I’m pretty sure that it’ll take just as long, if it even works at all, and that all of the “excess voltage” will just result in a lot of waste heat, possibly enough to be dangerous. You’d be much better off getting 18 volt batteries, or rearranging the panel wiring so the output is lower voltage.
The question can’t be answered exactly from the available information. If the 8.6A is a measured value then the 75/8.6 value is close, but it is very hard on lead acid batteries to fully discharge them, so it would be best to only run them down to 50% so they will be fully charged in about half that time. You need to subtract from the 8.6A any current drawn by loads while charging. The 120V loads count for more by a factor of about 10, because the inverter will draw 10A at 12V to power a 120V, 120W load at 1A.
The battery will pull the array down to it’s charging voltage, ~14-15 V, the extra voltage is dropped by the V/I characteristic of the array which varies with insolation and temperature, and in the resistance of wiring between the array and battery.
Get a Solar battery charger. If you aren’t matching the power output of the solar panel, you are wasting a huge percentage of your energy, and solar energy is extremely expensive. The charger will convert excess voltage to additional current, and will also not overcharge your battery, which is a real risk in a direct-connect setup.
What would happen if an 18 V battery had to power 12 V devices?
The array consists of 3 panels hooked up in parallel, each of which has 2 sets of cells hooked up in parallel. I don’t think we could rewire them to output lower voltage short of rewiring individual cells, could we?
Good reminder. Thank you.
Can you please explain this part?
We actually have one, a SunSaver 10 Amp, but I didn’t realize it could convert voltage to current. Are you saying it turns that 19V/8.6A input into a 12V/13.6A output (roughly)? That would be… amazing!
Depends on the device, but they’d probably run too fast, burn too bright, or otherwise do things they’re not supposed to, unless they have some sort of regulator on the input.
No, if it’s everything in parallel, then you’re already at the lowest voltage you can get. I was guessing (incorrectly) that there was something in series in there.
beowulff really has the right answer: Get some hardware that’ll step it down. I don’t know anything about specific models, but such hardware does exist, and it’s getting pretty cheap.
Starting from no load, (zero current) a solar panel will have a fairly high output voltage. This known as the open circuit voltage. If you directly connect the output leads, it will produce a fairly high current at zero voltage. This known as the short circuit current.
Since power (watts) is voltage times current, if either voltage or current is zero, you are getting no power at all.
So to get power out of the panel, you need to operate somewhere between open circuit, and short circuit. The behavior of the panel in this area is a curve, but the shape depends on the panel particulars, and the curve shifts toward higher current with more light, and toward higher voltage when cold.
Though the curves vary in shape, drawing more current will always cause the voltage to drop. As you go from open circuit (zero current) to short circuit (zero voltage) the power will increase to some peak level, then drop off again.
For a given amount of light, and temperature there will be one optimal operation point which you could either specify as voltage or current…given one, the panels output characteristic curve forces the other. This is known as the peak power point of the panel, and it is where you want to operate the panel.
Many panels are designed to be near peak power when charging a common battery (12, 24, or 48V) with average light levels at average temperature. This is why your existing setup works at all.
A lead acid battery has a charging curve. As you increase the current going into it, the voltage will increase a bit. Where this charging curve, and the panels output curve cross establishes the operating point of the system. So the charging voltage and current depend on:
The panel’s output characteristic.
2)The battery’s charging characteristic.
The amount of light shining on the panel.
3)The batteries current state of charge (the curve changes as the battery charges)
4)temperature of the panel, and to a lesser extent the battery.
A “Peak Power Charge Controller” attempts to match the panels output curve to the battery’s voltage needs, even as both vary over time. They do this by slightly varying the load on the panel and observing the change in output power. Rather than wasting the extra voltage as heat, they use it to produce extra current. They do this using switching regulator technology, which is fairly advanced Electrical Engineering subject matter I’m sorry to say.
Hmm. If the controller doesn’t do that right now, does that mean the battery is actually getting charged at higher-than-intended voltages or is the controller shunting it off somehow?
What I’m trying to do, basically, is increase the farm’s available energy as at low a cost as possible. They are a nonprofit CSA and funds are very limited. If the controller is not currently doing anything useful with the excess voltage, and if my math above was correct and it does indeed take a full 8+ hours to charge the existing battery under full sun… getting a bigger battery would do no good, right?
Am I correct in understanding that a peak-power tracking controller would deliver more current at the same voltage, enabling the farm to also upgrade the battery to something with more capacity without affect charge time (since the solar panels are outputting more than the current battery can take, the rest being wasted)?
The other thing I might just try to do (or do additionally) is increase end-use efficiency. They currently power an answering machine and boombox. If I can find less power-demanding devices, maybe we could bypass the battery issue altogether.
Your battery should be fine.
This is what the battery charge controller does. Not having the MPP controller just means that you are losing precious efficiency.
The PV panels are nearly always the dominant cost, so anything you can do to squeeze more watt-hours out of them will typically have a payback period of months rather than years.
The most bang for the buck is usually improving load efficiency. Inverters are at best 90% efficient, and often much worse if run at light loads, so if you can eliminate the AC loads and dump the inverter, that is a big win. If you can get a boom box with class D output amplifiers it will use about 1/2 the power to produce the same volume as a typical unit with class AB amplifiers. If you are running a computer, consider a netbook, and get a 12V input power supply rather than running a 120V supply via an inverter. (these are available via 3rd party, not from computer manufacturers).
Battery charge/discharge is another source of loss, so it is better if you can use the power as it is produced, rather than storing it in a battery…for example, if you have a solar powered well pump, you should run the pump when the sun shines and fill an elevated or pressurized tank…you should NOT charge a battery and just run the pump when you need water. Of course this is not practical for all loads, but it is something to keep in mind and take advantage of when you can.
The MPP charging units ARE a good way to improve efficiency, as they require no other changes, and don’t require any user interaction. You might pick up 20% here.
A larger battery is normally a good thing, as it is better (as I mentioned in my first reply) to use only a fraction of the capacity. If you are cycling your battery 20-100% each day, expect only a year or so of life. If you double the capacity so that you are cycling it 60-100% it may last around 4X that long, and waste less power doing so. There are diminishing returns: If you are able to keep your battery at over 80% charge, then there is not much point in going larger. A bigger battery will also provide some reserve to help ride through a few cloudy days. Maintain (water) your battery! There are nice AGM batteries (Concorde, for example) that require no watering, but these are not cost effective. For the same money, you can invest in a larger capacity lift-truck based battery do the maintenance, and enjoy longer life and better performance. AGM batteries are only an economical choice where it is not feasible to maintain a flooded cell battery…i5 sounds like this might be your situation??
Oversizing the battery jumpers and panel wiring is a good way to pick up a few percent efficiency. Electrical codes specify wire sizes that will stay cool enough not to be a fire hazard, which is nowhere near large enough to perform well in a low voltage system. If you drop one volt through the wiring in a 120V system, that is less than 1% loss, and not an issue. If you drop 1V in a 12V system, that is an 8% loss. A 120V 20A circuit needs AWG 12 wire. In a 12V PV system I’d use AWG 6 for a 20A charging or battery circuit…maybe 8 if it was a short run and I was in a cheap mood. Make sure any crimped and/or bolted connections are solid, and use good connectors if you must use connectors (I like Anderson Power Products hermaphroditic connectors for this type of use) If you must save money on wire, do it in the load circuits, as these seldom run at full (breaker rating) current, and you will likely notice if there is excessive loss (dim lights, etc), while a lossy charging circuit goes unnoticed.
Finally, if you have occasional heavy loads (laundry day once a week) it may well make sense to use a gasoline/diesel/propane generator to support those peaks, and only have enough PV for the base load. This is analogous to utility companies that run the base load on coal and nuke, and fire up the natural gas plants to cover the peaks. A generator can also be used to overcharge/equalize your battery occasionally.
I know this is GD, but here is some IMHO (well, IME actually) material:
Another way to improve efficiency is tracking mounts for the panels. DO NOT consider this if you do not have a gear-head tinkering type on site to keep them working, and seasonally adjusted. The panels represent a huge wind load, and the mechanisms must operate exposed to the elements…snow ice, rain, wind born grit and scorching summer heat. With regular inspection and maintenance, you can expect them to need repair in around 5-10 years. Neglect them and don’t be surprised if they don’t work in a year. I have repaired a couple of these, and it was never anything complex or even expensive, just clueless owners. The owner of these things needs to be, at a minimum, the type that changes the oil in their car themselves, but best would be the type that built the tracking mounts themselves: If you are mechanically inclined enough to maintain/repair bought ones then you are probably qualified to build them outright. Yes, they probably COULD be made maintenance free, but this would likely make them so expensive that you’d be better off just installing more panels.
The farm has no maintenance personnel or people familiar with electricity. Its budget for this project is about $100… and that’s stretching it. A track mount is absolutely out of the question… heh, before I got there, the panels were simply lying on some 2x4s and held down by gravity. A wayward gust of wind or earthquake would’ve devastated them. It was a very jury-rigged system (basically, the farm used to be affiliated with a local university, and the PV system was a student project operating on donated panels) .
But anyway, the current PV setup was working alright until the farm decided to add a solar hot water system powered by an electric pump. The pump draws an estimated 40-60 watts (out of an available 160 W or so from the array on a sunny day), and so we thought that the system should be fine, having neglected to actually calculate the power draw from the boombox and answering machine. I will go out and do that soon. Anyway, we added a spare PV panel to the array to bring it up to the 160 W, believing that would give us enough power overall, and it did, pump, boombox and all – until we had a few cloudy days in a row, which is actually very common – then the battery was entirely drained.
I need to go out there and measure just how much power everything is drawing. I tried to do this last time, but ended up blowing a fuse in the solar controller… making me believe I really don’t know what I’m doing I’m hesitant to stick the multimeter at other things for fear out of frying something that doesn’t have a fuse.
But I’m GUESSING that the boombox and answering machine don’t really draw that much power to begin with, and if we’re getting 160 W, there should be more than enough for all the devices on sunny days… meaning an 20-30% increase in efficiency may not give us that much benefit. It may simply be that the pump is still operating at full power even when it’s cloudy, which is silly because the water wouldn’t be heated up that much anyway and we’d be draining the battery unnecessarily.
I wonder… is there a way to hook up the system so that the panels can both charge the battery AND power the pump directly, but also so that the pump can’t draw power from the battery? The solar hot water setup is non-essential, and it’s ok for it not to work on cloudier days; the answering machine and boombox have priority and need to stay alive for several days.
If not, I might consider taking the battery out altogether. I’d change their voicemail setup to use Google Voice instead and just have them dial in from a landline-connected telephone requiring no additional power (or even their individual cell phones). As for the boombox… what are amplifier classes and how do I determine which class a particular unit is? Would it be printed on the actual boombox along with the UL labels and such?
you would be better to place a battery before the pump.
a diode in the answering machine and boombox circuit between that battery and charge controller would isolate that system.
DC wiring has to be done carefully to handle the loads and to last. getting some techniques and procedures down will help. done improperly it will lead to energy loss and to deterioration of the system. it would be well worth it to read up a little to know the techniques and procedures.
I’m hesitant to stick the multimeter at other things for fear out of frying something that doesn’t have a fuse.