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sighmoon

sighmoon

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Hi all,

Maplin have a special on solar powered trickle charges (£10), so I bought a couple, thinking I could connect them in parallel and install them as shutters.

But then when I opened the pack, it says that they're only for keeping the battery topped up, not for charging from flat. Why is this? They claim 1.5W output at 17.5 V - surely two or three of them would be ample to recharge the battery while we're off the boat.

S
 
Well you wouldn't charge a battery at 17.5v.....

Other than the fact that it would take rather a long time to charge with such small outputs, I can't understand why they wouldn't charge a battery. Its probably not a good idea to run the battery flat anyway, IMHO, and if they are constantly connected and you don't draw a lot of power then hopefully they will keep the battery topped up.

Do you have other means to charge the battery? How 'flat' do you normally let it go?
 
I would guess that a flat battery would present an unbalanced load so that you would get nothing like the panel's max rated output of 1.5W @ 17.5V. It may well, over several days put enough back into the battery to allow it to build up a bit. But would you really let it go completely flat anyway?

Something powerful enough to boost the battery from flat would have to have a regulator circuit to stop the battery cooking if left on charge for a long time (like over 10 hours perhaps).
 
Given that the output is at 17.5v, wouldn't the almost short circuit load on the panel from a flat battery drag the voltage below the minimum required to get it charging? Given it's miniscule output...

Just my tuppence worth...

Toe.
 
I wouldn't want the battery to go completely flat, no, but this would be the primary means of charging the battery. Really we only use it for lights and depth sounder, but there's talk of getting a 12V mini fridge.

[ QUOTE ]
Something powerful enough to boost the battery from flat would have to have a regulator circuit to stop the battery cooking if left on charge for a long time (like over 10 hours perhaps).

[/ QUOTE ]

Does that mean that a couple in parallel would be a bad idea?
 
Two of those at 1.5W each is not powerful by any means and you are not going to get a lot of current from that. What is the rated amps output? and what size is your battery?

IMHO, this isn't really going to be enough as a primary source of charging.... echo sounder may not use a lot, lights - depending on bulbs and length of use could add up, but a mini fridge will kill it.
 
[ QUOTE ]
Given that the output is at 17.5v, wouldn't the almost short circuit load on the panel from a flat battery drag the voltage below the minimum required to get it charging?

[/ QUOTE ]

No (in a word /forums/images/graemlins/grin.gif)

They will find a blance between the battery voltage rising (slightly) as you try to force current into it, and the voltage from the panel falling as it supplies more current. At the rated output, you get about 0.085 amps. This will be the peak output. In real life, you'll do well to get 1/4 of this (ish) - i.e. less 50% cause it's dark for "half the time", less another 50%, because the sun isn't always overhead, etc.

That amounts to about half an amp-hour of charging per panel per day. Suspect it isn't advertised as for charging batteries, as it would take about 6 months to charge a typical leisure battery! (Even ignoring the "self discharge" that the battery has - i.e. it runs down, even if you don't use it).

These may be enough to keep a battery topped up, and cope with occasional echo-sounder usage, but usage of lights, etc would need to be *very* sparing unless you want to take the battery home to charge it regularly).

If you're talking about a 12V "peltier" mini-fridge, the one we had drew 3-4 amps continuously - thats over 80 amp-hours per day (or 5 months' charging /forums/images/graemlins/frown.gif)

Andy

Edit: No problem with running a couple in parallel. Money might be better spent on a 10W one at the boat jumble (ca. £20). Fridge will still kill it, though.

JMKENT types faster than me /forums/images/graemlins/tongue.gif
 
The panels will present no problem to the battery even if left on perminent charge as the charge current from one panel would be less than 1amp . watts devided by volts is the theoretical maximum current.
 
Yes the maximum voltage of a solar panel is a bit confusing because they have an inherant internal resistance the voltage falls with load.
So the panel tries to deliver 17 volts but acheives only that of the battery. The rated current delivered by a solar panel is what it will deliver into a short circuit. (or flat battery) The voltage does mean that it will continue to force itscurrent into the battery even when battery is charged but for tiny panel that is no problem.
So the warning on the panels is to warn buyers not to expect too much.
If you had a tiny SLA 2AH batteery they would be ideal for recharge from flat.(but would need a regulator) Bigger battery takes longer but don't need a regulator. My solar panle on boat is only about 2watt but seems to do the job adequately for my usage with a small 14AH 12v Nicad battery. (in oz summer)
I would suggest if you reckon you have a good spot for these tiny panels then the current will be useful. considering you are presumably charging by removal at present. Certainly if you need more current then a larger panel is more cost effective.
 
The circuit voltage will be that of the battery bank, so call it 12V if the battery is flat. 2 panels in parallel will be fine so you have 3W / 12V = 0.25A charging current. This will drop to .23A if the battery ever gets to 13V, which is unlikely as this would only keep up with self discharge of all but the smallest lead acid battery, though will fair better with a gel battery which has v.low self discharge.

Bear in mind that the 1/4A will only be available for that portion of the day when the sun is perpendicular to the panel without clouds or rigging casting shadows. In the winter allow 1/4A X 4 hours (ie 1Ah per day) after you have cleaned the panel, and less once it is filthy and covered in leaves.

In the summer you should get double that or perhaps more, but 2Ah will look a bit sick if you have a peltier effect fridge drawing 50 - 60 Ah per day, plus other loads.

This summer I left my wife in Salcombe with 2 X 20W panels and instructions to run the engine for 30 mins twice per day to keep up with the fridge and long evenings aboard with friend and three kiddies. I also showed her how to monitor charging with the 80A alternator so I could modify the timings after a few days. Sadly she called on the second evening to report that she hadn't wanted to run the engine and disturb the neighbours. This was why the domestic bank was drawing no load from the alternator (a flat battery bank would easily drag the revs down below 1500 which was max charging load), this being due to the bank being so flat the voltage was too low to excite the charging circuit.

With your current arrangement, I doubt you will keep up with your needs, with a fridge there is no chance. What is the size of the battery bank, and what technology are the cells? Then we might be able to point you in the right direction for PV size that won't need regulation, but will do the job.

In answer to the title 'Cheap Solar'... 'No it isn't'!
 
I agree with your advice, but on a technical point:

[ QUOTE ]
2 panels in parallel will be fine so you have 3W / 12V = 0.25A charging current.

[/ QUOTE ]

I believe the units are 1.5W at 17.5V - i.e. 0.086 amps (1.5/17.5), or 0.17A for two in parallel. Although the current might increase a bit as you feed a lower and lower voltage, it won't maintain the volts x amps = 1.5 relationship. Solar PV cells are fundamentally current generating devices, and (secondary effects aside) you can't get more current without more light / changing the cell efficiency.

/picky /forums/images/graemlins/smile.gif

Andy
 
I have had a few of these trickle panels .... one for my car and another on boat ..... Neither actually did the job that well. Yes they maintained batterys for a few months - but eventually they did go down enough to prevent engine starts.

Second they are not weatherproof and the boat one even though sheltered - the "glass" over the cells crazed and greyed out ... a few cells cracked .... and all NOT because of some boot plonked on - they just were not suitable to be outside.

I still have the one in the car .. as long as I don't leave car longer than a couple of months - it is normally ok.

Normal price on these ... better to put price of 2 together and buy better ....
 
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