Newbie battery charging question

System

Hello all.

With some of the reading I have done, my understanding is that batteries (in a storage / backup system) should only be discharged to about 50% of their capacity. And should only be charged at about 50% of their discharge rate.

Am I missing something? This seems very inefficient to the point that the battery bank would have to be redundant, or be 2 or 3 times as large as what I was figuring.

Can anyone point me to some good info on this subject?
Thanks!
Ron

icarus

Re: Newbie battery charging question

http://www.batteryfaq.org/
http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

Batteries should never be left discharged for more than a day or two without fully recharging! A 50% DoD is a bit too much in my book for good battery longevity, but it is trade off between battery life and usability.

The above links will give you good charging specs for most battery types.

Tony

Kamala

Re: Newbie battery charging question

The lifetime of a battery depends a lot on the specific application. Many people on this forum use their PV system to power a full time residence. They cycle their batteries daily. Every day. Some, like myself, only cycle their batteries on weekends.

From the links Tony has provided you can see that if a battery is only discharged to 20% of capacity, it can be cycled 3000 times! If discharged to 50% it can be cycled 1000 times.

I will occasionally discharge my batteries to 50%. Never more than that. I use my system for about two-thirds of the year on weekends. Or about 70 days or cycles. It will take about 14 years for me to cycle my batteries 1000 times.

My understanding of charge rate is that it should be 5 to 13 per cent of the C/20 discharge rate. Or, for a 100AH battery, 5 to 13 amps/

HTH

Craig

sub3marathonman

Re: Newbie battery charging question

Kamala wrote: »

My understanding of charge rate is that it should be 5 to 13 per cent of the C/20 discharge rate. Or, for a 100AH battery, 5 to 13 amps/

HTH

Craig

I had followed those instructions too, keeping it to 5% of the C/20 rate. Then I talked with Serge at Rolls/Surrette, and he said it is better to be at 15% of the C/6 rate. He also said the Absorb time would be 270 minutes (4.5 hours) instead of my original setting of 450 minutes (7.5 hours) with the C/20 rate I had been using.

As far as why this way and not that way, I don't understand at all. And I don't understand what difference the time setting makes if the charge controller will switch to float at the proper time anyway. Why not give the charge controller as much time as possible and let it decide?

BB.

Re: Newbie battery charging question

13% at C/20 vs 15% at C/6 -- Probably the results are pretty close to the same (C/20 gives a higher AH battery rating).

The reason for the maximum rate is to prevent the battery from overheating... Also, for solar systems where we pay a lot of $$$$ for solar panels, for most systems it would be a waste of money to install more panels for a higher rate of charge.

Regarding the time for the "absorb" voltage... You are setting the point at which the battery charger thinks the battery bank is now charged and it is time to switch over to float voltage.

For a system that has 24x7 power for battery charging for standby emergency power--You don't want to "boil" (over charge) the battery bank (over charging will cause more water usage, and can cause plates to flake, and positive grid corrosion from the oxygen formed during electrolysis).

For a solar power system where you have 6-10 hours a day... You may end up with setting up two scenarios... 1. Summer use and lots of battery usage/cycling, set the timer to 4 hours. And 2. for winter, cabin is shutdown and no power usage, you might set the timer to 1-2 hours.

Monitoring your battery bank's state of charge (hydrometer+specific gravity, Battery Monitor, etc.)--will help you determine if your absorb timer is set properly for your usage/charging cycle.

Read more about your batteries here:

Deep Cycle Battery FAQ
www.batteryfaq.org

-Bill

System2

Re: Newbie battery charging question

Thanks guys; that information helps me better understand the charge side of things.
Ron