Charge controller reading higher than set voltage

Freewilley · April 2018

Note that I have JUST installed 2 new AGM batteries. Full River 400 6 volt.
Of course I have adjusted charger settings to manufacturers specs

My new setup is showing voltages of 15.1 at the controllers, even though the absorb setting is 14.7, and the system is in absorb mode.
It is a nice sunny day.
I am guessing that the voltage is higher due to RTS adjustments....is that right?

It is COLD here in Northern Ontario. This is the coldest I have experienced since I went off grid.
The temperature at the batteries is 11 Celsius, which is 52 F.
I have RTS installed.

Advice?
thanks!

Estragon · April 2018

Yes, you should see higher voltage after RTS temp compensation with cool/cold bank. Setting are referenced to 25°C IIRC. Colder = increased voltage, warmer = lower.

Estragon · April 2018

Still waiting for ice breakup in my part of NW Ont. I'm not there now, but batteries are likely ~6°C.

scrubjaysnest · April 2018

In the 50 deg F range mine typical shows around 15.1 volts. Absorb setting is 14.8 @ 77 deg F and voltage increase is 0.003 volts per deg C. RTS is connected to one battery about 2 inches from the second battery.

BB. · April 2018

Is 14.7 volts for charging recommended for Fall River AGM batteries?

Sounds a bit high, but if recommended by the vendor, then ok.

Bill

Freewilley · April 2018

Note: Surrette AGMs are made by Full River, so the specs should be the same, I believe.

Yes, I found a document on the outback forum from the manufacturer. 14.7 for absorb, 13.6 for float.
I did see that on this forum there has been advice that conflicts with the manuf rep down below. I think you and Vic have had different thoughts at times.

Here is what she wrote in 2010. (the chart she refers to is no longer accessible).
That is a long time ago and I wonder if the info is up to date?

Also, she says that 50% discharge is 11.6 "under load". Which raises a couple of big questions..
1. What is LOAD? 4 amps or 40?
2. If the 11.6 corresponds to my traditional 50% discharge on lead acid flooded of 11.9, what is the fully charged resting voltage? Is it 12.8/9 or more like 12.6? Rereading the doc, I guess the answer in her view is that flooded is 12.75 and her AGM is 12.9.

I had lots of sun and lots of absorb time today and floated for at least an hour. I think the resting voltage was no more than 12.6, so I am wondering what is happening with mine. These batteries are year old and have sat a bit (from a dealer who took them in). I need a refresher on what open circuit voltage is....the CC reading without any draw? As I mentioned, that was 12.6 at the best reading...

Will

FROM: Christine Johnson, Fullriver Battery, +562-665-5312, cjohnson@fullriverdcbattery.com

1. If the 4 hour time limitation is for the Absorb mode only, it should be fine. I was thinking Bulk plus Absorb time combined. Sorry about the confusion on this. Let’s try the 4 hour timer and after a few days to a week confirm that the batteries are properly charged by checking the open circuit voltage, which should be between 51 Volts - 52 Volts.

2. Okay, now I’m going to teach you more about batteries than you probably want to know :-). The open circuit voltage of AGM batteries is higher than wet batteries. This is because the open circuit voltage is proportional to the specific gravity (the approximate formula is specific gravity + 0.845 = the open circuit voltage per cell). The specific gravity of a wet battery is typically 1.275 per cell and an AGM is typically 1.300 per cell. So in doing the math for a wet deep cycle battery: 1.275 + 0.845 = 2.12 volts per cell x 24 cells (in your 48V system) = 50.88 Volts and for an AGM deep cycle battery: 1.300 + 0.845 = 2.15 volts per cell x 24 cells = 51.6 Volts. So I hope this clears up the difference between open circuit voltage of wet versus AGM. The calculations you did for our batteries by adding 1.2V was pretty much right. I have attached our official chart of SOC versus OCV and as you can see your numbers are almost exactly the same once you multiply the 12V by 4 (see chart below).

3. If you want to limit the discharge to 50%, the voltage cut off under load should be about 11.6 volts per 12V (so 46.4 Volts for your system). This is of course an estimate as the voltage under load relationship to depth of discharge varies with discharge rate, temperature, age of batteries etc. But 46.4 Volts is a good estimate.

4. Finally I wanted to comment on our recommendation of 2.45 volts per cell on the absorption phase of charge. The forums said that others recommend 2.3 -2.4 vpc but that would be correct for Gel batteries, not AGM. Gel batteries are super-sensitive to over-charge and dry out quickly. AGM batteries need to be charged more conservatively than a wet battery but more aggressively than a Gel battery. I have much more concern about under-charging our batteries than over-charging. It is necessary to make sure you use the 2.45 vpc, especially when you charge with a profile that goes from an absorption into a float mode, opposed to the IUI type in our charging document.

+ + +

So, to summarize, here are the final settings I'll be using for the Fullriver batteries:

Absorb: 58.8 v
Absorb time: 4 hours
Float: 54.6 v

BB. · April 2018

Sounds good. Just make sure that they don't bubble or vent or get too hot when charging/near the end of the cycle.

-Bill "not a battery engineer" B.

jonr · April 2018

Not that OCV is almost the same at various temperatures. Hotter is slightly higher voltage, at perhaps .2mv/°C/cell.

Freewilley · May 2018

Hey BB...
What would "too hot" be?
Right now they are fully charged (I think) as they are drawing about 200 watts to float. The ambient temp is 15 celsius and the temp at the top of the batteries is 17 c. At what point would I say, oh, oh, these babies are too hot!
I am going to have to do some experimenting to determine if they are fully charged....temp would be one, I know.

Also, I get a rough idea of the state of charge by removing input power and running a 1000 watt heater for 20 seconds and reading the voltage....does anyone have a more precise method? Or tips?

BB. · May 2018

Nominally, worry if 110-120F or so (again, check battery specifications).

Also, look at the float current... Typically it should be less than 0.1% or so (or even much less) for an AGM battery, fully charged, in good shape. For most Lead Acid type of batteries, if continuous float current is >2% -- The battery bank should be replaced. At that point, the there is a (still small) chance of a major battery failure (explosion, fire, etc.). (or check float voltage is not too high and causing high float current). Note that many AGM batteries have catalyst somewhere in the top (or in the "sealed" battery cap). The tops of the batteries will get somewhat warm if there is significant charging current causing gassing (a little gasing is normal, continuous gassing wears out the catalysts and can cause early life venting/failure).

Your test to take run a heater for 20 seconds at 1,000 Watts should take off the surface charge. And if you do consistently (every few months) and log the results, it will probably let you know when things start to go wrong.

-Bill "again, not a battery engineer" B.

Freewilley · May 2018

I am not sure how to calculate that .1%...can someone give me the formula?

Freewilley · May 2018

I see this on a rolls doc

End Amps: If instead of time the current is used to determine the end of the absorb cycle, usually called ‘End Amps’, set it to around 2% of C20 (ie. for a 415 Ah battery bank that would be8 Amp as the End Amps setting, when the current falls below this value the batteries are full)

Is 8 amps the setting for my 12 volt system? That is about 100 watts, so I think it is...

BB. · May 2018

If you have a 400 ah @ 24 volt battery bank, a 0.1% rate of charge would be:

0.001 * 400 AH = 0.4 amp float current

Remember 0.1%, move decimal point 2 places to the left or 0.001

And we usually use 20 hour discharge rate for our calculations:

400 AH capacity / 20 hour rate = 20 amps (draw 20 amps to take battery bank from 100% state of charge to 0% rate of charge in 20 hours... Battery dead). You may find other discharge rates. Pick something close (like 24 rate), for the calculation.

Nominally we suggest that you only use 25% to 50% of your battery's capacity, and avoid ever using more that 80% of battery capacity for long battery life. Most rechargeable batteries don't like being taken to dead.

Bill

Freewilley · May 2018

Sorry, I am confused.
I have a 12 volt system (yes I know that I did that wrong).

I believe Surrette says the end amps should be 2%.
End Amps: If instead of time the current is used to determine the end of the absorb cycle, usually called ‘End Amps’, set it to around 2% of C20 (ie. for a 415 Ah battery bank that would be 8 Amp as the End Amps setting, when the current falls below this value the batteries are full)

Since I have a 415 ah bank at 12 volts, is that 4 amps setting for End Amps?

BB. · May 2018

Technically, bank voltage does not affect your programmed ending amps.

1% or 0.01 * 415 AH = 4.15 amps
2% or 0.02 * 415 AH = 8.30 amps

Of course, bank voltage * bank current = Power--So a higher voltage battery bank will require more "Watts" to achieve the same end amps on a higher voltage battery bank (larger solar array and/or larger genset, etc.).

-Bill

Estragon · May 2018

> @Freewilley said:
> Sorry, I am confused.
> I have a 12 volt system (yes I know that I did that wrong).
>
> I believe Surrette says the end amps should be 2%.
> End Amps: If instead of time the current is used to determine the end of the absorb cycle, usually called ‘End Amps’, set it to around 2% of C20 (ie. for a 415 Ah battery bank that would be 8 Amp as the End Amps setting, when the current falls below this value the batteries are full)
>
> Since I have a 415 ah bank at 12 volts, is that 4 amps setting for End Amps?

To clarify, there are two different numbers involved here; end amps (2% of 415ah = ~8a), and float amps (~.1% of 415ah= .4a).

End amps is the point at which you terminate absorb, after which current is stopped until voltage drops to Vfloat (13.65v per recommendation). Current used to keep the bank at float voltage is lower than what's needed at absorb voltage (absent loads).

Watching current in float can give a warning sign that all isn't well with the bank (ie it starts taking more current to hold at Vfloat).

Charge controller reading higher than set voltage

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