Avoiding DeWalt BMS Meltdown (questions)

[jdbauman]

Hi All,

I think I want to build a bike with a 408 Front Hub Motor. I would like to use the Dewalt 36V battery packs without opening up the packs (so as not to void the warranty). I would like to combine the packs in a combination of Series and Parallel connections to get approximately 66V and 7Ah.

I know that others are doing this already, however there seems to be a problem of blowing the BMS unless you bypass it when discharging the battery. My question is, is it the increased Voltage or Amperage that causes the problem with the BMS.

If it is the Amperage, would I eliminate the problem by using the 36-72V (20 Amp) controller instead of the 36-72V (35Amp) controller.

Or

Is there another way to control the Voltage and Amperage to keep it within safe limits, so that I wouldn't blow the Battery BMS.

I would appreciate any helpful ideas on how to protect the battery BMS without alteration to the batteries. Thanks Guys!

Jer

 

[OneEye]

The individual packs are internally fused at 15A. You will need to parallel sufficient packs to avoid blowing one.

I don't know why folks are reporting problems with their setups with 20s?p (2 packs in series) setups. The battery has no idea what its external voltage compared to ground is, only the voltage difference between the two ends.I'm assuming they are either overamping them or simply overdraining one of the 20 cells in series and that is causing their problems. It might be a low voltage cutoff is not sufficient to protect a slightly weaker cell when you have 20 cells in a string and such a drastic end-of-discharge voltage drop.

 

[Jozzer]

It seems that by just using the + and - connections on the BMS output doesn't provide any protection at all. It will happily let the cells drop to below 2v (at least the only one a dared try did ). There are several other pins out of the pack though, perhaps it needs to be "jumpered" somehow?

 

[OneEye]

Correct,

the (+) spade connection is direct to cell, and one of the two (-) spade connections is "direct" to cell via the 15A fuse. Using these connections provides NO overdischarge protection. You have to provide a different method to protect from running them down too far.

Does anyone know the DeWalt recommendation for end-of-discharge voltage on their 10 cell pack? I've read a 2.0V minimum for a single cell, but with the precipitous voltage drop at the end of discharge I would imagine you need a cushion in a 10s pack to keep a weak cell from being pushed well below 2V before the pack reaches 20V.

 

[magudaman]

I thought when people where hooking up their pack in parallel using the spade connections is cooks the BMS. Something about higher voltage feeding back into the pack is not liked.

 

[OneEye]

Bizzare. I wouldn't expect that.

According to the schematic Doctorbass posted, the two spade connections mentioned don't even run through the BMS, just through the fuse. There is a third spade connector that is controlled by a transistor on the BMS.

Ross over on the revived visforvoltage forums (is that a forbidden subject btw?)is running 72V on 4 packs (2 series, 2 parallel) and doesn't report any problems. I don't know if he had some problems early on that he has overcome, but he seems pleased enough with his setup to be posting it on youtube and an alternative energy/environmental website.

I have heard crossed wire reports where they let out the magic smoke, but I had thought those were specifically wiring mixups.

 

[knightmb]

Ross over on the revived visforvoltage forums (is that a forbidden subject btw?)is running 72V on 4 packs (2 series, 2 parallel) and doesn't report any problems.

Nope, not at all. There are some that had a "bad experience", but this forum, just like the other represents a place to get like minds together to discuss these subjects in a public forum. I post over at visforvoltage from time to time and myself will respect the rules and such they have in place. So, I don't consider them to be the nemesis of this forum and we certainly discourage any bashing here (as well as other subjects).

This isn't really to side-track this topic, but it's good for me to state this every now and then. :wink:

 

[Doctorbass]

Bizzare. I wouldn't expect that.

According to the schematic Doctorbass posted, the two spade connections mentioned don't even run through the BMS, just through the fuse. There is a third spade connector that is controlled by a transistor on the BMS.

Ross over on the revived visforvoltage forums (is that a forbidden subject btw?)is running 72V on 4 packs (2 series, 2 parallel) and doesn't report any problems. I don't know if he had some problems early on that he has overcome, but he seems pleased enough with his setup to be posting it on youtube and an alternative energy/environmental website.

I have heard crossed wire reports where they let out the magic smoke, but I had thought those were specifically wiring mixups.

I personally tried 2s2p with dewalt pack and again tonight i had fun with that (2560W peak... and wonderfull acceleration) I eat some honda civic on the first 250ft

I never had problem!... I only use the direct connection on the battery post and not passing thru the BMS fuse or internal transistor by activating the output... About the Low Voltage Cutoff, i sinply put a voltage meter on the each parallel group to avoid overdischarging under 25V.

Doc

 

[jdbauman]

So let me see if I have this straight...

If I want to use DeWalt (36V Batts) in parallel, and do not want to make any mods to the batteries, then I need to make sure that I hook several of them together (the more the safer) so that the current demand from each of the batteries never exceeds the capacity of the 15Amp fuse in each battery. Is this correct?

So if I have 4 batteries in Parallel, and my total current demand is 40Amps, then I should be drawing only 10Amps from each battery. If I only have 3 batteries, then my current draw from each battery would be 13Amps.

If this is true (please let me know if it is), then to make sure that I do not harm my batteries, then I would be safer using a 36-72V (20Amp) controller, rather than a 35Amp controller, because I would only be requesting 20 Amps of current at any given time. That current would then be divided by the number of batteries I have to come up with the drain for each battery.

Is this correct? When hooking up in parallel, do the batteries always divide the current demand equally between them? or might one battery be supplying 15Amps and another battery only 5...

Thanks for all your help!

Jer

 

[Doctorbass]

do the batteries always divide the current demand equally between them? or might one battery be supplying 15Amps and another battery only 5...

Thanks for all your help!

Jer

depending on how you are lucky, your battery couls have all the same capacity over many years.. or not....

even if they are not the same capacity, the problem may occur and some battery could pull more than 15 A... in case of one or more of your packs would have a bad cell and that the current become low from those.. and more from the others...

to Really avoid this problem, just add some 10A blade fuse on each battery. the 10A from car usually allow 12 or 13A.. but will blow for sure at 14-15 if current is sustained.

by this way, you will protect your battery pack.

 

[jdbauman]

Would this device work for connecting 4 Dewalt batteries in parallel? Could you just plug in your batteries to 4 of the connectors and then plug in you controller to one of the connectors?

Jer

http://www.powerwerx.com/product.asp?ProdID=15761&CtgID=3823

 

[jondoh]

Hi Jer,

Maybe you should also consider e-moli. Maytag and I have been using these for some months without problems. In my opinion, they are perfectly suitable for ebikes for 20 amps (more if you have more in parallel). I personally there's a sweet spot at 48v and 20 amps for most e-bike applications anyway.

The problem I see with A123 dewalts are the touchy BMS and the fact that the cells don't hold as much energy.

one cell of A123 = 2.3 ah compared with e-moli = 2.9ah

The nominal voltages of A123 is also a bit lower than e-moli.

The A123 rocks e-moli at discharge rates but it ultimately comes to what you want. Do you want to further or faster? If further, go with e-moli (milwaukee electric). If speed is what you're really after, A123 is your best choice.

The attached video shows my e-moli pack. They're stock packs connected 2s3p. They can push my e-bike 28 mph and have a range of 20 miles at 20mph. Maytag has the same set up which can go 32 mph and 21 miles at 21mph.

Jon

 

[CGameProgrammer]

The DeWalt packs use 22-gauge wire for the main power leads, which is pathetically small, so I would definitely not use them for output. Luckily, the two leads are soldered to the batteries right on the top outer edge of the battery, meaning you can very easily solder something else there without even removing the BMS. I plan on using 12- or 14-gauge wire but will leave the existing wires in for the BMS to use when charging.

The cells are connected together with tabs; no wires are used among them, so there's no need to remove or rewire them.

I'm thinking I might remove the BMS (which isn't fastened to the pack at all besides with the wires used to connect it) and use molex connectors to plug it in dynamically with a pack. That way I save space by keeping it off the bike when riding. Since the 10 balancing wires are already hooked up in connectors, the only wires to disconnect are the two power wires, and possibly the thermal sensor which is simply taped on one of the batteries.

How important do you think the temperature sensor is? I'd like to just ignore it completely, which would result in the BMS always thinking the batteries are cool. Does it rely on temperature when charging? I'm assuming it does not since the sensor is only "connected" to one of the batteries.

 

[Doctorbass]

Guys, Always remember this ! :

Never unconnect the balancing harnes wire first!!! You need to unplug first the POS wire, then the NEG wire and finally the Balancing harnes wires.

Same for connecting. Always connect the harnes wire first. then the NEG and POS wires.

You could damage seriously the BMS if you forget that. I damaged two of those. The King of the dewalt BMS on Vis for voltage was the first to experience that.

Also, the ground wire from the BMS case only need to follow the NEG wire.

Hope that can help!

Doc

 

[CGameProgrammer]

I removed the balance connectors for the BMS of one of my batteries, then plugged them back in. I never disconnected the power wires. Do you think I messed up the BMS somehow? Why/how?

 

[CGameProgrammer]

Bump. Still eager to learn the answer to my question. I'd be surprised if it really mattered; the balance connectors are only used when charging... or possibly discharging, but the battery was doing neither.