Solar Panels in Parallel for Battery Charging

[pmlogg]

I have a pair of 1.5W solar panel trickle chargers, like the one in the image that I would like to connect in parallel to charge a 9Ah 12V sealed lead acid battery.  The battery is the power source for a 2nd hand Pulsar Dome rotation system.  It is normally charged by just one of the panels but I had a spare so with short winter days often not providing a lot of sunlight I thought adding a second would help.

I have fitted them to the outside of my dome, immediately above each other so they get almost the same solar exposure and each is fitted with a built-in blocking diode.

Can I safely connect them in parallel using a Y-cable or do I need to use any charge controller(s)?  The original system did not include a controller.

[Davey-T]

Probably better to use a controller, they're cheap enough, will monitor the battery level and have connection for load.

Dave

[Dr_Ju_ju]

Personally, I'd just add a diode in series with each panel, to ensure a fault in one won't affect the other, but then I'm a cheapskate....  

[symmetal]

At 1.5W they can only supply about 100mA maximum each to a 12V battery, so can be left permanently connected to the battery without issue. As they have a blocking diode built in you can just connect the outputs in parallel. If they were higher power solar panels that could supply 500mA or more then a controller would be recommended as Dave suggests.

Alan

[Anthonyexmouth]

are they actually doing anything? i've found the little panels are next to useless. at optimum angle it'll only give about 0.1A with losses and a fixed panel will give that for such a short time duding the day. 

[Davey-T]

I had one of those from Maplin that got rained on and packed up, took it back and they said it wasn't suitable for exterior use 

Dave

[Stub Mandrel]

As they have blocking diodes fitted they can be paralleled with no other protection needed.

Those cells are so puny (about 100mA) they will keep a battery topped up with no risk of overcharge especially as the current will be nowhere near 100mA into a fully charged battery. To charge your 9aH battery from flat they will take a week or two.

P.S - worth removing the LED, even if it only takes 10mA that's 10% of your charge current!

[pmlogg]

Thank you for all those replies. 

One of the panels came with the dome, accidentally included as I hadn't bought the motor drive -it was Pulsar's demonstrator. The other panel came with the 2nd hand system I bought - so both have lived outside and have been again for a few months, although not connected to a load.  The LEDs are blinking so the panels aren't dead but what amps they are producing I'm not sure. 

As the dome was already drilled and fitted with grommets for the cables, and I already had them, I thought I might as well give them a try. I just had to make a bit of bracket to hold the pair, using the existing bolt holes.  I do have mains power in the dome though so I could use a normal charger instead.  If they don't perform I'll just disconnect and do that - remembering to unplug before rotating!

I think I will try without a controller first - given the panels are so low powered, and have the diodes - really to avoid having to find extra space for the controllers and a means to secure them. 

I take the point about the LEDs but the panels are sealed so I don't want to open them up to get at the LEDs.

I'll report back when everything else is fitted and I've had a chance to let them do their thing.

 

[Davey-T]

9 minutes ago, pmlogg said:

The LEDs are blinking so the panels aren't dead but what amps they are producing I'm not sure. 

The LED on mine was blinking but no power output was measurable with an ampmeter.

Dave

[symmetal]

You can put an ammeter directly across the solar cell panel output and it will display the short circuit current which is a bit more than the current at maximum power output as shown by this graph. Open circuit it should give about 15 volts. Short circuit it should deliver about 120mA. Unless you've got Dave's panel. 

Alan

[Stub Mandrel]

If it's a blinking LED it will use a lot less power.

[pmlogg]

A while back I did put my multimeter on the panel outputs and did get a voltage but I didn't check the current, just the volts.

I will test V and A tomorrow if it's dry when I get home from work. 

The LEDs are blinkers.

Thanks

Peter

[sloz1664]

On 30/07/2018 at 15:51, pmlogg said:

I have a pair of 1.5W solar panel trickle chargers, like the one in the image that I would like to connect in parallel to charge a 9Ah 12V sealed lead acid battery.  The battery is the power source for a 2nd hand Pulsar Dome rotation system.  It is normally charged by just one of the panels but I had a spare so with short winter days often not providing a lot of sunlight I thought adding a second would help.

I have fitted them to the outside of my dome, immediately above each other so they get almost the same solar exposure and each is fitted with a built-in blocking diode.

Can I safely connect them in parallel using a Y-cable or do I need to use any charge controller(s)?  The original system did not include a controller.

Certainly something I am considering on the shutter system of my Pulsar Dome.

Steve

[pmlogg]

Well, not sure if I can recommend them. 

Today I measured voltage then current across the terminals.  One gave a stable voltage of 9.3V, the other averaging that, but jumping around.  Not sure if I am reading the scale wrong but set at max. 300 DCmA I only got a reading of 1.4 on each and 2.2 when connected in parallel.  Wouldn't that be just 20mW?  It was quite overcast, after rain but nowhere near dark.

Peter

[symmetal]

Yes, you got 20mW. I expect these only give their specified outputs in direct bright sunshine, but the readings you got don't look too promising. If you can't get at least 14V from them then they aren't going to do any charging.  Try again in sunshine to see what you get, before you consign them to the bin.

Alan

[Stub Mandrel]

I've had one of these. They are little more than toys, they need good sunshine to work.

My application was daily charging up a small NiMH battery pack for a weather station, it wasn't up top it even though the amount of charge needed was very small.

[pmlogg]

Looks like we are due some sunshine Friday and Saturday so I'll check again then.  But in the real astronomy season the number of hours of good sunshine each day can be pretty limited so not a dependable supply - mind you, the number of good nights when the dome is used can be quite few too!

[pmlogg]

As the BBC promised, some late sun.  Voltage bouncing about but 18-21V and 20mA.  So, more than an order of magnitude better but still under 0.4W rather than the theoretical 3.0W.  Definitely not enough to even maintain the battery let alone do any re-charging after a night's imaging. 

The search for an alternative has started.  I noted some properly waterproof models - NOCO BLSOLAR5 5W and BLSOLAR2 2.5W but they cost more than a mains trickle charger and would need re-jigging of the mounting on the dome.  I'm struggling to think of reasons not to use the available (and reliable) mains power rather than a panel,  other than a reluctance to leave a charger on all the time.

[Stub Mandrel]

This looks a better bet, a good deal for £25 if it does what its says on the tin:

https://www.lowenergysupermarket.com/renewable-energy/solar-power-kits/942/10w-solar-panel-kit-1-les?gclid=Cj0KCQjw-o_bBRCOARIsAM5NbINztO40W3NnLEZDHm_Ip9BIynpkV3N_uR3OR0E_CSZaB3fwXyd0QAkaAlN0EALw_wcB

[Davey-T]

I bought a 25 Watt panel connected to 12 volt battery to try and run a 2amp pet blanket, switched on / off at dawn / dusk by an LDR but it can't keep it running all night n the winter.

Dave

[pmlogg]

1 hour ago, Stub Mandrel said:

This looks a better bet, a good deal for £25 if it does what its says on the tin:

https://www.lowenergysupermarket.com/renewable-energy/solar-power-kits/942/10w-solar-panel-kit-1-les?gclid=Cj0KCQjw-o_bBRCOARIsAM5NbINztO40W3NnLEZDHm_Ip9BIynpkV3N_uR3OR0E_CSZaB3fwXyd0QAkaAlN0EALw_wcB

Thanks for that link. I had looked at similar (same factory?) ones but eventually found a description clarifying that the connection on the back is not waterproof, although the panel it self is claimed to be.  So I didn't have much confidence in that. Also I was uncertain how easy it would be to mount that type.  The one I listed is described as IP67 - the only panel I could find with an IP rating.  The reviews on Amazon seem good.

The draw from the vintage Pulsar rotation system is intermittent as the dome indexes periodically, not continuously.  It of course draws more power during slews. 

I don't have a feel for how many watts might be used during a session as there are no specifications for the rotation kit and Pulsar seems not to have any corporate memory.  I assume that even an extensive night of imaging would probably use less than if dotting about observing multiple objects.  Pulsar did intend for just a single 1.5W panel to be enough to re-charge during the day and trickle maintain but how users found that in practice I don't know. 

The standard battery was a 7.2Ah.  The one that came with the used system was a bit old and as access is very difficult when fitted to the dome I decided to buy a fresh 9Ah and fit it first.  That's the stage I'm at now.

[symmetal]

Another idea is a low power float charger permanently connected to the battery which you can just switch on when the dome is in use. This model is a reasonable price (3 different models are available), and it says when it switches to maintain mode it's a float charge.

Alan

[pmlogg]

I would have to do the opposite - dis-connect when using the battery as it will be dome mounted.   But that is the sort of mains charger that I meant as an alternative to using any solar panel.

[symmetal]

Ah! I didn't twig the battery rotated with the dome but as the solar panels do it kind of has to. 

Alan