AGM Absorb voltage range and temp compensation

nickdearing88

Hi everyone.

My existing 12v "unbranded" inverter that I was scammed into buying before I knew what I was doing died this weekend and I figured it's the perfect time to upgrade to a 24v system. My MPPT charge controller can do 12/24 volt and my small DC USB converters are rated for both 12/24 volt so the only change required will be the 24v inverter (just ordered the Samlex SA-1500-24 from our hosts) and battery reconfiguration.

Since I just spent $550+ on the new inverter, I'm going to keep stretching out the life of my existing "Universal AGM" batteries for awhile. I have 2-100Ah connected in parallel for 12v/200Ah, so I'll be switching those to series for 24v/100Ah. As soon as possible, I will be upgrading the batteries and adding panels for a slightly larger bank. Despite these being cheap batteries, and very poorly treated for the first 2 years of ownership while I was learning, they still operate with about 70% of their rated capacity.

When I reconfigure the batteries in series, I'm planning to switch my Morningstar MPPT to custom battery settings so I can customize the charging voltages, float time-out, etc. However, I see in the Universal Battery specs, it lists the absorb and float voltages as a range:

Absorb: 14.5v - 14.9v, Float: 13.6v - 13.8v

I figured the absorb voltage range the min and max when using temp compensation (included on my controller) but I see in the SunXtender manual, it gives their range "at 25 degrees C so I'm wondering if my thought is incorrect.

Based on the above voltage ranges from the manufacturer, what setting should I use in my charge controller that also automatically adjusts for temp compensation? In my area, the batteries usual temperature would vary between 20 F and 80 F so I also wonder if the default temp comp setting of -30mV / 12 volt / ºC would be correct?

westbranch

I run mine at 28.8 (max) ABSORB and 27.2 Float.... as important as Voltage is Absorb TIME is equally important...  I use a min of 2 hrs in summer and 4 hrs in winter , never gets to float till early March at best...

jonr

Your two batteries were in parallel and probably not perfectly balanced.  So they have different amounts of wear and may not balance well in series.  So I'd check voltage balance and if it's not right on, add a balancer (like the HA-01).

I'd use a good DMM to verify charging voltages and also to verify current at end of absorb.

Marc Kurth

First - Don't use the Concorde Sun Xtender or Lifeline manual for other batteries - virtually everything is very different than other AGM's. Often, radically different to the point of battery destruction. I would be happy to give you real examples if interested.

The range that you quoted is not the temperature compensated range. It is the permissible voltage range at a nominal 25C.
You should use the temperature compensation curve specified by UPG. Sorry, but I have never dealt with Universal so I don't know their details. If you can't find it, at least use the default until you do.

I can't even count how many AGM battery banks I have seen destroyed with the customer saying: "I had it set to AGM, so I know it was charged right......."     Their battery dealer didn't bother to teach them. Early battery failures are rare for us, because I educate the customer.

Marc Kurth

nickdearing88

Marc Kurth said:

First - Don't use the Concorde Sun Xtender or Lifeline manual for other batteries - virtually everything is very different than other AGM's. Often, radically different to the point of battery destruction. I would be happy to give you real examples if interested.

The range that you quoted is not the temperature compensated range. It is the permissible voltage range at a nominal 25C.
You should use the temperature compensation curve specified by UPG. Sorry, but I have never dealt with Universal so I don't know their details. If you can't find it, at least use the default until you do.

I can't even count how many AGM battery banks I have seen destroyed with the customer saying: "I had it set to AGM, so I know it was charged right......."     Their battery dealer didn't bother to teach them. Early battery failures are rare for us, because I educate the customer.

Marc Kurth

Right, I agree. The voltages I posted are from the UPG specs but I just noticed the "range at 25C" remark in the Concorde manual.

UPG has quite detailed specs: https://www.solar-electric.com/lib/wind-sun/45978.pdf but no remarks about temp compensation so I guess I'll stick with the default -30mV / 12 volt / ºC. Maybe I'll shoot them an email to ask for sure.

jonr

I looked at a few AGMs and most were in the range 3-4mv per cell per C.  So without an actual number, maybe 18-24mV for 12V.

One said 3.3mv for standby, 5.0mv for cycle use.

nickdearing88

jonr said:

I looked at a few AGMs and most were in the range 3-4mv per cell per C.  So without an actual number, maybe 18-24mV for 12V.

I emailed them but haven't heard back yet. I agree that 18-24mV/12v/C sounds more in-range. I think I'll go with that unless I hear differently from the manufacturer. Thanks everyone!

BB.

Don't forget the minus "-" sign.

-20 mV /12 /C

- Bill