I would be very surprised if DeWalt does not have a BMS on their Lithium packs, all power tool packs with Lithium that I have opened always come with a BMS which both checks every cell and turns off any current flow above max and under min voltage levels.
I agree that if you let LiFePO4 sit at 3.6V for a while, you are over-charging because you chose the wrong voltage level for your BMS, how is that the fault of the circuit? It is a design error. If you want to properly shunt LiFePO4 cells you should do so while keeping them at about 3.4V since there is almost no capacity above that voltage anyway. Oh and shunting should be a relatively short process: burn away the differences (the delta in the small self-discharge between cells) and as soon as all cells are at shunting level, the charging is disconnected and all cells can float to their resting voltage. Cor van de Water Chief Scientist Proxim Wireless office +1 408 383 7626 Skype: cor_van_de_water XoIP +31 87 784 1130 private: cvandewater.info www.proxim.com This email message (including any attachments) contains confidential and proprietary information of Proxim Wireless Corporation. If you received this message in error, please delete it and notify the sender. Any unauthorized use, disclosure, distribution, or copying of any part of this message is prohibited. -----Original Message----- From: Paul Dove [mailto:[email protected]] Sent: Thursday, June 18, 2015 6:28 PM To: Cor van de Water; Electric Vehicle Discussion List Subject: Re: [EVDL] Bicycle battery I did not say it was impossible. As long as you stay below the open circuit voltage it's possible. However, if you hold the voltage at 3.6 for LiFePO4 while shunting the current you will still be charging the cell. In essence all you are doing is modifying the charge procedure and loosing track of actual charge time. CC CV charging dictates holding the voltage and tapering the current which is not achieved in this scenario. I said its unnecessary. I am not really concerned about what engineers are doing in commercial products it's clear from the laptop fires the airplane fires the car fires etc. that there is still a lot of false beliefs about these cells. I know dewalt bottom balances their battery and has no cell BMS Sent from my iPad > On Jun 18, 2015, at 1:43 PM, Cor van de Water via EV <[email protected]> > wrote: > > Paul, > How familiar are you with electronics? > My wife's eBike has a nice pack with a built-in BMS that simply starts > shunting current above a certain voltage and presumably (but I can't > verify) also reduces charging current, so you can take care that the > shunting cells are receiving zero current and no longer rise in > voltage. The trick is to find a good voltage at which your BMS will > protect the cells. > That pack is very nicely top-balanced and I expect that many more BMSs > that you are not even aware of in daily appliances such as power tools > will all top-balance. > > Regards, > > Cor van de Water > Chief Scientist > Proxim Wireless > > office +1 408 383 7626 Skype: cor_van_de_water > XoIP +31 87 784 1130 private: cvandewater.info > www.proxim.com > > > This email message (including any attachments) contains confidential and > proprietary information of Proxim Wireless Corporation. If you received this > message in error, please delete it and notify the sender. Any unauthorized > use, disclosure, distribution, or copying of any part of this message is > prohibited. > > > -----Original Message----- > From: EV [mailto:[email protected]] On Behalf Of paul dove via > EV > Sent: Thursday, June 18, 2015 9:40 AM > To: Lee Hart; Electric Vehicle Discussion List > Subject: Re: [EVDL] Bicycle battery > > No one has convinced me that top balancing is even possible much less that a > BMS can achieve this. > If you hold a cell above the Open circuit voltage you are charging it > even if you are trying to shunt the current with a Mosfet > > From: Lee Hart via EV <[email protected]> > To: Electric Vehicle Discussion List <[email protected]> > Sent: Thursday, June 18, 2015 10:46 AM > Subject: Re: [EVDL] Bicycle battery > > David Nelson via EV wrote: >> That is a reason I don't use a cell level BMS. With a cell level BMS >> like the miniBMS there is a constant drain on the cells running the >> BMS boards and it is nearly impossible to make sure that each board >> uses the _exact_ same current regardless of voltage in the cell. >> Furthermore, boards like the miniBMS expect that you will balance at >> the end of every charge and last I saw, that balance voltage was at >> 3.6V or so which is over the theoretical 100% SOC level for LiFePO4 >> cells which leads to potential overcharging of the cells on every >> charge cycle. If you stop charging without the balancing taking place >> and/or let the pack sit for extended periods of time then the >> different current draw of each board working 24/7 introduces an >> imbalance in the SOC of the cells in the pack. > > Yes, this is a problem in the BMS market. Like quack medicine, it's full of > customers worried about a problem that they don't understand very well. So > there are marketeers only too happy to throw together some piece of junk that > purports to solve it. It doesn't have to actually work -- it only needs to > *sound* like it works, to separate the customer from his money. > > So some BMS work -- and some don't. The bad ones are so bad that they give > the whoel BMS market a bad name. > > I've written about this over and over... check my old posts. It mostly seems > to be of no avail, so I won't repeat myself again. Anything I say is > overwhelmed by the noise of the internet. > -- > The greatest pleasure in life is to create something that wasn't there > before. -- Roy Spence > -- > Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com > _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > For EV drag racing discussion, please use NEDRA > (http://groups.yahoo.com/group/NEDRA) > > > > > -------------- next part -------------- An HTML attachment was > scrubbed... > URL: > <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20150618/92 > 58c414/attachment.htm> _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > For EV drag racing discussion, please use NEDRA > (http://groups.yahoo.com/group/NEDRA) > > _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > For EV drag racing discussion, please use NEDRA > (http://groups.yahoo.com/group/NEDRA) > _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
