Just replace the resistor with a potentiometer. I prefer to use a throttle with a hall effect as the control, but that probably won't interface well with the Dewalt unit. Brushed controllers are cheap to obtain.

 

[quindiciottobre]

Thank you for your reply.

Yes, you're right. I connected a hall effect throttle with the BMS and in effect I had to put an opamp circuit to extend the 1-4V out of the hall sensor to the 0-5V input of the battery circuit.

I'm building a mini moto for my son. I will post some photo.

 

[The Mighty Volt]

I like this a lot. How confident are you that the bugs have been ironed out? Thanks and well done.

 

[kfong]

Not sure what you mean by bugs, I’ve been using the boards on my ebike and scooters for over 2 years now. The only problem you have to worry about is leaving the packs plugged in for over a week, especially if the packs are already discharged. This is because the Dewalt bms requires a small amount of power to work and is enabled once the packs are plugged in. Some of the newer Dewalt packs shut down completely, so it’s not an issue.

I don’t encounter this problem because I take mine out after each ride and just plug it into the Dewalt charger so it’s ready when I take my bike out again. Unlike Nicads, you don’t need to cycle these packs, so I usually have them topped off after each ride. There are already a lot of users on this forum who are using my interface boards.

 

[PeteCress]

Not sure what you mean by bugs, I’ve been using the boards on my ebike and scooters for over 2 years now....
... There are already a lot of users on this forum who are using my interface boards.

I'm one of the people using them, and have had zero problems. None, nada, zip, zilch, bupkis....

I'm also one of those people who can mess up a wet dream: if something can go wrong in use, I'll make it go wrong.

There's probably a job for me somewhere in product testing....

 

[Kevinator]

I've been asked to post some more details on how I mounted the interface boards.

Here are pictures of the battery tray I built. I used lexan for the carrier and a lexan spacer between the fiberglass board. The lexan spacer was needed to provide a gap for the screw heads, I would omit it if I could. Wood or aluminum channel could be used. It doesn't have to be as elaborate as my setup. The interface board is epoxied while plugged into a pack so the glue can set while in final position. Once done, the batteries just plug in and lock into position.

Great setup. Those battery trays are really nice! Are you making extras to sell? I'd be interested in a few.

 

[kfong]

I really wasn’t planning on making them since I would need to source the material. I can charge $20 per holder if you drop ship the material to me. Email me for the address. Each part is .5”x 3.25”x4” Leave about .25” extra in material on each side except for the .5” That will allow me to cut and clamp it. Each part will need to be setup, so there really isn’t any good way for me to automate it. Just choose the material you want to use. If you have to get a larger piece. I can just exchange material for services or mail the extra to you.

I've been asked to post some more details on how I mounted the interface boards.

Here are pictures of the battery tray I built. I used lexan for the carrier and a lexan spacer between the fiberglass board. The lexan spacer was needed to provide a gap for the screw heads, I would omit it if I could. Wood or aluminum channel could be used. It doesn't have to be as elaborate as my setup. The interface board is epoxied while plugged into a pack so the glue can set while in final position. Once done, the batteries just plug in and lock into position.

Great setup. Those battery trays are really nice! Are you making extras to sell? I'd be interested in a few.

 

[scriewy]

something i wanted to ask gazillion years ago (these torture me for a few months now), can few BMS's parallely be hooked to 1 interface board ? (i mean surely can be wired parallely but will they be activated or the interface board average resistors value will greatly change while current be passing through few BMS's, like the more different electrical components (from more BMSs) current passes the more it'l change.
i guess it's possible to use 1 BMS 1 interface board for 8s and what ever parallel number, but be more risky on the cells the more parallels.
when the BMS is enabled (1 BMS 1 interface board hooked) are the pos neg terminals on the BMS live ? (i mean can i grab them from the BMS terminals to which the interface board is plugged). or they become live only after exiting the interface board ?
like the flashlight interface jk1 is using, theoretically thats what most of dewalt tools do, activating the BMS ?
you think the dewalt charger activates the BMS as well in same manner and after that simply pumps the juice through pos neg of it's interface to charge as we do when we drain the pack after activating the BMS ?

also about the parallel charging mentioned with 1 charger, when pack is full the BMS shuts off (has HVC) or the charger shuts off sensing packs voltage ?

 

[kfong]

You need one interface per Dewalt battery. The interface board turns on the Dewalt pack and allows you to parallel the batteries without the batteries damaging each other due to difference in charge conditions. The interface board provides battery isolation, this means you don't have to worry about plugging in a fully discharged pack with a charged pack.

I have not explored how the charging circuit works. That information isn’t readily available from Dewalt.

 

[cassschr1]

Kfong, would this be of any help to the circuit===a zcat 2035-0930 from TDK some kind of snap on line filter, its not magnetic either, came from a rear projection tv by the color projectors. GUYS ,THERE IS NOTHING WRONG WITH HIS BOARDS. probably shouldn't post here but what does it do.

 

[scriewy]

and after the interface board activates the BMS and the power is open can i charge the pack with something like iCharger or GT A8 while using same current rate as dewalt charger would ?
this question looks pointless, as if id use those chargers i can simply rout out few wires and let the charger do all the work, the question is in case i want to stay in safe framework of warranty and do everything through the BMS.

 

[kfong]

Those snap on ones work with high frequencies, but I don't think they can provide enough inductance. That is why we wrap around them to create more inductance.

Kfong, would this be of any help to the circuit===a zcat 2035-0930 from TDK some kind of snap on line filter, its not magnetic either, came from a rear projection tv by the color projectors. GUYS ,THERE IS NOTHING WRONG WITH HIS BOARDS. probably shouldn't post here but what does it do.

 

[kfong]

To charge with a normal charger, you can tap off the Negative fused output instead of the Neg FET output on my interface board. This is a direct connection to the battery with a protective 15amp internal fuse incase you have a mishap. I don't recommend parallel them this way unless you know the packs are equally charged. As long as you keep the same packs from a run, it will work charging them all in parallel this way, but you will need to make a charge cable separate since you are bypassing the isolation diode.

and after the interface board activates the BMS and the power is open can i charge the pack with something like iCharger or GT A8 while using same current rate as dewalt charger would ?
this question looks pointless, as if id use those chargers i can simply rout out few wires and let the charger do all the work, the question is in case i want to stay in safe framework of warranty and do everything through the BMS.

 

[Kevinator]

I received my boards from kfong last week and the boards look great. Thanks for getting them out promptly!

I put an order in with digikey for the parts to build my boards. The premade one is working fine, and I can confirm these interface boards work with 28v dewalt packs. I haven't tested it on my ebike yet, but the interface board does power up the pack.

 

[Papa]

To charge with a normal charger, you can tap off the Negative fused output instead of the Neg FET output on my interface board.

Having just purchased a few 28v DeWalt packs, i'm also interested in your boards. However, having endured the "remove batteries - put in charger - reinstall batteries in bike" hassles on previous builds, I'd like to simply charge the batteries on-board. I'm planning 2p packs. Suggestions?

 

[kfong]

You can charge the batteries from the 15amp fused negative power and postive power connection. You would need to use a hobby charger that can charge 10S A123 cells. This would take a while since you will be charging them all at once. The charging harness will need to be removed from the boards since you are now bypassing the diode protection circuitry. This can be easily done by using a multipin connector for the charge port. When the charge connector gets unconnected the wires to each 15amp fused connection and positve connection now gets separated. The only drawback to this solution is the BMS is always on and can drain the batteries if left for long periods of time on some of the older Dewalt batteries.

The Dewalt charger can charge a pack in less than an hour. I still find this the fastest way to charge the packs. I have 4 chargers and can get back on the road the fastest this way.

 

[pwbset]

The Dewalt charger can charge a pack in less than an hour. I still find this the fastest way to charge the packs.

True, but these chargers are horrible at balancing so at least every so often leave them on overnight. I'm already seeing wild voltage swings between packs on the "one hour" charge method and one charger seems to want to bring my 28v pack to 30+v every time. Bad charger (or pack BMS)!! Anyway.. so ironic that the best cells available were mated with complete DeWalt junk. :|

 

[kfong]

I have not even bothered to balance them, as long as I'm getting the capacity the chargers seem fine to me. I think the balancing stuff isn't that critical with the A123's. I'm going on 2 seasons now and have yet to worry about the packs. They are performing consistently for over 2000 miles. If Dewalt was that concerned, I'm sure they would have put in a better BMS. This is their top of the line tool so they must have done enough testing. I use A123's for my RC stuff, and I don’t even bother to balance those. Lipo’s on the other hand I do balance.

The Dewalt charger can charge a pack in less than an hour. I still find this the fastest way to charge the packs.

True, but these chargers are horrible at balancing so at least every so often leave them on overnight. I'm already seeing wild voltage swings between packs on the "one hour" charge method and one charger seems to want to bring my 28v pack to 30+v every time. Bad charger (or pack BMS)!! Anyway.. so ironic that the best cells available were mated with complete DeWalt junk. :|

 

[pwbset]

I think the balancing stuff isn't that critical with the A123's. I'm going on 2 seasons now and have yet to worry about the packs. They are performing consistently for over 2000 miles.

Good to know thx. Admittedly I'm only a few cycles into my 28v pack ownership, but seeing resting pack voltages off the charger ranging from 28v up to 29.8v isn't inspiring confidence in me yet with these chargers. :wink: I will say that I sucked 8.8ah out of 4p packs yesterday and they still felt fairly strong so if I'm getting 2.2ah/pack already after only 5-10 cycles out of cells from 2007 I'm a happy kid.

 

[kfong]

Resting voltage won’t really indicate balanced cells. These batteries have a surface charge that gets depleted rather fast. If you have older abuse cells the surface charge drops rather quickly. Because of this the resting voltage can vary quite a bit. The load voltage is what is more critical of cell performance. After you deplete the surface charge and then measure the pack. You will see a more consistent number. They should hold steady 3.3-3.0 volts all the way to the end of cycle depending on how much load you are drawing.

 

[pwbset]

They should hold steady 3.3-3.0 volts all the way to the end of cycle depending on how much load you are drawing.

Again.. good to know thanks! Slurped 7.4ah this morning on the way to work and I just checked and after 6hrs the pack is sitting at 52.4v / 16s = 3.275v/cell. Works for me.

 

[The Mighty Volt]

Hi, do you have any interfaces available, and if so, how much are you charging for them. Please feel free to PM me. Many thanks indeed.