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Old 16-01-2011, 15:48   #1
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Need a Regulator ?

Many moons ago I recall seeing somewhere a rule-of-thumb about how big a solar panel can be compared to battery Ah, before a regulator is needed to prevent over-charging.

Can you oracles please enlighten me?

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Old 17-01-2011, 15:02   #2
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c'mon guys - don't hold out on me!

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Old 17-01-2011, 15:34   #3
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A regulator is needed if the PV output voltage exceeds the recommended float voltage of the battery. With the high quality AGM and gel-cell batteries the internal leakage is so low that very little current will raise the voltage above a proper float value.

The question is how much time can elapse above float before the internal gassing causes significant loss of electrolyte due to excessive gassing. That might not take more than a few hours in high temperatures.
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Old 17-01-2011, 16:02   #4
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From an nice article about solar charging; found here

"The basic rule of thumb is that if the panels approximate daily output is more than 1/60 of the bank’s rated amp-hour capacity, then a charge controller is probably necessary so that the panel doesn’t boil off the batteries."
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Old 17-01-2011, 16:05   #5
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Rule of thumb: 5 watts and below not required; ie, 10W+ panels best to have...
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Old 17-01-2011, 19:33   #6
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IMO, it depends to some degree on the type of usage the system sees. If the boat is left for long periods with no loads on the batteries, then yes, regulation seems prudent. In the case of a full time cruiser (like us) with 12 volt refrigeration, lighting, computer usage etc on a daily basis the requirement is less stringent. On Insatiable II we have 240 watts of PV plus an Air-X feeding 4x Trojan t-105's. Unless we are going to be off the boat for a period, we switch off the PV regulator. In periods of good sun activity, sometimes the batteries will get full enough that the voltage rises up to around 15.4 or so. Pretty high, but under those conditions the charging current is only a few amps and little gassing occurs. It is pretty similar to the recommended equalization charge situation. We have to add water every few months -- not an unreasonable deal IMO. Does it hurt the batteries? This set is now over 4 years old, has been abused by frequent deep (down to less than 12 volts resting) discharge, and they ain't dead yet! You judge...

Oh, and we have one of those much maligned desulphators on the system. Don't know if it has made any difference, and don't want to get into that bun fight, but it is there doing whatever it does!


Jim and Ann s/v Insatiable II lying Morning Cove, NSW, Oz
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s/v Insatiable back in MBTBC marina, waiting for next eye jobs to be done
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Old 18-01-2011, 11:45   #7
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I have seen a few rules of thumb, I haven't seen a simple rationale of how they are derived which makes them all a bit confusing. But all apply only for non sealed lead acid batteries.

For example this is the recommendation

"When using a small solar panel to keep a float (maintenance) charge on a battery (without using a charge controller), choose a panel that will give a maximum output of about 1/300th to 1/1000th of the amp-hour capacity. For a pair of golf cart batteries, that would be about a 1 to 5 watt panel - the smaller panel if you get 5 or more hours of sun per day, the larger one for those long cloudy winter days in the Northeast."

And here is West Marines

"As a general rule panels that produce less than 1.5% of a battery's rated capacity in amp hours don't require regulation. This means that a 1.5A panel is the largest you should use without a regulator on a 100-amp-hour battery. Regulators should generally be used any time you have two or more large panels connected to your batteries.
If you're concerned about damaging your new gel or AGM batteries due to overcharging, you can add a small, inexpensive charge controller. These controllers, also called regulators, are rated by the maximum number of amps in your solar array, and we offer a 10A and a 25A version. Regulators are ideal if you can't be onboard to monitor your electrical system."

Big difference.

If you seek out self discharge rates you will find some values for a lead acid battery of between 4-6% per month up to 8-25% per month.
Another big difference.

What you want to avoid is overcharging a new 100% charged battery being hooked up on a sunny day yet and the same time prevent sulphation on a older battery by allowing it to discharge.

I cannot see a way to square the circle, I would recommend an inexpensive PWM controller with a solar trickle charger as they are more efficient than an MPPT in this application.

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