Harm to my AGM Batteries with my max charge set point of my system

dagelt · February 2012

Hello again,

I have a 48 volt system and use AGM batteries and because of different factors of my system I run by max charge voltage at 53.6 volts most of the time. I would say once a month or 2, I let my solar charge controllers work there magic of raising the volts to 59 volts, going into absorption and then to float. then I turn my sell back on and 53.6 volts and away I go selling to the Grid.

I know this is the off grid part of the forum but you guys are the only ones that deal with batteries so that is why I posted it here.

Does anyone have much experience with AGM batteries and what harm I may be doing to them. I know this is very bad for flooded cell batteries but how hard is it on AGM's?

I will give more reasons of why I use 53.6 volts if anyone wants to know but I did not want to waste peoples time if I did not need to.

Thanks in advance, you guys have helped me before and I am looking for some more information before I totally trash my batteries.

Cariboocoot · February 2012

Re: Harm to my AGM Batteries with my max charge set point of my system

Don't take my word for it, but it sounds like you are habitually undercharging your batteries. Normally you'd want to hit 56.8 Volts for Absorb, and do it every day. Fully charging them only once a month is bound to shorten their lifespan, AGM or FLA. Keeping them in Float at about 55.2 Volts would be better.

I think you're slicing years off their potential life, but that's just my opinion and no one has to agree with me.

niel · February 2012

Re: Harm to my AGM Batteries with my max charge set point of my system

what exactly you may or may not be doing to them is depending on the manufacturers specs and charging parameters for your batteries. 59v if for a sunxtender would be too much as this works out to be 14.75v per 12v battery with no temp compensation factored in. sunxtender requirements state 14.2v-14.4v. if you are exceeding the max charge voltage speced for your battery you are popping open the vent and losing water that you can't replace. in the case of sunxtender it would be 4 x 14.4v = 57.6v maximally with 56.8v minimally. the eq i would not do unless the batteries are old and visibly measured to be too far off from one another. this is a near end of useful life thing imho and if newer they should be physically rearranged to allow low to replace high reading positions.

do not exceed the normal charging voltages for your batteries and use a battery temp sensor when applicable to compensate for battery temperatures that affect charge voltages.

the practice of floating the bank while selling the excess and giving a charge once a month is fine to do as most agms hold their charge well even after a few months. it may be good to use the batteries discharging down to at least to 85%-90% soc every few months too to exercise them and to be sure all is working well. like i said it depends on what battery you are actually using and what it requires for proper operation and that applies to the float voltage too.

Thomas · February 2012

Re: Harm to my AGM Batteries with my max charge set point of my system

I have heard a lot about being careful with batteries, what I have gathered is that these AGM batteries run golf carts cars, some portions of power grids, lawn mowers, washers , driers and a whole host of other real heavy duty daily stuff that requires indistrial speed charging from the grid. I think that for solar/wind application there is no better choice. I do get scared of the fact that water does leave the battery and cannot be replaced. There are varying numbers about that. I would like to know how much AGM s can be pushed before deteriorating overall longevity/performance.

niel · February 2012

Re: Harm to my AGM Batteries with my max charge set point of my system

if it pops the vents and loses water then you pushed too far. the more it loses the water the less capacity and battery life you will have. keeping within the max voltage specs manufacturers place on them is important for long life. other than this, and maybe a bit less tolerant to cold temps than fla types from my research, agms are otherwise far superior to fla types.

RCinFLA · February 2012

Re: Harm to my AGM Batteries with my max charge set point of my system

I think you are doing just about the exactly right thing.

The float setting might be a little low for AGM but, if you are selling, the batteries are being subjected to ripple current that will prevent any negative plate sulfation from lower then normal float levels. You are also avoiding excessive lead oxide build up on positive plates which is reversable but raises battery internal resistance which you don't want since it is the filter capacitor for your PV DC injection to single phase AC selling of power from inverter which is the source of the battery ripple current.

The once or twice a month taking to absorb level is no significant stress on battery. My system does this whenever there is flicker interruption in grid power which occurs in my area several times a month or more.

Thomas · February 2012

Re: Harm to my AGM Batteries with my max charge set point of my system

Yeh they don't like to work cold at all. I have heard some about Equilibrialize charging AGM, mostly do not. My set points are within factory specs. The EQ charge is +1v for 2 hours. Can I perform the operation safely? can I reduce the voltage by 1v and perform the operation on multiple batteries monthly?

How can I tell if the batteries are loosing water?

dagelt · February 2012

Re: Harm to my AGM Batteries with my max charge set point of my system

Thanks for the feed back everyone,

I use my system like an off grid system really with the plus of "NET metering". I use the batteries at night and charge them back up to 53.6 the next day because that is where my sell is set. It hits 53.6 at around 12pm and then it starts to sell. As the batteries charge the rest of the afternoon the amps that are needed to charge the batteries goes down and I sell more. So the batteries are held at this voltage but the amps are high at 12pm and as the day goes on it goes down.. kind of like a charge controller.

Again I was just wondering if the low voltage (53.6) was bad or good?

I have done some checking/testing on the weekends and if I turn off sell at around 3pm (this time of year) it almost always goes into float mode immediately. So I am guessing that the batteries are charged? I just did not know for sure what the low voltage (53.6) was doing to the batteries?

The reason I have to set the voltage to 53.6 is because of my wind turbines, if they go overvoltage it will not reset (start charging again) until the voltage is back down below 54 volts. So if I want to get any use out of my wind turbines I have to set my sell to 53.6 so they will turn back on.

Again thanks for the feedback and I am willing to learn more from you all.

RCinFLA · February 2012

Re: Harm to my AGM Batteries with my max charge set point of my system

dagelt wrote: »

I use the batteries at night and charge them back up to 53.6 the next day because that is where my sell is set.

Just use the grid when its available, including at night. The wear on batteries discharging them at night will result in sooner battery replacement. The amount of grid electric cost saved will not outweight what you will end up spending on replacing batteries more often.

BB. · February 2012

Re: Harm to my AGM Batteries with my max charge set point of my system

There is also something to watch when running GT with a Hybrid inverter... The inverter's DC input current is not "DC", but a 120 Hz series of current "hits" against the battery. You want to ensure that the during the current draw (i.e., AC sine wave approaching peak voltage/current), that the battery is not being drawn down below 12.7 VDC (50.8 volts)--Or you are discharging and recharging the battery a 120 times a second.

-Bill

niel · February 2012

Re: Harm to my AGM Batteries with my max charge set point of my system

RCinFLA wrote: »

Just use the grid when its available, including at night. The wear on batteries discharging them at night will result in sooner battery replacement. The amount of grid electric cost saved will not outweight what you will end up spending on replacing batteries more often.

i agree here with rc and only cycle the batteries once every month or 2 just to flex them and be sure all is functional.

dagelt · February 2012

Re: Harm to my AGM Batteries with my max charge set point of my system

Thanks for everyone's response, I just got my local COOP power company to do true NET metering ( 1 for 1) so I will be switching the way I do run my system but for the last 18 months I have been running like an off grid system. Thanks for the feedback on cycling the batteries each month to exercise them... I may not of thought of that.

Thanks again!

RCinFLA · February 2012

Re: Harm to my AGM Batteries with my max charge set point of my system

The cycling of the batteries once in a while helps to clear off excessive lead oxide build up on positive plates. Only needs to be 10%- 15% of rated AH used, (85-90% SOC) to do it.

The ripple current results from the single phase AC put out by inverter. Its current profile drawn from battery node is sine squared shaped (looks like full wave rectified waveform).

For 6 kW's from PV controller, the current delivered to battery node is nearly a pure constant 113 amps at 53 vdc. 6 kW's from battery to single phase inverter has a peak current of 160 amps and minimum near zero at the point of AC output voltage is zero crossing. When the inverter needs the peak 160 amps, the difference from the 113 amps, equals 47 amps, must come from battery. It is replaced into battery during the period when AC output voltage is near its minimum, zero crossing point in the sinewave. Average battery goes-in versus goes-out current averages near zero. Bottom line there is 47 + 113 amps, equals 160 amps peak to peak ripple current, going in and out of batteries, 120 times a second.

The batteries have to be big enought, AH wise, to take this ripple current without putting excessive stress on them. Our rule of thumb is about 100 AH minimum battery size for a 48 v system for each 1 kW of PV controller peak power injection. You may want larger AH capacity for off grid power/run time needs. With your approximately 1000 AH battery capacity you should be in good shape to take in 6kW's of solar power.

dagelt · February 2012

Re: Harm to my AGM Batteries with my max charge set point of my system

Ok RCinFLA,
So if I am understanding everything correctly you are telling me my battery bank is the proper size for my new net metering situation and leaving my inverter set at 53.6 volts and selling anything above it will work fine?

But once a month I should flex my batteries down to around 85% to 90% SOC (10 to 15% DOD) to keep them exercised and in good working condition.

and just use the grid at night since I am able to net meter (get 1 for 1 starting next month)

If I could have lobbied hard enough 18 month ago I would not have installed my battery bank but they were not going to budge.... now I have a lot of extra expense for no reason!!! Well not as much reason....... I guess when my power goes out I will have them to run off of.

again thanks for the advice!

Harm to my AGM Batteries with my max charge set point of my system

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