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I believe I have a schematic that will function as intended,
The circuit doesn't reflect this, but the switches will physically mounted together to activate on a single touch (basically making 1 switch out of the two), with the Drive switch being NO and the Regen switch being NC, therefore the circuit is in default Regen mode, until the switch is pressed, which would then put the circuit in Drive mode.
My issue, is that I don't want to dump 22V into my 11.1V battery pack, as each cell can only safely charge between 3.8-4.2V, which makes the safe charge voltage for my pack 11.4 - 12.6V.
So what is the easiest way for me to maintain 11.4-12.6V entering my battery pack from the motor?I was thinking of using a Zener diode, but the current is too high. I found a few sources saying I could use a Zener and NPN transistor together, but I am not entirely sure on that.
And I would very much prefer not to kill my self in the process!
Just to be absolutely sure... You are talking about 2 dual pole single throw switches always pushed in tandem (with no chance of being on at the same time), right?
Yes. However, I only used two switched because I wasn't sure how to do it with one.
As for telling you max current out of the motor, how do I determine this? Right now the device is setup to simply turn on and off. The motor terminals are shorted to brake the motor.I can measure the circuits current when battery is powering the motor, but I am not sure what I need to do in order to measure current out of the motor.I am not sure what else you need, but you mentioned the battery capacity... There are two 11.1V 3300mAh 20C batteries.
And off topic, Soeren, I just saw that you are from Denmark, I just visited there for the first time this past July. I spent 3 weeks in Copenhagen, beautiful city, unfortunately I didn't get to explore the countryside, so I suppose I need to make a trip back!
I definitely will not be discharging the batteries at their full 20C rating, I imagine I will hit a max draw of about 3C or 10A.The only motor specs I have are: 24VDC 13,000rpm @ no load.Physical size is approximately 2" diameter 3.5" long.
OK. So I read a bit about buck converters and buck-boost converters.You said I would need both. Can you explain a little more about how this works, and why this would be a safer way of charging Lipo batteries?
Series method described is not used to charge battery. Capacitor is used as additional power source to power motor rather than power source to recharge battery.
If the experts (you guys) say that going with the capacitor method would be simpler then any safe method of putting the charge into the batteries, than I will peruse this method..... hopefully with some more guidance from you all!
Hi,Quote from: stevet47 on January 24, 2012, 03:40:58 PMIf the experts (you guys) say that going with the capacitor method would be simpler then any safe method of putting the charge into the batteries, than I will peruse this method..... hopefully with some more guidance from you all!Simplest method is, as I mentioned previously, either nothing at all or a voltage limiter of sorts.With the motor in question, the net winning is extremely small and doesn't really make up for anything more complicated than a Transil or similar.(You'd still need rectifier diodes to keep the motor from draining the batteries when the generated voltage gets lower than the battery voltage of course).If it was a motor for a full size express train with miles of daily braking, it would be worthwhile sucking every drop of juice out of a regen system, but in your case, you might get a few more meters of running tops.Any kind of switch mode (SEPIC and Cuk topologies would be more suitable here than Buck/Boost anyway) will give losses that may, in this particular case, be more lossy than a voltage limiter and it would need to be designed to the exact specs (found by testing the motor extensively) to be able to output anything useable at all - lot of work for a marginal net win (if any, compared to a limiter).Capacitors would need to hold as much of the energy from regeneration as possible and since caps are leaky, this energy needs to be used within a short time.They need to be able to handle the max. voltage possible and they haven't got the kind of "regulation" that a battery will does.They need to be able to handle high ripple currents.The total value of the bank can be calculated as: C = A*s/VA = Amperes = secondsV = the tolerable voltage drop during sIf you eg. want to be able to get 1A from a capacitor during 2 seconds, with a maximum voltage starting at 25V and still being at least 21V at the end of the 2s period, you'd need a capacity of 1*2/4 = 0.5F = 500,000 µF with a rating of 35V (assuming 25V tops, it needs a margin) and able to handle ripple currents of at least 1A.Filling the cap, you still need to keep the voltage and current well within their ratings (same formula apply) and the short time you get the regenerative power will not be able to do anything but burp at it softly.If you want to go with caps, eg. to lighten the load on the battery, when the motor draws a high inrush current starting, you could charge the cap bank with a reasonable low current from the battery, but that's an expensive way of nursing the battery.I took some measurements on the motor I mentioned and although I had to run it on ~7V (it's a 9v motor), as the scope I used have a low and pretty unforgiving max input voltage of 30V that I wouldn't try exceeding (plus the motor whine is annoying at 20kRPM), but the timing will not be that far off (an inertial load will make the regen somewhat longer).As can be seen here (click for a larger pic), there's roughly 12µs of 110V to deal with. With your motor running at 22V, the voltage may be 300V to 350mV in a period close to those 12µs (plus what inertia brings), but as long as you don't tell the battery, it'll never know Even a pulse of 100 times that (i.e. 1.2ms) at 350V won't impress your batteries and will certainly not be dangerous in any way - it's simply too short a time that the impedance of even a flat battery will wart it off like an elephant would a mosquito.One thing is that the battery won't be hurt from this, another is that the charge you gain from a few brakes here and there wll be negligible and IMO not worth the hassle, unless made from an experimenters perspective on learning.If you feel you need to add something, a Transil (even a fast 1W zener would do) can be installed in 5 minutes and then you can spend the rest of the week (or how long it was) on something more useful (like practicing Ninja RubberNeckin' SkillsTM, building a table of empty Schotch bottles or whatever your favorite pass-time may be ).
If I am understanding you correctly, you are saying:1) This is a waste of time... ok, but I've gotta at least try.
2) You do not think I need any type of voltage/current regulation at all, and can use my original schematic? This is surprising to hear, based on the initial comments about how what I was attempting was very dangerous.
3) You think a Transil (that is a clamping diode, correct?), or even a zener would do the job, it I wanted it.
Safe charging of the Lipo batteries would be in the 11.4-12.6V range at anything under ~12A. So I don't think I have to regulate current, but I thought I would have to regulate voltage to within that range. You don't agree?
I am surprised to hear you say I could use a 1watt zener, I would think at only 1watt I would blow it up, can you explain how you determined 1watt is all I would need?