Backup charger for solar system?

[pilsebob]

Hello.
My family has a cabin we use for holidays. We have grid power but often the grid fails (in bad weather). We plan to put in a solar panel / solar charger and a 12 volt battery or two as backup so we can have basic lights and charging of phones (from 12 volt) and such at all times. We don't want to have double lighting so we plan on changing out all lighting with 12 volt led. I believe that the solar panel will be able to keep the battery charged all on it's own most of the time. But i would like to have a backup by using the grid to charge the battery if it falls below 70% or something. I am therefor looking for a batter charger (220volt - 12 volt) that only will start up if the battery voltage is very low and then charge the battery up and wait until it happens again. The times this might happen is when we are using the cabin and the charger must therefor work with a load while charging. My idea was that the grid connected charger won't come in conflict with the solar charger if it's done that way.

I wonder if anyone know of a charger that could do this? It would need to be in the 15+ amp range. I would also like opinions on the basic idea.

Best regards
Pilsebob

[cosmicray]

I have a charger / power supply (it's really both) from Iota Engineering. These have very clean output, and are (more or less) designed for FLA batteries. You may also want to read the NAWS forum, where renewable energy (batteries, solar, etc) are the primary topics.

http://forum.solar-electric.com/forum.php

[pilsebob]

Thank you for the tips. I will check out the forum you posted.
 I won't have a inverter in my setup only a charger.
The Iota Engineering charges look nice but it looks like it will start charging while the battery still has a high voltage (almost fully charged)

[cosmicray]

The Iota Engineering charges look nice but it looks like it will start charging while the battery still has a high voltage (almost fully charged)

FLA batteries (also called deep-cycle) can be discharged deeply, but it's better to only do that rarely, and to start charging again at 50%. The Iota unit can delivery any voltage between 13.6 and 14.8 (documented) and probably another 0.2V above and below those values. Most battery manufacturers have a preferred charging scheme/profile. They are all similar tho, unless you have something like AGM batteries.

There are times I think a 10 volume encyclopedia could be written on battery chemistry.

[SL4P]

Just curious...  I'm certainly not solar savvy.  This may be a stupid observation 

If you're primarily using solar / 12V for your minimal cabin light & power requirements, and expect to fallback to mains for those 'odd occasions' (to recharge) when battery is low.  Why not simply use a complete changeover to mains-sourced 12V for the required 'dark' time - rather than paying to recharge the batteries?

To me this seems like you're pissing away inefficiencies in charging and storage phases - simply to fill a battery that provides the same end result.  When the sun returns, your batteries will top-up again - for free.   

At the very worst case, you could recharge the batteries in parallel - while doing your normal cabin stuff - but I can't think of a reason why you'd pay for that extra power - other than to resurrect heavily discharged batteries.

[Lightages]

Solar charging and batteries is almost always more complicated than people realize.

Generally:

Lead acid batteries need at least 5% of their rated capacity in the current supplied to charge them in order for them to not die very quickly, and I mead die permanently. This is to ensure proper electrolyte mixing.

Lead acid batteries need to be discharged to somewhere around to 70% to 75% of their capacity regularly in order to cause the electrolyte to get mixed whilst charging.

Never discharge a lead acid battery below 50% of their capacity or they die a quick death.

Never let a lead acid battery freeze.

Once a lead acid battery has been discharged to 50%, make sure it is charged to at least 90% within 24 hours.

Now, given the above general rules, you need to actually think about what you want to do and how the batteries will be used. Most people think they can a 50 watt panel charging a 1000AH battery bank and they only use it for 4 days each month and therefore there is plenty of time for the batteries to get recharged the next time they spend a long weekend at the cabin. Well if you understand anything I have said above, then you see this will not work.

[mtdoc]

 I agree with what Lightages says for the most part.

While 5% charge rate is the bare minimum, the goal should be at least 10%  So for a 100AH (20 hour rate) deep cycle battery - peak charging current should be 10A ideally, 5A minmum
.
Not all FLA (flooded lead acid) batteries are deep cycle. In fact most are auto batteries meant for high peak current and not deep cycle.

Lead acid batteries need to be discharged to somewhere around to 70% to 75% of their capacity regularly in order to cause the electrolyte to get mixed whilst charging.

Flooded Deep Cycle lead acid batteries may benefit from regular discharges to 50% SOC but not 70 -75%.  Other types of lead acid batteries (eg. Absorbed Glass Matt -AGM) may do not need this since they do not have liquid electrolyte to mix.

Never discharge a lead acid battery below 50% of their capacity or they die a quick death.

  Contradicts earlier statement but in general I agree.  However , there are some types of Deep Cycle Lead Acid batteries designed for regular deep discharges as far as 80%  For example, traction batteries used in fork lifts.

Never let a lead acid battery freeze.

Once a lead acid battery has been discharged to 50%, make sure it is charged to at least 90% within 24 hours.


As far as the original question - if you are using deep cycle lead acid batteries  you will need a 3 stage charger that has a profile appropriate for your batteries. I agree that Iota chargers have a good reputation.

[Lightages]

I think you misread. 70% to 75% of their capacity is not the same as saying discharging them to the point of using 70% to 75% of their capacity. I agree, not as clear as I could have put it.

And as far as the other points, I specifically said "generally"

[mtdoc]

I think you misread. 70% to 75% of their capacity is not the same as saying discharging them to the point of using 70% to 75% of their capacity. I agree, not as clear as I could have put it.

Oops - my bad. You're right - I misread  - it's the old SOC or depth of discharge mix up problem..

[cosmicray]

Never let a lead acid battery freeze.

What is interesting about that, is the freezing point of the electrolyte is related to the specific gravity. A higher SG gives you more protection against lower temps.

A number of times this winter the morning temps here have been around 0c. Because of that, I am always making sure that my FLA deep cycles have a good SG level (as they are exposed to external temps). In fact, I have a pair out there charging right now. They should be close to 100% SoC before I shut them off for the night, and it will be 1c here in the morning.

Note the table on the right (pdf) ...
http://www.trojanbattery.com/pdf/WP_DeepCycleBatteryStorage_0512.pdf

[pilsebob]

Thanks for all the feedback.

SL4P - You ask a good question. The only reason for not changing manually is that i might want to put up a small surveillance cam. We just got the cabin and we don't know about the risk of theft yet. So i just wanted the possibility open. I could perhaps use a small ups for the cam alone. would properly run it for a good while.

Lightages & mtdoc - Yeah i was thinking perhaps 30% discharge (if i get a deep cycle battery). The idea was to allow the drain in the everings and night to most of the time not be to much for the battery and recharging next morning. (i was thinking 100 watt solar panel and 100-200 amp battery).

When it comes to cooled battery so is my problem the opposite. The cabin is located close by the equator. That way even a single solar panel would charge a fair bit in a day.

I could possibly hack a battery charger so it won't start untill battery voltage is "low" and stop recharge before the battery is at 100% (and let the solar charger do the top filling / maintanance charging. Anyone know a charger that might be programmable or hackable?

[mtdoc]

Lightages & mtdoc - Yeah i was thinking perhaps 30% discharge (if i get a deep cycle battery). The idea was to allow the drain in the everings and night to most of the time not be to much for the battery and recharging next morning. (i was thinking 100 watt solar panel and 100-200 amp battery).

A typical 100 watt panel will have an Imp of about 5 Amps.  This would be bare minimum for a 100AH battery.  I would suggest you double that to 200 watts (12 V panel with VOC about 18-20V)  per 100AH. (12 V nominal battery).

Being near the equator is good as far as year round solar insolation but not good as far as heat.  Remember panel output goes down as temps go up. Panels will have a temperature coefficient that you can use to estimate output in full sun at different temps.

[Porch]

About time I find a subject here I know something about. I live off-grid all the time.

You didn't say how many lights, and how long do you expect to be without power, and how much sun you get. So without those numbers, I can only give tips.

Running a pure 12V system is expensive. Todays solar panels run at 24V so they can be chained into larger arrays and keep the amps low. 12V panels are not made in bulk like the 24V. The price per wall for most 12V solar panels is around 2-4$ per watt. 24V panels are 0.50$ per watt. So you will easily be paying double for panels to stick with 12V. And most 12V panels are smaller wattage, so you need more of them, complicating the mounting.   

How do you plan to "convert" the lighting to 12V? Split the feed out at the beaker panel?  Having both 12V and 120V in the same panel is not up to code in most US states and not a good idea. Then you have to find 12V lights and convert fixtures. If you are doing a few, this adds cost.  Inverters are cheap, and end up costing less then the mess of a rewiring for a duel voltage cabin.

Here is what I would do, and have done on other places.
Get a Xantrex TR1524 inverter, or a used Trace DR series. Both basically the same inverter. I have an older Trace DR that has been in continuous operation for 12 years now as a online UPS system. Dead reliable and work perfect for this application.

The entire cabin wires into the OUT of the inverter, and the grid wires into the IN. Batteries wire into the battery post. Golf cart T-105 6V 220Ah are the best bang for the buck still, if short lived compared to sealed batteries that cost a lot more.
When the grid is up, the inverter passes though the power and switches into charge mode charging the batteries. When the grid is down, the inverter run the loads. It's exactly like a big UPS.
The batteries must be sized to handle how long you plan to run without grid power. Hours/Days/Weeks?

Now say you want to run solar. First let me say running solar while the grid is available silly in most cases. You are just using up your batteries. It's not cost affective when you have power already to the cabin.

However, to do what you want, aka, kick the inverter over to grid to charge the batteries only when they are very low, you use a voltage controlled switch. They are easy to find on amazon. Set it to turn on at around 40%SOC and off at 60%SOC. The switch controls a large solid state relay that is in series between the grid and your inverter.

Bingo, everything is robust and transparent to the cabin users and wife approved.

If you expect to be without power, but have lots of sun (the two generally don't happen at the same time that often), you can have solar panels as backup. Say if you are without power for a week. If the grid goes down, the panels will keep things going and the beer cold. When the grid is up, the panels are not doing anything, but with so few panels, you are not really missing anything. I would still not do the voltage controlled relay as I would prefer to bring up the batteries to full SOC as soon as I can in case the power cuts out again in a few hours, and the panels are not making any power due to a snow storm.

Let me recap. Doing a small off-grid solar system where grid power already exist is not going to save you money as the cost of replacing your batteries every few years, plus the cost of the system is going to get you in the wallet.
If you need a big UPS, then do a big UPS and float the batteries, saving them for when you need them.

 



 







   

[mtdoc]

About time I find a subject here I know something about. I live off-grid all the time.

Running a pure 12V system is expensive. Todays solar panels run at 24V so they can be chained into larger arrays and keep the amps low. 12V panels are not made in bulk like the 24V. The price per wall for most 12V solar panels is around 2-4$ per watt. 24V panels are 0.50$ per watt. So you will easily be paying double for panels to stick with 12V. And most 12V panels are smaller wattage, so you need more of them, complicating the mounting.   

A couple of points:

Yes, 12V panels  are generally more expensive but they still make plenty of them and no one should be paying more than $2 per watt for them

For example HERE are several 12V panels -most for well under $2 per watt. Some closer to $1 per watt.

12V systems still can make sense for very small systems.

True that the large panels that dominate the market are higher voltage and cost less per watt. These are made in vast quantities because they are targeted at the grid -tie market which dominates.  Battery based systems are a small percentage of the PV market.

Grid tie systems run at very high voltages with many panels per string in series.

However these panels are almost never true "24 volt panels"  their Vmps may be close to 30V but they will not usually run at a high enough voltage to adequately charge a 24V nominal battery system unless they are wired with more than one in series.

With the prevalence of MPPT controllers now - most people with medium to large systems run 24V or 48V systems. The inexpensive "grid tie" panels from 225-300 watts each with Vmps of 29-30V are wired in series with MPPT contollers "bucking" the voltage of the string down to the appropriate charge voltages.

[Seekonk]

I live five months of the year off grid in the northern climates and only use a single car battery.  PV solar runs my refrigerator, lights, hot water and pumping.  This is all controlled by a $6 Arduino which also operates 12V & 36V simple buck converters at MPPT. In total the electronics cost about $15. The whole setup is automatic directing power to where it is needed.  Scheduling allows me to run without a massive battery pack. It requires  just a little adjustment in life.  At home, on grid, my utility company tells me I use twice as much electric as my average neighbors!

It really takes a microprocessor to determine the state of charge of a battery.  Simply looking at battery voltage isn't enough.  At rest, with a small charge, or a small discharge will give quite different voltages.  You could get away with a fairly simple voltage monitor if you change the voltage in the evening hours.  Most commercial systems are really dumb and I wouldn't use them in my system if you gave them to me.  If you have even a little technical ability, try using something like an Arduino to automate your system.