And if you do this you want to use a really really beefy switch. You don't want to have the switch contacts welding themselves shut.
- David Herron, http://davidherron.com/
- David Herron, The Long Tail Pipe, davidherron.com, 7gen.com, What is Reiki
- Electrified Electra To
Hi Brian and others, I have swapped a known good BMS into the "repaired pack" and it charged fine on the Dewalt charger. So far I feel the charger is okay although I wish the low voltage rejection (that causes bad battery indicator) was set for a lower pack voltage. I want to build a 5-pack/1 charger setup like Brian's next. I don't plan on using the Dewalt pack as-is but rather a custom setup which I will eventually show here in this thread. Jeff K., Burbank, CA
Jojo - I think I prefer Chas's DPDT Switch Configuration to mine. Using his his nice symmetrical configuration, there is no way you could tie a live charger into one of your 36V Battery Packs while the motor is on. You also get a less confusing wiring setup.
Glad you discovered the problem with your Dewalt battery pack :) . Nice to salvage such a pricey investment.
Did you ever get your failed individual cells revitalized? Did it involve disconnecting the BMS module and then putting it in parallel with a good cell?
A new ebay listing you may want to check out to replace that burned out bms module:
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Hello Brian, thanks again for the connectors. I am hoping to parallel my dewalts to charge from a single pack like you have. I just have a query about this method. If you are connecting the balancing wires in parallel to a BMS that is designed to balance ten individual cells doesn't this mean that each balancing wire from the BMS is effectively balancing a group of cells - one from each pack, and therefore each of these cells could get out of balance with the others. Won't the BMS be taking an average voltage from each group of cells? I don't know a great deal about the physics of this nor how these modules balance the cells but I just wondered if it was an issue.
regards, Micah (greengadget)
Another Question for Mr Aerorider
Do you know if it's in anyway possible to modify the bms to charger more than 10 cells in series? say with a different charger?
Sounds like many failed BMS modules may only need their fuses replaced/bypassed. No need to replace the whole thing.
By tying all the balancing ports from all the batteries together in parallel, the paralleled cells themselves are self-balancing. On initially hooking one pack's main powers and balancing ports together, the charge gets redistributed between all the parallel cells until the voltages of all the parallel cells are the same. Thereafter, they always stay the same in lockstep during charging and discharging. [That said, before you hook them in parallel, it is a good idea to charge and balance your parallel packs separately using an overnight cycle on the Dewalt charger, so that the cells to be paralleled are not too mismatched when first connected. If you don't you might melt some connectors as these cells balance each other initially.]
Now, you are right that the BMS is balancing a group of cells in parallel at each voltage level, and there must be some repercussion.... I think the repercussion, is that if the series cell voltages get mismatched a certain amount in the parallel pack system, it will take take longer for the BMS system to balance them than if there was only one pack out of balance by the same amount, I think this will be factor of time directly related to the number of packs in parallel.
However, because the parallel cells also self-balancing in the first place, I don't think they will get as unbalanced as a single pack would in the first place, possibly a factor of unbalance inversely related to the number of packs in parallel, possibly resulting in taking the same time to balance the parallel pack in practice. I am hoping this is the case anyway... I don't have enough data yet to know for sure how unbalanced a 5 pack will get and how long it will take to balance the whole 5-pack.
I hope that is clear.
I am not sure if I understand your question. The BMS module of the Dewalt pack itself is really only able to work with 10 cells in series.
Now looking at it another way, if you took the Dewalt packs and disconnected the Dewalt BMS modules from them, you could use the parallel balancing ports built into the pack to charge them with any other charger you wanted, and you could in fact put two packs in series and charge them using a 72 volt charger that could balance 20 cells.
Does that answer your question?
Thanks for your thorough response Brian. Yes this does make sense to me. I had not appreciated that paralleling the cells would have a self balancing effect as you describe. What you say about the extended time taken to balence them also makes sense. I feel more confident about doing this now, and its good to know you have done this without any probs so far.
Has anyone used the 36v output from the BMS to shut down their speed controller when the voltage drops to minimum. I need to do this as my controller doesnt have this function built in, and it makes sense to use the bms as it should give the the cells the best protection. I guess a small relay would do the job though there are not many with a 36v coil, Can anyone suggest a suitable circuit? My controller is a 36v "Uni4" from Q4D.
Good idea to use the Dewalt BMS output to trigger something to protect your battery pack from going too low.
If someone can come up with something like what you describe, I'd also like to use it too....
I've been consider doing something a little simpler, but similar to your idea - Instead of turning off the power, I may use the voltage at the output of the Dewalt BMS module to light up a warning LED using a FET to let me know my battery is too low.
If and when I get my circuit figured out, I will let you know.
Thats sounds interesting. Basically I'm more interested in a low cost charge life po4 balancer than the dewalt cells themselves. I live in the UK and the dewalt packs are very expensive here after tax. But life po4 suppliers seem to be springing up all over the place for example (http://tinyurl.com/3agyws or http://www.goldenmotor.com/)
The problem is the BMS's are either unsuitable for ebikes, for example the golden motor lithium phosphate pack contains an unnecessary discharge BMS that might cause problems at high voltages or simply aren't available.
So it would be cool if the dewalt balancing modules could be used as a diy charge balancer. Especially if they could be modified to charge any number of cells in series, given a suitable charger.
The impression I get from your last post is that it is, so if I wanted to charger a 15s 48 volt nominal pack I could use 2 10 Series modules and leave 5 empty?
With this thread getting so long we now have a "a123 developer packs (continued) thread. Please enter additional post to the new thread at a123 developer packs (continued).
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