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AGM Battery Charge rates


Chris Hobson
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Charge rates on batteries is one of the last things I understood about solar.  Charging rates are expressed as 1C C/10 etc. They are based on the discharge rates of the battery.

C1 is the rate of discharge to discharge 100% battery capacity in 1 hour. C/2 or 0.5C (different notation for expressing same discharge rate) is 100% discharge over two hours and so on.

The rule of thumb is to charge at a rate of C/10 or 0.1/C (10% of battery capacity).

One of the advantages of AGM batteries is that they can be charged at a much higher rate than standard "wet" lead acid batteries. My batteries for instance can be charged at C/5 (double the recommend wet rate) for 2 hours and then C/10 for 7 hours. I do not discharge normally past a SOC of 70% so I charge at a rate of less than C/10. This is fine since in spite of drawing 1500W for 5 hours for the geyser my batteries are charged by mid afternoon. 

On Saturday night an appliance was left on and I discovered this on Sunday morning and the batteries had been drawn down to a SOC of 50%.  To compound my problems it was not a particularly bright sunny day on Sunday. I cranked up the charging rate to 0.2C and gave the battery bank a quick buzz well within their capacity.

This got me thinking. Am I perhaps reducing my AGM battery life by charging at such a low rate? One of the disadvantages of AGM batteries is heat build up (they have less electrolyte than vented and gel batteries (which helps with the heat transfer) and so my lower charge rate is an advantage in that respect. AGM batteries also stratify slightly but due to the fact that one cannot bubble the electrolyte I don't think it is avoidable.  Any discussion/pointers is welcomed.

 

 

 

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I think the basic principle is that when you charge, some of it causes chemical changes, and some of it is lost either in heat or some other parasitic process. The faster you charge, the more heat you make, the more you lose to parasitic processes. How fast you can charge will depend on the battery chemistry.

The basic problem is that chemical processes need time. If you push current in faster than those reactions occur, the rest just cause gassing (a parasitic loss) and heat. So it comes down again to the chemical composition.

I would think that the slower the better, because then you're giving those chemical reactions ample time, and this is obviously the recipe for the least heat loss.

I also have this gut feeling that there will be a minimum, but that's going to be in the milliamp range, because it would have to be so low that it is lower than the self-discharge of the battery.

What I'm not sure about is whether there is an optimum point where your parasitic losses will be minimised, and if it even matters in our context. I'm thinking it doesn't.

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Hahahahah. What I found is that not even the manufacturers agree on this. But yes I would do what the manufacturer states on how you should charge and discharge. 

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My gut feeling about the charge rate is pretty much... if you're in deep doodoo and need to charge these things fast... what's the absolute limit before tropical fruit starts hitting rotating interfaces :-) And then once you know that number, you ignore it and go nowhere near it :-)

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Very interesting topic.

My batteries (AGM Gel 200Ah @ C10) state max charging current of 51 A, but the most I charge them at is 30A.

I actually dont think I could even charge them at 51A unless I was pumping solar and Eskom into them at the same time as I only have 3000W of panels, and the most I've ever seen from them was around 2800W.

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Hi Viceroy 

If I remember you have 8 batteries (2 strings of 4). So your bank is in fact 400Ah so a C/10 charge rate would be 40A. You are right with a max of 2800W charging at 51A is going to be a push. At 30A you are being cautious, as I am,  and I was wondering whether this was detrimental.

Chris.

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1 hour ago, Chris Hobson said:

From my reading it appears AGM batteries do stratify slightly. How do you counteract that with an electrolyte trapped in a mat made of hollow glass fibres?

Skud hulle so bietjie? ;):lol::D

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2 hours ago, edmundp said:

I charge about 25A on a 340Ah agm pack. I just "boost" them a little from time to time to get rid of stratification?

Sent from my SM-N900 using Tapatalk

Is 25a not a wee bit low. 35a is 10%?

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2 hours ago, Chris Hobson said:

Hi Viceroy 

If I remember you have 8 batteries (2 strings of 4). So your bank is in fact 400Ah so a C/10 charge rate would be 40A. You are right with a max of 2800W charging at 51A is going to be a push. At 30A you are being cautious, as I am,  and I was wondering whether this was detrimental.

Chris.

You are correct Chris. I was just stating the individual battery specs :)

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From my reading it appears AGM batteries do stratify slightly. How do you counteract that with an electrolyte trapped in a mat made of hollow glass fibres?

Well try charging them at 50A and listen closely to your batteries. You will hear them still gassing ever so softly....

Sent from my SM-N900 using Tapatalk

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