Is there a maximum indefinite time period charge current that will not harm an AGM battery?

[sdold]

If you connect a current source to an AGM battery, is there a rule of thumb maximum charge rate (C/x) that won't harm the battery if left on indefinitely?

[SunEagle]

From what I have heard anything faster than a C/4 rate could hurt most AGM batteries and even a C/6 might be too much.

I would think a very slow rate (trickle charge) like C/20 would be ok for long duration but even that may dry out a sealed battery. Hopefully one of our "experts" will come by and provide and answer.

[Mike90250]

Yes, there is a rate, it's unique for each battery, and is in the specs, as a Float Voltage.
You could then derive the allowable current from measuring the current after a couple days on float. It may or may not apply for the life of the battery

[Sunking]

Generic answer is C/4 is safe for any AGM. However there are some made for extremely high charge/discharge rates. Example some UPS batteries can be hammered with 4C. Most of the hgier end batteries wil specify a minimum and maximum charge rate.

But remember the expense premium for AGM batteries. You need to know why you are spending up to 400% more for AGM batteries. Or at least you should know why because you are not going to like it when you find out it could have been done at 1/4 the cost. Loosing money is a great teacher. Almost as good as Fear.

One place you can justify AGM useage is what they were originally intended for, Aircraft where spills and leaks cannot be TOLERATED. One would think Lithium would be first choice for aircraft.right?. Think again. Just to many fires and smoke on aircraft using or transporting Lithium batteries.Risky stuff.

[sdold]

The subject line should have read "indefinite time period" instead of "continuous", sorry about that.

---Mod Note: Fixed It For Ya

[PNjunction]

The subject line should have read "indefinite time period" instead of "continuous", sorry about that.

In that case, you are basically talking about an endless float.

The worst thing you can do, and most manufacturers point out, is that a constant-current "trickler" that maintains a constant float current (while voltage is allowed to vary to maintain that current) is not desirable for most consumer agm's. You WILL overcharge with a CC float, even if you can't feel any heat, plates will slowly buckle, cases burst and so forth. Heh, Enersys knows how to detect the application of crap chargers. That really surprises a lot of guys who don't understand the harm a $5 unregulated trickler will do to an agm.

BUT, most recommend a "CV" or constant voltage float instead. Typically, the CV-Float should be limited to no more than 0.1C inrush. The battery itself will reduce the current drawn over time while in a CV float. Ideally, one should wait an hour after absorb is finished before applying float, but we're getting beyond the basics.

Rule of thumb for agm's if you really really want to charge it to full:

1) After absorb has finished, apply 8-16 hours of float at 13.6 to 13.8v at no more than 0.1C inrush. Even after the preceding absorb period has dropped to say .05C current, you'd be amazed how much more an agm wants to drink in, but it takes 8 hours to do so, regardless of the initial bulk/absorb charge rate. (basically, the recombinant action of absorb is not actually perfect and can only take you so far) Obviously, 8 additional hours is an issue for cyclic solar with agm - you have to take other measures to compensate.

2) After this 8-16 hour period of 13.6 / 13.8v float has finished, you can drop the float voltage to 13.2v for very long term storage like a year or so. Those systems that drop to 13.2v immediately after absorb, may not actually finish the charge, but maintain in a slightly undercharged condition.

3) Check with your manufacturer - Enersys for example will not allow for anything less than 13.6v float long term with their agm's to maintain the warranty.

4) Batteries are meant to be used, and not really long-term floated forever in the first place (unless they are telco types) - they need *some* activity once in awhile to keep them healthy.

Wheelchair users with agm's that pull the charge when the light goes "green", (absorb finished) and not allow for an additional 8 hour float period usually suffer very rapid agm deterioration.

[sdold]

Inetdog, thanks for fixing the subject line, I try to make the subject line detailed but forget to take a second look before saving to make sure it makes sense.

Thanks all, for responding. PNJunction, that was great info and I'm glad I asked this question because I was thinking of leaving a low-current (maybe 100 mA) trickle charger running on an AGM airplane battery. Those things are getting really expensive, I will go back to FLA on the next one but I want this one to last awhile. A lot of airplane guys are using something called a Battery Minder, I'll see what that does. I'm interested if anyone here has an opinion of them. During the winter I only fly once or twice a month.

[Sunking]

Steve it is important to understand how Current and Voltage sources work.

A Current Source, is nothing more than a DC Power supply with a fixed voltage and current limit. If we were to draw the circuit equivalent a battery with a series resistor. Let's say we have a 10 Volt 1 Amp regulated power supply. That supply can supply 10 volts up to 1 Amp or 10 Ohm Load at a maximum of 10 watts. If you understand Ohms Law you know I made a True Statement.

But what happens if I connect a 5 Ohm Load what happens? The power supply can only supply up to 1 Amp before the regulator folds back the voltage to maintain 1 amp. So what happens?

Well you are pumping 1 amp into a 5 Ohm resistor is what you are doing, exactly what it is suppose to do, be a Current Supply. You now have 5 volts @ 1 amp with 5 watts. What happens if I used a 1 Ohm resistor?

1 amp x 1 ohm = 1 volt and 1 watt right? Stil the same darn 10 volt power supply used to charge batteries. I can short it out, and now I have 1 amp x 0 Ohms = 0 Volts and 0 watts.

But what happens if I connect say a 20 Ohm resistor? Well I have 10 volts with 0.5 Amps and 5 watts. What is different? Only 1//2 Amp right. Why it is a 1 amp supply? Why did current go down?

Because Ohms Law says Current = Voltage / Resistance. 10 volts / 20 ohms = 0.5 Amps. To supply 1 amp into a 20 Ohm Rsistor requires 20 volts. All we got is 10 Volts Source pushing the current. 10 volts and only push 1 amp from 0 to 10 Ohms.

This is how every battery charger works and has many names to confuse you. Name like Float, Equalize, Absorb, Bulk, Trickel, and finally CC/CV (constant current/constant voltage'

So now lets go back to your question. It cannot be answered without a quest because you did not specify what the Current Source Voltage was. What would happen is the voltage would continue to rise until the battery voltage and source voltage are equal. When the Charge Voltage and Battery Voltage are equal no current flows and the battery Floats. If you set the Voltage to say 13.6 volts the battery fullyu charges up to 13.6 volts and floats waiting for you to use some day. The battery will bee fully charged up ready to go in like new shape. Set the voltag eto high and you burn the battery up eventually. Set it to low and you will make it a boat anchor eventually.

Now you should understand how every battery charge works. Only thing that changes is the Voltage Set Point. With the exception of Nickel batteries, every battery charger works in this way. Set it to the right voltage, and you can walk away for years. In fact I know it can be up to 35 years using pure lead Pb batteries. Some of my first batteries I ever installed are still in service and last year tested at 92% Specified capacity.of 9200 AH @ -48 volts.

[sdold]

So now lets go back to your question. It cannot be answered without a quest because you did not specify what the Current Source Voltage was.

High enough that the current could stay constant. For example I used to build NiCad chargers that ran at C/10 continuously, even when the batteries were full. Somewhere I had read that this was the highest rate that could be left on without worrying too much about shutting the charger off when finished. I think it was considered an "overnight rate". The DC was around twice the voltage of the battery, usually 20V for a 12V NiCad, and I'd use a 7805 wired so that the 5V was developed across a series resistor at whatever current I wanted. So it became a current regulator instead of a voltage regulator. I wondered if that method would screw up an AGM, it looks like it does, so I'm glad I asked.

Thanks all for the responses. I wasn't asking this because I wanted to charge an AGM with a current source. I just wanted to have an answer for people who ask me if they can connect a constant current source (like a homemade current trickle charger or unregulated solar panel) to an AGM. I knew C/10 was about the upper limit for a NiCad, but I wasn't sure if that would also apply to an AGM. I see now that it does not, so mission accomplished. Thanks!

Steve

[Sunking]

High enough that the current could stay constant. For example I used to build NiCad chargers that ran at C/10 continuously, even when the batteries were full. Somewhere I had read that this was the highest rate that could be left on without worrying too much about shutting the charger off when finished. I think it was considered an "overnight rate". The DC was around twice the voltage of the battery, usually 20V for a 12V NiCad, and I'd use a 7805 wired so that the 5V was developed across a series resistor at whatever current I wanted. So it became a current regulator instead of a voltage regulator. I wondered if that method would screw up an AGM, it looks like it does, so I'm glad I asked.

Steve I think you might have a few details mixed up.

True C/10 on NiCd is known as the over night charge, but it needs to be terminated after 16 hours or you will destroy the NiCd cell. I think you have that mixed up with Trickle Charging NiCd because they cannot be Float Charged like Pb. But Trickle is C/20 and less. At this level the voltage required to maintain C/20 is constant and temperature dependent.

None of that applies to Pb batteries. With an AGM and you have the recipe for Thermal Runaway and an Explosion, with wet batteries the voltage and heat keeps rising until all the electrolyte is electrolized and the battery destroys itself.

OK having said that there is a Constant Current you can apply to a Pb battery to maintain charge. But you have no clue what that current is nor do you care what it is. It is a moving target. There in no way you can detect what that current is, but it matters not what it is. You can do it with a Charge that provides up to 10 times the AH rating or 10C, makes no difference what it is as long as it is enough. Know what I am talking about?

It is called Self Discharge all batteries exhibit, even lithium. What is the discharge rate. Who gives a crap what it is, You do not have to worry about it. It is called FLOAT VOLTAGE, set your charger to the correct voltage, and the problem takes care of itself. FWIW the self discharge rate will be around C/100 or less. Makes no difference what it is. Hold Constant Voltage and the problem is solved. With AGM you need one more step to control things, TEMP CORRECTION to prevent THERMAL RUNAWAY. Higher the temp, the lower the voltage.

So the answer is there is not a high enough voltage you can apply to a Pb battery to maintain any meaningful Constant Current because you will destroy the battery. So the question becomes a MOOT POINT, you do not control the current, you control the VOLTAGE. All you can do with Current is limit it to some maximum value up to some specified voltage. Otherwise you have no control of current with respect to charging a battery. .

So what I am telling you is your question is Invalid. You control the Voltage. You can only use current to determine when to stop if you want a fast charge. Otherwise set the voltage and forget about it, current will take care of itself via voltage.

So the Answer you give people is F-NO you do not use a Constant Current source to keep a battery charged. I think what is throwing you is stupid Marketing terms and a lot of morons like Dax falls for. You hear the term word Trickle Charger or Battery Maintainer that supplies current to keep a battery charged. True, but at the same time complete hogwash. All they are selling you is a very small Batter Charge with a CC/CV algorithm like every battery charger ever made. Like th eBattery Tender is a 12 volt 1-amp battery charger. You select for 3 setting's Gel AGM, and Wet. All 3 use a slightly different FLOAT VOLTAGE and the power supply can supply up to 1 amp. Once th ebattery is fully charged up the current is way less than 1 amp

Just a play on words and terms. That 1 Amp 12 volt Battery Tender is a regular super fast charger for a 10 AH or less 12 volt battery. Barely a Trickle Charger on a 100 AH battery and incapable of charging it, Only MAINTAIN IT with up to 1 amp. There is no difference between a 1 amp charger and 10 amps other than the max amount of current they can supply.

In Telecom we use Trickle Chargers. They can supply up to 10,000 amps on a 1000 AH battery, No tonly can they Trickle, they even Charge. We cal them Float Chargers or Rectifiers.

Don't let morons throw you off course like Dax.

[sdold]

So what I am telling you is your question is Invalid..

Maybe, but I still got a lot of great info, so I'm glad I asked. Thanks to you and PNjunction for taking the time to write a lot of good info. All of your info is stick-worthy and I'll be reading it more than once. I think you are right about C/10 vs. C/20, we were probably over-charging our NiCads.

[Sunking]

Maybe, but I still got a lot of great info, so I'm glad I asked. Thanks to you and PNjunction for taking the time to write a lot of good info. All of your info is stick-worthy and I'll be reading it more than once. I think you are right about C/10 vs. C/20, we were probably over-charging our NiCads.

This should help Click Here.

As for Stickies already have one you should read, Bulk, Absorb, Float.

[PNjunction]

Maybe, but I still got a lot of great info, so I'm glad I asked. Thanks to you and PNjunction for taking the time to write a lot of good info. All of your info is stick-worthy and I'll be reading it more than once. I think you are right about C/10 vs. C/20, we were probably over-charging our NiCads.

Yeah, well I'm just standing on the shoulders of giants. Read their stickies first. I tend to ramble, so I don't try to write too many that are actually sticky-worthy.

My comments about current was limited to the float-stage after absorb for agm lead-acid. For bulk this is a different matter, so reading those stickes first will help. In a nutshell, most conventional agm's have twice the initial current charge capability than flooded (0.2-0.3C vs 0.1C) for instance. There is much more so grab an adult beverage, and start reading.

Re the Battery-Minder:

Aside from Tecmate-Optimate, a Battery Minder is the only other automotive-type charger that I will let touch my expensive agm's like Optima or Enersys/Odyssey pure-lead types. I can heartily recommend the Battery-Minder, but for best results get the model that matches your agm's specific voltage specs. Ie, my conventional Exide Edge dual purpose lives happily with a model 2012 for a 14.4v absorb. For Optima's or Odyssey pure-lead, the 2012-AGM with it's higher absorb voltage is preferred.

At first, most battery guys may be turned off by the manufacturer's belief in the desulphating frequency (not high voltage) process. You can safely ignore that if you want.

HOWEVER, what you get with a Battery-Minder is a *documented* and published algorithm, voltages that agree with a Fluke 87V multimeter, proper times and timeouts for bulk, absorb, float etc. Oh, and good build quality. Even rarer are instructions to neophytes on how to use a hydrometer to prove to themselves that they are making progress with flooded batts, rather than rely solely on terminal voltages. Of course temperature compensation probes are included with most, that do more than just being ambient sensors. If you are an aviation guy, they make models specifically for those too, to pass inspections and for overall safety as well.

Fanboy reviews and kludge-mentality freeloaders often ascribe things to these units that the manufacturer never says nor recommends. For instance, while it will do an admirable job of bringing back some dumpster-dive batteries, that is NOT what the manufacturer recommends you do, nor any sort of wink-wink nudge kind of way. They are adamant about doing your OWN measurements, and include instructions on how to do so - and most importantly, when to throw in the towel and not waste time with trash. Essentially, they are designed to keep good batteries healthy, and not bring back joke zombie batteries that should be recycled in the first place.

They have various models with different amperages, but nobody who knows batteries recommends them for REPETETIVE constant cycling charge of big batteries. For maintenance, top off, or using with small batteries as a primary charger, that's ok. In fact, their own solar charge controller recommends that those trying to revive batteries in trouble to do so with their AC charger first!

Basically, these guys are on the up and up, despite what others may believe about the desulphation freqs. They could drop that "feature" tomorrow, and I'd still recommend the Battery Minder as one of the very few chargers worthy of using with very expensive agm's. Get the proper model of course. Read the online manual first if you like. They've got nothing to hide.

Most importantly, let the manual speak for them, and not any rave fanboy or unsafe hacker reviews.

[sdold]

The airplane battery environment seems like a really bad one. A typical flight might be an hour or two, followed by two or three weeks or more of sitting idle, so I'm looking at ways to keep it charged without removing it each time. The battery is currently a Concorde AGM 12V. I've been thinking about using keeping the Battery Minder connected, but when you mentioned using the right charger for the battery, it prompted me to go look at their web page. They have an article here:

http://www.batteryminders.com/why-an-aviation-specific-charger-maintainer/

It says that an aviation battery uses a higher specific gravity electrolyte, and that a lower charge voltage should be used than with typical batteries, so an aviation-specific charger should be used. Mine's not aviation-specific, so if that's true I might be getting ready to use the wrong charger, I need to learn more about this. Thanks for the heads-up.

[SunEagle]

The airplane battery environment seems like a really bad one. A typical flight might be an hour or two, followed by two or three weeks or more of sitting idle, so I'm looking at ways to keep it charged without removing it each time. The battery is currently a Concorde AGM 12V. I've been thinking about using keeping the Battery Minder connected, but when you mentioned using the right charger for the battery, it prompted me to go look at their web page. They have an article here:

http://www.batteryminders.com/why-an-aviation-specific-charger-maintainer/

It says that an aviation battery uses a higher specific gravity electrolyte, and that a lower charge voltage should be used than with typical batteries, so an aviation-specific charger should be used. Mine's not aviation-specific, so if that's true I might be getting ready to use the wrong charger, I need to learn more about this. Thanks for the heads-up.

I would guess that disconnecting the battery and keeping it on a bench charger would be more work then necessary?

What do the other pilots do with their batteries?