AGM white corrosion from negative terminal

sandreas41

Hi, I have an off-grid small solar system, 6x100W Renogy PV, 6x100Ah AGM, Outback FM80 Solar charge controller, built in 2016 and working for 9 years.

I noticed one negative terminal is growing white crystals, and is spreading along the copper lug as a discoloration.

I read another discussion here, very informative commenters here, so am trying to ask some different questions.

I have read different reasons for this happening, 1. Too long without discharge, 2. Under charging for long periods of time, or 3. venting, shouldn't be possible unless the sealed AGM has cracked.

Either overcharge, or undercharge... which or both?

Re-sulfating techniques involve Overcharging.

The charge controller is configurable, Currently set to Absorb for 3.5 hours, and then float the rest of the solar day.

I would like to understand what happened, so I can prevent a re-occurrence. Thermal camera shows even temperatures. 

When I turn it off to dismantle and clean it, I'm not sure I have the right tools to test the battery in question, and it seems either way, after 9 years, it is probably best to replace them all, and probably the affected cable.

Should I adjust the Absorption time? Maybe alter for time of year, since it gets more energy than I am using now in summer, and has to be shut down for at least 2 months during winter for lack of sunlight.

Thank you, Stewart

SumPower

An undercharged battery can cause this on the negative terminal.

9 years with an AGM is a long lifetime! So you have done good in my opinion.

Overcharging usually causes corrosion on the positive terminal.

SumPower

Does your AGM battery manual has any information as to the highest and lowest voltages in a string?

AGM's that I have worked with have specified that the individual battery voltages as measured at the terminals should be within 5% of the average voltage in the string. This is used to find batteries that may have been damaged due to under charge, over charge, or just aged out ( gone bad just because of age).

Again you have 9 years on AGM's, which is a long life. Might be time to look for replacement batteries.

BB.

I don't know about the positive vs negative corrosion vs state of charge issues...

My typical assumption is the battery terminal(s) closest to the cell(s) that are "gassing" more (i.e., running at high charging voltages), typically because of one or more failing cells (shorted/low voltage). The gassing of hydrogen/oxygen also carries some battery acid mist out too.

So, my guess is the battery(ies) closest to the corrodded connection are probably over charging/failing cells.

Which brings into the discussion the question of your parallel connection of a bunch of batteries... With your present connections, the third battery from the left is where your main cables are connected to the bank. Which means the battery closest to the mains connections, will see the lowest resistance when charging/discharging, and will tend to get more cycling and high load/charging current vs the rest of the batteries in the ladder connections...

This site does a nice job of describing several different connections methods to better share current with a bunch of batteries in series:

http://www.smartgauge.co.uk/batt_con.html

Also, there are sprays, liquids, anticorrison felt rings, using grease, etc. that keeps the terminals from corroding too...

https://mechanics.stackexchange.com/questions/87500/do-battery-corrosion-pads-really-work 

Otherwise, checking for poor/dirty connections, cleaning corrsion, etc. is just one of the maintenance jobs that comes with battery banks... Corrosion especially for Lead Acid batteries.

Here is an assortment of liquids/paint/spray... You can get some of these from your local auto parts store.

https://www.amazon.com/battery-anti-corrosion-spray/s?k=battery+anti+corrosion+spray

Poor connections can overheat and cause other issues (voltage drop, melted terminals, etc.)... So don't let this continue (plus require your parallel string of batteries).

-Bill

Dave Angelini

Add to Bill, a bad seal on the battery post that can happen on old batteries.