Four Dewalt 36V with BMS in parallel?

[EVnewbie]

Hope everyone's weekend is well with no magic smoke,
I am getting all my electric and bike stuff together and want to build an electric chopper (Giant Stiletto) My plan is to use my 36/48V powerpack BMC motor with 35 amp controller to drive the 48 spoke 36mm wide double wall with stainless ferrel rim through 20 to 34T gears (5 speed) The gear reduction is almost figured out but I need battery power for testing and sorting the thing out.
Resting comfortable are four Dewalt "36V" A123 battery packs for my drill and saw kit. I would like not to make Frankenstein's monster out of the packs, the 36V tools are amazing and my wife would kill me. It must maintain the "WAF" or Wife Acceptance Factor so I wonder if I could just put 4 of them in parallel for a 32V 9.2 Ah pack. It would be great for the BMS in the battery packs to remain in use so not to have any worries.
The 35 amp controller will pull the 4 batteries in parallel down at a C4 rate but that is no big deal. Will the BMS in the packs shut down if connected with more batteries in parallel? The batteries are of late 2006 vintage and I understand Dewalt has made the newer ones "worse". If I go for four packs in parallel, do I need to install inductors between each one or do anything special to keep the BMS alive?
The four Dewalts are more for testing and getting all the parts that will fail, to actually fail until it is a smooth running machine. Once the chopper is proven, then I can move forward with either a 36 or 48V 20 to 30Ah LiFePO4 pack. My wife remembers every detail of my hot rodded scooter project of 2005-2006 and reminds me of those cost of those AGM 14Ah batteries. She demands BMS and knows what they do so I figure having four 32V packs of A123's finest around, she will note that I can build something without it becoming a money pit.

 

[dogman dan]

To be sure not to screw up, use the diodes to isolate the batteries. Keeping the wife happy is important stuff when it comes to funding future projects. The strain on the packs with 4 in paralell should be pretty low, about 2c, so that should be easy for tool batteries.

 

[GGoodrum]

Actually, I think you should be fine, without the diodes. I think Doc tested the BMS to about 19A, before the current limit kicks in, but you need to use the little circuit that simulates connecting this to the flashlight. The problem is I'm not sure how well the LVC function works on this BMS. The problem is that it is just too easy to get the voltage level down too low, to the point that the DeWalt chargers just flash an error code, and refuse to charge them. The DeWalt charger/BMS combo also does a piss-poor job of balancing the cells. All of mine got horribly out-of-balance.

One of my early setups (shown above...) used 8 of these packs, two in series and four in parallel. I tried bypassing the BMS, by using the outer negative terminal, but I just about killed some of the cells, over discharging them. Even being careful, it was just too easy to get the cells low enough that the DeWalt chargers would not charge them. I had to use a separate RC charger to get the pack voltage up enough that the DeWalt chargers would finally start charging them, but eventually, even that didn't work, as some cells were just too low. I had to take the packs apart and individually charge each low cell. In short order, I eventually gave up, tore all the packs apart and made 4p packs where I paralleled the cells first, and then connected them in series. I also added the LVC boards to protect them from ever becoming over-discharged.

-- Gary

 

[EVnewbie]

Thank you for the information, gentlemen!
I was thinking of putting a curcuit breaker on a custom box to hold the 4 packs. It will requiring shutting off the breaker to open the box to remove or put the packs back in. Thank you for the idea of the diodes, wonder what size I need...I am assuming it will be rather high in wattage. All 4 packs in parallel and in place should keep my BMS from tripping at 19 amps when the controller gets kicked on.
Been using these batteries for the last 2 years with my tools. They might have 20 cycles on them and are in very good shape. Read about the charger and Dewalt states to keep the packs in the charger for 8 hours to balance them about once a month under heavy use. I guess I can get another Dewalt charger so I can charge all 4 packs in about an hour or so and balance them on the weekends.
Anyone have any idea were I can get the female end for the battery packs? I am assuming the only output that matters is the negative and positive outputs so I can build a bar for them to lock down on if need be.
Now for my gear reduction problem... trying to get the party going and there is always a part missing

 

[momo]

Post by EVnewbie » Mon Apr 27, 2009 5:25 pm
Anyone have any idea were I can get the female end for the battery packs?

Good question, would be nice. Don't think anybody is using them. or i could be wrong.

 

[docnjoj]

Dewalt seems to not sell female battery ends purposely. Milwaukee does. Folks have cut up old flashlights (Dewalt) for the ends, but I think the price got out of hand.
otherDoc

 

[Doctorbass]

The dewalt BMS cut as soon as any cell reach the low point of 2.5V... and if the charger blink 3 led indicating that the battery is deffective, you just need to charge the lowered cells a bit above 2.5V and it will work again.

i'll check for you for this dewalt battery connector and ask my uncle at the service center. i should know tomorrow or monday..

I need to remind that the 19A i tested was limited by the load i applied on the battery.. I had not higeher load on hand when i did this quick test.. but probably with a lower resistance load, the current could have reached higher current..

I built my own dewalt battery testing harness that activate the BMS ( with resistors..) and i'll try hihjer load soon.

Doc

 

[64ragtop]

Would the observations and questions in this thread apply as well to the 36 volt Bosch "Fat Packs that
I hope to place in e-bike and scooter service???

Are the BMS and charging conditions similar in both the Dewalt and Bosch packs, or are there significant
differences that will require different approaches?

I plan to use a Cycle Analyst to monitor the battery conditions, so as to not over discharge the packs.

What would be a safe low voltage cut off point for the "FatPacks", please??

ATB

BC

 

[EVnewbie]

Thanks Doctor Bass!,
I would really like to just click the 4 or 5 packs in parallel and let the BMS protect them. I do have the (useless) Dewalt 36V flashlight to harvest one connector and one board from. If I get three more flashlights, will that allow the BMS to function for LVC? If I use the steering diodes across each battery pack, would this prevent the individual BMS's from shutting down because they can't "see" each other?
Been doing many calculations and I'll get another battery pack and charger from fleabay for a 32V 11.5 Ah pack using 5 Dewalt batteries. Just want a very reliable pack that will give me years of use until I can get some of those LiTi packs from Altairnano and an upgraded motor.
5 Dewalt packs would be my vision since I already have 4 of them, a brushless powerpack 35 amp motor with controller and the Giant Stiletto to install it in. Think I have the belt driven gear reduction thing figured out, now to make sure I won't smoke 4 or 5 Dewalt battery packs. They work so well with the drill and circular saw

 

[EVnewbie]

After reading this thread again, I just wanted to say "Thank You" to Doctorbass,
Since I use the Dewalt 36V stuff about 10 hours per year now (semi-retired now from military) It would be a shame to have 4 packs laying around with all that Lithium Iron Nanophosphate goodness laying around unused. I just want to sort out the pack, keep them isolated from each other, ability to click in and have the stock BMS work with the batteries. I'll test it with 4 packs then go to 5 packs for a nice 32V 11.5 Ah pack.
Seriously thinking of using an X5 5305 hub motor in stroke monkey mode with my powerpack 35A brushless controller. I require gears to change since I'll be heading to the mountains in the next year or so. The reason for the torque monster hub is I can change gearing driving the chopper mid drive to the rear 7-speed. If I get sucked into the vortex of more and more power, the X5 will provide!
Thanks for everyone's help with my attempt at 4 Dewalts packs staying intact. Much appreciated! 8)

 

[Doctorbass]

Hello John.. Here is what you need!

but you will need to remember that the max current per pack i recommand is 12A since the fuse is 15A so when paralleling them, if all BMS are activated ( all cells are still above the cut voltage) the current max you will be able to draw will be 12A x the number of pack in parallel... and

As soon as a BMS packs will disconnect you will need to substract 12A for the max current you draw on the hole pack group.

ex: 4 parallel all pack charged: 48A
one pack low and disconnected: 36A
two disconnected 24A... etc

To monitor that you can put a LED + a serie 3.3kohm resistor to it that is connected direct on the + and - of the BMS output( just before the shotkey diode you will put in serie with the +. By that way you will have a great monitoring to know how many packs are still ON to allow you to adjust your throttle to the max allowable current.

I jioned a schematic to understand how to connect your packs.. ( note that this chematic is made for serie/parallel packs so just modify it to keep the parallel section

Doc

 

[Doctorbass]

Here is the modified version i did for you! using 4 parallel pack :wink:

Doc

 

[EVnewbie]

Thank you, Doc!
My wife had this look when she saw the schematic. Then the "how much?" question about a minute later. "Uhhhh, probably $100" then add $250 for two more Dewalts packs and another charger. Of course she just wants to plug in the charger and now worry about it, etc. The boss told me to build the bike first and do the battery pack last. So for now, time to get the belt drive gear reduction pulleys, belt and freewheel crank project going. Then to the battery pack project to finish.
She is fearful of the Dewalt pack system, something about blowing them up and not being able to use all my power tools to fix a house or something. :lol: She still remembers the 36V 14Ah SVR AGM pack going out of balance and dying when I did the scooter hot rod project back in 05/06. Guess these projects involve technology, money and persuasion skills.