9 watt solar panel to charge a 12v battery directlyor do I need a rectifier?

[Clive_]

Hi,

I have some questions please...

Can I use a 9 watt solar panel to charge a 12v battery directly? Do I need a rectifier or is the loss back in to the solar panel too small to worry about?

Also, can I use a 24v alternator (permanent magnet motor) to charge a 12v battery - or is it just likely to cause damage to the battery?

Thanks,

Clive.

[Finlay]

it depends on the output voltage of teh solar panel.  the 24V alternator is different.  If it is a permanent magnet motor then the output voltage is speed dependant.  if it is an alternator from an engine it could still have a clamp diode mounted on it which limits the output voltage to a bit over 24V.  If it is a plain old DC motor then you can use it, but beware of overvoltage on the battery, since you could damage the battery if the motor produces too high a voltage by spinning too fast.

A catch all solution would be to use a DC-DC voltage converter.  These are available which accept input voltages from 5-36V and regardless of the input voltage, output a voltage of 12V or so (you will want a little more than 12V to charge a 12V battery (is it a lead-acid battery?)).  Such voltage converters are available in a format that typically is mounted on a PCB and put in a box with associated input / output sockets.   Not too difficult to make if you are handy with a soldering iron, but if you are not confident with this then you ought to be able to buy an out-of-the-box (already in-the-box) DC-DC converter.  You will just pay a bit for it.  I don't know of any readily-available ones, but that is not to say they don't exist.  Maplin electronics would be a good place to start.

You really do want to provide the right charging voltage to the battery though.  You will make a more efficient charging solution and also not risk damaging your battery.  Hope this helps...

[Clive_]

Hi,

Thanks for the reply.

The battery is a 12v one from Maplins – looks like it is usually used for robot or model construction. It weighs too much for most other applications. It is a 12Ahr battery; so drawing 2amps an hour I should get 6 hours (but don’t!)

The solar panel is 9 watts and running a meter on it shows an output of around 20v in peak conditions. It is rated as a 12v 9w but seems to vary. I have seen it produce anything from 1 – 6 amps but it is usually closer to the 2.5a mark.

I put the panel in sunlight for 6 hours two days ago and it only charged the battery around 20%. This was in good sunlight too. Because the charge was so poor, I started to worry that the panel was either not putting the charge in to the battery or it is taking it back out when cloud comes over (though I thought it was dependant on light overall so find this hard to accept). I periodically metered both battery and panel to see what each was doing. The panel seemed to continually give good readings (20v 3-5a; when metered with leads unconnected to the battery) and the battery seemed to be taking on a charge to start with (0.2v at start, then 3.8v an hour later), but then seemed to drop. Later in the day it gave better readings but the fluctuations have confused me and left me suspecting the panel is withdrawing the power as well as adding it at other times. Though, firstly I do not know if this is possible and secondly are unsure of the best way to combat the problem? I did notice at one stage that the volts had increased but the amps had decreased since the last reading (an hour before)!

The motor I mentioned is to add to the battery charging array. I have not selected one yet but see more 24v available than 12v ones and assumed that generating 24v as standard would increase the chances of sustaining a 12v charge (regardless of conditions). Whereas a 12v, I assumed would need to work harder to compete?

Have you any suggestions please?

Clive.

[billi]

The solar panel is 9 watts and running a meter on it shows an output of around 20v in peak conditions.

hope i am right but you have to look at the AMPS of your panel , so a 9 watt panel  does not mean to push 9 watt into your battery  even on ideal situation

More likely it delivers 0.4 AMPS at say 22Volt (= 8.8 watt) , so at 12 volt *0,4 amps only 4.8 watt remains

the rest is wasted

Billi

[Clive_]

Hi,

Thanks for the reply.

I was with you when you said 0.4amps at 22v giving the rated 8.8watt (which means I must have read the meter wrong) but don't understand the "4.8 watt remains so is wasted" part. I take it that as the battery is 12v then it can only accept 12v at any given time and thus only accept half of the available amps?

If my understanding is correct, have you any suggestions please?

Clive.

[martin]

Let's start at the beginning - first of all your poor battery!
Unlike "chuckaway" batteries, about the worst thing you can do to a rechargeable battery like yours is to flatten it anywhere near fully out - in some cases a very few cycles like that can kill it stone dead....... A good simple "rule of thumb" is to remember "10%"......
Don't charge the battery at more than 10% of it's capacity (100 amp/hr battery, 10 amps charge current)
Don't DIScharge the battery at more than 10% of it's capacity (100 amp/hr battery, 10 amps DIScharge current)
Ideally aim to discharge 10% of the battery capacity before recharge (for maximum longevity) - I'd aim to never take it below 50%

Then the pv panel - if it's a genuine "9w at 12v" panel, it is designed to charge a 12v battery, not that it gives 12v (if that was all it gave, it wouldn't charge) - if you put your voltmeter across the panel when it's in the sun, and not connected, it should show something like 20v. My suggestion would be to get even a cheap charge controller which will have the necessary diodes built-in - it'll stop any charge leaking back out, and will stop the battery frying when fully charged.
I suspect you're probably connecting the meter wrongly - if you have the panel connected to the battery (preferably using a charge controller), the voltage will probably be anywhere between a few volts and 15 volts, the amperage should be around .5 to .8 amp.........
If the poor old battery is "pancaked" (flat as a...), it'll take probably take 4-5 days of full sun to get it back up to full charge using your 9w panel (on a good day at this time of year, you should get around 6 times the panel rating in total over the day - around 3 amp/hrs taking losses into account..........)

[Baz]

Maplin used to do a gizmo for charging a second (caravan) car battery which might do the trick.
http://www.maplin.co.uk/Module.aspx?ModuleNo=37762 is one item but not really for sealrd batteries.
or http://www.maplin.co.uk/Module.aspx?ModuleNo=37767 looks a bit better.

If your panel is one with a car ciggy lighter plug on it then it will have the diodes built in.

[stephendv]

You need a charge controller not a rectifier.  The controllers that Baz linked to look the part, just make sure that they can accept up to 20V input (or whatever the Voc rating of your panel is).

[martin]

Or you could use the "house brand" one - http://www.navitron.org.uk/product_detail.php?proID=38&catID=127
I'm using one with 2x5w pv panels feeding a 12 volt 26 amp/hr battery running the electric fencer unit for my chooks, and it works a treat! - when checking this week, the voltage across the battery in late afternoon was around 13.2 volts - happy battery!

[daftlad]

Also, can I use a 24v alternator (permanent magnet motor) to charge a 12v battery - or is it just likely to cause damage to the battery?

These guys use a 24 volt motor to charge 12 volt batteries.
http://electricpedals.com/how-it-works/
ta ta

[Clive_]

Hi,

Thanks to all for your help so far 

I will pop in to Maplins and get the charge controller you mentioned - the second one does look better and is only £2 more expensive; plus it gives me the ability to get another battery somewhere along the line without needing a second controller.

Yep, as we all suspected, I was reading the meter wrong (or used the wrong setting) because when I tested it earlier it gave the sort of readings that you mentioned. It has an upper limit of 20v but seems to hover around 18v so I assume that will be ok.

As for the battery: with my current configuration I think there is no immediate danger of me over changing it (  ) and I use it to power an inverter so that I can run 240v from it. The inverter has a cutoff when the battery reaches around 9v to save the battery from being crippled etc.

So, if I get myself a charge controller, I will be able to charge the battery more effectively and not need to worry about leakages?

Going by the idea that the inverter will shut off at 9v, in reasonable conditions, how long do you think it will take (using the above 9w panel) to get the battery back up to a reasonable voltage again? I ask so the I have a benchmark to go by - if things work out much different to your estimates, I know that I am doing something wrong or the panel is not performing properly etc?


Thanks again for all of your help.

Clive.

[martin]

"If the poor old battery is "pancaked" (flat as a...), it'll take probably take 4-5 days of full sun to get it back up to full charge using your 9w panel (on a good day at this time of year, you should get around 6 times the panel rating in total over the day - around 3 amp/hrs taking losses into account..........)"

from my above post.............

ps, running the battery down regularly to 9v will kill it very fast indeed - probably tens of cycles only........

[Clive_]

Hi,

Ok, so 3 amps per hour with a max of 12 amps (battery is a 12amp per hour) - I should get it up to full charge in around 4 hours from near flat.

As for the inverter: so I need to make sure I have also got some 12v stuff running off the battery too so to draw the battery down on a regular basis (but not too flat etc)?

Thanks again for your help 

Clive.

[martin]

Methinks you're taking the michael............. had you actually read my post, and the reiteration of the relevant chunk, you'd know that the sum total for a day's output would be around 3 amp/hrs, which means 4-5 days to fully charge the battery if the sun shines, and you take no power out of the battery at all..........

IF you are serious, do some simple sums on what you're taking out of the battery, then you'll have some rough idea of what's going on.......

Taking your battery of 12amp/hrs, I'd design to take no more than 1.2 amp/hrs in a day, 6 amp/hrs total maximum - which if you're using an inverter would be something like a 15 watt bulb for one hour (or 5 hours if you don't want your battery to survive very long)

(Are those the sirens of the National Society for the Prevention of Cruelty to Batteries?)

[Clive_]

ha ha ha ha ha...

Sorry - I did wonder what you were talking about - I thought when you put 3 amp/hrs you were saying that there should be an intake of 3 amps per hour lol. That's why I asked a second time lol  ha ha ha...

Ok, so lol, my 9w panel will give me a booming 3amps a day at best. I need a regulator - the second one that Maplins sell that can connect to a second battery at a later date might be a good choice and I need to restrict usage of the battery to 1.2 amp/hrs in a day, 6 amp/hrs total maximum. If somethings goes wrong and I completely flatten the battery then I need to expect to wait about 5 days before it can breath again!?

I have been running my laptop and an LED projector off it in general (only a couple of times overall) but intend to use it as a temporary power supply when camping. It will only be boosting phone chargers and sometimes a computer but that's about it.

I want to encompass some form of wind generator (small one) to assist the panel but without making the alternator myself are a little restricted (cost is an issue so eBay motors are out of it for now   )

Thank you for your continued help lol

Clive.