Passive Car Battery Trickle Charger

3 - 5 seconds is a pretty pessimistic estimate too, like you gotta run the starter for 5 seconds to start this car:

Not what you want but maybe something someone else can use.

Have a look for a " buck- boost " regulator. They are mass produced for use with solar panels. If the solar panel is only producing 6 volts it boosts it up to 12 volts. If the panel is producing 18 volts , it reduces it to 12 volts.

Dan

Yup you are right. I was thinking of making/ buying a similar gizmo and I could imagine using it for all sorts of charging jobs. And for those a current limit may not be right. (Feel free to claim I moved the goal posts.. 'cause I did.)

George H.

That is *incredibly* optimistic! --

"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

It's important to bear in mind that a fully charged lead/acid car battery's output voltage is a shade higher than its nominal 12V. A 12V battery with an output voltage of 12V is partially discharged. So one diode drop in the wrong place and it's back to the drawing board. --

"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

[snip signature]

[snip signature]

You are correct. In your proposed circuit, you will have 12.4V available from the zeners. But then there is a diode drop of .7V from there to the battery. That would be an 11.7V float voltage.

The typical float voltage for a flooded lead acid battery is 13.5V.

Funny I've never hear that term before. Anyway, it's interesting and I'll probably build it and test it against my own which I've now completed. Just need a bit of sunshine to test it!! Just a couple of queries: I can't quite make out what you've written directly under the 5V zener reference diode? Also, are R3 and R4 intended to be equal value? And I'm guessing any old jelly bean single-ended op-amp will do? Thanks!

--

"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

I've just measured the discharge current on a Victron SPM04030 1200 panel and it was less than 5 mA at 12.6v, rising approximately linearly as I increased the voltage up to 20v. This was in an ambient temperature of about 10C with the panel in darkness.

Other technologies may have a different leakage rate or it may increase badly at higher temperatures, but with this panel in these conditions there didn't seem to be a need for a series diode. It generates nearly

20v in Winter sunshine, so if you did need to add a series diode, a normal power type with 0.6v drop would be perfectly adequate.

This particular panel is rated at a nominal 30W but actually delivers about 1.5 A into a 12v battery in moderately good daylight. it came with a hefty permanently-wired reverse-voltage protection diode inside the connection box.

--
~ Liz Tuddenham ~ 
(Remove the ".invalid"s and add ".co.uk" to reply) 
www.poppyrecords.co.uk

Is this your latest fixation, then? Posting videos of cars starting up in icy weather? That chap was really gunning the starter motor. They aren't designed for extended use and neither are the solenoids that switch them in. It's a toss up between the solenoid and the motor as to which will fail first with an idiot like that over-stressing them. Lovely car like that, too. Shame. --

"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

You need to define what you mean by "modern cars". To me a modern car is anything manufactured post-1960. And I have no idea what "SoC" refers to. --

"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

My 40W panel measured 9 mA at 12V and 25C. In darkness of course.

I assume, then, that you bypassed the diode to make your measurement?

5V bandgap. R3 and R4 should divide the battery voltage down to equal the 5V reference.

Well spotted. In that case I'll insert more diodes in the series chains to compensate. What a lot of diodes! Crikey. --

"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

That seems about right - a leakage current like that isn't going to discharge the battery overnight.

No, the diode is there to act as a short-circuit if the panel is connected to the car battery the wrong way around. I connected the panel to a variable power supply the right way around, with a low-value resistor in series, and measured the voltage across the resistor.

This panel came with just a watertight terminal box containing the kamikaze diode and no wires or plugs. I use my own modified IEC connectors for 12v supplies in the van and on portable batteries for P.A. work (non-interchangeable with the mains!), so I fitted it with one of those connectors. Because the connectors are polarised, the diode is redundant, but it can stay there as an insurance against the day when the unthinkable happens.

--
~ Liz Tuddenham ~ 
(Remove the ".invalid"s and add ".co.uk" to reply) 
www.poppyrecords.co.uk

I think you are mis-understanding the polarity of the diodes in the solar cell. Put a resistor across your panel and measure the polarity.

George H.

I connected the

Nah it isn't. The e.g. Honda Civic starter motor is only rated for 1kW. The worst-case upper bound on a starter energy consumption is the max rated cold cranking amps times 12 volts times the cranking time; even pessimistically you're only talking couple 10s of kilojoules to put back.

Here's a paper, mathematical model of a single piston 150 cc engine (you'd have to extrapolate for a 4 cylinder 1.5L or something)

Peak current draw tops out at 80 amps and it's down to about 15 in under half a second.

Modern cars can also detect if the 12 volt battery SoC is running a bit low and preferentially direct power to top it back up it's not entirely at the mercy of whatever the idling engine and free-running alternator feel like doing.

Yes. As it stands now you need 40 components. If you have them, yours is a well-stocked junk box. I would look for a better way, but it's your project.