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Old 07-04-2010, 04:44   #1
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Solar Charging for Dual Batteries

I am a weekend sailor only, and don't have massive power requirements. (I have shore power, but don't use it and won't use it until I get an isolation transformer.)

I have a 40watt (I think) solar panel, with a 5amp Projecta controller which only charges up the wet lead acid engine start battery.

I would like to be able to hook the solar up to charge this battery and my second battery, a deep cycle 120Ah house battery. I have read somewhere however, that you shouldn't try to charge two different types of battery at the same time, as the lower charged battery will pull down the higher charge - thus shortening their life.

My question is, can I have a solar charger regulator that automatically charges both batteries (one after the other) and then maintains a float charge? If so, will this setup still work with the battery switch (1, 2, both, off) in the off position (which is what i use when during the week) or does the switch have to be set to both?

PS Sorry if this has already been asked and answered, I did a search but couldn't find an answe.



PS I'm very raw with all this electrickery, so don't spurt your coffee over your keyboard whilst laughing hysterically, if what I just suggested defies the laws of physics....

I plan on living for far so good.
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Old 07-04-2010, 05:57   #2
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What voltage does the solar panel charge at? because a VSR between the batteries might be an answer.

Ours runs between the engine start and the domestic bank and links in staight away when the engine is charging. The solar panel (45w) is connected to the domestic bank, but I have noticed the led light on the VSR is on quite a lot spliting the charge. I suspect their is sufficient surfce charge to hold the VSR open when the engine is stopped and the solar panel then keeps the VSR open until night time and opens it again when the sun comes up. VSR connects at 13.7 volts and disconnects at 12.8v

We ditched the 0-1-2 rotary switch to fit the extra on / off switches.

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Old 07-04-2010, 06:03   #3
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I think the best way would be to use two charge regulators, one to each battery. And then connect the solar panel to both regulators. Depending whether or not the charge regulators are reverse bias protected (I believe most are, but some cheaper ones might rely on the solar panel's inherent reverse bias protection), you might need to connect a diode on each regulator on the solar panel side (anode to solar panel, cathode to regulator) to prevent back-feeding one battery from the other throught he regulators since the solar panel will be wired to both regulators.

The reason you want to connect the diodes on the solar panel side of the regulator is that the solar panel can easily overcome the voltage drop of the diode, but if connected on the battery side the regulator will charge to a certain voltage and the diode voltage drop will cause the battery to be lower voltage.

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Old 08-04-2010, 04:55   #4
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What I have done on numerous boats is use a dual battery regulator. The primary circuit is to the engine battery. Once the voltage is to a set level, the regulator shifts over to charging the house battery. It will monitor the engine battery and revert back to charging it if the voltage drops.
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Old 08-04-2010, 06:15   #5
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Keeping it Simple

Thank you for all the input.

I think the dual battery controller seems to be the easiest solution. Just remove present solar controller and replace it with a dual controller.

Morningstar Sun Saver Duo seems to fit the bill, with a separate digital readout.

I plan on living for far so good.
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