Car Battery Solar Trickle Charger

Hi there

I'm a newbie here but have been reading some of the threads on solar, batteries, chargers etc for some months now.

I have got a grasp of the types of 'ah' needed to fully charge batteries with solar panels etc but I am looking for some feedback in terms of is this a good idea or am I wasting my time?

I have a Subaru Forester with huge sun roof.
I don't drive my car during the week and if I do use it, it is for short journeys at night, lights on so the battery does not get a good charge, even at weekends it still does not get much use.

Once every couple of months I run my OZ Charge on 'rejuvenation' mode.

I have a spare 20 watt solar panel and was wondering if I keep this in the sunroof and attach it to a Solar Charge Controller (like the Projecta SC005 $50) will this help maintain my car battery?

I know it will take forever to charge it (the panel gives out about 1ah) but that is not the aim, all I want to do is keep a small current 'trickle charging' the battery as I believe this will help combat the battery from sulphating and keep it in a healthy state.

So would this work, would it be a benefit or is there another solution.
My car is parked out doors ad will get a good 5 hs + of sun most days

All advice is welcome


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Reply By: Battery Value Pty Ltd - Friday, Jul 09, 2010 at 15:01

Friday, Jul 09, 2010 at 15:01
Interesting proposal.
The question arises what's more likely to happen: sulphation due to under-charging, or increased electrolysis of water and positive plate corrosion due to over-charging.

Your current battery charge/discharge pattern looks like the battery is being udercharged slightly, although your bi-monthly maintenance charging certainly helps it maintain a reasonable good state of charge. I'd simply do this on a monthly or even fortnightly basis.

A 20W solar panel can pump an average of 67Wh into your battery per day (don't know how much solar energy is getting absorbed by the tinted sun roof though).
A back of the envolope calculation reveals this is enough energy to split 8 grams of water into it's constituents.
So you might have to top off your battery with water a bit more frequently.

Best regards, Peter
AnswerID: 423474

Follow Up By: Dagil - Friday, Jul 09, 2010 at 15:51

Friday, Jul 09, 2010 at 15:51
Hi Peter

Thanks for the reply.

I had thought about over charging that's why I want to use an Automatic Solar Charge Controller like the Projecta SC005 which says it cuts out when the battery is full. (if it gets full)
It states that it prevents the battery from overcharge.

The reason I thought of this method was that the new generation 'smart chargers' state that you can leave them connected over a long period of time and they will trickle charge and cut out when the battery is full - same as the Projecta SC005.

I have also noticed that some places like Supercheap sell small panels you can plug into a car cig socket without a regulator so I was trying to improve on this.

But maybe thats a whole different ball game?


FollowupID: 693928

Follow Up By: Battery Value Pty Ltd - Friday, Jul 09, 2010 at 16:04

Friday, Jul 09, 2010 at 16:04
Hi David,

sorry, overlooked your solar regulator - yeah no probs with this whatsoever.

Ciga ligher sockets are only switched on while the ignition is on, or in acc, don't know about Subs though.

Best regards, Peter
FollowupID: 693929

Reply By: Going Bush - Friday, Jul 09, 2010 at 18:37

Friday, Jul 09, 2010 at 18:37
I had a 1990 Fairlane that I only drove once a month or so. The battery would always be flat as the always-on electronic stuff , alarm / radio memory ect ect which only draws 50mA or so would be enough to flatten it over a few weeks. I put a 20w panel & reg on the rear parcle shelf (thru tinted windows) and I never had another flat battery (even in winter) & that same battery lasted another 4 years until I sold the car.
AnswerID: 423492

Follow Up By: Dagil - Friday, Jul 09, 2010 at 22:05

Friday, Jul 09, 2010 at 22:05
Hi Going Bush,

Thanks for that, so far I have not suffered a flat battery but that is, I suspect, because I charge my battery using an OzCharge on a regular basis but this is becoming a drag.
I'm going to get a solar regulator now and wire it up, it's a shame not to utilise the big sunroof window, the solar panel I already have sits in there nicely.

Seem strange in this day and age why solar is not used more in cars.
I've also been thinking of a solar powered vent in the car so on hot days the fan kicks in without flattening the car battery and there is airflow through the car!

Anyway first up is getting the panel wired up.



FollowupID: 693979

Reply By: drivesafe - Saturday, Jul 10, 2010 at 06:57

Saturday, Jul 10, 2010 at 06:57
Hi Dagil, depending on what model of Forester you have, you will probably have either a 110 amp or 140 amp alternator.

Either way, you have more than enough alternator capacity to run your headlights, sound system, A/C, a couple of driving lights and still be able to charge a low battery during night driving.

In your case, the night driving is not so much a problem, it’s how long you drive for.

If your managing upwards of 30+ minutes a drive then your probably keeping your battery in good nic but it also depends on how often you do the 30+ minute drives. At least once a week is needed.

BTW, Subaru are one of very few vehicle manufacturers’ who actually list the size of the alternator in their owners handbook so you can check to see exactly what size your model has.

As to your solar panel, anything from 10w solar panel size for every 100 Ah battery size, will charge a battery, slowly, but it will charge the battery.

So your proposed set up will mean your should not need to use the battery charger any more.
AnswerID: 423538

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