Absolute Newbie question about batteries

System · June 2011

Hi All,

I am definitely a newbie when it comes to this stuff. I currently have one 30watt panel, a 240 amp hour battery, 12 volt 30 amp charge controller, and a 400 watt inverter. I am using this to power my outdoor lighting for the summer. Currently, I am powering six lights and they last for about 6 hours with my current battery. Everything is working as planned.

I want to add another battery in order to add some more lighting. The timeline of 6 hours is fine. My question is how do I wire the batteries. I believe from my research that it is simply a matter of wiring them in parallel, (positive to positive, negative to negative), but I wanted to be sure I understood correctly before I attempted this. I also wanted to make sure that my charge controller and inverter would not be impacted.

Any guidance would be very much appreciated.

BB. · June 2011

Re: Absolute Newbie question about batteries

You are correct... Adding batteries in parallel adds "Amp*Hours" to the storage capacity of your bank.

If you added batteries in series--it would increase the voltage of your bank and you would need a new inverter to work at the higher voltage (i.e., two 12 volt batteries in series equals 24 volt bank).

Adding batteries either way will add storage capacity--but adding in series causes you to (usually) buy new equipment to operate at the higher voltage.

Back to your system... a 30 watt panel on a 12 volt 240 AH battery.

Adding battery storage/capacity is probably not your issue right now--Not having enough solar panel for charging is going to be a problem.

Normally, we recommend around 5% to 13% rate of charge for a battery bank, assuming 0.77 derating for solar charger and panels:

240 AH * 12 volts * 1/0.77 derating * 0.05 rate of charge = 187 watts minimum
240 AH * 12 volts * 1/0.77 derating * 0.13 rate of charge =486 watts rough maximum for cost effective solar charging

Right now, you run the risk of keeping the battery below ~75% state of charge for too long of period... Basically, you should get the battery recharged above 75% state of charge after a few hours or a day--Otherwise, the battery will sulfate and loose capacity--eventually loosing enough capacity until it does not store enough energy for your needs.

Normally, we look at the loads first... Average current * hours of use; or average watts * hours of use (Amp*Hours @ 12 volts; or Watt*Hours).

Then we size the battery bank to supply 1-3 days of storage to 50% maximum discharge. And then size the array to properly charge the battery bank (based on battery bank capacity and AH/WH of energy used per day--and the amount of sun you have in your area).

In general, a large battery bank is more trouble than it is worth. Look at adding solar panels first. (after you measure/estimate your daily loads).

-Bill

Cariboocoot · June 2011

Re: Absolute Newbie question about batteries

Welcome to the forum.

Now it's sarcasm time!

If you're managing to keep a 240 Amp hour battery charged with a 30 Watt panel you are performing a miracle none of the rest of us can manage. Okay, enough nastiness to the newcomer.

Seriously, that panel would manage a couple of Amps at best, which would barley keep ahead of the self-discharge rate of the battery. You must be using another charge source. If not then the battery is slowly going down in capacity, which you will not notice until it's too late. The old formula of "recharging over enough time" has been discounted as being too hard on the battery's lifespan.

I mention this because you want to add more battery capacity. The one you've got should be good for up to 1400 Watt hours. Or 240 Watts worth of lights running for the six hour time frame. If you add more battery capacity you will exacerbate the charging deficit problem. What's worse, old and new batteries don't mix well, resulting in rapid shortening of the life of the new battery.

If you're wondering, you really need 220+ Watts to recharge that battery on a daily basis, which is the recommended procedure.

But to actually answer your question, when adding batteries in parallel (positive to positive, negative to negative) the Amp hours go up and the Voltage remains the same. If you do this you should make the charger and inverter connections to the (+) on one battery and the (-) on the other, to keep the current flow as even as possible through both batteries.

System2 · June 2011

Re: Absolute Newbie question about batteries

Was expecting some sarcasm, LOL. Like I said, complete newbie.

Thanks for the input, I think I understand what I need to do. Sounds like I would be better off adding a couple more panels and buy new batteries.

I am a little confused though. Yesterday, when I hooked up everything, the battery was registering at 12.9 V. When the lights switched off, it read 11.1 V. After 3 hours of direct sunlight, it is now sitting at 12.2 V. If I am understanding everyone correctly, the current setup will result in a slower and slower recharge, and eventually result in complete and irreversible depletion. Correct?

Cariboocoot · June 2011

Re: Absolute Newbie question about batteries

Actually it will appear to be a faster recharge, as the plates get covered in sulphur and the real capacity of the battery diminishes.

With the right amount of panels, that 3 hours of sunlight would have the battery up into Absorb and possibly fully recharged. To get familiar with batteries you should read through the FAQ's: http://www.windsun.com/Batteries/Battery_FAQ.htm

Also, to properly size the system you need to know the Watts of the lights and how many of them in addition to how long they're on. This adds up to the total Watt hours, which is the magic number for determining battery capacity and array size for recharging.

BB. · June 2011

Re: Absolute Newbie question about batteries

Basically, yes....

Just like having a 20 gallon gas tank, use 8 gallons and replace 6 gallons, and continue to do that--and eventually you will run out of gas.

Lead Acid batteries, kept for long periods of time below 75% state of charge will "sulfate"--which means that the lead sulfate crystallizes--and the crystallized lead sulfate no longer participates in the charge/discharge cycling. Crystals begin forming after just a few hours below 75% state of charge.

A couple of things to read:

Deep Cycle Battery FAQ
www.batteryfaq.org

Properly and fully recharging your battery (over 90% state of charge several times a week) is very important for long battery life.

Measuring the resting voltage (no charging or discharging for 2-3+ hours) of a room temperature battery is one way of estimating state of charge... Basically around 12.7 to 12.8 volts is fully charged. ~12.4 volts is 75% state of charge and ~11.5-11.6 volts is around 20% state of charge.

You don't want to take a battery below ~20% state of charge permanent damage may occur.

-Bill

icarus · June 2011

Re: Absolute Newbie question about batteries

To carry Bill's analogy one step further. It's like y ou have a ten gallon tank, you use 8 gallons, but when you. Go to refill, the tank is now only 9.8 gallons, use 8, then it becomes 9.7 gallons, soon enou it becomes 1 gallon.

Tony

mike95490 · June 2011

Re: Absolute Newbie question about batteries

icarus wrote: »

To carry Bill's analogy one step further. It's like y ou have a ten gallon tank, you use 8 gallons, but when you. Go to refill, the tank is now only 9.8 gallons, use 8, then it becomes 9.7 gallons, soon enou it becomes 1 gallon.

Tony

Ah, the old "vacuum collpases the tank syndrome".

Absolute Newbie question about batteries

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