L16 Batteries Voltage Drop

Stuart

I have DEKA L16 6V 370AH batteries 4 in series (2 sets of 4 in parallel) that have been installed for 3 years. They are charge daily by solar panels and a couple times a year by a generator. By sun, the controller goes to float within a few hours (26.8V) and the batteries without controller on have 6.5V on each battery.

But in the morning the battery bank is at 21V. The battery voltages in one series bank are 6.5, 4.3, 6.5, and 4.4V. In the other series string the voltages are 6.5, 4.9, 3.9, and 6.5. All specific gravity  readings are (good (1.275). Controller indicates capacity of 93% 78F.

What is going on?

Any insight would be helpful. If sulfation, I thought was due to not reaching full charge but the system appears to be charging just fine.

littleharbor2

How often, and when was the last time you equalized them?

Voltages that far apart are an indicator of bad batteries.....Ya think?

You could make a single string from the 4 with 6.5 volts if the other 4 are shot.  What are your absorb and float voltages set at?

Stuart

Equalized 14 days ago.
Float 26.8V, Absorb 30V, Eq 30.6

Yes, I will probably make a new bank with the good ones but I hope to understand why the batteries went bad after 3 years and what can I verify to ensure the other 4 and 4 new will not fail. 

Thank you for your reply

Photowhit

Stuart said:

But in the morning the battery bank is at 21V. The battery voltages in one series bank are 6.5, 4.3, 6.5, and 4.4V. In the other series string the voltages are 6.5, 4.9, 3.9, and 6.5. All specific gravity  readings are (good (1.275).

Seriously doubt that they are all 1.275 and all the same! Did you check each cell on each battery? Each battery has 3 caps and each serves a single cell, they don't intermix! Pretty much impossible to have all the things you are saying be true, short of someone adding electrolyte instead of distilled water. Even then it would be dang hard to get them to read the same across the board.

BB.

Adding to what Photowhit said...

How often/deeply are you cycling your battery bank? And what temperature do they (mostly) run at (room temperature, 95F or what)?

3-5 years is sort of OK for inexpensive Golf Cart Batteries. Yours should be 5-7 years (assuming a warm climate), reasonable operation, and good maintenance.

• Distilled (or deionized) water (do not use tap/well water, only or use clean/filtered rain water)
• normally do not add acid/electrolyte to cells (just water)
  
• do not let the plates ever get exposed to air (low electrolyte levels)
  
• do not let batteries cycle "often" below 50% state of charge
• do not let batteries go below 20% state of charge ever
• do not let batteries set for days/weeks/months below ~75% state of charge
• ideally charge batteries around 5% to 13% rate of charge with solar
• Hold ~29.5 volts for 2-6 hours "absorb" state (longer if deeply cycled, shorter if shallow cycles)

Given that you have batteries in series that are both "good" and "bad"... Does not make sense unless they were treated differently (short life).

What is the SG of the cells after discharge? Generally, resting voltage and SG together (low SG is low resting voltage).

-Bill

Stuart

Photowhit thanks for your input. Yes, each battery cell (under each cap) was tested and the values were very similar ( I used an EZRED temp compensated hydrometer). I agree it is odd that the SG is high and the voltage low?? Only distilled water has ever been added. 

Bill thanks also for your insight.  The batteries do not discharge more than 85% on a sun day and on a series of cloudy days the generator is used at 80% (only a few times per year). Yes it is strange that two batteries is a series string are good and two bad on both strings. The SG in the morning when the batteries voltage has dropped is still close to 1.275.  

BB.

Stuart, you are checking each cell of each battery (i.e., 6 volt battery has three independent cells).

If you are "shallow cycling" (15% or less usage of stored capacity)--Some folks/companies recommend turning the charging off for a day and let the battery go down to below 85% or even below 75% state of charge at least once per month.

There are batteries designed for near 100% float duty (i.e., for UPS systems where you have one or two outages a year). And then there are deep cycle batteries that actually do better if cycled more deeply and at least once per month.

Otherwise, I do wonder a bit if your hydrometer is having a bit of a "moment" (everything is showing 1.275 SG).

Hydrometers do stick (need to rinse several times with distilled water after usage before putting away), floats crack, and after charging, there can be bubbles in the electrolyte.

There is always the old saying...

Segal's law is an adage that states: "A man with a watch knows what time it is. A man with two watches is never sure."

Segal's law - Wikipedia

https://en.wikipedia.org/wiki/Segal's_law

A second glass hydrometer may (or may not), help you figure out what is going on.

Otherwise, some links to read:

http://www.windsun.com/Batteries/Battery_FAQ.htm
http://www.batteryfaq.org/
http://batteryuniversity.com/

Eric/Westbranch posted a link to a 1922 battery repair manual. A very interesting read and look back almost 90 years at technology and mass production (near the end are some factory photographs).

Antique battery info (1922) (thread)

And here is the direct link to the table of contents:

THE AUTOMOBILE STORAGE BATTERY ITS CARE AND REPAIR

Despite the title, also includes information on storage batteries too (Farm Lighting Batteries).

The above links are from our FAQ thread:

http://forum.solar-electric.com/discussion/4426/working-thread-for-solar-beginner-post-faq/p1

-Bill

Photowhit

Most auto parts stores will have a $5-7 glass hydrometer that will at least give you some confirmation.

Estragon

@BB. I really like Segal's law.

@Stuart - assuming, for the moment, the SG readings are correct, how are you reading voltage exactly. Touching MM probes to terminals, lugs, or ? If you wiggle the probes around a bit, do they vary? Does digging the tip of the probe into terminals a bit make any difference?

In my 48v bank, SGs vary from ~1.265 to 1.28, and no amount of EQing seems make it go away. I'd be a bit surprised if a 3yr old bank had identical SGs.

Wbuffetjr1

Any further info on this situation?