AGM Bulk charge time & voltages

[Rotary]

Hi all,

I'm running 2 X 260Ah AGM batteries wired in series @ 24v.

Cycle use of the batteries are specified as
14.4 - 14.7v (28.8 - 29.4v @ 24v)

Float / standby specified at 13.4 - 13.8v (26.8 - 27.6v @ 24v)

I have some questions;

I have programmed my mppt bulk & absorption voltages at 29.4v. should I back off a little or is using maximum specified voltages recommend?

2nd question;

I have calculated the absorption time using the formula .42 x C20 / nominal current (rolls tutorial on YouTube)

I have therefore inputted the maximum absorption time into my mppt of 180mins (max time allowed on my controller)

What should the bulk charge time be set to? I currently have this set at 120mins. I cannot find a definite answer to this online and I believe battery manufacturers don't tend to specify this parameter.

Any help / advice appreciated

 

[RCinFLA]

I have programmed my mppt bulk & absorption voltages at 29.4v. should I back off a little or is using maximum specified voltages recommend?

You are destroying your batteries if they are being held at 29.4v float level.

AGM's are electrolyte starved and sealed so you have to be very careful of overcharging. They cannot afford to vent electrolysis broken down water, hydrogen and oxygen gas, from their valve regulated pressure relief port. Up to a point, internal gases will recombined.

AGM's are compression supported plates made of pure lead. No support grid like normal flooded lead-acid batteries. Their glass mat is soaked with a higher sulfuric acid concentration electrolyte compared to flooded lead-acid battery which gives them a slightly higher charging voltage requirement. Discharging looks for sulfuric acid so AGM's are on speed drugs for discharging, but charging looks for water which is in lower concentration in AGM's driving the greater charging voltage requirement. They have about 25% lower over-potential voltage (terminal voltage slump with discharge current) compared to flooded lead acid battery for nominal cell currents.

They are still lead acid batteries so have similar limitations of all lead acid. Newer AGM's have carbon additives that reduces sulfation crystal formation size improving recharging ability from partially sulfated battery.

For AGM's the best way to terminate charging is current taper level. This is not a viable method in most PV solar charge controller applications especially with an inverter running simultaneously.

You should not exceed manufacturer recommendation for absorb and float voltages. Running 29.4v for absorb and float is drastically going to shorten battery longevity by boiling off electrolyte. I don't know why you are talking about your absorb time setting as with float voltage set to absorb voltage you have an infinite absorb time (assuming PV available).

Best absorb voltage and absorb time depends on your ability to maintain enough charge on battery which is dependent on your PV system and power usage.

Sustained average float voltage should not exceed 13.8 vdc or you will vent electrolyte. The absorb voltage level depends on recharge time necessary. Lower absorb voltage (14.2v) is better for longevity of battery but increases recharge time. Absorb time depends on absorb voltage setting. Higher absorb voltage should have shorter absorb time setting. I would not use longer than 2 hours. It also depends on your average charge bulk current. Lower bulk current needs less absorb time, but there will be longer charging time to get to absorb voltage. For 260 AH AGM battery you should have bulk current less than 25% of 260A, 65A and it should be less than 15% above 13.5v battery voltage.

13.8 v is where electrolyte water electrolysis breakdown starts to greatly increase so more time spent above this voltage means greater likelihood of venting which will reduce battery longevity.

 

[Rotary]

Thanks for taking time to reply RC and thanks for the detailed information.

Sorry if I was not clear..

Currently;

float charge voltage is set to 27.6v

Bulk & absorption set to 29.2v (reduced slightly from 29.4v in my original post)

I was referring to absorption time as this is an adjustable parameter on my epever mppt, the entries are pictured below. I believe my mppt classifies Boost as absorption and Bulk as Equalise.




battery spec:

 

[RCinFLA]

Absorb time should be only long enough to fully charge. Question is how do you know when fully charged. Current taper off would tell this but is not available for normal PV operation. If you have a Columb counter battery monitor it will give you an indication of lack of fully charging if low current load battery voltage drops faster than AH consumed indicates.

The absorb time should be based on what you see for average bulk current during charging. If due to your PV size you have lower bulk current than you don't need much absorb time. If you have a lot of PV so you are pushing 50 amps of bulk current then two hours max should be fine.

For PV systems you have to be careful of repetitive absorb recycling. This can be controlled by the Boost reconnect charge voltage setting. You don't want to reinitiate a full absorb cycle (what they call boost) every time a moderate inverter load slumps the battery voltage. This is aggravated if you have too much cable voltage drop with inverter load current. Better to have SCC cables go directly to battery terminals to avoid cable voltage drop of inverter load current.

By reinitiating an absorb cycle too often you stress the batteries more.

 

[Rotary]

Thanks again, much appreciated! My mppt has a visual display which indicates SOC so i will have a play with the settings and see how i get on

 

[mikefitz]

The mppt SOC display is not very accurate, it's a guess based on voltage, not the actual energy in the battery. This is assuming you have a Epever or Renogy unit.
It would be useful if a link to the controller and batteries were given.
OK I see its an Epever controller.
Boost charging volts is the same as absorption volts, the charger puts power Into the batteries until the target of boost charge volts is reached. It then holds the voltage constant for the boost duration, before dropping to float volts. If the volts drop below boost reconnect, the unit reverts to full charge power until the target is reached, and then enters boost duration.
During the solar day, conditions may cause the unit to enter boost duration (absorption period, constant voltage), several times. To prevent over charge, the total boost duration is limited to a maximum of 3 hours.

The only settings on the charger that determine AGM battery charging are;
Boost charging volts...........absorbtion voltage
Boost duration............absorption period
Boost reconnect voltage
Float voltage

Equalise volts and duration are not used with AGM batteries, set duration to zero

The main issue with charging AGM batteries with a small solar system and an Epever controller is that its not idealy suited for the task. Termination of the absorbtion period based purely on time may not correctly charge the battery.

However it is what it is, so I suggest:
Dont use too high a charge volts, try 28.8 or 29.0 volts as boost voltage.
Use a float volts of 27.6 if the system is in use, charged and discharged each day.

Mike

 

[Rotary]

That's very helpful thanks Mike,

My controller is epever Triron 3210N (100v max VOC / 30A)

Batteries are 2 X Lucas LSLC260-12 wired in series @ 24v 260Ah

 

[Rotary]

If anyone has a link to a suitable battery monitor it would be appreciated. I was wondering if it's worth buying a victron monitor and shunt although I see cheaper ones on Amazon/eBay.

Does a battery monitor & shunt have to be connected to a fully charge battery to provide a reference point or can they somehow detect accurate SOC at any charge level from the start?

 

[mikefitz]

What country are you in, easier to provide a link if known. The Victron smart shunt is the best value for accurate , (well near accurate, as none are perfect), readings .
You set the battery capacity and then have the option of allowing the monitor to 'sync' to 100% SOC during a charge cycle, this will occur on each full charge cycle, or if you know the battery is fully charged, you can set the SOC .

With suitable settings in the SmartShunt you will get a much better idea of battery condition thus allowing ideal management to of the system .

4. Configuration

www.victronenergy.com

The default settings are for lead acid, charged via an AC charger. When using solar changes are needed to prevent early sync to 100%.

Set the charge voltage to 0.2 volts less than boost volts. Keep the others as default until you gain experience with the system .

Expect to adjust the tail current to a lower value and perhaps the charge efficiency. Lots of discussion on the net on these settings.

Mike

 

[Rotary]

Thanks Mike, I'm in the UK. As my batteries are connected in series should i also be using a battery balancer or can i get away without one?

 

[mikefitz]

Best price seems to be on Amazon in the UK for the smartshunt.
If you mean a mid point balancer then I would see how things progress as you cycle the battery pack.
The smartshunt can monitor the mid point, details,

https://www.victronenergy.com/upload/documents/Manual-SmartShunt-EN.pdf

Mike

 

[Rotary]

Thanks for your help !

 

[Substrate]

Some precautions:

Did you just slap these two batteries together in series without charging each one up to full first with a suitable 12v charger before placing them in service as a 24v system? You want to start out with each one as identically charged as possible.

Are you actually reaching the absorb stage at all? This will depend highly upon how much of a discharge you put on them daily (you did say cyclic and not standby / intermittent) and your actual solar power and most importantly, your *solar insolation hours* at your location. Ie, Sweden is very different in usable hours than one who lives on the equator.

In other words, homework needs to be done as to how much you discharge each day, and how much you are able to put back according to your solar-insolation hours available. Which are far different than just sunrise to sunset! (early morning and late afternoon hours are usually discarded in calculations).

Typically most people actually sulfate their agm batteries in solar use, especially when they artificially limit them with absorb-time calculations that don't match real world. Usually they are babied to death, rather than overcharged. Unless you are dealing with diy'ers that like to play with junk batteries to start with.

Here's the deal: You don't set any absorb-time at all, and set the system for bulk charge voltages full-time!

WHY - this *can't* be correct? !! ??? !!

IF YOU ARE DAILY CYCLIC, then programming your controller so that there is either unlimited absorb time, or simply making absorb and float voltages the same, is because of these two facts with solar:

1) Unlike an ac-powered charger, the SUN SETS on you and stops any harmful lengthy overcharge at say your 14.6v absorb (cv).

The example here is that if you set your controller for say 2 hours only for absorb, and the sun is weak / cloudy to begin with, you actually need more time than that. But no, your system dropped to "float" early on with the time limit.

This is why the simplest action - IF YOU ARE DAILY CYCLIC - is to just set your CV absorb to unlimited, or if there is a forced timeout to float, change your float value to the same higher CV. Ie, the old "BULK=ABSORB=FLOAT" method of perhaps 14.6v for them all.

It gets highly contested, but that only comes from those who think in ac-powered charger terms, and not in real-world solar situations where the sun itself drops below the horizon, and pulls the switch for you so there is no damage.

 

[Substrate]

Shorter version:

Since it takes a *minimum* of 8 hours in CV/Absorb to fully charge an agm (typically even more), when you are daily cycling, even if you have only discharged by a small amount, there is not enough time in the day to properly finish a CV/absorb.

It is called "walking down" the capacity. Every cycle, when you can't fully devote 8 hours or more to CV, it leaves a small amount of capacity that eventually hard sulfates and reduces the capacity.

So in most places of the world, that would mean you need to be in CV/absorb as soon as the sun comes up more or less.

So ... being a daily-cycler makes it super easy. Set absorb AND float to 14.6v And yes, you are temperature-compensated right? So now it doesn't matter since not only do you not get 8 hours of absorb in, but you have simply done the best you can before the sun sets by not "dropping to float" too early or unnecessarily at all being daily cyclic (either by letting absorb time be unlimited, or fooling the float value to be the same as the cv value)

Those who have problems with this are typically those who just slap batteries together, never getting them a full charge equally to begin with, or are using second-hand / bad distributor old stock or trash.

 

[Rotary]

Thanks for your input Substrate

The batteries i'm using are brand new and i ensured that they were fully charged before connecting them up. Thanks to Mike / RC i've now installed the victron smart shunt which seems to be working well although i'm still playing with the settings. This time of year on a clear day i'm getting near 9-10 hours of sun on my solar array (1.4kw peak) so in terms of charging capability i'm assuming it to be sufficient. However i'm still to make a clear judgement on that pending setting up the shunt properly. I will hopefully get the batteries on the smart charger this week to get a clearer understanding.

My main concern at this stage is that my SCC (tracer 8420AN) seems to be stuck on float. I have posted a seperate thread in the charge controller section. I'm trying to understand the logic that the Tracer uses to re-enter boost charge. I've set the boost reconnect voltage to 24.4 but when i dip to that voltage the SCC doesnt seem to enter boost again. I've switched the SCC off this morning and back on again and its entered boost charge again.

My post is here;

epever Tracer Boost reconnect logic

Hi, Can anybody explain to me the logic my Tracer 8420AN uses to enter into boost charge. I have the boost reconnect voltage set to 24.4v (i'm running agm batteries). I understand that the controller should enter boost charge when the batteries run down to 24.4v but this is not immediately the...

diysolarforum.com

 

[Substrate]

Doing AGM with solar is always tough to get right - sometimes you simply can't and accept the faster than normal degradation if you are daily cyclic. That is, getting that last one-percent fully charged is the walk-down killer.

Simply put, ideally you hold the agm at your CV/absorb until current drops to 0.005C. As the battery ages, that actual current value rises (watching to see if while trying to reach that low current, there is a 10-minute stall that seemingly never gets better - you raise that to perhaps 0.010C) before stopping - but this is with a constant AC charge. Hard to do with solar.

If you drop out of absorb too soon and have no way to extend the CV time, then some will compensate by raising the float voltage to it's highest value like 13.8 / 13.9 to catch the most of the limited time before the sun sets.

Anyway, this is probably overkill for this application. Just saying - if you ever have the chance to actually allow your agm's to get very well charged, and sit all day for just once a week or biweekly, or at least once monthly - to slowly attempt to get that last 1% charged with what appears to be ridiculously low charge current - so much the better.

Remember thisis daily cyclic. If you are not daily, and have the luxury of a few days off (like weekend warrior), while the rest of the week the bank just charges like normal, then conservative settings will do ok.

 

[mikefitz]

Comments on the issues.

Knowing the application, actual solar power, location, daly loading, battery make/type, would be useful.

The Epever/Tracer range of solar controllers and their rebranded units have serious limitations in many real life installations. Unfortunately intenet exposure and attractive cost results in wide use of the units .

Alternative controllers offering complete user control of charge parameters ( absorbtion duration and termination by tail current) and records of daily performance, such as the Victron Smart Range, cause minimal grief to the user.

AGM batteries, in my experience, rarely give a useful service life in leasure applications. The continual undercharge and resulting walk down of capacity, can make the battery unusable within a year or two.

My advice is to use the system as, accept that the AGMs have limited lifetime, and save up for a 24 volt lithium battery.

Mike