kold kanuck said:
Just reread this thread from the beginning (for the 3rd or 4th time) and would like to thankyou for a well documented build. This has been most helpful to me as i'm in the process of putting together a build list very similar to what you have done here and have found your links and building decisions to be great information in making up my own design parameters, along with the advise and input that others have contributed to this great source. A few questions if i may? In an earlier post you inquired as to wire size for batt. and phase leads - any conclusions?? Is there such a thing as too big ? Also do the lipos need to be charged immediately after use or is it possible to build a battery pack that you can ride to work, let sit for 8-9 hours ride home and then charge? A large pack is more desirable for me because it halves the charging equipment(less charger = more money for batts = more fun
) and provides a good range for weekend exploring. Any thoughts or suggestions greatly appreciated and i cant wait to see your little magic keyswitch/lvc/throttle intrface/ect. come to life. thanks for your time.
Thanks for your comments and questions. Hopefully this build thread isn't driving people nuts with the long and changing thoughts.
I have learned a great deal while researching this project, much of it from reading ES and from comments other members have made in this thread as well as elsewhere. Thanks to all who contribute here, and to those who ask questions that make us think.
Another source of helpful information is Venkat Srinivasan. His blog is at
http://thisweekinbatteries.blogspot.com. I have also had the pleasure of meeting with Venkat over lunch and discussing some of the finer points of Lithium batteries. Venkat is a materials scientist who researches the behaviour of the materials in batteries. I told him about ES so we might even see him on here at some point, but he is a busy guy.
Taking your questions out of order, let's talk about lithium batteries. You will find that Venkat's most recent blog posting (which is actually on Earth2Tech, but is linked from his blog) he discusses his perception of the "laws" that regulate lithium and other batteries. Pretty much all commercially successful batteries are operated beyond their electrolyte's thermodynamic stability window. What this really means is that, at full charge, the electrolyte is being decomposed. Now, in lithium batteries, this is a built in mechanism that "wears" out the battery starting when it is manufactured. Even if it is not used, it will eventually wear out due to this mechanism. We have no way to replace this lost electrolyte other than buying a new cell. This process is very much a function of the cell voltage. Leaving a battery on the charger "cooking" at the full charge voltage, or leaving it set at that high voltage actually is harder on the battery than leaving it at a lower state of charge. Manufacturers of lithium batteries usually state that 50-70 percent is the best state of charge for storage (and low temperature is also best).
Note also that this same concern is one of the explanations for the long life of Lithium Iron Phosphate batteries. They operate at slightly lower voltage so they have a much longer electrolyte life. So if your battery is more than capable of making your round trip, and you leave it for 8 hours in that 50-70 percent charge region you will actually extend the life of your pack as opposed to charging it immediately. And correspondingly charging it to 90% instead of 100% will increase life. The best plan would be to recharge to 50-70 percent when you get home. Only charge to 90-100 percent when you are about to make another trip. Otherwise park it at the optimal storage state of charge.
Can wire be too heavy - Well, No and Yes. In terms of performance, we want the heaviest wire we can fit on the bike. But the point of diminishing returns can be exceeded, and the weight and bulk of over-heavy wire can be problematic on an ebike. You have to decide how much current you are going to run, and work from there.
In my case, I have decided to go with the 6x10 motor to reduce this current. Motor current saturation will occur with this motor at about 45 amps. Beyond that efficiency drops, so ideally I would like never to exceed this current in the motor. Battery current will be that value or lower, depending on the back EMF vs supply voltage and PWM. So I need wiring that can handle 45 amps.
There is a nice wire chart here: http://www.powerstream.com/Wire_Size.htm This chart indicates that, for chassis wiring, #12 is rated to 41 amps, and #10 to 55 amps. Now we know that the wire into the controller is #12, so there is one choice already made. The wire into the motor is about #14 or #16, I should measure that. But it reportedly is empirically adequate in the 6x10 winding (hard to melt), and not so adequate in the 9x7 winding which saturates in the 60 amp neighborhood.
So one argument says that #12 is adequate, and even #14-16 seems to get by. We don't spend a lot of time at full current. If we did our batteries would not last very long. But climbing an extended grade can stress too-light wiring a lot.
Another factor is the connectors. They are often weaker than the wire. A natural wire weight to use is the max that your chosen connectors can handle. You might not want to go that far. Anderson SB-50's can take #6. This is very convenient when you are paralleling several batteries and you have #12 or #10 coming from each. But running #6 from there to the controller, and then stepping down to #12 may not be worth the expense and physical difficulty of routing the thick cable. It might make sense to make any long run a few gauges heavier than the controller's #12, say #10 or maybe #8. But the benefits will be pretty small. #12 is pretty adequate here.
I'm planning to use #12 or #10 for the heavy stuff. It is probably slightly overkill but not that big and heavy. #8 or #6 would be pretty heavy. I would look carefully at whether that was practical before choosing it.
