Lawrence, note that a car with DC Fast Charge capability will request a specific max voltage and current from the charger, you cannot just short 2 packs together. One pack may be at 400V while the other empty pack is as low as 300V and all you are getting is a big spark, a blown pack fuse and possibly destroyed BMS if it cannot handle the surge current of around 1000 Amps. Most non-Tesla packs have a pack fuse of around 300A, and as low as 225 for an early Leaf. Also, when the BMS sees a current higher than it requested or not confirmed by the charger, it will drop the contactors, ending the charge and if an excessively large current is flowing at that time, the contactors may be destroyed because at high current they have very limited operational cycles for breaking, as low as 1 for high enough current. Besides, the two vehicles may not have the same number of cells, I have seen commercial EVs with 84 cell and with 108 cell packs, not wise to try and short those together. Lastly, if you use a high resistance cord to limit the initial current when the source pack is full and the destination pack is empty, then most of the charging will happen at a disappointingly slow speed: say for example the packs are at 300 and 400 Volts, this is the extreme case that you want to manage and let's be generous: you manage to get the resistance figured so that you get a peak just up to 200 Amps, which in the cold will instantly destroy the destination pack, but let's say both packs are warm. This means 1/2 Ohm of resistance. Before you have transferred 10% of the charge though, the two packs are probably at 360V and 375V so now you are getting 30 Amps of current for what is essentially a Level 2 speed of charging of 11kW (48A on a 240V circuit). Now taking all that hacking and making it a standard solution requires that you have a 2-headed DC Fast Charger with no need for a grid or battery supply, only a DC/DC converter to adjust the output to the voltage and current that the destination vehicle requests. This is not a big unit, with Silicon Carbide power electronics the size of the converter is smaller than your EV motor controller, definitely something that you could throw in the trunk of a roadside assistance EV... Much better than the paltry 30 Amp generator that AAA had on a few experimental roadside trucks, 10 years ago. Cor.
On Thu, Mar 20, 2025, 8:46 PM Lawrence Winiarski < lawrence_winiar...@yahoo.com> wrote: > Maybe for vehicles with close enough voltages, all that's necessary is to > connect the packs together to equalize them? > > I know it sounds crude, but really having a big enough resistance in the > charging/jumping cable could work to an advantage here, because you can > still transfer a lot of energy and only waste a portion of it. The > amount of energy you are transferring is Bat volt * amps and the amount of > energy you are losing is Cable Volt * amps. > > So connecting a 400 volt car to a 360 volt car means you are wasting > 10%...that means your 100 kw cable has to disappate 10kw, which is a lot, > but for an emergency jump situation probably do-able and maybe it's just > instantaneous, because the voltage sag and charge increase, so maybe the > 400 goes to 380 during the heavy discharge and the charge side goes up to > 380 and all the sudden the cable works just fine. > > I know diddly about Tesla's and what the voltage characteristics are > during charge and discharge, but I'm 100% sure you could make a jumper > cable that would work (obviously a high resistor would work....just very > slowly). So the question would be in optimizing it to equalize to packs > and still stay cool enough and fast enough to be useful. > > > On Thursday, March 20, 2025 at 01:03:33 PM PDT, Cor van de Water < > cor.vandewa...@gmail.com> wrote: > > > Lawrence, > That will work, provided that you supply DC, else you are limited to the > onboard charging speed of the receiving car, which will typically be in the > 6 to 11kW range, for the overnight fill. > The only way commercial EVs would be able to exchange high power, is when > both vehicles connect to a boost charger, which uses one (V2X) DC > connection as power source, changes the voltage to the level required by > the second car and performs a normal DC Fast Charge into that car, while > doing the normal station things like adjusting voltage to meet the > requested current or limits it to the available power from the source car, > keeping in mind limits like minimum remaining charge configured for the > source, so it can still get where it needs to after the charge. Note that > input and output voltage levels will likely cross over during the charge, > so it will likely require a special architecture or a sequence of for > example a boost converter to take the source voltage and create >500V, then > a buck converter to generate the ~400V for the receiving EV. > Cor. > > On Thu, Mar 20, 2025, 12:26 PM Lawrence Winiarski via EV < > ev@lists.evdl.org> wrote: > > Yeah, the pack swap has lots of prob if you aren't swapping your own pack, > but it got me thinking..... > Maybe there might be a merit for something like this idea : Charging a > stranded EV You need a gimongous generatorto charge an EV in 5 min and a > little generator would take hours or even days, but perhaps a way of using > an 2nd EV to "recharge" a stranded EV you could do it super fast also > > > On Thursday, March 20, 2025 at 12:04:56 PM PDT, (-Phil-) via EV < > ev@lists.evdl.org> wrote: > > This has been tried before, even by Tesla. People don't want to risk > getting a pack swapped in that has been abused, has high degradation, > etc. The only way it can work is if you lease the pack, and while that > model has had success in China and the EU, it's not worked here at all. > > I think Tesla only ever had a handful of takers willing to pay the extra > premium over supercharging (which was then free), and it was only used in > the LA <--> SF corridor and Tesla promised to give your original pack back > when you returned! > > It's been tried also by others, and it's always failed in the US and EU, > only in China has it had any success whatsoever, and I bet it won't last. > > On Thu, Mar 20, 2025 at 11:55 AM John Lussmyer via EV <ev@lists.evdl.org> > wrote: > > > On 3/20/2025 11:10 AM, Lee Hart via EV wrote: > > > But it would (gasp! choke!) require EV manufacturers to standardize > > their batteries. > > And have all "charge" stations have large, mechanical, complex stations > > that are likely to need lots of maintenance, and possibly attendants. > > AND still have huge power requirements to recharge all the swapped packs. > > _______________________________________________ > > Address messages to ev@lists.evdl.org > > No other addresses in TO and CC fields > > HELP: http://www.evdl.org/help/ > > > > > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: < > http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20250320/fab575fb/attachment.htm > > > _______________________________________________ > Address messages to ev@lists.evdl.org > No other addresses in TO and CC fields > HELP: http://www.evdl.org/help/ > > > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: < > http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20250320/c79563df/attachment.htm > > > _______________________________________________ > Address messages to ev@lists.evdl.org > No other addresses in TO and CC fields > HELP: http://www.evdl.org/help/ > > -------------- next part -------------- An HTML attachment was scrubbed... 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