Passive Car Battery Trickle Charger

Just turn it over and let it run for 10 min once a week which is better for the powertrain, also

Far from true. Running the car cold like that puts both gas and water into the crankcase.

Lead-acid batteries lose very roughly 1%-3% of their capacity per day. (See e.g.

.) The capacity of your average car battery is roughly 3 MJ, so figuring a couple of hours' peak charging per day (probably optimistic for England), the peak charging power required would be on the order of

(0.01 to 0.03)*3E6/7200 ~ 4W to 12W, which isn't that horrible for a small solar setup.

Cheers

Phil Hobbs

This is a commodity item. Schumacher SP-200

You don't need anything more than this for the average personal vehicle. I think everything is miniature in UK anyway.

I wonder if you need the resistors and zeners? Maybe just reverse protection? Let your battery be the voltage ref. Well maybe something to stop the battery from frying the solar panel if someone puts the polarity backwards. (The backwards FET and R trick?)

I looked for a solar charge tear down, but found nothing good. For ~$40 you can buy one.

George H.

That seems kinda complex.

Suppose that every day at dawn, you make a decision to connect the solar panel to the battery, or not, for the rest of the day. I think there could be a simple circuit that does that. That circuit would have basically zero power dissipation.

--

John Larkin         Highland Technology, Inc   trk 

The cork popped merrily, and Lord Peter rose to his feet.   
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

Oh boy, if the simple circuit does not involve a human, I'd like to see it. I did like CD's 6.2 V zener, with a 0.6V pn diode I think you get near zero tempco. ... But I guess you'd like the reference tempco to follow the battery. This says you should reduce float voltage with temp.

GH

That's rated 2.4 watts. Might be marginal, especially if it doesn't get a lot of full sunlight.

A 10 watt version is about $100. That would need solid sunlight for a couple hours per day.

--

John Larkin         Highland Technology, Inc   trk 

The cork popped merrily, and Lord Peter rose to his feet.   
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

I'll scribble something up. Gotta drive back to SF now.

Maybe some other electronic designers could have a shot at it meanwhile.

--

John Larkin         Highland Technology, Inc   trk 

The cork popped merrily, and Lord Peter rose to his feet.   
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

That would make a nice change from "oh, you might as well just buy one of those from China" which I typically get! This is supposed to be a design group, after all. ;-) --

"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 did try the bare-bones approach to begin with, but the solar panel was able to fry even a heavy-duty Land Rover battery before the end of February even. I suppose I could mask-off 2/3 of its surface area to reduce the amount of sunlight it gets, but that will cripple it except on the brightest days. It's also not a design solution!

Yeah, but for ~$0 and some junk box components I can build one. And have a bit of fun thrown in, too. Don't get much fun at my age. --

"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

Design isn't just about sorting out which components to tie together and in which way; you also have to think about the suitability of a solution to the problem posed IN PRACTICAL TERMS!

As I stated elsewhere, that's where your solar idea falls down as it's still a corded device.

Sure, but for me I'd start with whatever I could buy and pry that apart..

But yeah for home brew you have to limit the current or the voltage. (or a uC to monitor both.) The easy way to limit current is the size of the panel. You could put a current limit in... but that seems to limit the solar charger. So a voltage limit. Are you going to compensate for battery temperature? You could make something that turns on and off with ~0.5 V of hysteresis that might cover any temperature variation.

George H.

Just a bang-bang on/off regulator should work. When the battery voltage is low, connect the solar panel. Maybe have a little hysteresis.

--

John Larkin         Highland Technology, Inc   trk 

The cork popped merrily, and Lord Peter rose to his feet.   
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

I'm not exactly sure what a "bang-bang on/off" regulator is, though, John. Digikey doesn't appear to stock them. Nor Mouser! :-) You don't reckon my original design is worth building then? Not even with a few tweaks?

--

"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

Does it involve a latching rely? How much current are those things good for? George H.

Nonsense. I had a 15V - 400 mA wall wart that I enlisted as a car battery topper and worked fine. It would not just prevent the battery from losing charge, it would charge it up if it were depleted, slowly of course. The v oltage was regulated by the battery, in effect being a constant current dev ice. I even went to the point of adding a jack to the truck wiring so it w as easy to connect.

The ideal circuit for this application is to use a pFET as a switch through an inductor, letting the battery manage its own voltage. Run the switch s o as to maximize the current. A simple dithering algorithm would maintain a maximum power transfer. Make a small adjustment to the pulse width and m easure the current. If it drops reverse the direction of the next change a nd maybe double the size. When the change is not downward, set the step si ze to minimum and retain the direction. Very easy to do in a low power MCU . Heck, probably not hard to do in a discrete circuit, but not worth the e ffort.

An even simpler circuit is to just use a resistor to limit the current if y ou can size the solar cells to not overcharge the battery when it is full. I believe a diode is required to prevent the battery from discharging thro ugh the solar cells.

--

Rick C. 

- Get 1,000 miles of free Supercharging 
- Tesla referral code - https://ts.la/richard11209

ed

He means like your heating thermostat. When the battery reaches a full cha rge disconnect the charger. When it droops, reconnect it. Other than over

-voltage the battery will control the voltage itself. The solar cell will set the current, but not optimally. That's what the switcher does, load th e solar cell to produce the most current into the battery. I think I would use a solar panel with a lower voltage than the battery and let the contro ller boost the voltage as required. Using a panel with a higher voltage me ans it has to be disconnected when the voltage falls, or use a buck-boost. So maybe a 9 or 10 volt panel.

As someone indicated this should take into account the temperature of the b attery. That can be by measuring the temperature or by simply setting the voltage threshold to the lowest voltage that is safe at all expected temps, but... that might not provide enough charge under all conditions. It's no t like the battery has to be 100% charged all the time. It needs to have e nough charge to not sulfate and start the car.

--

Rick C. 

+ Get 1,000 miles of free Supercharging 
+ Tesla referral code - https://ts.la/richard11209

Error: You must specify an action

You can build a constant current source from a LDO linear regulator and resistor. Work out what level of "trickle charge" does not fry the battery and build a LDO constant current source to send no more than that much current to the battery.

You might lose out on overcast days, but you'll prevent frying the battery on bright sunny days.

I.e. (simplified):

in |---| out +---------|LDO|------+ +| |---| R solar |gnd R panel | | -| +--------+----+ | |+ | battery | |- +-------------------------+ The solar panel will need to create a voltage greater than the sum of Vbatt, the LDO's regulated voltage, and the LDO's dropout voltage. So you'll want to use a low voltage LDO regulator to keep the max needed from the solar panel down to a reasonable amount.

Omitted is any reverse protection for the LDO and/or necessary bypass caps for the LDO to prevent oscillation.

You have to compute a suitable R from the LDO's regulated voltage to produce the desired charge current that will flow into the battery.

Dunno what those ratings mean exactly but you'll never see them in practice . I measured it delivering just 17mA into a 12.2V on the battery on a sunny day. That's enough to keep it topped. I've been using three (SP-200) of th em for over ten years and never got close to a battery problem. I can leave a vehicle parked six months and they spin right up when I start them. The main thing they're good for is not to keep the charge topped so much as to prevent sulfate build-up on the plates. Apparently you don't need a lot of current to achieve that. If you let the battery discharge for too long, the sulfate makes the battery unsalvageable, it can't be re-charged ever again . The batteries I use are in the 100Ah range.

on"