AGM batteries - do they demand a minimum charge current?

[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]

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.

[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]

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.

Yes. I have 8 run in series for a 48v system. I dont know if I understand...I would think everyone would want to charge thier bank in one day and would size thier array accordingly.

[inetdog]

Yes. I have 8 run in series for a 48v system. I dont know if I understand...I would think everyone would want to charge thier bank in one day and would size thier array accordingly.

Exactly right, everybody will want to charge their bank in one day IF they started the previous day fully charged and only drew their battery down by 20%. But sometimes you will have a cloudy or rainy day (or two) and may get your batteries down to 50%. At that point it would be very nice to have a panel array that could bring it back up at least to 80% (bulk all the way) the next sunny day, but not necessarily take it to full charge.

So the size of the array should at least match the expected daily load, plus some extra for charging inefficiency. Beyond that, there are various tradeoffs to make and different people will make different choices. For example some people would take care of the cloudy days by using a generator to bring the batteries back to where the panels could handle them again.

[PNjunction]

Ah bingo - 190 * 0.3C = 57ah max. Do you really go to 50% DOD all the time? How much PV do you have? Still, the 2 hour insolation will hurt.

I think the generator would be a wise move as you are now covered with a Plan-B should any point of failure crop up in the solar system - usually at the worst time.

Maybe think about just a quality 48V charger as a maintenance tool - with appropriate setpoints of course - to give them a nice charge once in awhile between cycling. Even though I have 4 hours of solar insolation, and can blast the Odysseys, they never REALLY get to 105 - 115% or so unless I had 16 hours of float to play with.

So failing that, I just devote one whole solar day every 2 weeks maybe to nothing but float-charging since my solar setup actually does better than the automotive "smart" style chargers and might help offset the cycling loss a little bit. Since float currents are so low, I have been able to go beyond the normal solar insolation hours for an hour or so on each side of the charge window, so that helps when it's "float day" here. Of course you have to be able to withstand the night before....with a generator, no problem!

[Sunking]

The most up to date, best charge controllers out there still cannot recharge my battery fully .

Tom what does that have to do with fully charging your batteries?

MPPT charge controllers are 95 to 98% efficient. There is no more improvement that can be made. That last 2 to 5 % is insignificant. The key to getting the batteries fully recharged is the DESIGN. If your system cannot fully recharge in a day under normal use means you do not have enough panel wattage.

[PNjunction]

With foot in mouth, I proudly say good-bye to "float-days".

Heh, I was all set to argue this, and then I stumbled across a chart / doc from Rolls that clearly showed the error of my ways:

http://support.rollsbattery.com/solution/categories/687/folders/10488/articles/4345-agm-charging

During absorb / float, I'm constant-voltage, not constant current. Man, I have a hard time keeping that straight.

[inetdog]

With foot in mouth, I proudly say good-bye to "float-days".

Heh, I was all set to argue this, and then I stumbled across a chart / doc from Rolls that clearly showed the error of my ways:

http://support.rollsbattery.com/solution/categories/687/folders/10488/articles/4345-agm-charging

During absorb / float, I'm constant-voltage, not constant current. Man, I have a hard time keeping that straight.

Particularly interesting to me was that a constant current charge rate of .01C should not be maintained for more than 8 hours, while .007C (not that much lower in absolute terms) could be maintained indefinitely. (More properly, .007C is the expected current when the cell is at Float voltage (2.3), while .01C can take the cell voltage up to 2.55 and eventually cause damage.)

That is a fine line indeed!

[894tom]

Tom what does that have to do with fully charging your batteries?

MPPT charge controllers are 95 to 98% efficient. There is no more improvement that can be made. That last 2 to 5 % is insignificant. The key to getting the batteries fully recharged is the DESIGN. If your system cannot fully recharge in a day under normal use means you do not have enough panel wattage.

For some reason, I do not get updates/replies from solarpanel talk. I didnt know all these replys were on my thread.

To answer your question SK. I have an OutbackFM80 (arguably one of the best from different articles and thread I have read) For the sake of your question, let's say I have 2736watts of solar producing the recommended 57A/H rate I need to recharge my 190A/H AGM battery. My battery is discharged 50%. Its a beautiful sunny day on December 21st in Mid State New York (2.0kWh/m 2/day). Will the Outback be able to recharge my battery to 100%? Thats what I was saying. If you think it will, I have completely misunderstood what the Deka rep was telling me and this is great news!!

[Sunking]

OK Tom let's see if we can get you on track OK. Let's forget about Amp Hours right now, they are not important for this discussion. What is important is Watt Hours.

I assume we are talking a 48 volt 190 AH AGM battery right? If that is correct then 48 volts x 190 AH = 9120 Watt Hours. So 50% is 9120 wh / 2 = 4560 Watt Hours is what you will have to replace. To do that with an AGM battery you are going to have to generate 4560 / .8 = 5700 Watt Hours at the panel terminals to overcome the losses in the CC and AGM charge efficiency.

OK no wwe know what has to be done all we need now is the Sun Hours which you state is 2 Hours. So all we have to do now is factor out the time element hours to find the panel wattage to do that. Watts = Watt Hours / Hours = 5700 watt hours / 2 hours = 2850 watts. You said you have 2736 watts right? You are close enough to say you have just enough to get the job done. Keep those cable distance short and conductors large enough and you can improve the efficiency enough to get it done.

No back to the battery issue. 57 amps charge on a 190 AH AGM battery is NO PROBLEM, just use a temp probe to monitor the temp of the batteries. If it is cold there in December, heat will not be much of an issue.

[Roil]

Gentlemen

I reread alle the nice post on this thread again today. I must admit that I am not sure if I understand your views fully. Let me try to explain where I am coming from.

To me it sounds like we are have two different conserns.

1) what is the minimum Amp charge a AGM batteri typically is build for.

And

2) in a daily cyclic use of a off grid solution what kind of recharge time can we alow ourself to get the batteries back into a fulle charge state.

The typical off grid situation we experience in Norway is a cabin where usage is weekends by intervals of 1, 2 or more weeks.

As in my examples I have a charge capasity of 130Ah on a good day down to 25Ah on a bad (yes 12V, and average charge voltage of 13, so watthours should be in the range of 325Wh - 1690Wh) ......
Why should I not try to capture as much of that solar energy as possible?

Let's assume my weekly production consist of 2 good and 5 bad days giving something in the neighborhood of 5Kwh. My 400A batteries contain only approx 5Kw and a 50% discharge Is therefor recharge in less then a week. And this with only a charge capasity of C/20.

So If I visit my cabin every 3rd weekend I miss the opportunity to capture and store 10Kw of energy. That's a shame isnt it?

I do ofcorse understand that if I tripple my batteribank to 1200A my charge rate will drop from C/20 to C/60, but so what if the low resistant AGM batteries can cope with it?

Let me have your views on this

[894tom]

What is important is Watt Hours.

57 amps charge on a 190 AH AGM battery is NO PROBLEM

See...now thats what I have been thinking all along. I definetly had a miscommunication with the Deka rep. I'm gonna go out on a limb and guess its my fault

I actually only have 2280w in solar panels (47.5a/h)....so I will have to expand...but it was the overall concept I was trying to understand. Thanks for getting me back on track. Basically, I think the cheaper way out is to turn off the coffee maker a half hour sooner than usual, and skip the 5oclock news .

[Sunking]

See...now thats what I have been thinking all along. I definetly had a miscommunication with the Deka rep. I'm gonna go out on a limb and guess its my fault

I actually only have 2280w in solar panels (47.5a/h)....so I will have to expand...but it was the overall concept I was trying to understand. Thanks for getting me back on track. Basically, I think the cheaper way out is to turn off the coffee maker a half hour sooner than usual, and skip the 5oclock news .

Actually Tom there is a good point you were driving at. Even though Flooded Lead Acid and AGM are both in the same family, they have completely different charging and discharge characteristics. And if you do not know what those characteristics are can bite you in the butt. Especially for folks like you up north with very short winter days. The big difference is the INTERNAL RESISTANCE of the two batteries. FLA have higher internal resistances which limits the maximum charge rate, and they also have a minimum charge rate to keep them from Stratifying. So FLA batteries have a window of C/12 to C/8 charge rates. FLA have another negative characteristic related to the higher resistance in the voltage folds back significantly once you get below a C/8 charge rate. Which means you don't take a 12 volt 100 AH FLA battery and connect it to a 1000 watt inverter and expect it to work at high power levels.

AGM's on the other hand as a general rule can be charged at C/4, and many as high as 1C and up. They can also be discharged at very high rates. Where this comes as a benefit to you Yankees with very short winter days is they can take the very high amounts of current to get charged quickly with you rshort winter days where a FLA cannot. Example a 1 Kwh/day system in Tuscon takes a 200 watt solar panel to generate 1 Kwh/day in winter and on a 12 volt system is 15 amps on a 180 AH battery is perfect C/12 rate. As for you would take would 750 watts or 63 amps @ 12 volts which would fry even a 400 AH battery. Solution use an AGM.

[PNjunction]

Sorry - old thread but now that I'm nursing an 18 month old neglected Deka / East Penn agm that only had a factory charge, I paid a little more attention to the 1927.pdf file tech manual I found while searching:

Valve-Regulated Lead-Acid (VRLA) - EAST PENN

and noted that in their tips for longest life that one should "Use the highest charging current available, (up to 30% of the 20-hour capacity per hour) while staying within the proper temperature-compensated voltage range."

They did not specify if this was particular to only deep-discharge situations and it seemed like a generic recommendation.

Ok, so this may be impractical for solar, and specific to the East-Penn, but like the posts above since we are not dealing with FLA stratification issues, lower levels of current are ok, they just aren't optimal. How much less optimal I don't know.

From an anecdotal standpoint, with other agm's that I have with a maximum current limitation, I just try and push as much as I can without going over the limit.