Battery Charging/Sizing Solar System (new to solar)

20 Posts
Jun 22, 2011 04:36 am
Battery Charging/Sizing Solar System (new to solar)

Hi all,

I just got my first solar system all setup.  I bought a used 110 watt Mitsubishi solar panel and a morning star sun saver 10 charge controller.  I have these connected to a NAPA Marine/RV Group 27 battery.  They couldn't tell me the AH rating however I suspect it is approximately 100AH.

My question is about charging this battery.  When the battery is approaching full the charge controller starts pulling back by design however this is allowing my precious solar power to go to waste.  How do I fully charge this battery yet waste little power?

I have come up with an idea however I am not sure if it will work.  My idea is to setup a second battery hooked to a second charge controller and to enable that controller whenever the primary battery is at 14.1 volts or above.  That seems to be the point where the charge controller really starts pulling back.

EDIT:  The second charge controller would be on a switch/relay and would be in parallel with the primary charge controller.  Both controllers would be getting the solar power at the same time.

Is that something that could work?  How do all you solar pros out there keep from wasting this power?

If I increased the size of my battery would that prevent this pull back on the charging?

Just a FYI for anyone that reads this.  Off my 110 watt panel I am able to power my laptop all night, run LED lighting, amateur radios, fans, and charge all my portable equipment.  I have been doing this for a few weeks now and in the morning I have still only used 40-50% of the battery.  (All loads are DC to avoid the use of wasteful inverter).  I actually have to try hard to use all this power.
« Last Edit: Jun 22, 2011 04:43 am by Travis Austin »
446 Posts
Jun 22, 2011 08:29 am
Re: Battery Charging/Sizing Solar System (new to solar)

Travis, no need for a second controller. Just add the second battery to the first in parallel.(neg to neg, pos to pos). Try to buy the same battery. Basically you are adding more storage for power from your panel.
 The panel you have puts out around 8 amps so you probably only get 50 amp hrs a day, which covers your usage. Any excess will contribute to recharging your loss and keep the extra battery fully charged.
20 Posts
Jun 22, 2011 08:57 am
Re: Battery Charging/Sizing Solar System (new to solar)

If I just add a second battery wouldn't it start pulling back from the solar panels at the same voltage?

I want to be able to put the final acceptance charge to the battery for its health but I want that excess solar to go somewhere when the controller is doing its PWM thing.  The solar panel is sitting there with all this power available and nowhere for it to go.

I would imagine that if I simply added the second battery the charge would take twice as long in the bulk charge mode but then when it hit the PWM part of the charging cycle it would pull back just as it does now.

I am still new to all this so I hope that I am explaining this correctly.
446 Posts
Jun 22, 2011 02:01 pm
Re: Battery Charging/Sizing Solar System (new to solar)

Travis, usually when designing an off grid system you size  your battery bank to have enough power for several days in the case of no sun. So adding the second battery would compensate for this situation and the panel you have would use the excess power created (wasted in your terms) once the sun comes back out to charge both batteries back to full state.
20 Posts
Jun 23, 2011 02:13 am
Re: Battery Charging/Sizing Solar System (new to solar)


Thanks for the quick replies.  There is little doubt in my mind that my battery is undersized for the amount of power that I'm creating daily.

Ok lets say hypothetically that I add the second battery and now both batteries are approaching full charge.  Is the controller going to pull back to the same degree as it is now?

Currently what I'm seeing is that as the battery is approaching ~80% charged the controller goes into a constant voltage mode.  When it goes into this mode the current pulls WAY back.  I know that this is to protect the battery during the acceptance part of the charge. 

I suspect (don't know) that no matter the size or number of batteries that its going to pull back to the same degree when it reaches this voltage.  Is that correct?

My goal is to be able to put in this acceptance charge and still be able to use the excess power.

I'll give an example.  Lets say the controller is in the acceptance part of the charge and I start to use a small load such as one of my amateur radios (~30 watt load).  I see that the power comes directly from the sun and that the controller compensates for the additional load by allowing extra current.  I can tell this because the voltage doesn't sag much below the 14.4 charging voltage.  I want to be able to harness this extra power that it has available. 

I'm not always going to be around to use this excess power directly during the day as I am typically at work.

My crazy idea is to upgrade to a new set of batteries (370AH golf cart batteries) and to use the battery I have now as a diversion load of sorts.  When those golf cart batteries start to approach full I'll have the current battery sitting @ ~50% charged just waiting for that extra power.

I was going to create a circuit that once it saw the voltage going to 14.2 or above it would engage a secondary charger (my own design) and that secondary charger would be current limited so it doesn't disturb the primary charge controller.

If the sun falls behind a cloud the secondary controller would see the drop in voltage and would start pulling back on current so the primary controller would get the voltage it needs for the acceptance charge.  It would keep pulling back on the current all the way to turning off the secondary controller if need be.

When the sun comes back out from behind the cloud the secondary controller would be able to pull more current without causing the collapse of voltage.  It will continuously monitor the voltage to make sure its 14.2 volts or higher.

I am thinking this would allow me to use 100% of my solar panels abilities.

If simply increasing the capacity of the battery bank would achieve the same thing I'm not opposed to that.  My concern is that if I get a larger bank of batteries I will be really tempted to use even more power.  If I start using more power then the likelihood of getting a full charge during the day drops way off and the batteries never get fully charged and possibly cause damage to them.

Thanks again for the quick replies and humoring my many questions.
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