Solar powered battery sustainer

Keep the car charged without manes voltage during months between drives.

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The car battery loses 16Ah every 2 months it isn't driven.  Letting it drop & recharging it every 2 months isn't a solution.  Borrowing $4 million for a house with a real garage isn't affordable, so something had to be done to top it off without manes voltage.

A $16 pile of solar cells

didn't work so well. They were each rated for 60mA at 5V. Wiring them as 5S2P, they gave 15V 75mA in direct sunlight. They gave 3.5V 3.5mA in the shade.

The car burns 270mAh every day, requiring 3.6 hours in direct sunlight to recharge. Being parked in direct sunlight would destroy the paint. Open circuit voltage goes to 30V, so the solar would need protection diodes to avoid blowing up the car if the battery was disconnected.

The lion kingdom was left with a pile of solar cells after a failed attempt to make a car battery sustainer. Enough solar cells to do the job would end up costing more than a new battery.  Any electrified car port costs millions in today's money. It's hard to believe there was ever a time when houses were only 5x an average annual income, but lions still remember. It's been 20 years since the multiple became 10x & it's never gone down. The average electrified garage is now 15x the average salary. Lions believe the multiple will keep rising as long as the global workforce continues expanding faster than demand for labor.

Solar power is a pit of despair for lions without money. It's amazing they can light up a small LED in ambient light. It is believed they would be better than nothing at sustaining the car battery, despite only making it a hair less flat.

Up close, the cells are a messy bodge of smaller silicon shards.

After contemplating different fates for the solar cells, the decision was made to finish a minimal battery sustainer by tapping into the ODB2 port.  Don't forget to whack in shottky diodes to prevent the 2 solar cell strings from feeding each other or draining the battery.

This included a teardown of the Metromile.  The Metromile has never been torn down before.  There was no tamper protection, so after a few warnings about the unit being disconnected, it resumed functioning after being reassembled.

The battery was fully charged on 2/22.  Then it was driven twice until 3/5.  The solar panel went in on 3/8.  

The lion kingdom decided to recharge it again on 4/24.

Given that it was only sitting for 1.5 months, it might have gone to 11.5Ah if left for 2 months.  The solar panel was generating power at least at a rate of 5Ah over 2 months.  Another $50 of these cells would bring the rating to 12W & be required to indefinitely sustain it.  Once above $30 though, it's cheaper to get a 25W panel instead of 12W of loose cells.  The loose cells would be written off as a low cost test of viability.  It's a case in point for the cheap option always leading to paying twice.

A $50 total is still cheaper than a new battery & lions are so tired of reinstalling the battery every 2 months, it might be worth it.  Still perplexed on what the solar panels could be used for after commuting resumed.  A car battery only takes 2-3 Amps of charging current.  Driving it only a few minutes isn't enough to charge it.

25W panel with custom stand.  Making the panel stand up in a car was a big deal.

The 25W panel makes enough power in the shade to power the Metromile & the car's engine confuser.  It stays above 12V & the bar graph oscillates from the metromile pinging GPS.  

  • Ratchet toilet roller

    lion mclionhead11/05/2021 at 03:53 0 comments

    The next idea was a ratchet section paired with the spring section, to dial the spring to just the right tension.  The ratchet mechanism was quite complicated, but seen as cheaper than printing a corkscrew.  

    Trying to make a timelapse of the printout was problematic, so sections got printed once in timelapse mode, thrown away, & again without timelapse mode.

    Timelapse mode created blobs on 1 side.  The blobs impacted the nozzle & broke the parts off the bed.

    The thinnest cylinder was plagued by voids & eventually got more material added to compensate for the voids.

    It solved the problem, achieved just the right pressure for any height, but definitely was lacking.  That ratchet mechanism was complicated.  The ratchet desperately needs a stopper to keep from falling out & bumper to make it extend in a straight line.  

    The adhesives bond rubber, but not PETG.   PETG facing rubber needs holes to take up adhesive.

    A corkscrew would definitely be simpler than the ratchet & probably use the same amount of material, but it would be harder to use & harder to look at.  1 section could have a corkscrew.  Another section could have a tab stick into the corkscrew.  

  • Giant toilet roller

    lion mclionhead11/01/2021 at 07:07 0 comments

    So the pool noodle compressed instantly & the solar panel fell back down.  The next attempt was a giant toilet roller.  

    Made from the cheapest spring currently made.  The lion kingdom wondered who bought plastic toilet rollers for any use other than recycling the spring or who mail ordered a plastic roller from 3 miles away & waited 10 days.

    The PETG pieces were surprisingly strong.

    Some rubber from sandals was epoxied on.

    A previous attempt had joints, but these proved to be 2 extra pieces requiring alignment rather than any improvement.

    Unfortunately, the spring had the right tension at only 1 point.  There was much shimming of the spring length to make it stand up at 1 point, but  the spring never reached an ideal tension.  It was seen as the easiest to install because it would only require 1 paw.  In reality, it takes 2 paws to fight the resistance of the spring while positioning the ends.  Because of the variety of alignments required to stand the solar panel up, a better system would use a ratchet.  

    Another idea is to make the solar panel fold like a book.  It probably needs a bigger area to apply force.

  • Standing a solar panel up in a car

    lion mclionhead05/13/2021 at 03:42 0 comments

    The Ecoworthy was moved to the back window since that seemed to get more sunlight in the morning. 

    Current logging would find the optimum position, but you've got a shunt resistor, an op-amp, feedback resistors, an ADC.  All the resistors have to be schmick to withstand the temperature changes in a car.  Another idea was making a timelapse movie, but the camera needs to withstand high temperatures.  

    The battery was never recharged from manes voltage after going to the Ecoworthy, so the results with the Ecoworthy were never fully known.  It definitely wasn't dead.  The car seemed to start as fast as a fully charged battery, after months between drives.  

    After much debate, the optimum way to stand up the panel was viewed as a pool noodle.  The front has the instrument cluster to prop it against.  The back has the seats to prop it against.  The front passenger side would be problematic.  The lion kingdom had some 20 year old pipe insulation which could be bundled.  It's unknown how it withstands high temperatures.

  • 25W panel

    lion mclionhead05/04/2021 at 19:27 0 comments

    $33 later, there was the mighty Ecoworthy.

    To save money, it was just connected to an LM1117 to limit the voltage to 14.5.  That has a 1V drop at low current & 1.5mA quiescent current.  

    Lacking a dummy load, it was easiest to time the charging of a capacitor.  It took around 20 seconds to charge a 36000uF cap to 14.5V in a shaded area.

    According to LT, it might have been making 5-10mA in the shade at 13V.  There's a brief time every day when the windshield has direct sunlight.  That's when most of the charging would happen.

     In sunlight, it made 1.5A at 1.5V & the cap charged instantly.  Its peak efficiency is at 18V, so an ideal charger would charge a cap to 18V & discharge just enough current to keep it at 18V.  

    A simple PETG enclosure followed.
    Then some TPU bumpers would keep it from scratching the car.  A simple trucker's hitch lashed the bumpers down.

    Some worthless socks propped it up.  The bumpers definitely made it look more upmarketed than the standard solar panels we see in parking lots.

    Helas, in the shaded windshield, it only made 13V with 1.5mA load.   It might be dirt on the windshield, less indirect light under the roof, the load, the panel being rated for lower voltage.  The battery was stuck at its starting voltage of 12.7, whether the panel was fully sunlit or in the shade.  The smaller panel must have generated under 1mA in the shade.

    There is no direct sunlight in the summer.  The sun is too high.  Maybe current logging would give more data, but it's a lot of work for what it is.  The lion kingdom doesn't have any other budget besides a $33 panel & linear regulator.  It's easier just to wait another 2 months.

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Brian wrote 11/06/2021 at 00:51 point

This is awesome! I was just thinking of a similar thing to top off my plug-in hybrid. It would be something like those sun shades that unfold and you put in the front window to keep the car from getting so hot in the summer, but with solar cells. Or maybe just put solar cells on the roof of the car? That might look funny.

$4 million for a house with a garage? Where in the world is that? Move somewhere less expensive.

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Lightning Phil wrote 11/01/2021 at 10:05 point

Had fun with a 2007 Prius, where the traction battery also lost some energy over time and became flat.  Didn't drive it for a couple of months and the car wouldn't work and stated "add fuel" despite having some in the tank.  So charged it up slowly with a high voltage PSU and away it went. 

Its 12V battery goes completely flat after 36 days if left locked and about 3 days if left unlocked.  Hideous power draw!  Solved the problem by getting rid of the car.

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