More battery charger, voltage shunt reference.

Hi all, this is mostly a continuation of CD's solar battery charger thread. I was thinking of using a power zener as a shunt reference... but this has some problems. I think #1 is that as the zener sucks up the power it will warm and go up in voltage... which is exactly the opposite of what I want. So how about a zener turning on a power npn. I think someone mentioned this idea in the previous thread, but I couldn't find it in a quick rescan. Maybe the TL431? Anyway I drew this,

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npn across the rails, with base feed by zener and some series diodes and maybe led and ?10 ohm base R. Then two or three diodes from base to emitter to protect b-e when it turns on. And maybe a collector resistor. ~50 ohms 3-5 watts I'd like the shunt to turn on at 13.5 Volts at room temp. with a negative tempco.. less voltage at high temp. I'd like some knob (pot) to tweak the voltage. I was thinking if I used diode connected transistor (as part of diode chain) I could put a little bit of R in the c-b connection... that seems like a beta dependent tweak. Which might be OK if beta goes down as the temperature goes up.

But maybe there is a better way. Ideas?

George H.

Reply to
George Herold
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People used to do that on motorcycles, add a power zener to reduce overcharging boiloff.

I think #1 is that as the zener sucks up the power

Just a modest zener and a big transistor or Darlington should work. But it needs a blocking diode to the battery, to avoid a couple of hazards.

The series base resistor is prudent.

Reply to
jlarkin

I I thought of a Darlington, but then there's not enough base current to turn on the indicator led. I guess I can just put that above the collector.

Oh I didn't show the series Schottky to the battery. That hardly makes any difference in the charging current.

George h.

Reply to
George Herold

Oh boy, I think I'm liking the Darlington idea. On hot sunny days, I want this to reduce the float voltage...with D's, I've got two b-e voltages going down with temperature increase of the pass element. (one man's thermal runaway is another's control :^) George H.

Reply to
George Herold

That tempco is kind of accidental, but maybe you can make it work. I'd suggest a more deliberate circuit, maybe like my bang-bang thing, with a proper temperature sensor.

Seems like overkill, though. How about a solar panel and a diode?

Reply to
John Larkin

There's three phases to be considered: firstly, if the battery is not fully charged, is the unlimited current into the terminals (assuming a solar cell source won't need limiting) phase. Second, when the battery reaches full charge, change to a 'float' constant-voltage-limited state. Third, every night will interrupt the solar power source, so a low-drain 'wait' phase will interrupt charging.

The 'float' state is to be entered when the (possibly temperature-dependent) fully-charged voltage threshold is detected, and occasionally re-checked during solar-power interruptions (once a day is often enough, so that's easy).

For an indicator, I'd like to see a moving-needle meter, with 10-15V expanded scale, which will only cost a few microamps (a photographer's light meter only has a few cm^2 of solar-cell to run it).

I dislike the power zener idea, because it could fail shorted, and it can be more expensive than, say, an eight-pin microcontroller.

A full, elaborate system would include a switchmode for low-solar-cell voltage boost charging, and an ambient temperature sensor. And, a minimal system is just a solar cell with backflow-prevention diode.

Reply to
whit3rd

Modern SLAs tolerate a modest constant current charge. So a small solar panel and a diode should keep one topped off.

Reply to
John Larkin

Oh I bought and tested a solar panel... it needs a voltage clamp... or a bang-bang switch thing... that's how my 2 amp charger works bangs between 12.9 and 14.2 V Oh could also make a with opamp and pass element. (I think that was your first circuit.. with optional hysteresis)

George H.

Reply to
George Herold

So I got my small riding lawn mower battery... ~1/3 the size of a car one. I fully charged it. And then hooked it up to the solar panel. It got to 14.4 V (at ~100mA or so) and I stopped. It's possible this panel would work fine with a bigger battery. Most the batteries I'll be charging are not attached to modern cars. There is no drainage (except internal), till I turn the key/switch.

George H. I'm going to try the zener thing.. I still need a voltage tweak. ~300mV or so. (+/- one Schotkky)

Reply to
George Herold

You do you. But thanks. This is only the float charge... ~13.5 V According to Bat. U. it needs less temperature dependence. (Max I for panel is 200mA.)

George h.

Reply to
George Herold

That's a battery cooker! --

"Andrey Semyonovitch really was rather stupid; he attached himself to the progressive cause and 'our younger generation' from enthusiasm. He was one of the numerous and varied legion of dullards, of half-animate abortions, conceited, half-educated coxcombs who attach themselves to the idea most in fashion, only to vulgarise it and who caricature every cause they serve, however sincerely."

- Fyodor Dostoevsky

Reply to
Cursitor Doom

Maybe, maybe not; my old bike light used an alternator to charge the lead-acid

6V battery, and the first battery lasted over a decade, though the rectifier was a voltage doubler (12V nominal output). While the bike's moving, the battery cools just fine.

A 1 square foot solar panel generates as much electric power as the heat of a 3x3 inch bit of direct sunlight onto the battery. Free air may suffice to keep it cool.

Reply to
whit3rd

Current, after it performs the charging chemisrty, evolves hydrogen and oxygen and blows up batteries. Modern SLAs have catalysts to recombine modest amounts of gas back to water, and a relief valve for extreme cases. That's why we don't have to keep adding water like we used to.

A small current, around enough to overcome self-discharge, won't hurt a good battery.

It's basic that a small solar panel can't make much power. Power cooks things, not voltage.

Reply to
jlarkin

Yup, but you'd be surprised how much power these solar panels can generate. The one I have is only A4 size, but if orientated correctly will supply 10W on a sunny day. And with a Voc of nearly 22V. If you leave that connected with one rectifier diode on a "fit and forget" basis, it *will* cook the battery in short order. I think it's just semantics over the exact meaning of "cook" which seems to be the issue here.

--

"Andrey Semyonovitch really was rather stupid; he attached himself to the progressive cause and 'our younger generation' from enthusiasm. He was one of the numerous and varied legion of dullards, of half-animate abortions, conceited, half-educated coxcombs who attach themselves to the idea most in fashion, only to vulgarise it and who caricature every cause they serve, however sincerely."

- Fyodor Dostoevsky

Reply to
Cursitor Doom

BTW, I breadboarded your design today and it looks promising. Cheers! --

"Andrey Semyonovitch really was rather stupid; he attached himself to the progressive cause and 'our younger generation' from enthusiasm. He was one of the numerous and varied legion of dullards, of half-animate abortions, conceited, half-educated coxcombs who attach themselves to the idea most in fashion, only to vulgarise it and who caricature every cause they serve, however sincerely."

- Fyodor Dostoevsky

Reply to
Cursitor Doom

The surface area of an auto battery is about 400 square inches; at 10W, it certainly won't overheat thermally (25 milliwatts per square inch).

Reply to
whit3rd

On Saturday, 6 March 2021 at 19:12:21 UTC-8, snipped-for-privacy@highlandsniptechnology.com wrote: ...

... Not quite true - in both lithium-ion and lead-acid battery chemistries higher terminal voltage causes unwanted chemical reactions that don't occur at the same power level when the battery terminal voltage is lower.

Lithium-ion in particular is very sensitive to overcharge, even storage at high SoC activates irreversible side reactions.

Reply to
ke...

Semantics again. I'm not talking about 'cooking' it thermally but rather 'boiling' the electrolyte. Cue big argument over the exact meaning of "boiling" in this context. ;-)

--

"Andrey Semyonovitch really was rather stupid; he attached himself to the progressive cause and 'our younger generation' from enthusiasm. He was one of the numerous and varied legion of dullards, of half-animate abortions, conceited, half-educated coxcombs who attach themselves to the idea most in fashion, only to vulgarise it and who caricature every cause they serve, however sincerely."

- Fyodor Dostoevsky

Reply to
Cursitor Doom

As noted, modern SLA car batteries recombine excess hydrogen and oxygen. Nothing will boil if 10 watts is applied for a few hours a day.

Voc doesn't matter when the panel acts like a small current source.

Reply to
jlarkin

The bang-bang regulator? Did you Spice it?

Reply to
jlarkin

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