Successful de-sulfate / equalisation of lead acid batteries

[Sverige]

I thought I’d record the process I have recently followed to improve the performance of a couple of lead acid car batteries which I’ve been using in my off grid set up. Unfortunately I didn’t measured the capacity or internal resistance of the batteries before and after, but I can see that my efforts have been successful because the self discharge of the batteries is much reduced now.

On each battery I applied 15.3V from a server PSU with max 45A current supply capacity, while monitoring current drawn. Beginning with just a few minutes of this 15.3V treatment I alternated with using a proper “off the shelf” car battery charger of around 14V for longer. So in other words the battery was taking a short burst of 15.3V high current, then a longer trickle charge at lower voltage.

At the start of this, both batteries were absorbing just under 20A at the higher voltage, but this current declined as the charge progressed. After something like 3 hours overall, most of which time was at the lower 14V, the batteries were accepting under 2A when connected to the higher volt PSU.

The effect on the battery performance of this equalisation has been quite marked, as both batteries now maintain their voltage overnight, even when a small load of 1-2A is being drawn. Previously they would sink from 12.4V down to 11.9 or 11.8 overnight, even with the lightest of loads, so self-discharge was excessive.

Now someone will perhaps tell me I’ve taken a terrible risk and my SLA batteries might have exploded, but I hope not to have to repeat this procedure anyway and the benefits seem worthy of a mention. The batteries were around 10C because of the generally low ambient temps at this time of year where I am. I didn’t notice any increase in their temp during the charging, but in a warmer climate this would of course be something to be monitored more closely.

 

[snoobler]

Manufacturers of SLA/AGM batteries explicitly state equalization should not be done. The reason is simple - you can't replace electrolyte, and "sealed' doesn't mean they won't gas. If they gas, they lose capacity. Period. Even if some desulfation occurs, you've permanently reduced the capacity and life of the battery.

The good news is that you likely didn't do any equalization. You actually did a proper temperature compensated charge.

14.8V is a common AGM charge voltage... at 25°C. At 10°C, temperature compensation is +0.45V, so 15.25V, i.e., to get a full charge at 10°C, a battery that charges at 14.8V at 25°C would need to enter absorption at 15.25V. You're .05V over... probably not an issue.

Temperature compensation for FLA/AGM is -.005V/°C/cell.

This is why Will (and battery manufacturers) recommend temperature compensation. If you're not charging at 25°C, then you're either under or over charging.

Here's an example:



My absorption cycle started at 60.35V at 8:51am and ended at 59.00V at 11:30am. You can see the corresponding temperature change below - about 65°F to 82°F.

If you don't have temperature compensation on your charger, you should get it.

 

[Sverige]

Thanks snoobler, that’s interesting info and you may be right that I wasn’t equalising, however something has dramatically improved the ability of my batteries to hold a charge.

I take your point about manufacturer recommendations, however if the batteries were previously showing much reduced capacity and excessive self discharge, then the impact of some loss of capacity if the electrolyte were to gas becomes somewhat moot.

My SLA batteries are old car start batteries and just a temporary solution while I have some LiFePO4 cells on a slow boat from China.

 

[snoobler]

This is how rumors get started and anecdotal evidence as "data" runs amuck.

Numbers don't lie. This isn't a matter of I "may be right." I'm right. Your lower temperature necessitates a higher charge voltage to achieve full charge based on the data presented. This is lead-acid batteries 101.

If you're not consistently charging to the proper absorption voltage AND to the appropriate tail current, you're not fully charging your battery. A 10-15% improvement in actual charge can make a very noticeable difference in perceived performance.

Since you documented 2A @ 15.3V, that's the fully charged point for a 67-200Ah battery (1-3% of capacity).

The only conclusion supported by data is that your performance is improved by actually fully charging the battery. There is no evidence equalization occurred based on documented temperature, voltage and current. Other "data" obtained presents only anecdotal evidence as you're only going on an end voltage and a current of "about 1-2A" - which represents a huge potential error. 9-10A is going to be 10% off. 1-2A may be 100% off.

If you have a battery monitor and can establish a given amount of capacity results in a given end voltage, and then the subsequent discharge results in a higher end voltage after extracting the same capacity, one could claim specific benefit.

The reality is that charging an AGM to only 14V is unlikely to get it above 80-85% charged. You've discovered the benefits of proper charging.

The conclusion here isn't that equalization of AGM batteries is good. The conclusion is that properly charging batteries is good.

 

[RCinFLA]

Normal SLA batteries are not quite the same as AGM or Gel sealed lead acid batteries. Unlike AGM or Gel they have a good amount of electrolyte reserve. AGM and Gel cells have very limited electrolyte.

Equalizing will convert water in electrolyte to hydrogen and oxygen that will escape. This results in lower electrolyte level and higher acid concentration (higher specific gravity). Problem with sealed case is you cannot replace loss water.

Higher acid concentration supercharges lead-acid battery but reduces its lifespan. Equalization can knock off harded sulfate crystals exposing blocked plate surface. Either of the two or both is likely the cause of observed improvement.

Water fracturing (bubbling) starts at about 13.6v for 12v battery and gets very vigorous above 14.5v. Better to keep up with normal charging at less then 14.5v, ensuring batteries are brought to full charge at least once a month to prevent sulfate hardening. Equalization is hard on battery.

 

[Sverige]

<snip>

If you don't have temperature compensation on your charger, you should get it.

I’ve now purchased a better charge controller which has temperature compensation, so hopefully I will see improved charging results. My batteries are at around 16.5C this time of year, so I anticipate voltages will be a little higher than the 25C typical. I’ll keep an eye on that.

 

[rin67630]

Manufacturers of SLA/AGM batteries explicitly state equalization should not be done. The reason is simple - you can't replace electrolyte, and "sealed' doesn't mean they won't gas. If they gas, they lose capacity. Period. Even if some desulfation occurs, you've permanently reduced the capacity and life of the battery.

The good news is that you likely didn't do any equalization. You actually did a proper temperature compensated charge.

14.8V is a common AGM charge voltage... at 25°C. At 10°C, temperature compensation is +0.45V, so 15.25V, i.e., to get a full charge at 10°C, a battery that charges at 14.8V at 25°C would need to enter absorption at 15.25V. You're .05V over... probably not an issue.

Temperature compensation for FLA/AGM is -.005V/°C/cell.

This is why Will (and battery manufacturers) recommend temperature compensation. If you're not charging at 25°C, then you're either under or over charging.

Here's an example:

View attachment 25683

My absorption cycle started at 60.35V at 8:51am and ended at 59.00V at 11:30am. You can see the corresponding temperature change below - about 65°F to 82°F.

If you don't have temperature compensation on your charger, you should get it.

Hi Snoobler,
i am currently conceiving a low-power software SCC with software MPPT for regions with unfavourable irradiance.
Mainly the available current will be too weak to consider any limitation, I just will feed the maximum I get into the battery.
When the sun shines in the winter I should get the very maximum possible onto my 100Ah FLA batteries.
It will never be for more than 2-3 hours.
So my theoretical upper limit will be 14,5V + temperature compensation?

During summer I am more likely to get close to absorption, however never more than 4 hours at C/20 + 2 hours before and after at C/40
should I consider anything else than 14,5V + temperature compensation?

 

[snoobler]

Hi Snoobler,
i am currently conceiving a low-power software SCC with software MPPT for regions with unfavourable irradiance.
Mainly the available current will be too weak to consider any limitation, I just will feed the maximum I get into the battery.
When the sun shines in the winter I should get the very maximum possible onto my 100Ah FLA batteries.
It will never be for more than 2-3 hours.
So my theoretical upper limit will be 14,5V + temperature compensation?

During summer I am more likely to get close to absorption, however never more than 4 hours at C/20 + 2 hours before and after at C/40
should I consider anything else than 14,5V + temperature compensation?

I'm not sure I completely understand your configuration/intent, but the following seems relevant:

Low current charging of FLA encourages sulfation requiring more frequent equalization. C/20 is the absolute minimum rate to hope to avoid this. If you're in absorption and voltage limited, it doesn't matter, but if you're in bulk and below C/20, it's a risk. If C/20 is your maximum, then being below that seems probable.

Failing to attain 100% SoC with each charge cycle further increases sulfation.

If you're typically operating above 80% SoC, the above influences will be diminished, but the need for periodic equalization will still be present, and it sounds like your solution can't accommodate equalization given the 14.5V limit.

 

[rin67630]

I'm not sure I completely understand your configuration/intent, but the following seems relevant:

Low current charging of FLA encourages sulfation requiring more frequent equalization. C/20 is the absolute minimum rate to hope to avoid this. If you're in absorption and voltage limited, it doesn't matter, but if you're in bulk and below C/20, it's a risk. If C/20 is your maximum, then being below that seems probable.

Failing to attain 100% SoC with each charge cycle further increases sulfation.

If you're typically operating above 80% SoC, the above influences will be diminished, but the need for periodic equalization will still be present, and it sounds like your solution can't accommodate equalization given the 14.5V limit.

The problem is however: where to get the energy?
The design is for off-grid instruments running all year outside.
The battery must also accept a couple of days below freezing, which is a no-go for LiFePo.
During a longer winter period with casted clouds all week, I just have nothing left to do that full charge...

 

[snoobler]

More solar.
Better SCC design.

 

[rin67630]

More solar.
Better SCC design.

That's easier to suggest than to provide. I becomes increasingly difficult to get a permit for erection from the communities, the larger the panels are. Everything gets more complicated, before all the mechanics! and vandalism / theft is not the tiniest problem.
A better SCC... That is precisely what i am aiming to realize. It must be efficient and have a very low power drag.
hope to get your support with your experience to improve the software.

 

[snoobler]

Certain designs have unavoidable constraints. Only you know those. I suggest them because they're the easy answer, but there are clearly other restrictions. If that's the compromise you have to make, then that's what it is.

 

[Deleted member 9967]

Lead acid has been around for over 100 years.
It i no longer speculation. there are facts about these batteries.

I would listen to snoobler if I were you.
Just some friendly advice.

 

[rin67630]

Lead acid has been around for over 100 years.
It i no longer speculation. there are facts about these batteries.

I would listen to snoobler if I were you.
Just some friendly advice.

Hmm... I am currently listening to the experts as much as i can, I am just designing on a less used path.
There is not that much literature for off-grid all-year outdoor grass roots instrumentation available, excepted the dumb weather stations that can afford to sleep quasi all the time.
I am open to every suggestion...

 

[Deleted member 9967]

Hmm... I am currently listening to the experts as much as i can, I am just designing on a less used path.
There is not that much literature for off-grid all-year outdoor grass roots instrumentation available, excepted the dumb weather stations that can afford to sleep quasi all the time.
I am open to every suggestion...

There were books like mother earth news out since the 1970's that talked about grass roots rigging of things. So even that has been around for at least 50 years.
Most people complicate it endlessly.
What it comes down to is you have an input. Solar or generator.
You have a storage. Batteries.
You have an out put. Your stereo or lights etc.

I say all of this because snoobler went out of his way with charts etc to give you some excellent advice and you said "you may be right".
Stick to the facts and you will be fine.
Fact, input, storage, output.
That is mostly it.

 

[rin67630]

There were books like mother earth news out since the 1970's that talked about grass roots rigging of things. So even that has been around for at least 50 years.

Thank you for providing references...

 

[Deleted member 9967]

Thank you for providing references...

Mother earth news no longer exists. So it is difficult to provide links etc.
As for links to snoobler. Just scroll up.