GUIDE: Modifying the DeWalt A123 charger to not be useless

Batteries, Chargers, and Battery Management Systems.
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Post by CGameProgrammer » Sep 10, 2007 12:11 pm

It would be really great if you could monitor the voltage for each cell while discharging to see what happens. At 20A it'll take you 7 minutes to drain the battery, so you can discharge and watch each cell, turning it off when any cell voltage gets too low if the BMS doesn't do it itself. Then we'll know exactly what effect the BMS has on discharge.

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Post by fechter » Sep 10, 2007 1:52 pm

That sure is a mighty big resistor you got there Doc 8)

I guess if you were running a brushed motor, you could use the PWM as your controller.
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Post by Doctorbass » Sep 10, 2007 11:36 pm

+++++UPDATE+++++


Officially, THE LOW VOLTAGE CUT OUT WORK WELL !!!

I just connected the same circuit as shown above and i watched the voltage drop on the main terminal by monitoring the output POS and NEG(I monitored the neg from the from the 15A fuse that is not passing thru the mosfet)

The other NEG was for sure.. connected to the resistor!

I got:

24.2V cut out Voltage and the current faded to zero from 26.5V to 26.0V at 19A.

At 7A i got a cut out voltage of 26.5V

At 24V, the current decreased completly and the voltage rise a bit after that up to 26.97V

That make sense since the dewalt charger detect bad pack at 25V(when the 3 led flash together)


NOW YOU CAN OFFICIALLY:

=== USE THE BMS AS LOW VOLTAGE CUTOFF===


I posted a video here:
http://www.youtube.com/watch?v=vituFUmEnU8


:wink:

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Post by CGameProgrammer » Sep 11, 2007 7:47 am

That makes sense; the increase in voltage is simply because it's no longer under load. Anyway good test and informative video; it's nice that the BMS smoothly lowers current for a few volts. Also nice to know the "official" low voltage -- 26V no-load.

Today I used my eight DeWalt battery packs for the first time, using stock packs but with bypassed BMS, in a 2s4p configuration (20s4p in terms of cells). Initial voltage for each pack was between 34.8 and 35.6 (it varied). After using up 5.2 Ah (out of 9.2), I tested the voltage of each battery pack:

32.7
32.8
32.8
32.8
32.8
32.8
32.8
32.8

And that was without the BMS. So these things really do self-balance impressively; I think Doctorbass is right, it's safe to hook individual cells up in parallel and charge them as one. Of course I didn't measure each cell, only each pack, but given the above results, I think the odds are high that the cells are in balance as well...?

By the way, with each pack pushing 10 amps (40 total), the voltage under load was 31V per pack initially, and was 30.5V per pack at the end. So minimal sag, but the true voltage is much lower than the stated 36V.

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Post by OneEye » Sep 11, 2007 3:48 pm

Thank you. A beautiful test! Applause to Doctorbass for jumping in and running a solid test for us.

I like your final conclusion the BMS will step in and reduce power and eventually cut off the pack as it nears a low voltage condition. It's also nice to see there isn't a 15A limit on the pack in its pristine condition: you can avoid the 15A fuse on the outer spade terminal by correctly hooking up the other pins of the BMS, raising the output to almost 20A.

To clarify for me... does the BMS shut down the pack temporarily if the load attempts to draw too much current? or does it provide a current limiting mechanism instead?

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Post by Jozzer » Sep 11, 2007 5:15 pm

Yes, good work Doc, without a decent load to use on the workbench I never would have got to the bottom of it. As it was I had a full evening tryng to balance a bad pack before it went out.
With the cutoff so low, it must be very easy for the system to flatten the lowest cell completly unless it IS monitoring each cell too. When an imbalanced pack gets run down on a 1A load just to 29v on a poor pack with no bms, usually 9 cells are over 3.2v, and 1 cell is less than 1v (unless watced). When put back in the charger the dewalt charger refuses to charge, despite overall pack voltage to be above what you mentioned was the cutoff for the charger. Did you by any cance try it with a pack with 1 cell not fully charged?
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more charging current

Post by rf » Sep 19, 2007 5:17 am

Has anyone looked into cranking up the current on the Dewalt charger to charge a couple packs simultaneously at a faster rate?


Richard

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Re: more charging current

Post by Doctorbass » Sep 19, 2007 8:02 am

rf wrote:Has anyone looked into cranking up the current on the Dewalt charger to charge a couple packs simultaneously at a faster rate?


Richard
That would be difficult. that cahrger pulse the current to the batt and the circuitboard is not built to hold more current than 3A. You can cahrge many pack with one and having only one of the pack with the BMS, but the time to cahrge them will be proportional to the number of pack.

Also, the internal BMS of the pack is not built to be able to balance and protect the cells with higher current.

By my experience, balancing cells always give better results when charging at low rate.
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Post by fechter » Sep 19, 2007 9:21 am

Do you think you could parallel more than one charger? Certainly it would work if you had parallel packs and split them for charging. I'm just wondering what would happen if you kept the cells in parallel and paralleled some chargers. This tends to work OK for lead-acid chargers.
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Post by Doctorbass » Sep 19, 2007 9:53 am

Like i said, this charger pulse the current to the battery pack fonction as the level of charge is reached. The dead time between the pulse is used to measure the cells voltages and it re-adjust the cahrge rate if it is needed. This cahrger have more than two pin to connect to the battery. it have 8 active pins for some digital/analog feedback between the BMS and the charger. Because of that it would not be suitable to join two cahrger together and partalleling these pins. (for exemple, you would not try to plug two parallel port from a PC with an Y to use two printer in the same time with one computer!... digital data comunication can't be paralleled.

You can use only ONE BMS on a dewalt charger. I already found on the internet a guy that tried to parallel two BMS together on the input pins... he blowed those.... that was on the RC universe or RCgroup forum if i remember...

By paralleling ONLY the cells on a BMS out pins would simply share the current thru the cells and avoid problem. I thing it have a constant current sens so the load on the balancer pins and the out of the BMS to the battery pack are not overloaded.

If your goal is to cahrge faster than 45min ot 1h for one pack, then use a RC charger with balancer with at least 10s or buy a switching powersupply like Condor built (they are excellent) and chose the optioin with V and I linit adjustment... then buy an external balancer with 10s.

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Post by CGameProgrammer » Sep 19, 2007 1:11 pm

How about amplifying current between the BMS and the battery pack? So the normal amount of current goes through the charger and the BMS, but it gets amplified before being sent to the cells themselves. If that could be done, it would be a great way of taking advantage of the cells' ability to be quickly charged.

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Post by GGoodrum » Sep 22, 2007 2:34 am

So, I've now read this thread three times, and I think I know what to do to in order to enable the LVC function, and allowing current loads of 19A, but I have a few questions.
  • First, if a pot is used, connected between pins 1 and 7, with the center connected to pin 5, what is the value this pot needs to be?
  • The inductor does not need to be used, right?
  • Once the cutoff occurs, will it reset automatically if the voltage rises back up above some value, or does the circuit need to reset in some fashion?
I want to add this circuit to this mount:

Image

One thing I'm also wondering about is what will happen if the BMS is used in this fashion with packs in parallel? Should the Schottky diodes be used to isolate these, or does it matter?

Great stuff, really. I've been wondering what the heck the BMS does in these packs. All I've done for the last year is cut these off, and strip out the cells.

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Post by GGoodrum » Sep 25, 2007 1:28 pm

I have another question. I assume you can use two BMS-enabled packs in series, as each LVC function in each BMS will work independently to protect each pack. What about if these series connected BMS-enabled DeWalt pairs are paralleled, in 2p, and/or 4p configurations? Would it matter? If it is a problem, would using the Schottky diode isolators help?

Normally, when you parallel two packs, the voltage ends up being the average of the two. I always assumed this would be roughly analogous to having the level of water in two containers equalizing, if they are connected. To make that happen, some water has to flow from the higher level container to the lower level one. I would think the same sort of thing happens with two paralleled batteries. Is this the case, what happens if you parallel two BMS-enabled DeWalt packs? I guess another way of asking the question, can you recharge the cells through the switched connections? I would think not, but what happens if you try?

Interesting. :)

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Post by OneEye » Sep 25, 2007 2:23 pm

I thought it was more because the current in each parallel system would be in some way proportional to its ability to supply that much current at that voltage, so a 'weaker' battery that would normally experience a high voltage drop at moderate loading will not carry as much current, it will only contribute current at the rate available for the shared voltage. In the same way, the stronger cell will be contributing a larger share of the current as it can provide this current without as much voltage drop.

The neodymics schematic is a good jumping off point for a serial/parallel 72V system. You should be able to add more parallel branches based with the same basic philosophy.

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Post by GGoodrum » Sep 25, 2007 3:21 pm

OneEye wrote:The neodymics schematic is a good jumping off point for a serial/parallel 72V system. You should be able to add more parallel branches based with the same basic philosophy.
Yep, that obviously works, so adding the diodes is probably a good thing. I'm just curious if it is really necessary.

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Post by rf » Sep 26, 2007 12:09 am

GGoodrum wrote:
OneEye wrote:The neodymics schematic is a good jumping off point for a serial/parallel 72V system. You should be able to add more parallel branches based with the same basic philosophy.
Yep, that obviously works, so adding the diodes is probably a good thing. I'm just curious if it is really necessary.
The resistor network is the more important part in my opinion. It allows you to use the BMS to shut off battery output when voltage of any cell drops below minimum -- thus keeping the battery from damaging itself.

The diodes protect you from dumb mistakes, like not charging all the batteries evenly.

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Post by GGoodrum » Sep 26, 2007 1:09 pm

I'm still confused about a couple of things with the BMS. First of all, where is the fuse on the unswitched negative terminal? I took the BMS out of one of the DeWalt case carcusses I have laying around (about 15-20, leftover from the RC and eBike packs I've made...), and there is nothing visible. It must be internal. If so, what's the point of having a fuse if the whole BMS is trashed if it blows?

I'm also confused about whether or not the values for the resistors actually control the current limitation function. If the values for R2 and R3 used in the Neodymics PDF are used (7.7k and 8.5k, respectively...), I assume the PWM circuit will reach 100%, but will the current be something less than 20A that Doctorbass was able to achieve, or do you really need to use the 7.26k/7.28k values?

I was going to try a 10k pot in series with a 5k resistor, but what I'd really like to do is just use two fixed resistors, but this will be hard to do if values HAVE to be exactly what Doctorbass came up with in his test.

Finally, which negative blade terminal does the charger use to supply the main charging current, the switched one, or the unswitched/fused one?

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Post by OneEye » Sep 26, 2007 1:28 pm

When wired as discussed there is really no current moving through the unswitched spade of the pack, it merely serves as a ground reference for the rest of the circuit. Hopefully the risk of blowing the fuse is limited. The 15A fuse on the unswitched terminal is an SMD fuse buried in the potting. Doctorbass has dug down to it on a pack he blew, and there is a picture on one of his posts showing its location. The fuse can also be bypassed by shorting the unswitched terminal to the metal of the pack casing, and adding a fuse external to the pack.

https://endless-sphere.com/forums/viewtopic.php?t=1427

NeodymicsEngineer points out in a different thread the drill trigger changes the relative values of the resistors, so there is probably a bit of play beyond the values posted. Maybe you can play around with some resistors that are close in value to Doctorbass' final values and let us know your experience.

I haven't seen an answer to your third question.

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Post by GGoodrum » Sep 26, 2007 2:40 pm

Okay, this helps, thanks. I'll try the 5k resistor/10k pot idea first. Hopefully I can come up with a couple of readily available fixed values that work.

I'm also going to skip the schottky diode isolators for now as well. With the protection the BMS provides, even if one series pair ends up with a bad cell the worst that will happen is that the other series pair(s) will have to work a little harder, and will discharge faster.

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Post by rf » Oct 02, 2007 8:46 pm

GGoodrum wrote:Okay, this helps, thanks. I'll try the 5k resistor/10k pot idea first. Hopefully I can come up with a couple of readily available fixed values that work.

I'm also going to skip the schottky diode isolators for now as well. With the protection the BMS provides, even if one series pair ends up with a bad cell the worst that will happen is that the other series pair(s) will have to work a little harder, and will discharge faster.

The original resistor values seem to work just fine. The intent is to peg the MOSFET at full output, which they seem to do. When the BMS enable goes high the MOSFET and raw outputs have the exact same voltage.

On the diodes -- not sure how the MOSFET outputs will take to being put in parallel or series together. Haven't looked into that yet, and may not. Diodes are available for a couple dollars a piece. Cheap protection against a variety of problems and mistakes.

The thing I don't like is the CycleAnalyst shuts down when the last battery hits cutoff.

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Re: GUIDE: Modifying the DeWalt A123 charger to not be useless

Post by rf » Apr 22, 2008 10:17 pm

The best way to make the DeWalt charger useful is to clip the power cord right next to the case and use what's left as a doorstop.

Richard

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Re:

Post by ejonesss » Oct 05, 2008 6:32 am

i thought you said that the internal bms would not work or be good enough?

here you say it has an lvc i am confused.

can someone set the record straight.

GGoodrum wrote:I have another question. I assume you can use two BMS-enabled packs in series, as each LVC function in each BMS will work independently to protect each pack. What about if these series connected BMS-enabled DeWalt pairs are paralleled, in 2p, and/or 4p configurations? Would it matter? If it is a problem, would using the Schottky diode isolators help?

Normally, when you parallel two packs, the voltage ends up being the average of the two. I always assumed this would be roughly analogous to having the level of water in two containers equalizing, if they are connected. To make that happen, some water has to flow from the higher level container to the lower level one. I would think the same sort of thing happens with two paralleled batteries. Is this the case, what happens if you parallel two BMS-enabled DeWalt packs? I guess another way of asking the question, can you recharge the cells through the switched connections? I would think not, but what happens if you try?

Interesting. :)

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Re:

Post by ejonesss » Oct 05, 2008 11:26 am

the stock bms does seem to have an lvc ( i did a test where i have a pack apart and i removed the 2 caps off the cells so that the taps are not making connection and tried enabling the bms and it would not work soo i re connected the end caps and it worked).

i have not found out the cut off point yet but i think it is 2.5 volt per cell.

a video on youtube
[youtube]http://www.youtube.com/watch?v=vituFUmEnU8[/youtube]

shows the lvc in action on a 36 volt battery and at 24.3 to 24.2 the load is cut off

the math 24.2 volts divide by 10 cells is 2.42 volt.

2.42 volt per cell times 8 cells for a 28 volt pack should put it around 19.36 volts.

the test load is a 24 volt 250 watt scooter motor salvaged from a razor mini chopper motorcycle/scooter.

the math may vary slightly since the actual cut off is difficult to determine with standard meter .





OneEye wrote:Turns out not to be a parallel charging setup as I had originally thought, so sorry this wound up in the wrong thread. Hopefully it proves useful all the same.

Running through the BMS, the neodymics setup isn't drawing more than about 12A from the pack at any time (2s2p), and I don't know if they confirmed the BMS will provide a cutout for a pack at low voltage or a cell that is below the recommended cutoff voltage. If it does that would be great.

Essentially this could be a warranty-proof way to use the packs. You are not discharging them more than spec, and are interfacing with them the same way a power tool does.
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Re: GUIDE: Modifying the DeWalt A123 charger to not be useless

Post by GGoodrum » Oct 05, 2008 4:54 pm

Old posts, old info. A lot of us are a lot smarter now. Trying to use the DeWalt packs as is, along with the internal crap BMS module, will not yield a very satisfying experience, in my opinion. Feel free to prove me wrong. :)

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Re: GUIDE: Modifying the DeWalt A123 charger to not be useless

Post by ejonesss » Oct 05, 2008 5:58 pm

GGoodrum wrote:Old posts, old info. A lot of us are a lot smarter now. Trying to use the DeWalt packs as is, along with the internal crap BMS module, will not yield a very satisfying experience, in my opinion. Feel free to prove me wrong. :)

i think i will just wait for the board kit to come out and get the parts so i can get something better
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