Eric HertzDoes the "real world" align with physics for anyone, or is physics amongst the biggest hoaxes of all time?
The previous log detailed a string of luck, building a charger for my new "house battery".
Today I tired of its flakiness and invested in a lower resistance.
Believe it or not, there are still physical stores where such can be purchased.
I did math and came up with some extremes. Say the alternator's putting out 15V, and the battery's severely discharged at 11V... 4V difference. My lighter outlet can handle 20A... 0.2ohms should do well, and *way* better than the 0.75 from before.
(Oh yeah, so my 1:1 estimates last time were obviously disproven recently, as I drove at least a couple hours and barely got a half-hour's use. How'd it 1:1 last time?!)
So, the store's resistors are 2W max, and my extreme case is 80W(!)... I ain't using 40 parallel resistors, this is an extreme case.
Limiting factors included the available values and stock. I ultimately came up with 10 2.2ohm 2W resistors. In parallel with my two parallel 1.5ohm 7W resistors we should get 0.17ohms... close 'nough. Then I did some math, again, and these wattages should be fine for normal use. (The loaded inverter cuts out at 11.75, anyhow, not gonna drain it to 11! And a car regulator's like 14.2V, not 15).
So, I rig the thing up, 12 [big] parallel resistors, now... and try it out... This time a current-meter installed...
and I'm getting dang-near 1:1 again, right off the bat.
dang battery won't run the inverter+load, last I tried,... and it's charging at the rate I determined for the old 0.75ohm setup, (AND tested, that first time!)
OK.
So, there's that.
Now, watching the charging current, some other interesting findings... like... the heater fan drops the charging current, while driving 60MPH, by more than 10%. Headlights by 5ish. Brights vs. Normal, barely noticeable change in charge current. Braking... holy shizzle, 50% or more. 60MPH doesn't seem much different than 20, as far as charge current. In fact, I think it went *down* slightly after I got on the freeway from the parking lot. Oh, and jiggling the plug in the socket varies the current by about 10% as well.
I suppose with 0.2ohms and a couple/few volts, it should be expected that even a minor fluctuation in supply voltage would affect the charge current so dramatically... But... wow.
Definitely making me wonder if my alternator could handle a [nearly]-directly-coupled second battery. Or, I spose it's equally-plausible there's some resistance between the regulator and the accessory-distribution... bar(?).
Regardless, if my prior math was right for 0.75ohms, and with 0.17 ohms in actual resistors giving charge-currents calculated for 0.75... then there's about 0.6 ohms in the system. That seems like a lot to me. Think about the effect that'd have on heavy loads! 125W (output) inverters are pretty cheap these days, at 10A in, 0.6ohms would drop 6V?! and 60W lost in the wiring/connectors?! No, worse, 'cause now it's closer to 6V, so nearing 20A, and 120W in wiring! Som'n ain't right, here...
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Lots of things normally get swept under the rug. That's how we even get to Ohm's Law and stuff, right? I mean, it isn't like you're doing Maxwell's Equations on a model of all the vehicle systems.
In the case of braking, you have a "brake booster" with a vacuum pump, in addition to the brake lights coming on.
In the case of headlights, the high beams don't make a lot of difference because you have so many different bulbs powered at the same time, all around the vehicle, when that subsystem is turned on. If you had LED bulbs, it would be a smaller change.
I believe there is a hoax. The hoax is that when teachers use absolute words, it is because what they're saying is sciency and true. So like, for physics undergrad students, they just lie to them for three years straight, and their senior year they get an introduction to what the professors think is really true. Feynman talks about that in his "lectures on physics" recordings; they teach simplified (read: incorrect) models to first year students because they can understand those models and work with them, and then second year students get another simplified model, etc. And those models also generally follow the pattern of discoveries in physics. And engineering students never get taught anything but the models.
In the end though, the formulas that make up the "real" physics knowledge are entirely non-intuitive; Feynman insisted that even he couldn't intuitively understand his theories even believing them to be well-established! He was just so good at the math, his brain could give him results so fast that it appeared to others as if he had an intuitive understanding. This is why in the end, if you're doing something real-world, IMO it is folly to worry about physics. Instead, use Ohm's law; Kirchhoff's laws; positive current flow; etc.
So I say, trust Kirchhoff's laws, but also remember that it only needs to be "effective resistance." I've often been surprised by the effective resistance of batteries. And surprised again by effective resistance of a boost converter vs input voltage!
All of that said, you could also build a MOSFET-based current source and get the voltage overhead down to ~0.6-0.7V. But you'd have to parallel a few. And if you set up a switch, with your existing system on the other side, then you can switch back to the resistors for the final top-off. Ultimately, it might be less work to get some suitable heavy wiring and connect to the normal vehicle charging circuit. Maybe second hand jumper cables or something. If you can install the battery in the engine compartment, then you could use regular home wiring to get the power to the back, and battery cables for the actual charging.
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I dig it, RP. Great response!
I did, indeed, go the 'engineering' route (or maybe more like 'technician'), as opposed to higher-level physics... using ohm's law almost exclusively. I tried to account for some things in the system at-large, such as resistance in my long too-small-gauge 12V feed, etc. But, in the end, I obviously didn't account for much, considering the brake-lights, etc.
It does seem to work well 'nough as-is. But I guess that was mostly luck, rather'n my excellent design ability ;)
MOSFETs seem like a decent longer-run idea, along the lines of a current-source (or at least regulator). I hadn't thought of paralleling them.
The de-facto/well-established under-the-hood system definitely seems best, and probably easiest. But fitting one more thing in there is a bit daunting.
But I really dig those insights re: teaching-process, Feynman, etc. Thanks, yo!
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Electricity always takes the path of least resistance... ;)
EDIT: apparently there's some increase in resistance in that circuit, or the difference just doesn't amount to much.
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LOL, these electrons took one look at Kirchoff's Laws and said "f it, dude. let's go bowling."
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...or there's an additional factor that's being left out. As entertaining as it would be for them to go all Big Lebowski on you (I /really/ need to watch that film, one of these days!) -- my money is on the additional factor sort of thing.
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