Solar Panel Wired Directly to Battery Bank

Not clear on your system, but you need a charge controller between the solar panel and the battery to control the charge current.

They make chargers/controllers that automagically switch from mains to solar as the source for charging.

You want the controller to utilize the sun when possible, but switches to the mains when there is not enough solar power.

Here is an example of one such charger. There are many others.

I probably didn't explain the new system very well.

2 x 125 watt 30v solar panels are wired in parallel, sending 250watts of power to the Tracer 10A MPPT solar charger controller.

The Tracer charger controller the converts this 30v supply into the correct voltages to charge the 12v battery bank (The different voltages dependent upon the state of charge of the battery bank - the charger will either supply 14.6V for bulk charging or 13.2v for float charging if the batteries are full)

The batteries are connected to a 2kw inverter which is connected to the house supply via a DPDT switch. (The 2kw {4kw surge} inverter is an overkill to allow for induction motors starting up in fridge, freezer etc - we actually consume less than 0.25kw every hour)

So there is no way of connecting the little 30watt 17v panel. into this system.

There is no way this small system can power a house 24/7, even with our modest power consumption. The plan is to switch over from the mains supply when the sun is on the panels, & switch back mid afternoon allowing the battery bank to return to full charge by nightfall.

The system is being installed to deal with 12hr+ brownouts we experience regularly here in The Philippines. In a brownout the solar panels will provide most of our power we consume during the day, & the batteries will carry us into the evening if needed (At least that's the plan!)

So the extra 30watts from the small panel would be a welcome addition, if it can be done safely. At the moment it is connected directly to the bank, together with a mains charger. After a brownout the charger is turned on & the little panel is left connected - no problems have occurred. The question I am trying to resolve is if I can safely carry on with this practice with the new MPPT charger.

Hope that clarifies my situation.

It sounds like you need something like a transfer switch.

I suggest you study up on your batteries and the amount of cycles you can expect to get from them if you match your solar panel output to your charging needs. you simply cant start and stop all the time and expect the batteries to last anywhere near their potential. you should only be charging them after they've become somewhat depleted or you'll burn up your cycles prematurely.your 30 watt panel doesn't seem to have a home right now but you really didn't give enough information, but I think you could have made better selections when planning out your system, sounds like it will work but you'll be going broke replacing fried batteries prematurily.. get enough wattage to cover the amount of expected sun exposure you'll need to charge your batteries fully in a single cycle. the true "heart of your system" should be the charge controller, not a cheap panel. good luck

Great point. Battery packs should be sized so that they only deplete about 75% at worst case. You can go deeper, but it shortens the battery life.

If leisure type LA batteries, they can accept a lot more of such abuse than say car batteries....but cost more of course!

Another approach is to simply put only the things that are essential on the solar system and let those things that are more conveniences, or that you could do without during a power fail, tied to the grid.

This way you do not need an automatic transfer switch, but you could still install a lower cost manual transfer switch is needed. The manual switch would not automatically transfer your solar powered items to the grid. You have to flip the switch yourself.

First problem you have is that the Tracer 10 amp MPPT won't handle the 250 watts of solar input.

An MPPT controller has the ability to convert most of the input wattage to amperage at the prevailing battery voltage with very little conversion losses.

So you can expect that 250 watts to be converted to around 18 amps at 13.7 volts and over 20 amps if the batteries are down close to 12v.

You haven't stated whether the 30 watt panel's 17 volts is the Vmp or the Voc.

If it is the Voc, then you can connect it straight to the batteries with no ill effects.

Even if it is the Vmp, the roughly 1.5 amps that the panel can supply will do little to affect a 225Ah battery bank.

There is no reason why you can't connect the 30w panel directly to the battery whilst the other panels are connected via a suitably sized controller.

As for returning the batteries to full charge by nightfall:- assuming that you discharge your batteries to 50% Soc, you will need to replace around 125Ah. You will be lucky to average 15 Ah/hr over the entire afternoon (and that's in good weather). So 125/15 = 8.33 hours, and once the batteries reach around 85% Soc the amperage will be cut back even further. How long are your afternoons?

The next bit about "the solar panels providing most of the power during the day and the batteries carrying you into the evening". If the 250w solar panels are supplying the house load, and you say you use 250w of energy, where does the excess energy come from to charge the batteries?

Usually, a diode is already in the + line from a Solar panel to charger/Battery to prevent back feeding the panel. If is not already installed, you must add one, correctly sized for both reverse voltage and supply current. ALWAYS!

The diode (if installed correctly) will allow the solar panel to supply current ONLY WHEN ITS OUTPUT VOLTAGE IS HIGHER THAN THE BATTERY VOLTAGE.

Depending upon the voltage difference (Solar panel voltage must be higher than Battery) between the charger/battery and the Solar panel, that will control, up to the maximum the panel can supply, the charge current being supplied.

When the solar panel output voltage (after the diode) is the same or lower than the battery voltage, no current will flow in either direction.

There are available circuits that will take a low voltage and pump it up till it exceeds the battery voltage and will therefore charge, but usually at a far lower current.

I personally would first determine that this blocking diode is correctly installed/working and simply connect (observe polarity correctly) it across the battery via suitable (3A sloBlo for example) fuse or CB, rather than get rid of it...but at 30 watt, its not really going to make big difference in the scheme of things.....max 2.5 amps when new.....old could mean a lot less....check!

Solar panels get really hot in the Sun, which ages them with a couple of years of use. Try touching one on a hot day, but take "burn" cream with you!!!

Really new/modern solar panels have water cooling on the back, cooling water which can also can be used to heat a hot water/heating system via a heat pump or similar.

These are the ones to buy and use today, but usually expensive (I am told, assuming patents have not blocked their widespread usage).....maybe a little bit of DIY design and planning could resolve that cheaply!!!

The aging of the solar panels will be drastically reduced when cooled, saving money in the long run as well as helping to supply hot water.....

Just a bit of clarification here.

The blocking diode has nothing to do with controlling the output of the solar panel with respect to its voltage versus battery voltage when connected to an MPPT controller because the two are not directly connected together.

A blocking diode is generally an integral part of most controllers and so another one is largely unnecessary and in fact can be a liability with its inherent forward voltage drop, it is not required at all for an MPPT controller.

Provided the panel is producing power, the controller will regulate the amount of charge that is passed to the battery dependent entirely on its charge algorithm.

Solar panels do get hot, but they are generally guaranteed to lose no more than 5% capacity in the first 5 years and no more 15% of their original capacity over 25 years. The biggest problem with heating is the loss of efficiency of the panel, it falls by about 0.5% for every °C rise that the panel attains above 25°C

The use of both air and water micro pipe cooling of the rear of solar panels has shown that PE conversion efficiency increases by only about 3% whilst power conversion efficiency can increase by up to 15%. Considering the cost, the process has dubious economical value at present.

Actually, I never recommended an extra diode, I was just making sure one is in and how then the 30W can be connected directly to a 12 volt battery and almost forgotten....without a charger/controller being used. One must be certain that no back-feeding can take place, don't you agree?

So it was not needed to reiterate, unless of course you believe the OP is stupid, then it might be a good idea of course! He seems to be quite clever to my mind.

On such a large battery, even when fully charged, a 30w panel is unlikely to overcharge in normal usage anyway....and if the owner goes on holiday, he could disconnect it....just to be safe. Or he could add a small controller/charger just for that panel.

I would just combine it with the others....

Many thanks for the helpful replies

Solar Eagle- Great diagram, good to see that my proposed layout follows this schematic exactly. The only fly in the ointment is the old 12v (17v) panel. Itcannot be combined with the new array as the new panels are 24v (30v). The old panel is a flexible unit left over from my sailing days. In the worse case scenario, if it can't be used in tandem with the new system, I'll connect it to a separate 12v battery & run a 1A 12v LED garden lighting system off it.

Fredski- You are right of course, more solar panels are needed, If this was intended to be a 24/7 solar system & I were going to deplete the batteries by 40-50% on a daily basis then in, a 24hr cycle, to recharge the 100AH drained would a require a solar charger with a 30A charge capacity & an array that could provide those 30A; plus enough power to feed the house supply system whilst the batteries recharge (another 20A). In short I would need to triple my array size & charger size.

The idea behind this system was not to deplete the batteries, but to use the system for 4-6 hrs a day when the sun shines. The maths in my head was along the lines of 'if I am using around 200wh & my panels are providing 250wh (or 280wh if that little panel can be incorporated) then even with system inefficiencies the batteries will pretty much be dormant until an evening brown out occurs. (Fortunately most brownouts are in the day)

The cycle of the batteries for the last 3yrs, has been to remain fully charged for a week or 2 (with the little panel acting as a trickle charger), then being depleted in every brown out. I was rather hoping that this system would protect the batteries from this heavy depletion. What I didn't realise was that the batteries should be depleted by 30% or more before recharging.

My plan would deplete the batteries on a daily basis by less than 10% before being recharged by the afternoon sun. I didn't understand that this is bad for batteries!

A.H. On the face of it a transfer switch would seem like a good idea, but as we are only using 0.2 to 0.25kwh I don't think it is really needed

Thanks for all the help so far (although we still haven't answered the original query)

if you have a better understanding of your system, I'm happy, good luck

you might further your understanding here..http://www.solar-electric.com/deep-cycle-battery-faq.html#Lifespan of Batteries

My tendency would be to repurpose the 30W panel - i.e. Remove it.

If you leave it connected to the batteries and the bank gets fully charged by the big panels by mid-morning, the little panel could still overcharge the pack

Even if you got another small controller, it would be fooled by the larger controller bulk charging the bank and just switch to float making it not much use.

True, but most unlikely.

Maybe if away on holiday and the batteries get charged and charged....Just a thought!

Read my recent post, then if you will, you can use it on 12 volt batteries, assuming that you have the Batteries connected for 12 volts output.

If you are using 24 volts, then ignore my posts.

Hi Kebang, the little panel will never harm the battery, in fact after the big panels have finished their charging cycle the little one will help 'condition' the battery plates, giving them longer life. There will be no harm from leaving the little one connected.

Also with lead acid batteries there is no need to deeply cycle, even 30%, the batteries, that is required for Nickel Cadmiums batteries, not lead acid. The life of a lead acid battery is very tied to the depth of daily cycle.

You don't exercise lead acid batteries any more than absolutely necessary. Their life is measured by the depth of discharge and the time they're left discharged. So charge them at the optimal 10% of amp-hour rating as soon as possible with a clever charger that has a stepping and float function. This information was learned from years of electric wheel chair repair, perhaps the hardest service lead acid batteries see. Rare for their batteries to last two years, which is pretty close to 700 cycles. Best source I've found: http://batteryuniversity.com

Just a thought:-

LA Batteries should never be "Deep cycled", though the leisure versions handle it far better in the long term.

Car batteries can be severely damaged by one deep cycle discharge, ask almost any car owner!

Though the length of time between discharge and re-charge plays a huge role, the shorter the time discharged the better.....as in starting and then running a car engine!!

Any other battery type will also be damaged, no matter what the chemistry is.....there are minimum cell voltages that should not be gone below depending upon battery type. The problem being in a battery with more than 2 cells, one will be the weakest.

If this single cell goes below that bottom voltage limit (the total output voltage may still be above the total bottom limit!), then it will be reversed charged and damaged - forever!! It will get weaker and weaker....

Such damaged cells have a lower capacity and when being charged in a battery, are "full" first, further charging (to get all cells fully charged!) means that energy will be wasted as heat in the fully charged cells, further damaging them.....

Lookfar & Johnny - I was thinking that as the little panel can only supply less than 1% (in Amps) of the batteries capacity it wouldn't harm the batteries, my concern was any effect it could have on the contoller (also connected to the batteries.....). I am leaning to leaving it connected & closely observe what happens.

Bwilko - The solar panels short circuit amperage is 4.25A, so the 2 wired together in parallel equal 8.5A, or 85% of the controllers input limit. The controller also has a 10A charge limit,which it cannot exceed, but my interpretation of this (please correct me if I'm wrong) is that when the panels are only performing at 60% efficiency (Cloudy days) then 10A should still be making it through to the batteries...

The plan is to monitor the state of battery charge each day to learn the most judicious time to revert to utility power. For example if I find that at 2pm the battery voltage is 12.3v or less I switch then giving the batteries 3hrs charge time. If (Most Unlikely!) they are 12.4v or over I stay on solar power a little longer....

Ormandotvos, thank for your contribution, with your work history your advice is invaluable

Fredski, thanks for the link, I'll continue to try to learn.

Whilst the Isc may be 4.25A per panel, that is at zero volts, so no watts.

If you were connecting the panels to the batteries via a PWM series type controller, then the maximum current into the batteries would be the sum of the individual Imp's of both panels, ie about 8 amps, the panel voltages would be pulled down to the terminal voltage of the battery which will be way below the Vmp of a 30v panel ie. 12.4v x 8A = 99.2 watts, that's utilising only about 40% of the panel capacity.

An MPPT controller however, does not connect the panels directly to the battery, it matches the load impedance, so the panels can maintain their Vmp and Imp and stay at their maximum power point irrespective of any other load or the Soc of the battery.

The MPPT controller then converts that 30v Vmp at 8.33 amps (250 watts) into 13.7v at 18 amps (250w less a bit for inefficiency), or any other combination of prevailing terminal voltages and charge currents that equal the 250w panel capacity.

The controller has to be able to handle that higher current coming out of it regardless of the input current. A 10 amp controller will be capable of handling only about 150 watts of solar and this should be confirmed on the specification sheet for your unit.

If the controller restricts the current to 10 amps maximum, then you are wasting almost 50% of your charge capacity.

At 12.4v rested, your batteries are down to about 70% Soc or 67Ah used, that's around 850w or a bit over 3 hours at your stated 250w pattern. For how long will you really be draining them?

Three hours charge time at say 15 amps (even less in the late afternoon) will return just 45Ah into the bank, so it will end the day at only 90% Soc. Repeated the next day and you finish at 80% and so on. Stay on solar power longer as you suggest and you will end the day with even less capacity.

Thanks for explaining Bwilko.

I didn't realise that a 12.4v meant my fully charged 225AH battery bank had provided 850 watts. That is roughly my consumption over 4 hours. You obviously know a great deal more about this subject than I do so please correct me if the following statement is wrong:-

"During the same 4hrs the battery bank has been charging at 10A (replacing around 500watts) so my battery bank reading will be higher than the 12.4v I guessed at. My maths suggests that at 2pm in the afternoon I will need to replace the remaining discharged watts (around 350watts)."

Here in The Philippines the day length is around 12hrs, 6am-6pm the discepances between the seasons are no more than 45mins.

The bottom line is that this system is being installed to cope with long Brownouts, any free daily electricity is a bonus! My rough & ready thinking at the beginning of this project was that on 'brownout day' the panels will provide around 1kwh & the batteries a maximum of 2.5kwh. giving us up to 14hrs power. Obviously the day after 'brownout day' is spent on utility power whilst the batteries are recharged (both by solar & mains charger if necessary)

Maybe I'm a little confused at your intentions here, but your statement:-

"During the same 4hrs the battery bank has been charging at 10A (replacing around 500watts) so my battery bank reading will be higher than the 12.4v I guessed at. My maths suggests that at 2pm in the afternoon I will need to replace the remaining discharged watts (around 350watts)."

seems to indicate that you expect the 250w of solar to both supply your 250 watt power requirements and charge the batteries at the same time. This would appear to be an unreasonable expectation.

Whilst the day length may be 12 hours, the peripheral 2 to 3 hours either side of noon will not provide the intensity of light that you get at noon.

Even in the Philipines, unless you have a mechanical tracking array, you can expect no better than 7 peak sun hours per day in good clear weather, so a more reliable figure for solar contribution would be in the vicinity of 1.5kWh per day.

Your comment:-

"My rough & ready thinking at the beginning of this project was that on 'brownout day' the panels will provide around 1kwh & the batteries a maximum of 2.5kwh. giving us up to 14hrs power"

Drawing 2500wh from your battery bank is roughly the equivalent of 200Ah. This is just not feasible over the projected time frame from a 225Ah bank.

Provided you can recharge the batteries from mains supply the next day or sooner, then you will get some benefit from the solar during brownouts (Really blackouts because you hopefully won't be running mains at the same time), but nowhere near the figures that you are suggesting.

I wish you luck on your project.

Bwilko - Lets assume we start with a full battery bank. If my power draw is less or equal to the power output of the solar panels surely my battery bank remains full? If that is the case when I switch back to utility power the battery bank remains full. I'm perhaps misunderstanding your post (& I apologise if that is the case) but you seem to be subtracting my power draw from both the battery bank & the solar array output.

Andy - I know there is a blocking diode installed in the panels, thanks for all the other info

If you start with a fully charged 225Ah bank, then you have at most 1.4kWh of energy available for use before the battery is down to 50% Soc, and that's only if you draw it down at no more than the C20 rate for that bank. Any higher than that and Peukert's law begins to drastically affect the equation.

You can go to a lower Soc, but battery life will suffer greatly and voltage depression under heavy load (motor starting etc.) could cause problems for the inverter.

Provided you use no more than the panels can produce, then you will not be discharging the battery bank.

The panels can supply around 1.5kWh per day, but this can vary with available insolation, so you will probably dip into the red occasionally and deplete some of your battery capacity.

My understanding from your earlier post was that you were expecting to draw down 3.5kWh per day from this system (1kWh from the panels and 2.5kWh from the batteries), and as you can see from the numbers above, this is not likely to happen.

Sorry for the misunderstanding, In a brownout I'm happy for anything over 2kw. (From the combination of array power & stored battery power)

I had to read up Peukert's law. It's a shocker & should be repealed! The real size of my battery bank at my current draw drops to around 160AH & to remain in the 50% safe zone I can only use for 3.5hrs!

The bank is old & will soon need replacing - looks like a 400AH bank is on the cards.....

I'm glad you have seen the light.

Think seriously about LiFePo4 batteries before buying anything new.

They are far superior to LA and the prices just keep getting better and better.

They charge more rapidly, hold their voltage until almost flat, and can deliver a real punch without Mr Peukert even getting a look in.

Google them and you will get plenty of good info.

They can be a fire hazard.

Typically, AGM (Absorbent Glass Matt) batteries are the battery of choice in solar systems.

Golf cart batteries are another substitute. Deep cycle marine batteries are not a good battery for solar.

If the battery needs venting, make sure that it vents outside.

I think that you may be confusing LiPo, Lithium Polymer batteries with the much safer LiFePo4 technology. Whilst they do have a lower energy density and slightly lower voltage than the polymer type, they are far safer.

LiFePo4 batteries do not pose a significant fire risk and are fast becoming the battery of choice for stand alone power supplies due to their high drain without significant voltage depression and fast charge capabilities, they are also becoming very popular for use in recreational vehicles due to their lower weight as compared to lead acid batteries.

Very cool. Thanks for the clarification.

I think it would be very hard to find LiFePO4 batteries here in a very rural area of the Philippines! (I couldn't get Deep Cycle when I put the UPS system in a four years ago, though thankfully that's not the case anymore)

nickle iron is the way to go, but not so available.

Nickel?

http://www.vias.org/kimberlyee/ee_24_12.html

Your spelling was wrong, I know about the batteries....

no surprise therehow about if I just say Edison?

Spelt correctly! Compliment!!

actually last night I refreshed and updated my info on this style of battery for solar applications. they are the bomb! America doest make them anymore but ?China has a few manufacturers that have upgraded the design but it's amazing some of these monsters are in excess of a century old and still in daily service, AMAZING, heavy no question but what a cool piece of tech that has withstood the ages.

@Fredski can u elaborate more please ? whats this type of battery ? its name , etc

thanks

Nickel Iron are dinosaur technology and unless they can come up with a way to improve their charge and discharge rates, they will remain that way.

They particularly suffer from thermal runaway under constant voltage charging, which is what most intelligent chargers absorb and float at.