Correct Float Volatge for AGM batteries

[Sage Oldmann]

I'm learning so much on this forum. Thank you everyone for helping me. I wish I had come here BEFORE buying any of my solar equipment. I could have saved my self a lot of trouble.

I do have a battery question. I have a 30 amp PMW charge controller that has no adjustments available except the set point for the float voltage. I've read that AGM batteries can be set higher than the factory controller float setting of 13.8 volts. Is this true?

If I can raise it to say 14.1 would that mean my batteries would be at a higher starting voltage each day, giving me a little extra operating time before dropping to 50% or do they settle down to 12.7 volts or so after charging?

[inetdog]

The Float voltage is designed to push just enough current into the battery to make up for the self-discharge current of the battery. That takes more voltage than just matching the rested terminal voltage for 100% charge.

[Sunking]

AGM varies depending on which alloy and paste is used in the construction. With Solar voltae set points don't mean a whole lot because sola ris a really poor source of power for a battery. A AC 30 amp charger will put out 30 amps until eternity. Solar system can only do that for an hour or two. That may or may not be long enough. 99% chance it is not long enough. So it ends up there is no a voltage high enough to ever get you recharged in a day.

Start with 13.5, wait till the end of day, a couple of hours after sunset, and measure the OCV voltage right on the battery term post. If you see 12.6 volts all is good. If less crank the voltage up. Repeat each day until you can turn up the voltage no more. If you cannot achieve 12.6 volts at rest 3 hours after sunset, to damn bad, you do not have enough panel wattage and a poorly designed system.

1 in 100 will see it rest at 12.7 or more. That is because they have a properly designed system with enough panel wattage to replace what they use in a day. For those back off the voltage a bit until you rest at 12.6. But do not rest, next week well require another adjustment. You have a new part time job with no days off or excuses accepted. Fail to comply and you rbattery will die. No one cares. Well one person does. The guy who sales you batteries is tickled pink you destroy your batteries. Trust me that battery salesman wil not tell you how to stop destroying batteries. He has no interest in doing so.

[inetdog]

For those who are new to the TLAs (Three Letter Acronyms) of solar, OCV = open circuit voltage. Measured when possible after the batteries have not been loaded or charged within the last few hours. Also called resting voltage.

[Sage Oldmann]

For those who are new to the TLAs (Three Letter Acronyms) of solar, OCV = open circuit voltage. Measured when possible after the batteries have not been loaded or charged within the last few hours. Also called resting voltage.

Thanks for the clarification. I was going to ask.

[PNjunction]

One thing to keep in mind is the operational needs. Are you cyclic or float/standby?

For a cyclic environment where you are doing daily cycling, then an agm if it is too big, the panel is too small, or the solar-insolation is calculated incorrectly, your best bet is to avoid float altogether for an agm. Either that, or raise your float voltage if the controller forces it to come on too soon.

A big problem with AGM is that they get undercharged more often than over-charged in a cyclic solar setup. We've discussed this in another thread, but wanted to point this technique out for your notebook later on....

[PNjunction]

I think it's time to simplify this whole end-game with agm ...

Many manufacturers and dealers don't understand the deal with float in regards to time and voltage, so we'll look at how to do it with an AC charger first which has plenty of time, and then you'll see why we run max-smoke with a solar controller!

The usual thing misunderstood about float for agm is that it serves TWO purposes, not just one.

1) To actually finish the charge no matter how fast you get through absorb! Typically .05C (C/20). Since absorb recombination is not 100% efficient, we need some hours of float afterwards to truly finish.

2) Maintenance / parasitic loads obviously thereafter.

To actually finish the charge so you don't walk down the agm in capacity and performance with repeated very small amounts of undercharging that leads to hard sulfation in a cyclic situation, #1 is extremely important! It takes a minimum of 6-8 hours!!! Physics won't make it go any faster. Solar - gulp!

For maintenance, voltage matters too, but is again misunderstood. This is a CV float, and NOT a forced CC "trickle" !!

Here's the rundown for agm floating after absorb is truly finished (typically C/20) that serves the TWO purposes and we are assuming the use of temperature-compensation! :

13.8v 6-12 hours max to finish the charge. Too high for long term maintenance. Cracked, swollen cases etc. Pull after 12 hours max.

13.6v 12-24 hours to finish the charge. Ok for many months of maintenance / parasitic loads if left on.

13.2v Indefinite, but that does not actually finish the charge in the first place! Ideal for 6+ months of maintenance, BUT you MUST ensure that you have actually finished the charge at either 13.8 or 13.6v first prior to placing it into long service tendering.

The need to use float to actually finish the charge (and at 13.6 - 13.8v for "finishing"), and needing at least 6-12 hours to do so is the reason we use "max smoke" in a cyclic solar application, where our time is extremely limited.

This means setting both your absorb and float voltages the same, ie 14.6v (or whatever your manufacturer states.) If you can't set your float voltage this high, at the very least set it to the max, like 13.8v. With cyclic solar, you are always just trying to do the best you can with a compromise, and ideally find some way to get in a REAL long float on a regular basis.

That means if you don't absolutely HAVE to do 24/7 cycling, but can shut down for a day once in awhile to pull some ridiculously small float current for hours, when you have a 1K array at the end of it, then do so! Your agm's will thank you.

Astute readers of manuals will note that earlier Pb EV applications with much the same time limitations incorporated the IUI protocol, basically running the voltage up to 15.5v for about an hour, but that can ONLY be done IF your absorb is truly finished! Do it too soon, and you just spin your tires with heavy gassing. Currently no commercial chargers / controllers do this to my knowledge, and some manufacturers may immediately void your warranty. That leaves an extended float as your sole option if you want to get a truly full charge.

The moral is if you can find a way to get in a long float, then do so, as often as you can.

[PNjunction]

I guess one good thing about the "finishing float", is that if one is truly past the absorb stage, this low-current float may be able to be accomplished outside the normal solar-insolation period - poor weather etc - so it still pays to do it even if you think that something as small as 70 milliamps can't be doing any good. It does!

[Mike90250]

only your battery mfg has the right voltage specs for your batteries, different brands vary by half a volt, which is enough to ruin a bank in a week or three.

Large lead acid batteries cannot be recharged in 3-5 hours. It takes 3-5 hours to complete BULK, and another 3 for Absorb, 6 hours minimum, So with off grid use, it's always a contest to try to get batteries properly charged at least once a week even in summer.

Recycled batteries use purified lead. Recycled or Virgin, the lead is good, it's the user and application that shorten life. Car batteries are not suitable for deep cycle use.

[Guest]

I guess generalizing the charge time of batteries depends on the type of batteries used. In my case I have AGM batteries that can be charged much faster due to their ability to accept a high amp rate. I personally use a trimetric that I use to know how long it takes me when I am boondocking. As I also have a set of 6 volt batteries that do take longer to charge to reach 100% but for the most part I have no problems keeping them charged with the system I have using what I described in the post above.

For example charging a 100AH battery at 50% DOD with a 25A charger would take (50/100) x 100 divide 25) + 2 = 4 hours to reach full charge as a 10 amp charger would take 7 hours.

As when I keep the battery bank above 70% I can reach almost 100% daily and then do a longer charge when I can.

[PNjunction]

I guess generalizing the charge time of batteries depends on the type of batteries used. In my case I have AGM batteries that can be charged much faster due to their ability to accept a high amp rate.

So do I with quite a few pure-lead Optima's, Hawkers and also Odysseys. The ability to hammer them is what drove me into the LiFePo4 arena.

BUT, no matter how fast you charge pure-lead's, you STILL need at least 8-12 hours of float to actually finish! It does not matter how fast you bulk/absorb them, to get the full life out of them, you need to do either:

1) a long float to finish or...

2) adopt the IUI charge algorithm. (1-2 hours of 1-3A CC, allowing for voltage to rise to 15.5v or so AFTER absorb is TRULY finished. Preferably after a 30 minute rest if hammering them. No consumer charger does this. Used mainly for very short turn-around cycling.

If you don't do this, you won't get the best life from them, since the .05% that doesn't get recharged adds up cycle after cycle hard-sulfating eventually and walking the capacity down.

In other words, even if you hammer your Odyssey or other pure-lead agm in only1 hour, you can add 8-12 hours of float to that figure if you want to do it *right*.

This inability to TRULY charge an agm is why it can sometimes be rough in the vehicular or solar world with typical short lifetimes. If you want more than 3-4 years in a vehicle, throw a maintenance charger on it when you can. Ideally when you come home overnight, but that isn't too practical for many.

Daily cyclic solar needs some sort of additional float charging PM routine too (unless you can cook up an IUI from your controller), or lifetimes will be shorter than expected.

[Sunking]

I personally use a trimetric that I use to know how long it takes me when I am boondocking. As I also have a set of 6 volt batteries that do take longer to charge to reach 100% but for the most part I have no problems keeping them charged with the system I have using what I described in the post above.

Hate to tell you, but your Trimeteric, Volt Meter, and any gizmo that tells you SOC is WRONG and incapable of you telling you what the SOC is on any operating system. It is not possible for any device that uses voltage to do that. Nor can you use any form of Coulomb Counting Device except on a few types of Lithium batteries and they have to be Calibrated frequently.

With AGM you are pretty much SOL unless you can afford a Battery Conductance Meter. With FLA you can measure with great accuracy, but only with a Temperature Correcting Hydrometer.

The only thing you can tell with a Voltage device is the Ball Park SOC on a fully rested OCV battery. That means disconnected and left to rest for several hours to bleed off surface charge and come to ambient temps.

I can take a fully discharge AGM or FLA battery, put it on a charger and it will read 100% SOC when in fact it is DEAD. I can take the same battery, fully charge it to 100% charge, put a load on it and it will read 0% or DEAD when in fact it is fully charged.

[PNjunction]

As when I keep the battery bank above 70% I can reach almost 100% daily and then do a longer charge when I can.

And that's really all you can do with AGM with solar. Do the best you can do. FLOAT that bad boy as long as possible even if by all indications (coulombic or otherwise) say you are done. You truly aren't. OR, like many pure-leads can do, do an EQ on them once in awhile if you know what you are doing.

For non 24/7 daily cycling, a long float can actually be achievable - or at least a day's downtime while the it just floats and simmers with the panels basically doing almost nothing. But the benefits of doing that are not usually realized by agm owners.