Solar Panel / Electrics Question

So long as the controller produces a slightly higher voltage than the battery, it will charge the battery. A diode can prevent reverse flow. You may need something to prevent overcharging of the battery. The presence of a cigarette lighter plug is just a method of connecting the wiring. If everything else is good, it will present no problems.

To the OP:-

Take Tornado's comments seriously, they are all important. A simple charge controller is not expensive....

Do make sure that the socket is fused and that the panels cannot deliver more current than the plug and socket can handle, generally around 8 amps if I remember correctly. Remember that the wiring, for even 8 amps, needs to be properly sized...check.....

There are better types of socket that are physically similar, so they can be replaced without a lot of work.

Remember that bad connections, sockets and wiring all get hot in use.....but it can be SUPER hot so be careful!! Dripping wire insulation is a good tip!!

You did not mention your battery voltage so I guess we are all talking about 12 volts....right?

That will depend on the voltage compatibility of the panel/controller set with that of the yacht, and the output current from the controller at that voltage.

Cigarette lighter plugs/receptacles are rated for use up to 24v DC and generally 10 amps, but realistically most should not be used to carry a continuous current in excess of 8 amps as the central contact spring tension will reduce with heat causing further heating, less spring tension, and eventual failure. Fused ones tend to fail even more rapidly.

This then brings you to the maximum charge current. It is a bit pointless simply considering panel wattage, as the charge current to the batteries will also depend on the type of controller/regulator in use.

For example - Assume a 12v electrical system on the yacht.

If the controller is a series PWM type, then the panels would have to have a maximum power voltage of around 18v each and would be connected in parallel, with a combined maximum power current of 8 amps. A 140 watt panel combination would handle these figures and will in effect provide around 95 watts max charge power (8 amps) to your batteries at 50% SOC.

If the controller is an MPPT type, then the maximum power voltage of the panels could be any value that the controller can safely convert to the correct charging voltage for the 12v batteries. The panels could be connected in series for less line losses. The MPPT controller will also boost the charge current above that supplied by the panels, and so that would need to be allowed for in calculating the total panel capacity. A 100 watt panel combination would now be the choice, and max charge power would still be around 95 watts (8 amps) at 50% SOC.

Increasing panel wattage from the above figures will result in higher current flows, so make sure the plug and receptacle can handle it

You just need a charge controller like "battery tender" and to coat the wire connections with silicone sealant....You should know the output in watts and volts of your solar panels....they should optimally be around 18v, if this is the case they should be wired in parallel...

Your combined wattage, add the panel outputs together, should be under the max rating of your controller....

You forgot safety blocking diodes, at least for the panels as not all have them built in....

My concern would be the ease of unplugging a 'hot' solar panel to put in a USB charger for phones etc. - Would open circuit damage the solar panel?

Also, getting a 'good' contact with a lighter plug is often a challenge. Screwed connections might be better.

"My concern would be the ease of unplugging a 'hot' solar panel to put in a USB charger for phones etc. - Would open circuit damage the solar panel?"

No it won't, and neither will short circuit.

Some controllers require 12v battery supply prior to connecting the panels, but this is only to get the controller working correctly, it won't damage the panels

Thanks, Spades.

Do you want to short circuit the solar panels?!!!!!! The cigarette lighter socket you are going to use to connect to the power system is a female fitting. The power source from the panels will therefore be a male plug such as used for charging devices while in a car. With that the two contacts from the power source are going to be exposed and very likely to simultaneously touch a metal object such as a compass, eating utensils, metal pen, a piece of stay, the buckle on a PFD, a screw driver used connect fittings or innumerable other things. This then shorts out the panel conductors. This happens and you are buying and installing another set of solar panels. This is the same as using a male electrical plug to supply power to a socket. VERY VERY DANGEROUS!

A possible solution would be to use a set of connectors with a male and female set of contacts on both the boat electric system and the same for the solar panels. The positive on one end is recessed and the negative is sticking out adjacent to it. The other connector uses the recessed for the negative and the one sticking out for the positive contact. They can not be connected in reverse. When unplugged they can not short and when connecting they can only be connected in the correct way. They are also at least water resistant.

Spectro Wiring - 9651P

Features:

• 2 Pole Flat
• Wire gauge: 16

This is the type of connectors used on "Battery Tenders" and have worked very well for non-technical people for many years. Not stupid proof but close.

Good Luck, Old Salt

"This then shorts out the panel conductors. This happens and you are buying and installing another set of solar panels."

This is not so...a solar panel has a distinct short circuit current (Isc) which cannot be exceeded and will not damage the panel in any way. A solar panel is designed to work best at very close to its Isc.

A solar panel reacts very differently to other power sources when short circuited. When the panel is short circuited, the panel voltage immediately falls to zero..Ohms law tells us that Watts = E x I. With E at zero, panel power is also zero.

Some solar controllers actually work by short circuiting the panel to shunt current away from the battery.

If so, Why are firefighters trained that there is up to 500-600 volts across some sections of a large multi panel system and touching both sides of this source could kill them? This is especially dangerous when wearing wet turn-out gear and carrying metal tools. Also the training includes if the circuit is shorted it will induce a current large enough to possibly explode the panel and cause it to ignite.

Generally the procedure is to have someone familiar with the particular system and strong knowledge of what to do shut the system down and disconnect each individual cell or at least group of cells from the "buss wires".

I acknowledge that all volunteer firefighters are not Albert Einstein or Bill Gates but there is more highly intelligent people than many would suspect (especially when we tear a house apart to find additional extension of fire). None of the "tech" individuals, including a solar panel installation company owner, I have spoken to has ever mentioned the shorting situation noted.

Confusion? Clarification please.

Good Luck, Old Salt

Placing a human body between both polarities of a solar panel could in no way be considered a short circuit, the high current flow may certainly cause injury to the person as power will be dissipated in the resistive tissue, but this was not the topic of the discussion.

The discussion was regarding possible damage to the solar panel in the event of short circuiting the panel and, whilst there is no doubt that a low resistance circuit may indeed result in large current flows which may induce damage to other parts of the circuit, it will not damage a good panel, and that was the essence of your earlier statement.

Next point is the "exploding" solar panel due to short circuiting. While the term exploding is a bit extravagant, a panel, or rather cells on that panel, can burn and crack. This occurs due to a phenomenon known as "hot spot heating" which is where an already poorly conducting cell in a series array is subjected to a very high current such as occurs during a short circuit event.

The bad cell tends to limit the current flow through the good cells which then produce a higher than normal voltage. This high voltage then forces the bad cell into reverse bias, and virtually all of the power produced by the good cells is then dissipated across the bad cell which causes a huge build up of heat. The more cells in series - as is the case with a high voltage array - then the larger that heat build up. Bypass diodes built into solar panels are provided for the purpose of limiting this effect. Good panels in the array will not be damaged.

A short circuit current test is a common one to be performed on solar panels in order to test their integrity, a pretty dumb thing to do if the panel were to be destroyed in the process.

A google search of solar panel short circuiting will not find any credible evidence that doing so will damage the panel.

I assume your last point is regarding my comment about some solar controllers using shorting as a method of charge control. These devices are commonly called on/off shunt controllers. Here is a link to one such device, here is another that explains the process.

One other point that I should make is that the maximum power current (Imp) of a solar panel is very close to its short circuit current (Isc). The Isc is the maximum current that the panel can produce, and if the circuit componentry is designed to carry the Imp, then it will easily handle the Isc without damage.

A typical 12v 200w solar panel would have an Imp of 11.12 amps and an Isc of 11.89 amps, not a big difference.