AGM batteries and voltages

Les Nagy · November 2010

Hello all.

After dealing batteries sold fraudulently as new fresh stock batteries for 8 months, I finally got my order delivered from a US supplier. The new batteries are UPGI UB8D.

http://upgi.com/Themes/leanandgreen/images/UPG/ProductDownloads/45964.pdf
http://upgi.com/Themes/leanandgreen/images/UPG/ProductImages/45964.jpg

As you can see from the specs PDF, the voltages for charging are:
Bulk/Absorb 14.5-14.9
Float 13.6-13.8

With the wide range recommended in the PDF, I emailed UPGI and asked for their recommendation for the voltages. One of the engineers responded and told me he would use 14.8 for Bulk/Absorb, 13.8 for Float, and -20mv/°C. I have been using these recommendations on my Xantrex XW-MPPT60-150 for just over a month now.

I have no reason to doubt UPGI and their advice, but I have noticed a couple of things that make me think I might want to drop these voltages a bit.

1. I have noticed a very slight hydrogen sulfide smell in the area of the batteries.

2. I have also noticed a very light bubbling sound (or what I think I can hear) coming from the batteries during the last stages of the charge cycle.

I assume that these are signs that the voltage might be too high. Am I correct?

Thanks for any feedback.

Les Nagy · November 2010

Re: AGM batteries and voltages

I just heard back from the engineer at UPGI regarding my concerns and he agreed that the voltages should be dropped. He suggested that I drop to 14.6 volts for Bulk/Absorb. He made no mention of changing the float voltage but I also dropped it to 13.6 per battery.

I would still like to hear from others on this.

BB. · November 2010

Re: AGM batteries and voltages

Yea, probably dropping to 14.5 or 14.6 is a bit better. Anything near 13.6 or 13.7 is probably fine.

Have you had a chance to measure your specific gravity of your battery bank at the end of the charging day? And the next morning?

Getting the battery above 90% state of charge every day or every couple of days is probably a good start. I would not try to get it to 100% charge every day. That is running near to over equalization where excessive charging causes plate shedding and oxygen corrosion of the positive grid (oxygen formation from electrolysis).

Play with the Absorb voltage and absorb timer (1-3 hours or so?) to get the battery > 90% at the end of a sunny day with light loads.

It is approaching summer for you--so a shorter absorb time is probably appropriate as you have lots of sun... When you head into winter--a longer time is probably a good idea.

Here is a nice thread on battery charging (one of many).

Another issue that I am reminded from that thread--Battery Temperature... Basically, for every 10C rise in battery temperature, about 1/2 the life is lost. (2OC over 25C standard, 1/2 * 1/2 = 1/4 the life--and conversely, keep batteries below 25C will help give them a longer life).

So, keeping the batteries on the cool side on a hot day will be helpful too.

-Bill

Les Nagy · November 2010

Re: AGM batteries and voltages

Bill:

Thanks for the reply, but I think you kissed a couple of details. I have AGM batteries. It is not very easy to measure specific gravity in AGMs.:D

The batteries do get above 90% every day already.

And I have plans to insulate my container. As it is now they get as high as 27°C.

BB. · November 2010

Re: AGM batteries and voltages

Forgot the agm batteries--measuring the resting voltage and using a battery monitor.

Check the battery caps. Usually, there is a small metal catalyst in the cap. If that area is getting too hot--then you should dial down the absorb voltage/time. As the batteries age, if some caps stay cool then the catalyst has failed and it may be time to find replacement caps. (not cheap)

On person with catalyst caps used the heating of the caps (generating gas) to indicate the battery is fully charged.

-Bill

Les Nagy · November 2010

Re: AGM batteries and voltages

I did not know about the catalyst getting warm thing. My batteries do not have caps, but rather a large plate glued over the top. See this image:
http://upgi.com/Themes/leanandgreen/images/UPG/ProductImages/45964.jpg

I assume the catalysts are under the plastic top.

BB. · November 2010

Re: AGM batteries and voltages

Too bad--I could not find any technical drawings of their batteries.

-Bill

niel · November 2010

Re: AGM batteries and voltages

you might try going to 14.5v bulk and 13.6v float. stay at the bottom end of their specs and see if things are ok there. to hear anything on an agm is not a good sign. i do find it odd that you said you heard the noises when it was on the tail end of the charge. let us know if you still get any of these symptoms yet on the lower voltage settings.

RCinFLA · November 2010

Re: AGM batteries and voltages

Watch the absorb time. The default on Xantrex MPPT controller is 180 mins. You can also back down the default -27 mV/degC temp comp setting. Make sure equalize setting is off.

AGM's should not require more then 45-60 mins of absorb. If you are smelling outgassing then it is running too long.

Outgassing on AGM's is a no-no as losses cannot be replaced. Repetative outgassing can cause the pressure vent to become leaky allowing even more outgassing.

Les Nagy · November 2010

Re: AGM batteries and voltages

An interesting thing happened today. I used the batteries down to around 80% last night and then went to bed. When I checked things around 13:00, the Tri-Metric was showing 90% charged, and the XW CC was still in bulk mode with the batteries sitting at around 54.8 volts. They never got higher and the Tri-Metric measured a full recharge.

Part of the cause of this was probably the high usage of power during the day which kept the available current to the batteries on the low side. But I did not expect there to have been sufficient current for the Tri-Metric to show a full charge while the system never came out of bulk.

Opinions?

mike95490 · November 2010

Re: AGM batteries and voltages

Les Nagy wrote: »

An interesting thing happened today. I used the batteries down to around 80% last night and then went to bed. When I checked things around 13:00, the Tri-Metric was showing 90% charged, and the XW CC was still in bulk mode with the batteries sitting at around 54.8 volts. They never got higher and the Tri-Metric measured a full recharge.

Part of the cause of this was probably the high usage of power during the day which kept the available current to the batteries on the low side. But I did not expect there to have been sufficient current for the Tri-Metric to show a full charge while the system never came out of bulk.

Opinions?

Your loads are on the wrong side of the shunt?

Les Nagy · November 2010

Re: AGM batteries and voltages

No, the wiring is fine. But I just remembered that I had changed the efficiency of charge from 94% to 95% in the Tri-Metric, that could be it. It could also be that I missed a seeing a transient load making the batteries drop below the 52V I have set in the charge controller to trigger a new charge cycle.

hillbilly · November 2010

Re: AGM batteries and voltages

Les Nagy wrote: »

An interesting thing happened today. I used the batteries down to around 80% last night and then went to bed. When I checked things around 13:00, the Tri-Metric was showing 90% charged, and the XW CC was still in bulk mode with the batteries sitting at around 54.8 volts. They never got higher and the Tri-Metric measured a full recharge.

Part of the cause of this was probably the high usage of power during the day which kept the available current to the batteries on the low side. But I did not expect there to have been sufficient current for the Tri-Metric to show a full charge while the system never came out of bulk.

Opinions?

Just wondering what you mean by the tri-metric showing a "full charge", do you mean just the AH counter/ %full reading? I wouldn't trust that reading entirely all by itself, it's really more meaningful as a way of having a quick glance at the general battery level when discharging or during bulk charging.

Les Nagy · November 2010

Re: AGM batteries and voltages

hillbilly wrote: »

Just wondering what you mean by the tri-metric showing a "full charge", do you mean just the AH counter/ %full reading? I wouldn't trust that reading entirely all by itself, it's really more meaningful as a way of having a quick glance at the general battery level when discharging or during bulk charging.

Yes, I understand that the Tri-Metric is not the ultimate authority on the state of the state of charge. It indicated 100% charge and only 10-20 amps left to full charge. This was big enough of a discrepancy from the state of the XW-MPPT60 that it made me wonder how this could be.

The voltage curve behaved as it should for full batteries during use.

I guess it is possible to pump enough current into the batteries during the day even if they don't get out of bulk to almost fully recharge them.

hillbilly · November 2010

Re: AGM batteries and voltages

I think that it is way better at telling you that the batt's have gotten a "full charge" if you set the charged parameters for the voltage and current levels. Check the days since full charge reading and rely on that, rather than the %SOC reading, to determine if the batteries got a full charge on any given day. Otherwise you can easily put in enough AH to add up to show a high %SOC reading without ever getting the batteries to a high enough voltage; especially if you have a lot of on/off loads during the charging process.
The other thing you can do if you wish to be able to verify later on just how high the voltage got on any given day is to reset the voltage high reading, then check it later and it will show you the highest voltage that was reached since being reset. Kind of a handy thing sometimes.

Les Nagy · November 2010

Re: AGM batteries and voltages

The batteries were still making some light bubbling noises even with the lowest recommended voltages as stated by UPGI. I have now set the charger to AGM battery type from custom with the following defaults:

Bulk: 57.2
Absorb: 57.2
Float: 53.6
Comp: -84mV/°C

I will update with results tomorrow.

niel · November 2010

Re: AGM batteries and voltages

"Comp: -84mV/°C"

that isn't right for a 48v bank. at 4mv/cell/degree c and 24 cells for a 48v bank this is 96mv. be careful that it's not looking for that, but the per cell mv which is 4mv.

Les Nagy · November 2010

Re: AGM batteries and voltages

niel wrote: »

"Comp: -84mV/°C"

that isn't right for a 48v bank. at 4mv/cell/degree c and 24 cells for a 48v bank this is 96mv. be careful that it's not looking for that, but the per cell mv which is 4mv.

The recommended value from UPGI is -20mV/°C per battery, for their AGM batteries. The default value for AGM batteries in the Xantrex chargers is -21mV/°C. I see that Concorde recommends close to the -24 mV/°C per battery that you state.

I really hope I can get a concrete answer from the manufacturer. It seems that there are quite few discrepancies.

Les Nagy · November 2010

Re: AGM batteries and voltages

After a few days with the standard AGM settings in the XW MPPT60-150, the batteries are operating perfectly an without any overcharging symptoms such as noise or odor. A typical day for the batteries now sees them getting down to no ess than 49,4 volts, and a half day of bulk with about 90 minutes more of absorb and then down to float.

At this point I could not be happier with the Universal Batteries AGM UB-8D. All that remains to be seen is if they last 10 years as is possible with AGM batteries that are never run run down below 70% charge,

niel · November 2010

Re: AGM batteries and voltages

my sunxtender is 7+ years now. the lifespan will depend on many factors, but i believe 10yrs can be had by your batteries.

RCinFLA · November 2010

Re: AGM batteries and voltages

The Trimeter 2020 has two register values that determine when the "full", 100%, charge state is reset.

There is a voltage register, that if not changed, is factory default of 57.6 vdc. This should represent slightly below the absorb voltage level.

There is current taper register, that is factory default of "OFF", meaning only the voltage level is used to reset to 100%.

This current value represents the lower end of current taper off during absorb phase of charging. It should be set to about 10% of the bulk charging current or not less then 2%-3% of A-H rate of battery. The value of this setting needs to take into consideration the available bulk current (phase 1) charging current used. It is default set to 'OFF' because in case of solar charging, with slow charging, may never meet a minimum current taper off number.

Both the voltage and current values must be met for greater then 20 seconds to cause unit to reset to 'full'.

Resetting only on Absorb voltage will likely prematurely reset the 'full' point unless a slow charge current is used. If your charge controller has a absorb current complete setting (where it drops to float voltage). then Trimetric should be set to a slightly higher value then charge controllers absorb complete current.

For the % full reading, there is a charge efficiency register that is default set to 94%, representing the derating put on charge current compared to discharge current. 94% is probably a good number for AGM's.

The % of full calculation is like 'dead reckoning navigation'. Without a reference point once in a while the errors will accumulate over time. The 100% full reset is this reference point.

Les Nagy · December 2010

Re: AGM batteries and voltages

RCinFLA wrote: »

The Trimeter 2020 has two register values that determine when the "full", 100%, charge state is reset.

There is a voltage register, that if not changed, is factory default of 57.6 vdc. This should represent slightly below the absorb voltage level.

I have mine set to 53.0 vdc.

RCinFLA wrote: »

There is current taper register, that is factory default of "OFF", meaning only the voltage level is used to reset to 100%.

This current value represents the lower end of current taper off during absorb phase of charging. It should be set to about 10% of the bulk charging current or not less then 2%-3% of A-H rate of battery. The value of this setting needs to take into consideration the available bulk current (phase 1) charging current used. It is default set to 'OFF' because in case of solar charging, with slow charging, may never meet a minimum current taper off number.

I have mine set to 8.0 amps. This way when the charger is in float mode and the current drops below 8 amps the 2020 has a good indication that the battery is full.

RCinFLA wrote: »

For the % full reading, there is a charge efficiency register that is default set to 94%, representing the derating put on charge current compared to discharge current. 94% is probably a good number for AGM's.

The % full calculation is like 'dead reckoning navigation' without a reference point once in a while the errors will accumulate over time. The 100% full reset is this reference point.

I have played with this efficiency setting and 94% does work out to be the closest. Charging efficiency varies because of many factors and varies during the different phases of charging. As you said, it is the least accurate display on the Tri-Metric.

I only wish that the XW series equipment were more accurate in the lower ranges of power use and charging.

AGM batteries and voltages

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