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

[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]

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]

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.

[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]

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

---Mod Note: Fixed It For Ya

[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.

[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

[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.

[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?