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Battery overcharging/undercharging


Chris Hobson
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I have spent a fair amount of time researching battery charging. The Axpert settings for AGM batteries (56.4V for bulk/absorb and 54.4 V for float - 48 V system) are low compared to some of the recommendations on the web (the observation about opinions and orifices springs to mind). What I like about these settings are that they are below gassing voltage up to about 30oC.

 

Chronic overcharging and undercharging are responsible for shorten battery life - overcharging leads to gassing and loss of electrolyte and under charging leads to _ _ _ _ _ _ ? (sulfation maybe?).

 

My question is this lets for argument sake say that the bulk setting of 56.4V is 1.5 V too low. The absorb phase will be entered too early and the the batteries will enter float once the current tails off. Surely at a voltage of  0.23V/cell above 100% charged during absorb and 0.15V/cell during float one is still charging the batteries I have seen my batteries regularly on float drawing less than 1% of capacity. I believe at that sort of current the batteries are fully charged. Does undercharging in a solar system occur where the settings may not be 100% correct (too low) and float is only achieved late in the afternoon and the batteries may still be taking in 3-5% of capacity?

 

The one time I was tempted to go to 57V for absorb (documentation for my batteries reckon 13.6 - 13.8 V for float and 14.6 -14.8 V for cycle service) within 5 minutes I could taste the sulphur in the air. I immediately reset to 56.4 V.

 

Anyone found ideal setting for AGM batteries?

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Hmm, It depends from battery to battery and not only type. I have used the custom settings on the MPPT controller as well as on my inverter. The Narada Rex-C batteries I have recommend absorption of 55.2v and float of 54v, which is lower than the AGM setting on the victron. I sugest you get hold of the battery you are using's spec sheet and set up your charger from that.

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Hmm, It depends from battery to battery and not only type. I have used the custom settings on the MPPT controller as well as on my inverter. The Narada Rex-C batteries I have recommend absorption of 55.2v and float of 54v, which is lower than the AGM setting on the victron. I sugest you get hold of the battery you are using's spec sheet and set up your charger from that.

 

Ditto, set according to the batteries' datasheet.

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I have 12V 260Ah CB Solar AGM batteries. They weigh 74kg and cost R3500 each so I am wanting to protect my investment. In spite of my double Zener regulator they are out of balance for an hour or so in late absorb.

 

The data sheet for them is 

 260Ah (3).pdf

 

The absorb voltage is unclear to me from the above documentation.

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The data sheet for them is 

 attachicon.gif260Ah (3).pdf

 

The absorb voltage is unclear to me from the above documentation.

 

They only give the float charging voltage - nothing mentioned about bulk charging current or voltage, and nothing mentioned about absorption voltage. The graph is also for "standby use". You'll have to try and get more info from the suppliers / manufactures / web since you're cycling them daily.

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Absorbtion voltage 58.4V

Float 54.6V

 

Edit: Your absorption voltage is the Equalisation / cycle voltage. That is the voltage at which your bulk charge is done. Also set your max amperage to no more than 10% of your battery bank total amps ie 26 amps if you are using 4, 52 if you are using 8 etc.

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Hmm, the pdf gives that as the cycle voltage. AGM batteries does not have an equalisation voltage, as this damages the battery. Therefore they give it as cycle/equalisation voltage. If your charger cannot turn equalisation off, you need to set the voltage the same as your absorption / bulk charge voltage. See the equalisation and cycle service per the spec sheet. 14.6-14.8VDC per unit. 

 

Edit: Preset 48v battery charges also usually have a 4v difference between Absorption and float charge values, or 1v lower per 12v battery. So where your absorption voltage is between 14 and 14.8v, your float voltage is between 13 and 13.8v

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Hmm, the pdf gives that as the cycle voltage. AGM batteries does not have an equalisation voltage, as this damages the battery. Therefore they give it as cycle/equalisation voltage. If your charger cannot turn equalisation off, you need to set the voltage the same as your absorption / bulk charge voltage. See the equalisation and cycle service per the spec sheet. 14.6-14.8VDC per unit. 

 

Edit: Preset 48v battery charges also usually have a 4v difference between Absorption and float charge values, or 1v lower per 12v battery. So where your absorption voltage is between 14 and 14.8v, your float voltage is between 13 and 13.8v

 

I do apologize, I've misread the "Equalization and Cycle Service" as just service, as in the periodic "Service" / "Equalization process".  It is strange though that they even mention Equalization for VRLA batteries.  :huh:

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Hi Widowmaker, hope you don't mind me abusing this thread but I'd love to get some advice on my batteries.

I have two parallel banks consisting of 4x100 Ah Vision 6FM100LP-X batteries. The only datasheet I could find had very little info on it. Is it a normal flooded lead acid battery or is it AGM? What should my charge settings be? At the moment I have bulk charging voltage at 58.4V and floating at 54V. I don't see an equalization setting on my inverter (Axpert MKS 5kVA).

Thanks

Cobus

Sent from my iPad using Tapatalk

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I have 12V 260Ah CB Solar AGM batteries. They weigh 74kg and cost R3500 each so I am wanting to protect my investment. In spite of my double Zener regulator they are out of balance for an hour or so in late absorb.

 

The data sheet for them is 

 attachicon.gif260Ah (3).pdf

 

The absorb voltage is unclear to me from the above documentation.

Hi Chris

 

Where did you get these batteries at that price?  Sounds like a great deal...

 

Regards

Mark

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Rubicon - I know they kept them for me. I do not know what the new price is. In hindsight I would probably buy more `smaller batteries if I was to do so again. They are a bitch to move and the battery box is built out of scaffold planking to accommodate the nearly 300kg of batteries. There is the advantage of fewer wires etc and they are compact. 260Ah of smaller batteries would occupy a larger space. 

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  • 1 year later...

@Chris Hobson, I think I've got my set up about right, but I really don't want to mess up my batteries and was hoping you would take a look at my settings.

I've got 8 12V 100ah batts in two parallel strings of 4 (data sheet Rocket ESC100-12.pdf) attached to an Axpert 5kva. I'm using a bulk charge rate of 58V and a float rate of 54.8V.  My BMV has program 2 set to 54.6V with a 3% tail current (program 3) and a 10 minute charge detection time (program 4). I set it up based on advice from this video

My 3kw PV array has no problem getting the bank to full charge daily. The temperature around my batteries is often up to 35C. I'm charging OSO solar only at 30A (the max current on the data sheet.) I'm leaning toward staying on inverter program 1 SOL which switches me to grid, but only takes my batteries down to about 85%. I can go all night on SBU, but I drop to a SOC of about 55%. That's a bit lower that I want to go.

Thanks. 

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1 hour ago, stuvo said:

@Chris Hobson, I think I've got my set up about right, but I really don't want to mess up my batteries and was hoping you would take a look at my settings

Hi Stuvo no problem. This thread comes out the Ark. It does show that we all started somewhere. Here I was asking advice and it's only a pleasure to pay it forward.

Your datasheet's  Bulk/Absorb voltage recommendation is a range between 57.6V and 58.8V. At 25°C this is  above gassing voltage which is 57.36V so I would opt for the lower end of the spectrum since the Philippines is tropical (just north of the equator). At temperatures of 35°C gassing voltage is about 56.3V. Operating at 57.6V rather than 58.8V means that you will go to absorb sooner and stay in absorb longer. Since you have 3kW of panels I don't think you will really notice the longer absorb time. It might influence you on a cloudy day but since the batteries are your biggest investment lets try and extend their life as much as possible. Float I would set at 54.4V (the lower end of the spectrum again with your higher temperatures in mind.

The voltages need to be compensated for the higher temperatures. I see Manila's average daily temperature is 30°C in May. The battery's datasheet gives a temperature compensation of 30mV/°C for bulk absorb and a 20mV/°C compensation for float. So at 30°C Bulk would be 0.6V lower at 57V and float would be 0.4V lower at  54V. At 35°C bulk would be 56.4V and float would be 53.6V

You can charge at up to 90A if you want to, since you have 3 strings of batteries and each string's maximum charge is 30A. Since you are charging OSO you are in fact limited to just under 60A. (3000W  of panels divided by battery voltage). With three strings I would use busbars for battery cables. Like the example below.

5835907b5621e_EqualResistanceperString2_wm.jpg.c13ea63451bfefd30e91fa5ae99eb009.jpg.917a8cc6becb183f2dc34c14cda67fba.jpg

Three strings would look like this

58870e46702c6_Screenshot(16).png.1cc521f668508d06f067da55db0158c0.png

I would also use SOL if it takes you batteries down to 85%. That gives you fair usage of your batteries without hammering them.

Now finally on to the BMV. Unfortunately each set of batteries is slightly different and it is just a matter of adjusting and seeing whether the SOC on the BMV is occurring at the appropriate time. Monitor how much charge your batteries are absorbing during float. With  a 300Ah battery bank I would say your batteries are full when the absorb about 150W or less. I have seen as little as 70W on my 260 Ah battery bank but that is after 2-3 hours on float. Look at BMV and get a feel of what constitutes 100% SOC. A charge of 3% or less of your Ah rating is considered by some to be 100% SOC (9A for a 300Ah battery bank). I find my batteries it is closer to 1%. If your BMV is showing 100% SOC too soon lower the tail current by 0.1% and observe for a couple of days. You may find for instance 2.8% is too short and 2.7% too long (as I did in my case). If so set to the higher value and then extend charge detection time.

I presume you are using the midpoint function of the BMV. Since you have a 48V system lower the alarm threshold (Program 47) to 1%. 2% is fine for a 24V system. Program 48 has to be then set to lower than program 47. I have mine set to 0.7%. The reason is that a given deviation will only register ½ the percentage deviation since the battery bank is operating at twice the voltage. For example a 0.5 V difference between the top and bottom of a 24V  system would translate to 2.1% deviation. The same 0.5V deviation which if it is the result of one battery is just as serious would only register as just over 1% deviation and would go unnoticed if program 47 is not adjusted. There is a detailed explanation on page 31 of the BMV 700/2 manual.

There is an awful lot to digest here so if anything is unclear just ask.     

 

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