Passive Car Battery Trickle Charger

That's not so simple with modern cars. With some of the ultra new ones

- anything made in the last 12 years for example - you need to enter some security code in every time you reconnect it, which requires the code AND some fancy piece of hardware to interface with the car's OBD socket. Everything's a massive PITA with modern cars - and it's only getting worse and worse and worse.....

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"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

So you're claiming that 20Wh per day is insufficient on the basis that

You have a "modern car"? I find that hard to believe..

Sounds like something Volvo would implement they seem to like making cars that are very hard for anyone but the dealer to work on. Nothing very interesting happens when you disconnect and re-connect the Chevy Volt's 12 volt battery (it still has one, under the trunk) even the clock and radio presets are maintained, amazing 21st century technology!

A lot of newer GM vehicles have a "Transport mode" the user can enter which significantly reduces the standby load on the 12 volt battery. On my car it's activated by:

Press "Start/Stop" button Activate the hazard flashers Press the brake pedal Press and hold the Start/Stop button for 15 seconds Display will show car has entered power-saving mode

Just put voltage sensor that turns on a load when the battery is full. this reduces the heating of the solar panel (versus just disconnecting it)

I though that was a signature, but no, it's part of your message.

Yeah a current limit is OK, but if your battery is ~1/2 discharged then you can only charge it at the Imax rate.

GH

It might work, but it's awfully complex.

Bang-bang here means a circuit that connects the solar cell to the battery, or doesn't.

I scribbled this in the car:

Most any low power opamp will work. None of the parts should get warm.

You might get away with just connecting the solar panel to the battery through a diode.

I played with it a bit, but it's really not necessary. A simple bang-bang reg would work as well.

What would be the point? If you have excess energy it costs nothing to run a regular relay.

I have some here that are good for 15A

My proposal would be the "super zener" circuit from the TL431 data sheet (figure 30) perhjaps with a thermistor in the bias chain to match the thermal voltage profile of the battery and a power resistor in the collecrtor circuit to reduce heating of the transistor. (but it still needs to be able dissipate up-to 1/4 of the solar panel power) keep the thermistor and the power parts thermally separated.

There's probably a way to introduce hysterisis to further reduce heating in the transistor, taking the bottom of the divider from a "tap" on the power resistor perhaps.

Just use up the extra energy. There's no need to block it from leaving the solar panel.

Yeah, lose all your presets on the radio, clock time and who knows what else! No, that's a pita and doesn't solve the self discharge issue of the battery.

Car batteries self-discharge. Some references say 5% per month. Some say 1% per day. Some say 4% per week.

Many thanks, John. I'll take a look in the morning.... --

"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

A number of late-model GM cars and I expect other brands too have a "transport mode" that puts the 12 volt bus into a low power consumption state because nowatimes cars often tend to sit around a while before they make it to a dealer, and in transit to the dealer, and then on the dealer lot before they're sold.

The CarMax near me has what looks like at least a thousand cars sitting around any one of which may have sat there for some time but could be sold at any time online, they'd probably want to avoid having to do battery-swaps or juice up batteries regularly if they can avoid it

The OP's opening premise was (quoted above in context): "handy to keep a battery topped up".

A reasonable presumption from the phrase "topped up" is that the battery is already nearly fully charged and so it would never be "1/2 discharged" and the Imax rate would not be an issue.

The original poster was Cursitor Doom. Any circuit that he might use has to be not only fool-proof, but also ingenious-idiot-proof, which is even more demanding.

Years of coping with the ingenious ways final test came up with to make their job less tedious has made me pretty pessimistic.

A trickle charger isn't going to work because of acid stratification:

Believe, me, it's a lot better than finding you have a flat battery when you're trying to make that important hospital appointment with no time to spare....

--

"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

In a car from the 90s, sure. In modern times it depends on the car some have weird security systems for the infotainment head unit that want a code to make sure they haven't been stolen but in a lot of cars made in the past decade what you lose from a battery disconnect is "not much", it's not like the good ol days when it was almost guaranteed stuff went bonkers when you disconnect the 12 volt for ten seconds and your ECU now thinks it's in a mail truck. Nothing devastating happens in either my Volt or my buddy's 5th gen Ford Ranger when you do this.

FRAM is pretty cheap nowadays it's non-volatile like Flash but doesn't degrade like Flash and is being used in automotive applications more all the time

I think people got odd ideas about how long it takes an exhaust system to get up to "full operating temperature."

A 4 cylinder engine will draw maybe 250 amps for 3 - 5 seconds to start, even a crappy alternator at idle can restore that in like 1 minute. Then you have another 9 minutes to top off the battery