Thread: 12V caravan battery charging

'...............should the two inwards solar ports be the panels rated V output?................'

Not quite sure where you're going with this Pedro, but here goes..

A solar panel has two 'rated' outputs that are important here.
Voc as the name suggests is the open circuit (no load) output voltage of the panel. For a nominal 12 volt panel this should be around 22 volts when it is angled directly into good sunlight with only a voltmeter connected to it and with any series connected protection diode disconnected (or add 0.6 volt to compensate).
Vmp is the voltage at which the panel delivers maximum power to the controller and hence to the load. Around 17 volts for a nominal 12 volt panel.

Solar panels have, in effect, an appreciable 'internal resistance' which means voltage is dropped internally in the panel as output current increases which means the panels output voltage varies with output current. The solar reg 'manages' this to give a useful regulated output.

Another solar panel parameter is Isc or short circuit current and is a measure of the capacity of the panel. A solar panel is the only power generating device that I know of where you put a short circuit on it to measure its output. Don't try this on any other power source or you'll end up with smoke, sparks, expense and a WTF just happened .

The reason I mention this is that when measuring Voc you have zero current (open circuit) for the maximum possible voltage of the panel and for Isc you have zero voltage output (short circuit) and maximum current.

The actual working voltage of the panel lies somewhere between these two voltages (zero and Voc) and is determined by the load the reg applies to the panel, the amount of sun on the panel and the temperature of the panel. With an MPPT (Maximum Power Point Tracking) reg I would expect the voltage (in good sunlight) to hover around the panels Vmp mark when charging. For a non MPPT reg such as your Morningstar I would expect the voltage range to be wider, say 15 - 18 volts under load, but that's really a guess.

So I guess the answer to your question is really a no but could be a yes or a maybe under certain conditions.

For an array of panels that have mismatched specs, no diodes and partially shaded it gets very messy (not saying this is you) and impossible to comment on reliably.


Sorry Pedro for the long winded explanation but I'm not sure what you're after here, I hope it helps more than confuses.

Deano

Originally Posted by bee utey

i think you should learn the difference between writing division and multiplication.
Energy is measured in Joules. One Joule is one watt-second, not watt per second or watt per hour. One watt of power drawn for one hour or 1watt-hour (not watts per hour or w/h) equals 3600Joules. One kWh (not Kw or KW/H) equals 3,600 000 joules of energy. Your energy retailer charges you for kWh, try reading your bill closely! Amp-hours (Ah not aH or AH or A/H) are useful only for a battery at a known voltage but not a lot of use for calculating energy unless you have a plot of the voltage over time.

Amps per hour is totally meaningless, amp-hours per hour has some sense but basically waffle ...

As I said before, it depends on your back ground. If someone worked in a power station, they'd probable (I don't know) want to know how many Joules are in a piece of coal and how many watts they could get out of it but when dealing with batteries every one I have seen have a AH rating in the specs. That AH rating is how many amps you can draw from the battery at an hourly rate between the battery's fully charged voltage and minimum voltage before it's damaged.

Now I know it depends on the battery type as Lipo batteries that I use frequently are designed to provide there full capacity in times less than 5 minutes but AGM batteries are often designed to provide their capacity over 20 hrs. Now I using the capacity faster usually means you wont get the capacity from the battery as some of the energy is lost as heat. Like my 7 AH battery will only deliver 4.5 AH if the power is drawn at 4 amp. Now I'm sure if you were that way inclined you could measure the amount of heat emitted from the battery in Joules, convert it to watts and then to amps and likely come up with 2.5 AH but there's no need to do that because the manufacture has already provided this info in the spec sheet.

I understand what you mean about requiring a voltage plot to understand the energy but things like fridges usually draw a constant current as the voltage drops so does the wattage.

Something else I've noticed is cheapo solar regulators usually only estimate the battery capacity by the voltage which I suspect is pretty useless but mid-range regulators show a Amps in and Amps out count. This is a bit more handy but doesn't consider how much is lost in heat. I guess if your batteries are charges or drained at less than the 20 hr rate then that's not too much of a problem.

Anyway, back on Topic..

Where are you up to Pedro?

Originally Posted by joel0407

I understand what you mean about requiring a voltage plot to understand the energy but things like fridges usually draw a constant current as the voltage drops so does the wattage.

NO, as the voltage drops, so does the CURRENT.

Hi joel0407, a couple of points

Re electrical current flow, measured in Amperes or amps for short is analogous to water current flow. Think of water flowing through a pipe, you could measure it's flow rate in litres per second (for example) similarly you measure electrical current flow through a conductor (pipe) in 'electrical energy units' per second or simplified as amps. So to refer to electrical current flow in amps per second is to say 'electrical energy units' per second per second which is doubling up on the rate term (per second) which is already defined in the term amps.

"................but AGM batteries are often designed to provide their capacity over 20 hrs..................."

maybe so, but the reason battery capacity's are given this way, typically at a 5/10/20 etc Ah rate is to let the user know the capacity of that battery under different discharge conditions. The 20 hour rate is commonly used, dare I say as a standard and allows the comparing of 'apples with apples'.

"............................... but things like fridges usually draw a constant current as the voltage drops so does the wattage....."

Not so. Fridge manufacturers who have 'dual voltage' fridges quote power figures for their fridges and the 12 volt current figures are twice that of the 24 volt figures which is how it should be.


"..............cheapo solar regulators usually only estimate the battery capacity by the voltage which I suspect is pretty useless but mid-range regulators show a Amps in and Amps out count. This is a bit more handy but doesn't consider how much is lost in heat ................"

Not only that, remember the input voltage and current to the reg varies as does the output voltage and current depending what 'stage' of charge the reg is at. So the current measurements here are not particularly useful either.
Remember the whole aim of the game here is to transfer power from the solar array to the load so a solar reg that gives power in / power out figures is comparatively much more useful.


Deano

Originally Posted by joel0407

Anyway, back on Topic..

Where are you up to Pedro?

7 hours of uninterrupted sun today (so far) and the V meter (on the last batt) now reads 12.95
started from 12 .19.

Originally Posted by DeanoH

Remember the whole aim of the game here is to transfer power from the solar array to the load so a solar reg that gives power in / power out figures is comparatively much more useful.
Deano

lifes funny,,
I "upgraded" to the Morningstar thinking I'd done enough (panels + batts) to not have to worry about
inwards/outwards amps.
my old one gave each days total current input,,,,,
The newer (mines all of 12months old) morningstars give inwards V and batt V on a screen.

sigh,,

Originally Posted by Pedro_The_Swift

7 hours of uninterrupted sun today (so far) and the V meter (on the last batt) now reads 12.95
started from 12 .19.

Sounds bloody good to me Pedro and I'm not talking about the voltages !

We're heading North in the OKA in a couple of weeks for a bit of a dip into the Gulf country, then West via Darwin, Kakadu etc heading for the Great Sandy Desert and eventually the WA coast. Can't remember when I last saw 7 hours of interrupted sun . Where are you, roughly ?

12.95 V is terminal voltage and not really indicative of how charged the batt. is but heaps better than 11.95 V

Overnight 12.19 V so about 60 % with everything else OK. Fridge is a three way I assume, and on gas ?, be funny if the pilot had blown out and it was running on the batterys , err.........perhaps funny isn't the right word here.

..................."so a solar reg that gives power in / power out figures is comparatively much more useful. "

Voltage in figures lets you know if the panel and reg is working and voltage out lets you know that power is getting to the battery and its terminal voltage so not all bad, beats mucking around with a multimeter in one hand and a XXXX in the other.

What was your old reg ? It's interesting that you didn't go MPPT but the Morningstar you've got is a ripper, I particularly like the 'sense' facility. I used Morningstar regs. for many years on the small solar stuff I did for Telstra and they were very reliable. The only one I had that (partially) failed had a bolt of lightning into the equipment it was powering that totally vaporised the cable leading away.

Internet seems to be working OK.

Deano

Originally Posted by DeanoH

........ then West via Darwin..........

.........stuff I did for Telstra ........

Deano

You'll have to look me up when you around and we can have a drink and talk about old Telstra days.

Happy Days.