justin_le wrote:paultrafalgar wrote:
Justin, could you improve the regen capture by using an ultracapacitor as intermediary, with an ultracapacitor such as JCG uses here:
viewtopic.php?f=6&t=7511&start=0&st=0&sk=t&sd=a&hilit=JCG
So even if an ultracap would allow me to do a maximum regen current of say 20 amps. Yes, I would be able to do more regen and come to a faster stop, but no, I would not get any more energy back. In fact, I would recover quite a bit less energy than had I come to a more gradual stop at 6 amps regen using just a battery to absorb the energy. So the idea that an ultracap could help 'improve' the percentage of recaptured energy is quite false in this case.
With respect, I couldn't disagree more. When you plug that battery pack in at night (regardless of its type), you won't be using a constant current of 20 A to charge it, and there are lots of reasons why you wouldn't. I think that people are forgetting that the battery is an electrochemical device, the emphasis being on
chemical. Don't forget that when you charge or discharge a galvanic cell, there are chemical processes being carried out in several places within the cell, each of them occuring at a finite rate and according to a specific set of electrochemical kinetic rules which are extremely complicated and difficult to describe in equation form without making several assumptions. I'm not going to preach equations here, but all of us knows that the actual energy put back into the cell is quite dependant on charging current (among other things) for a reason.
ZapPat brings up an important point that rings true.
ZapPat wrote:battery behavior really varies a lot depending on what you are using (chemistry, quality, temp,...).
The chemical reactions at the electrode surfaces can’t happen instantly and there will be a chemical concentration gradient between the electrode surfaces and the bulk electrolyte established involving things like boundary diffusion layers and natural convection (even in “gel†electrolytes). The chemical conversion boils down to two main steps.
1) Charge transfer. This is the faster of the two processes, involving very short transport distances at the electrode/electrolyte surface and surface electron transfer.
2) Mass transfer. This is the diffusion of ionic and molecular species to or away from the electrode surface (either the supply of material for the electrode or the exhausting of reacted chemical species). This is a much slower process than bulk diffusion, and mass transport in an electrolyte is often more than one order of magnitude slower than charge transfer on the surface.
There are a whole host of effects to consider alongside the (dis)charging process depending on the cell chemistry. Intercallation in Lithium cells, electrode crystallization or filming, gas production, etc. Even the often quoted “charging efficiency†can be a tough number to rely on. Personally, I consider the concept of charging efficiency itself to be weak, I’ll refer you to the following discussion:
http://www.smartgauge.co.uk/chg_eff.htmlThere are efficiencies associated with cell charging under specific conditions (trickle, constant current, constant voltage, pulse, float, and burp charging, and so on), each with their assumed or calculated time constants. Typical charge transfer time constants might be less than a minute, and a mass transport time constant could be several hours for a cell with plenty of Ah. Certainly, this is the case with the large capacity batteries in ebikes discussed here.
All of these things lead to the real reason people talk about damaging batteries with regen. Using regen from your ebike is best described as “random charging.†Constantly varying potential and concentration fields within the battery, and variable electrode process environments. But
it’s not all about damage. It's not even all about "internal resistance." Simply put, the electrode and electrolyte processes will be continually varying in efficiency.
solarbbq2003 hits the nail on the head here:
solarbbq2003 wrote:I guess also justins % graphs are based on current measurements via shunt? Would it be correct to assume the % graphs given will be slightly overestimating the actual % stored in the batts?
For the reasons I mentioned above, the Cycle Analyst will not by itself give an accurate accounting of the real, (re)usable energy put back into a galvanic battery by regenerative braking. It's not enough to measure the current and multiply by the individual slices of time. And forget about motor winding losses for the moment. Your battery, which even has charging losses when plugged into the wall on a 0.1C slow charge, will have a higher charging loss if you quick charge at 0.3C. Things will get worse with a fast charge at 1C. If you really want to kill the charging effectiveness, ride around with it and subject it to constantly varying charging and discharging loads!
I'll close with this - an ultracapacitor, while containing chemicals for things such as the dielectric layer, does not involve a chemical reaction at the electrode surface. No reaction diffusion time constant, nor is charge transfer between surface species is required. If I send 10 A for 14 s to my 140 F capacitor, I will expect (neglecting the very low resistances involved) an increase of one volt in the capacitor, with the associated energy storage that depends on the squares of the initial and final voltages. Send in 20 A for the same time, and it's two volts. 40? Give me four. The best news is that I can expect the Cycle Analyst to accurately track power and energy coming and going, nearly regardless of the rate of electron supply or withdrawal.
An ultracapacitor would
absolutely help improve the percentage of recaptured energy when compared to a galvanic battery.
