Comments on: How long would it take to charge a 212 amp hour 12 volt deep cycle battery using a 60 watt solar panel?

By: roderick_young

roderick_young — Wed, 10 Mar 2010 08:43:59 +0000

I’m also of the opinion that grid-tie would be unwise, not because it’s inherently a bad idea, but because you appear to have some fundamental misconceptions of how such systems are implemented.

With a few notable exceptions, most grid-tied systems do not use batteries. Also, when such a system is installed, it is generally with much more than 60 watts of panel. A special kind of inverter is used, generally compliant with UL 1741. There are now micro-inverters made by Enphase, but even those are intended for more than 60 watts. And you would need proper permits from your local jurisdiction, and permission from your power company.

Perhaps what you meant was that you were just intending to have a stand-alone battery system. In that case, I would say your panel is a little small for that size battery – you would typically want to charge at at least 1/20 or 1/30 of the battery’s capacity, C. So you would be looking for 7 – 10 amps of charging current. A 60-watt panel puts out 5 at most. It would be better to have two of those panels.

At last, to your actual question. It would take about 2 weeks of good sun to charge that battery from flat, or 1 week to replenish the battery from 50% up to full charge. Of course, one should never drain a battery flat, even a deep cycle one – that ruins the battery forever.
References :
Previously installed a grid-tied system. No batteries, though.

By: Richard B

Richard B — Wed, 10 Mar 2010 08:14:59 +0000

Three cheers for Irv S gird power is DANGEROUS!
References :

By: Irv S

Irv S — Wed, 10 Mar 2010 07:58:59 +0000

You can play with solar cell/ battery/inverter systems all you want,
but DON’T EVEN THINK about a "DIY grid tie".
If your frequency doesn’t match well with the grid the results
are not pretty.
There’s a lot of energy available at the ‘grid’ side of that tie,
and it’s not likely to be forgiving of mistakes.
References :

By: rickson

rickson — Wed, 10 Mar 2010 07:41:59 +0000

These are rated on open circuit voltage times short circuit current.

They generate about 20 volts and give about 3 amps.

So, taking this 3 amps as a starting point and assuming a totally flat battery, it would take about 212 / 3 or about 70 hours to charge a battery like that. Maybe 10 days at 7 hours a day.

That is a huge battery and not a very big solar panel, so maybe some adjustments need to be made especially if you get a lot of cloudy days.

You would have to have a look at the actual load and duty cycle to see whether to get a smaller battery or a bigger solar panel.
References :

By: Ecko

Ecko — Wed, 10 Mar 2010 07:02:59 +0000

A typical 60W panel has a 4A short circuit current from spec in first link below. This is most likely about 3.5A of peak sun charging current.
Depending where you live, you may get 2 to 5 hours of peak sun charging current a day. You should be able to find this figure on the internet for your area. It is the number of equivalent full sun hours (that the panel will provide 3.5A). It will take all the daylight hours to achieve this equivalent. Lets say you have 3 hours equivalent, so you will get 3.5A x 3h = 10.5Ah per day average.

The battery is 212Ah, so it takes (1.4 x 212 = 300Ah) to charge it. How many days is this? Divide 300Ah by 10.5Ah per day to get 29 days. This assumes no power lost from the battery over that time. Basically the battery (depending on type) could lose almost that much charge over a month from self discharge, so this panel would just keep a battery of this size more or less fully charged if there is no load.

With an MPPT (Maximum Power Point Tracking) controller this figure could be improved, as the charge is derived from the maximum power, not the maximum current, and could provide more current than the maximum from the panel (using internal electronics).

If you want a grid tie system you need a higher voltage to operate the grid tie inverter. You don’t need batteries, that is one of the reasons for using such a system. These inverters typically require 150-500V from the panels. This is for good reasons of efficiency and cost. Look up "Sunny Boy" (a German model popular in the US and Australia and Europe at least) to get an idea. This inverter adjusts the load automatically to use the input voltage that gets the maximum power from the panels (MPPT tracking). You need 10 x 12V panels in series to get this voltage. That means you can deliver about 600W to the inverter. Usually smaller systems target 1KW as the power from the panels, using bigger panels, or 24V modules etc. The design starts with the total wattage of the panels, as this is the high cost item. A 1KW system may not export much power at all.

I recommend you research this much further, or get some sort of consultancy with someone familiar with your region before you go ahead. There are plenty of references on the web. The wiring is at voltages similar to household voltages so you should get a suitably qualified electrician to do that work.

I know it seems disappointing, but the power from a solar panel is small compared with household use, especially as the full power only occurs a few hours a day. For example, the standby power from your appliances alone will exceed the 60W rating of the panel, so it makes economic and environmental sense to eliminate that by turning things off at the wall outlet. This panel could perhaps power such a small thing as a network router on a 24 x 7 hour basis.

Depending where you live, solar hot water may make more sense than an electrical system, as it represents more energy saving due to higher efficiency, and also is more practical for a DIY project.

You may get government grants, Renewable Energy Certificates (RECs) etc. to offset the cost of some systems in some regions. Hope this helps.
References :
http://buy.maplin.co.uk/Free_UK_Delivery/60W_Solar_Panel_Kit_97378_v2.htm

By: Bill W

Bill W — Wed, 10 Mar 2010 06:27:59 +0000

There is theoretical and actual. You are looking for actual.

The following is theory:
Power (watts) = volts x amps
You will have a 60 watt panel at 12 (or more) volts
60 = Amps x 12; Amps = 60/12 = 5
Your 60 watt solar panel is putting out 5 amps maximum
So it would take 212/5 = 42 hours to a full charge

That is theoretical. You will now have to apply some fudge factors and real-world experience. My experience comes from boating and using deep cycle batteries – but not using a panel. My seat of pants guess is that you double the number above. So maybe the battery would charge up in about a week in summer. Here are some considerations:

How many hours of sunlight per day will you have?
The charge cycle is not 100% efficient – maybe 75 – 80% ?
You will not empty the battery completely each cycle.

Check out some of the solar energy websites, such as the one below.

Hope this helps.
Bill
References :
http://www.sustainablevillage.com/resources/essays/simple_sizing.html