AGM batteries - do they demand a minimum charge current?

[PNjunction]

No big deal - we just manage our losses, or increase the efficiency of our loads. We accept the fact that we are going to replace batteries, but due to the way we cycle them, we may only get 4 years instead of 5 out of our agm's for example. We know up front we are wasting their absolute capacity. If we put them on decent chargers once in awhile to get a REAL good extended charge, we might be able to reduce that capacity loss a little bit. Real-world use, instead of laboratory-perfection. Of course we should meet a reasonable ratio.

Good reminders - Sun Xtenders / Lifeline 0.2C minimum
Trojan AGM (diagram 6) 0.2C
Odyssey 0.4C minimum

Maximums? Deka agm's and others appear to be about 0.3C max. Odysseys and others much higher maximums - it all depends so check the manufacturer's limits.

The key thing I learned here with the Odyssey is that this minimum is intended for deep-cycling - more or less going beyond a 40-50% DOD. If we don't go there, these minimums may be lessened a bit as long as it still fits into your charging timeframe. Lesson - don't go below 50% DOD otherwise you'll definitely need to meet the minimum requirement with either more solar, or perhaps a genny and/or charger. And in the case of the Odyssey, you CAN go below the recommended minimum when deeper than 50% DOD, but it is going to cost you in doing so - it walks down the overall capacity right at the outset. I wouldn't do it on a regular basis, but if you have to and have no other choice, it can be done at a cost.

The Genesis EP/XP application manual is the only one I've found that shows a chart of degradation in capacity when using currents below the minimum recommendation. Sun Xtender/Lifeline talks about it in their manual. Deka East-Penn just states that one should strive to get close to the maximum inrush allowed for best life.

And unless our day is always perfectly bright, there is a good chance of being below the minimum once in awhile if we take it below 50% DOD. Looks like I'll be buying batteries a bit more often. I'm starting to think that even general-purpose UPS-style agm's would do well if one tries to design for at least 0.2C - remembering that for them, 0.25 - 0.33C is the max. Yet I haven't found any charts or studies about them at very low bulk rates. I suspect that they will also walk-down the capacity below 0.2C, but we never really notice before retiring them.

Are you using 8AGC2 Deka's ? What is the configuration? Are you running a bunch of serial-parallel connections? Is your charging time-frame only one solar-day? Maybe the more experienced battery guys here can take a look and see if you are reasonable enough.

[894tom]

What is Ah on these Deka batteries?

190a/h

I contacted and spoke with a very helpful Deka rep.whos specialty is solar battery usage. While I used the correct equation, I imputed the wrong parameters. The recommended a/h charge rate for my battery bank is 57a/h. Thats a 2,736 watt array. Depending on how much I deplete the bank in a given day, I will definetly still need a regular combustion engine generator.

The more I learn about solar power, the more I understand the current technology in the solar field its kinda like polishing a turd...ya put a bunch of fancy electronic on the same basic fault (batteries in general). Even with a large recommended solar array capable of charging the the best FLA, AGM, or Gel batteries out there, God still controls the seasons and weather conditions. The most up to date, best charge controllers out there still cannot recharge my battery fully (from a greedy 50% discharge) on a short north eastern (USA) winter day. Until battery techo becomes affordable and easly produced, solar is really not practical from an off grid standpoint if your goal is to be fossil fuel free.

[Roil]

Ewww. I recently purchased 8 6v AGM Deka golf cart batteries. I was wondering the same thing as the OP in terms of minimum charge rate for an AGM. I currently have around 21a/h on a good solar day. This was good for my FLA golf cart batteries...however, I am concerned I need to add more panels for these batteries. From what this is saying...38a/h minimum? 47.5a/h max? for the new agm bank? PS...the battery bible link is no longer any good.

What is Ah on these Deka batteries?

[894tom]

No it does not mean there is no minimum. The answer you are looking for lies in the manufactures Operatos Manual. For example Concord Sun Extender AGM line up page 19 of 40 specifically states:



Trojan uses the same .2C minimum.

The battery Bible is not going to tell you a minimum because it would be irresponsible for them to do so because it differs from 1 manufacture to the next.

Ewww. I recently purchased 8 6v AGM Deka golf cart batteries. I was wondering the same thing as the OP in terms of minimum charge rate for an AGM. I currently have around 21a/h on a good solar day. This was good for my FLA golf cart batteries...however, I am concerned I need to add more panels for these batteries. From what this is saying...38a/h minimum? 47.5a/h max? for the new agm bank? PS...the battery bible link is no longer any good.

[PNjunction]

Yikes, getting my companies and products mixed up... I bet they won't like that!

I have to admit that when a company produces quality docs, like Enersys / Concorde (Crown?) / East Penn (Deka?) , it makes me gravitate towards them before I even hit the battery store. Crappy or non-existent docs make me look the other way even if the price is good.

[Sunking]

I think I may have an answer from Enersys. I looked at their tech manual for the "NP" series and unlike the big-boys like the Lifeline and Sun Xtender ...

FWIW and FYI Enersys is one of the Big-Boys in the battery manufacture market, one of the largest if not the largest. Crown who makes the Sun Extender product line is a pretty small fish. For a consumer like yourself you are mostly exposed to consumer grade battery manufactures of the automotive and consumer electronic manufactures. When you cross over into the commercial and industrial market area names like C&D, Enersys, and East Penn are not familiar to you which is understandable because they do not advertise in the consumer market media. Not many consumers have much use for something like C&D MCT II battery which is a 2 volt, 4000 AH, 650 pound cell. These are what you find in the Telecom, Data, Utility, Rail Road, Mining, and Submarine industries.

[PNjunction]

For the ups-style agm's, I think I may have an answer from Enersys. I looked at their tech manual for the "NP" series and unlike the big-boys like the Lifeline and Sun Xtender products which demand .2C, there basically is no minimum input for these smaller general purpose batts in the Genesis line. However, that doesn't mean it will be practical at rates lower than .15C for the ups-style stuff we are using.

You can check it out here:

http://www.enersysreservepower.com/catalogInfo.asp?id=215&brandID=3

Ok, so the Ritron isn't an EnerSys / Genesis NP model, but it put things into perspective for me with my Powersonics.

The .15C minimum comes from the cyclic / recovery charge use data in the manual, and there is even a lower value, but that is for a topping charge during storage / initial receipt. According to one of the charts, you can go down to .001C - almost nothing - but what good is that for cyclic use?

So If I had to recommend a practical minimum for THESE general purpose ups-style types in cyclic use, it would be .15C - but to take advantage of agm, I'd shoot for a faster rate close to, but no more than .25C. All this data is obviously for EnerSys / Genesis batteries, but I would imagine other manufacturers in this sector would be close.

[PNjunction]

I see the C/20 rate as kind of a minimal - but far from optimal rate, so I suppose there is no official standard (help me out here if there is)

I look at it from a solar-insolation viewpoint from back of napkin engineering - real world will be worse. (somebody jump on this if this is way out of line!)

Example --
Battery: 100ah
Panel output: 5a (C/20)
Max DOD: 50%
Capacity needed to replace: 50 amps
Efficiency offset added to recharge time: 1.75

So, with 50amps needing to be replaced it would take: (50 / 5) * 1.75 or, 17.5 hours. With only 4 hours in an average solar-insolated day, you are looking at nearly 5 days to fully charge! So for a weekend warrior, this might do (without any loads during charge!) , BUT spending so many days below full charge is not healthy for the battery. This also assumes you actually DO get 4 hours of solar insolation - not likely for many...

So while it works, long-term battery health would be my major concern at C/20 with a 50% DOD. I'd really not want to go there - cutting the DOD down to 25% naturally gets you down to taking 2 1/2 days or so.. If it was serious, I'd either bump up the panel capacity, or reduce my loads to cut my battery capacity in half perhaps just to get up to full charge more often ... and not forgetting to add my real solar insolation hours into the mix.

[Roil]

A C/20 charge rate applied on my 400Ah batteri bank is spot on where I am with my 20A, so that give some confindent that I am atleast not alone

Is C/20 a kind of minimum for "brandless" "limited spec. Documentation" AGM batteries we can agree on?

[PNjunction]

All this being said, if they do have the same demand as Concord or Trojan batteries I might have too small solar panels.

From the Ritar specs at the bottom of the pdf, they seem to indicate a max of 0.3C just like my Powersonic ups-style agm's. I used the C/20 minimum for AGM based mostly on a practical time limit to get it fully charged without taking too long to do so. However I try to push it to .25C (C/4) or so since I'm using a time limited opportunity-charge scenario while portable.

Someday I'll get serious with Concordes or Trojans to take advantage of the much higher charge rate if I can afford enough panel to support that capability.

[PNjunction]

Quick apology to all - I have corrected my errant postings showing C/20 minimum for FLA when it should be C/12. What makes it worse is that I had read Sunking's postings previously, yet continued to propose the wrong value.

[Sunking]

Roil wish I had a good answer for you, but I do not concerning your options for battery choices. Concord and Trojan are exceptions. As Inetdog points out AGM batteries do not have a stratification issue and normally do not have a minimum charge rate. IMO and I am certain many others share a C/5 minimum charge rate just is not economically feasible, and shatters acceptable design practices. It would eliminate needing to know your area solar insolation.

For example if you needed 1 Kwh/day with a winter insolation of say 3 Sun Hours. Battery size is fixed at 5 days or [5 x 1000 Kwh] / 12 volts = 416 AH. Standard design practice would require a 500 watt panel using a MPPT controller, and 666 watts with PWM. But to generate a C/5 charge rate we are now talking 1200 watts with a MPPT controller, and 1700 watts with a PWM controllers. The only place that makes sense is in extreme cold climates where winter insolation rates fall to 1 hour or less. Guess where that brings us back too. Use AGM in extreme cold climates.

[russ]

Sometimes if the supply of an item is limited you have no choice but any reputable first class company (just eliminated 90% plus of the China companies) will supply good documentation.

[Roil]

No it does not mean there is no minimum. The answer you are looking for lies in the manufactures Operatos Manual. For example Concord Sun Extender AGM line up page 19 of 40 specifically states:

For repetitive deep cycling, chargers should have an output of at least .2C (C/5 or 20 amps on a 100 AH battery). If the output is less than this the cycle life of the battery can be negatively affective.

Trojan uses the same .2C minimum.

The battery Bible is not going to tell you a minimum because it would be irresponsible for them to do so because it differs from 1 manufacture to the next.

Always strong and clear Sunking - thank you.

The challenge is of course that not all manufactures does supply the information we need. Most Off grid Sunpower suppliers here in Norway sell "own" brands which in fact is something else and very often a low price Chinese brand. That's at least the situation in my case. After some inquires to the supplier I got the attached specification for my set of Ritar RA12-200D batteries - and yes they are made in China ;-( - and I struggle to find any statements of minimum charge current. Interestingly enough the batteries are relabeled and sold as 290Ah (with 100h in small letters below) and considering 100 hours the 290 is probably true, but in the real battery-world they are in fact 200A batteries.

All this being said, if they do have the same demand as Concord or Trojan batteries I might have too small solar panels. With two of these batteries I got 400Ah and 0.2C they demands a charge current of 80A. I do have a Victron Multi 3000/120 - so I can charge them with 120A current but typically they get discharged by 50% in the weekend, and then recharged with a 20-22A current from my set of Kyocera 135W and old 65w panel. On nice days they charge around 130Ah in total. So depending on the weather they could be fully recharged in two days, or it can take a week, or even more in December.

Anyway 20A is only 25% of the possible required charge current, and even though I am planning to buy two more of the Kyoceras, I would still only be at 50%.

This would in fact demand 4 x 140W panels just to keep 1 x 200Ah battery nice and sound - Very few in our local solar panel industry promote this ratio between batteries and panels, but there knowledge in this area is probably very very low . My old setup was 3 x 115Ah FLA's from Exite and the same 65W panel as mentioned above - those batteries truly suffered from too low charge current.

I have attached the specifications if any of you are interested

[Sunking]

I have search the different treads and found many statements of minimum current for FLA batteries, but none conserning a minimum current for AGM batteries.

Does this mean that there are no such minimum current?

No it does not mean there is no minimum. The answer you are looking for lies in the manufactures Operatos Manual. For example Concord Sun Extender AGM line up page 19 of 40 specifically states:

For repetitive deep cycling, chargers should have an output of at least .2C (C/5 or 20 amps on a 100 AH battery). If the output is less than this the cycle life of the battery can be negatively affective.

Trojan uses the same .2C minimum.

The battery Bible is not going to tell you a minimum because it would be irresponsible for them to do so because it differs from 1 manufacture to the next.