Simple Back Up System - Best panel/controller for 12V?

bonedoggie · January 2013

I've looked through many of the posts here trying to find an answer to this question, but have not been successful.

Apologies if it has been asked/answered before and I did not find it.

I am making a simple 12V back up system for charging a single deep cycle battery.

Is there a "sweet spot" for most cost efficient panels for this purpose? Ideally, such a panel would be able to charge/top off the battery with a day's worth of sun. This batter would power an invertor that runs the electric bit of our gas heater in the house. I hacked a system together during Sandy that allowed us to heat the house when without power for 11 days and I would bring the battery to a friends house during the day to charge it back up. I'd like to be able to charge it here with solar only. Also, comments on the best reasonably priced controller for this set up would be appreciated.

I looked at this Instructable for some tips, but it looks a bit underpowered.

http://www.instructables.com/id/SPREE-Solar-Photovoltaic-Renewable-Electron-/?&sort=ACTIVE&limit=40&offset=120#DISCUSS

Thanks in advance, and feel free to just point me to a link or two.

Jon

Cariboocoot · January 2013

Re: Simple Back Up System - Best panel/controller for 12V?

Welcome to the forum.

The missing bit is: how big a battery? 12 Volts yes, but they come in different Amp hour capacities.
If you want to recharge it in one day, go for a panel & controller combination that can manage around 10% of the battery's capacity.

For example:
Crown 12 Volt 95 Amp hour battery; http://www.solar-electric.com/repoba12vo95.html
Solartech 140 Watt panel (7.8 Amps); http://www.solar-electric.com/sosp140wamum.html
Morningstar Sunsaver 10 Amp PWM controller; http://www.solar-electric.com/ss-10.html

In most cases that panel & controller would recharge that battery on a good day, providing it hasn't been discharged too deeply.

Higher Wattage panels are cheaper per Watt, but have 'odd' Voltages requiring more expensive MPPT type controllers to enable them to work efficiently with battery systems. For a very small system this route usually isn't worthwhile; better to spend a bit more on the panel and a lot less on the controller.

niel · January 2013

Re: Simple Back Up System - Best panel/controller for 12V?

the big problem with solar is that the weather interferes with it. not every day has full sunshine so on the not so sunny days you would still need to take it out to be recharged elsewhere unless you made allowances by doubling or tripling the capacity to account for such times. increasing battery capacity does does also increase the charge requirements too.

i had a longer more confusing explanation here, but scrapped it as i'll explain it slightly differently. firstly know that you normally should not drain a battery below 50% and when planning a setup this needs to be accounted for. in an emergency that wasn't planned for such as what you experienced then you do what you must.

i'll use a 100ah battery as the example and will assume that you will not exceed 50% of it so 50ah is used over the course of the 24hr day. you now need to recharge it over about a 5hr period max. (this is about the highest hours of full rating sunshine available to most people). this means 10ah to be delivered over 5hrs or a 10% rate of charge. now there are losses here so one may wish to go with 3 or 4 hours instead of 5 if the said battery can take the higher rate of charge. at 3hrs=16.67a and at 4hrs=12.5a. i know you may still be getting 5hrs of rated sunshine, but there are losses to be accounted for and don't forget that the absorb charge stage may take a couple of hours too. now we generally recommend between 5% and 13% on charge rates and 12.5a on a 100ah battery is 12.5%. some batteries can take a far higher charge rate, but you must check with the manufacturer to be sure on a particular battery and some fla types may need more maintenance as a result.

so what does this mean for a pv rating you might ask? the current is the constant we look for on pvs as some have various voltages. for a cheaper pwm cc the voltage will be around the 17v -18v area and can be slightly above that to be 18.5v or even 19v if you like as the current will still be the rated current on the pv. on average the newer 12v pvs will be about 18v or so. this could range between 17v x I = w and 18.5v x I = w. I in this example might be 12.5a for the 4hr selection or 17v x 12.5a = 212.5w to 18.5v x 12.5a = 231.25w depending on the voltage of the pv in question.

an mppt controller would take better advantage of the excess voltage present in most pvs and can accommodate the wider ranges of voltages seen these days with the advent of 60 and 72 cell pvs. 12v pvs are usually 36 cells. mppt ccs are generally able to give about a 10% increase in the current on average using a 12v pv, but circumstances vary and that could rise much higher to even 30% on some rare occasions.

gee i tried to simplify this and i fear you may still be confused due to the ins and outs of it all. hope this helped some.

Blackcherry04 · January 2013

Re: Simple Back Up System - Best panel/controller for 12V?

The best short term back-up system wouldn't involve solar at all in my opinion and I have 8.2 KW of it. My back-up is a couple batteries, a Charger, Inverter and a Honda EU2000. As was said, you'll never be able to put back enough with a couple panels to make a short term system viable. With 20 gallons of gas you'd be able to keep it all going for a week. With the gas in my cars, I could last another 2 weeks. After that , I'd start cutting wires, maybe for some long term PV.

The panels are to weather dependent and if they are comprised, your out of business.

bonedoggie · January 2013

Re: Simple Back Up System - Best panel/controller for 12V?

Thanks All,

This battery is the one I have in house:

http://www.sears.com/diehard-marine-deep-cycle-rv-battery-group-size-24m/p-02827494000P?prdNo=3&blockNo=3&blockType=G3

I considered the Honda generator solution, but am somewhat reluctant to deal with that approach. I prefer to not have the noise and fuel. That said, I would consider it.

The battery backup would still be useful for much we do around the house and allow for charging of devices, etc..

Given this and the time windows described by the other poster, what example panels would you all recommend as a good value for the power balance?

Jon

Cariboocoot · January 2013

Re: Simple Back Up System - Best panel/controller for 12V?

Note that battery is a Marine/RV 'hybrid' deep cycle, not the kind usually used for RE. In the long term its performance would be unsatisfactory.

In the short term it's only 80 Amp hours. You are looking at a maximum of about 380 Watt hours AC. That will not run much for long. The recharging would be pretty much the same as my previous example.

The inverter-generators are not very noisy, nor do they consume a lot of fuel. As far as Watt hours per $ goes they are much more economical than a battery + inverter.

A lot depends on what you want to keep running when the power goes down. Refrigeration is a good idea. That one battery would not keep a 'frige going for a day, but the generator would (shut down at night, leave door closed, it will be fine by morning).

My cabin system which can provide about 2.5 kW hours per day and runs everything May through October, cost $8,000. That's not including the $1,200 Honda EU2000i that is still needed in case the sun doesn't shine or some extra power is required. A battery back-up system can be done cheaper than that, but you will not get 1600 Watts continuous power as you would from the gen for the same money. Not even close.

bonedoggie · January 2013

Re: Simple Back Up System - Best panel/controller for 12V?

Thanks so much.

It appears I was short sighted to expect to cover our needs in a power outage without a small generator. It does appear the Honda EU2000 is the way to go.

BTW - We do already have a grid tied 4kwh system, but of course in an outage that is of no help given the power needs of the Sharp Invertor.

Jon

Blackcherry04 · January 2013