Battery charger with solar panel

Easy enough to do with a comparator, see

"OverAndReverseVoltageProtection.pdf"

on the S.E.D/Schematics page of my website. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 

     Thinking outside the box... producing elegant solutions.

gm prodded the keyboard with:

I've done exactly that using an LT8490 and the circuit on the data sheet. My panel is a 300W 42V 8.5A. feeding two 110 Ah wet cell leisure batteries.

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Best Regards: 
                      Baron.

Wouldn't that be a pretty inefficient design? The panel output voltage is dragged down by the shunt regulator wasting power both in the solar panel and the shunt regulator. You can find any number of converter circuits on eBay and elsewhere for cheap. Or you can build your own like Baron. The LT8490 looks like a pretty Cadillac chip at over $15.

I found one guy who is already making boards and sharing the design.

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Rick C

The devil is in the details.

You seem to be focused on the isolation diode loss.

Are you in a location where you can reliably get 75Wh/day out of a 15W panel in winter?

Depending on the time of load, you may be running the battery flat every day. Not a good idea.

Charge efficiency is not 100%.

Overdischarge management?

Overcharge management?

Depending on available insolation, time of load and the characteristics of your panel, you may be better off with a way bigger battery, way bigger panel and a MPPT controller. The MPPT is quite simple to build with a microcontroller. It also solves all your charge management issues.

Looks like the ideal solution to keeping my pick-em-up truck battery charged, since I drive it only every few weeks.

And in Arizona we certainly don't lack for sunshine ;-) ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 

     Thinking outside the box... producing elegant solutions.

Do you really need 75Wh/day?

For $12, it's hardly worth walking all the way to the lab to turn on a soldering iron so you can spend way more than that making one.

I was talking of others... I plan to buy that the next time I'm near the Mesa Harbor Freight store. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 

     Thinking outside the box... producing elegant solutions.

I believe that is only 1.5 watts. That's barely enough to trickle charge a car battery. I've used a 2 watt, 15 volt wall wart to trickle charge a car battery and that's just barely enough running 24/7.

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Rick C

Argh. That's an amorphous silicon solar panel. Low efficiency (about

7%) but cheap to produce and works in low light and shade: In full sunlight, open circuit voltage = 20v and short circuit current = 0.1A or 2 watts.

I have two solar panels similar to the Harbor Freight model. I used them in the distant past to keep the battery charged when I had a collection of dead vehicles parked in front of my house. A dead car in front is considered a status symbol and indication that I own the house as no landlord would let a tenant collect dead cars. During the winter, I had to top off the batteries every week with a boost from a conventional battery charger: as the solar panels were insufficient. I eventually went to a 12v

1.5A "battery maintainer" powered by the AC line. They worked quite well until someone stole both of them.

I also have an ICP Solar 04021 Battery Saver Plus 5w, which is about twice the size of the Harbor Freight model, and allegedly delivers twice the power: Oddly, I've never tried to use it.

Incidentally, you can purchase a ready made solar powered battery maintainer. Just add a solar panel: Or, get it with a 5w, 10w, or 15w panel built in: (near bottom of page).

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Not sure why you are multiplying these numbers, but assuming this is the potential power output from the panel is a faulty deduction. Is this just a convention for rating a panel?

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Rick C

How can you tell it's just barely enough?

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Google "MPPT"

A very simple charger can be constructed with a few current limiting resistors and forward biased diodes. The reverse breakdown voltage of the diodes must be larger than the maximum battery voltage so that the battery does not reverse discharge into the solar panel, after sundown.

Ideally, the maximum charging current should be 700 mA, i.e., (battery AH rating)/10.0 -- current limiting resistors ensure this, as well as ensuring that the maximum forward current of the diodes is not exceded. Also note that the solar panel power output rating is what has been measured under ideal testing conditions of the manufacturer -- on site daily power output will vary will depend on that day's weather conditions.

My truck draws 9mA. As I recall 6mA is the radio. All I need is that plus whatever self-discharge is going on.

Nope. You're correct but I didn't want to get into the details. The data sheets sometimes use the simple arithmetic I used. However, the real power output is about 20% less. The difference is called the fill factor. Measure Voc (Voltage open circuit) and Isc (Current short circuit) and plug it into the calculator below. Multiply and you get Pt (theoretical power), the maximum power that an ideal solar panel can deliver. Multiply again by the approximate fill factor and you get a more realistic value for Pmax (maximum power): Everything else causes the power output to decrease. Output drops when the panel gets hot, the angle of the sun is less than perpendicular, dirt on the glass, voltage drop from series diodes, reflections, weather, clouds, panel aging, etc.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Looks like a simple PWM controller and that's fine but that little shunt regulator will have to dissipate almost 100 watts itself. In fact, 12.0V for a 12V battery is basically dead anyway and if limited to 14.0 volts, the battery (lead acid) will die an early death without getting a decent absorb charge once in a while.

Linear Technology is smarter than this ! Maybe that circuit was submitted by a student or something ?

boB K7IQ

You clipped my example. It would take a full day to charge a slightly run down battery. If it was fully down it would take three days. I've measured the current it supplies and I see very little difference when it is run down or fully topped off.

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Rick C

The self discharge is not trivial. Or I should say the trickle current to maintain a car battery. Without rereading all the post to find what size battery you are charging, it *is* a regular car battery, right? Or do you have one of those sealed gel cells?

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Rick C

Huh? I quoted all that arrived here.

"barely enough" is, to my mind less than twice the self discharge rate.

If the vehicle can sit for 4 weeks (without the battery being fully depleted) and then recover on that charger in only 3 days, I'd say it's got at-least 8 times excess capacity (for for the task of preventing unattended discharge)

That is not unusual in a constant-current charger.

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