'Dark' current for small solar panels?

[mitaux8030]

Hoping someone can help with an issue we have. I'm wondering how much 'dark' current a typical small solar panel (20 watt / 24 volt) would have and would it be worthwhile removing a blocking diode, or perhaps even getting rid of the solar charge controller all together? Let me explain the situation...

Our electric gate system is draining the batteries quicker than what they're being charged by the solar panels, with the result the system goes flat after about three weeks. With three 20 watt / 24 volt panels charging two 12Ah batteries (in series to give 24 volts), I'd have thought it was enough to cope with even a typically cloudy Tasmanian winter. Even the manufacturer guidelines says that 2x20 watt panels ought to be enough. It has recently become apparent this is not so. I could just add more solar panels, but with the array already a square metre in size, I don't want to turn the front gate into a solar farm.
Looking at how the system is used, the 'standby' current drawn by the solar charge controller & gate controller accounts for the vast majority of the energy consumed.
The gates are used typically four times a day: drawing 1.3 ampere for 1 minute each open/close cycle, equivalent to 1.3 ampere for 4 minutes per day.
The standby current averages out at 90mA (it alternates between 80 & 100 mA, as the gate controller status LEDs constantly flash) for 1440 minutes a day.
For a rough estimation: 1300x4=5200 (gate operation), and 90x1440=129600 (gate standby). So standby uses 96% of the energy consumed in a 24 hour period - obviously that's where the greatest savings can be had.

So where to save? First is the flashing status LEDs on the gate controller - they're not needed unless programming or troubleshooting the system, so they can be disabled. Probably can save 20mA there. The simple solar charge controller also has a 'status good' LED that constantly shines. Since it's hidden away inside a weatherproof box, it too can be disabled, another 10mA saving. So standby current can go down to 60mA, down from an average of 90mA, that's a 33% saving. But I don't think that saving will be enough to stop the batteries from being eventually completely flattened in the darkest depths of winter (last August, it rained for 29 of 31 days!)

It occurred to me that the solar controller itself probably wastes some energy, and may not even be needed. The controller has three functions: 1) cut off the batteries if they get too drained, to prevent over-discharge. 2) cut off the batteries if the voltage goes too high ie: prevent over-charge and 3) it has a blocking diode to prevent 'dark' current discharge, I measure a 0.65v drop in voltage from the panel to the battery. The battery cut-out is achieved by a relay, probably just one relay for both over & under charge.
From what I've read, some people think that for small installations like this, it's better to not have this blocking diode as it 'wastes' more energy in that it prevents the battery from getting full voltage from the solar panel, versus the overnight discharge current through the panels. But then others say it's definitely worthwhile. Who to believe?

So I have three potential 'levels' of energy saving: definitely disable the LEDs to save that 33%, perhaps get rid of the blocking diode to get the batteries that little extra voltage (but accept a night time discharge through the solar panels) and ultimately, I'm wondering if the entire solar controller can be dispensed with and just connect the batteries direct to the solar panels and let them get full voltage, with no losses and save on the energy consumed to hold in the battery cut off relay (but then have no over or under charge protection). Good idea or not?

[Uncle Fester]

When the sun shines for a long enough period you will fry the batteries with out the regulator/charger.
You should be able to remove the diode if it is indeed in series. The charger will prohibit discharge through the panel.
Cut the LEDs if you like or drive them with a resistor 10x higher than originally, so you can still see them at night, which might be useful.

Edit: you have 60W on panels. On a dark cloudy day you should get 4-6W out of them.
Roughly 150mA including losses would be minimum and might not sustain the standby for 24 hours

If it worked before but not now:
Might be something wrong with panel wiring or you have a faulty battery.
How old are the SLAs?
Some don't last more than 3 years and a lot less if they get deep discharged, despite the term deep cycle.

[mitaux8030]

Ah, think I see the problem then. 4-6 watts out of a 60 watt array on a cloudy day, but the 'day' of usable sunlight lasts 6 hours. I was working on the assumption of 15-20% of output / 10-12 watts on a cloudy day. In the depth of winter, we perhaps only get one in 15 days of full sun, so to be conservative I should only budget for 6 hours at 4 watts charging daily. Going on those figures the system only has about 40% of the capacity needed to keep up with the average daily energy draw (at least in winter, anyway). I'll make the LEDs manually switchable for fault finding purposes, otherwise they're simply not needed for everyday operation. Even so, this 33% improvement in the standby current by getting rid of the LEDs only gets about half way to balancing out the energy budget. Another improvement I have heard of is replacing the standard silicon blocking diodes with Schottky types, with a lower forward voltage drop. Might have to investigate that one.

BTW, the system is fairly new, only installed about two months ago. SLAs seem good, though one definitely discharges deeper than the other - might have a weak cell in it. I should mention that the solar charge controller is a very simple device, it only has a few relays inside - one to cut off the battery if it's voltage gets too low, one to cut off the solar cells when the batteries are fully charged, and a blocking diode. Otherwise, no regulation or other smart function is going on inside it, as far as I can tell.

Are there any other ideas for efficiency improvement?

[peteramjet]

Are there any other ideas for efficiency improvement?

Run power back to the grid - far more efficient

But seriously, if the gate is only needed during certain hours (ie, not overnight) organise for the gate controller to be shut off using some form off timer. This will prevent the power hungry gate controller from drawing excess current when it's not being used.

(Edit: If the solar controller is also drawing too much current when there is no power being generated then a timer may be able to be used to disconnect this from the batteries during hours of darkness.)

[mitaux8030]

Been looking at Schottky diodes to replace the more common silicon diode that (probably) lives inside the solar charge controller and came up with two very suitable candidates: 'cool bypass' diodes (which they're not really, they're actually a power MOSFET switch, configured as a diode): SPV1001 and the improved SPV1002 with slightly lower Vf. They look like just the ticket to save an extra 400-500mV of forward drop compared to the silicon diode.

But then I see that MPPT controllers for low currents eg: 5 or 10 amp - perfect size for my application - are ridiculously cheap. So that opens a new set of questions: would the 10-25% efficiency gain be offset by the power used by the MPPT controller itself? Remember we're talking a steady drain of <100mA, which is enough to flatten the batteries over a three week cloudy period as things stand now. The gains afforded by the MPPT could very quickly be negated in this small system if the MPPT device itself chews >30mA to do it's job. Has anyone had experience with a MPPT in a very small system like this (60 watt array, 12Ah 24 volt SLA bank) and was there an overall net benefit?

[lsemmens]

Of course, you could always do it the easy way. Pull up to the gate, sound horn, and gate opens automatically. But only if there are rug rats, or cooks, around.

[mitaux8030]

Of course, you could always do it the easy way. Pull up to the gate, sound horn, and gate opens automatically. But only if there are rug rats, or cooks, around.

That system worked OK, but mysteriously stopped working when ever it was raining or very late at night. Never did figure that one out...