Any value in occasionally deep-discharging AGMs that are otherwise in float service?

[BackwoodsEE]

I have an Outback Radian GS8048A inverter/charger connected to eight Rolls-Surrette S6-460 AGM batteries in series. It is a hybrid setup that operates in grid-tied mode but can continue powering my critical loads panel for a while if the grid goes down. So far it's working great, maintaining my batteries at float voltage with less than an amp of average charge current while inverting the rest of any DC current provided by my two charge controllers. Any solar power not consumed by the loads on the critical loads panel gets backfed to the main panel to run my other loads (water heater, stove, washer and dryer) and, if there is still an excess, back to the grid where my shiny new net meter will count down instead of up.

While the grid is available--the vast majority of the time--the batteries stay at float voltage with only minor deviations. My question is whether I should be letting them discharge occasionally and then recharging them with a bulk and absorb cycle. I can initiate this just by switching off the 50A breaker for the Radian at the main panel and then turning it back on after my FlexNet battery monitor reports SOC getting down to 50%. (It's a Coulomb counter that should follow a single discharge down from 100%/float SOC accurately enough.) If the sun is shining during the day, it might take me a few days to get down that far. For recharging, the Radian's charger will kick in once AC voltage is detected from the grid and selling AC won't commence until the charger has gone through its full bulk-absorption cycle, possibly with help from the solar charge controllers.

I've read comments from various anonymous Internet experts online about "exercising" deep-cycle batteries but am a bit skeptical about that. Seems to me that keeping a lead-acid battery at its float voltage should present the least wear and tear of any other possible usage. Any thoughts from members of this august group? Sunking ?

[Sunking]

My ears are burning.

I think I can help you out a bit. Both FLA and AGM need cycled, and or topped off, just different time frames. Your question is a moot point in a cycled application right? Think about that. They get cycled everyday or quite regularly. So let's skip cycle applications and focus on your application, Emergency Stand-By where the battery stays in Float Service. Make sense?

FLA batteries require a Bulk/Absorb charge every 6-months to prevent build-up of sulfation, AGM's once a year because they are less prone to sulfation build-up. That is it, simple KISS.

Now if you want to throw in a deep discharge for your FLA or AGM battery before you top them off, then discharge FLA to 50%, and AGM to 80% DOD. Since they are rarely used a good deep discharge does not harm anything and just confirms capacity loss. That is the meat on the bone, capacity.

Now if we are talking Storage without a charger. Top FLA's once a month, and AGM's every 6-months. FLA have considerable higher self discharge rates than AGM. Thus is why FLA needs once a month vs 6 months for AGM while in storage. Again not your application, but it ties in for the same reasons.

So once a year when you are board, in the mood to watch paint dry and grass grow, discharge your AGM's to 50% DOD and measure Amp Hours. That way you will know what your capacity really is and can track it. When you get to 80% of rated capacity, they are toast and need replaced. That may sound silly but here are the facts. It may take 4 years for your batteries to degrade to 80% capacity. Once that point is reached, it is a very fast ride to dead. We are talking months, not years any more.

Hope that helps.

[Mike90250]

thanks sunking. I'm still hunt & peck with left hand, till my right grows back together, week 4 and it's doing OK

[Sunking]

Mike get well soon. You have been in my nightly prayers. Try holding onto cold aluminum cans.

[BackwoodsEE]

Thanks. I think what I'll do is an annual or semi-annual discharge to where the Coulomb counter calls it 50% SOC, then disconnect the battery for 12 hours or so, and then measure the open circuit voltage. Then reconnect and let the battery do a full bulk-absorb recharge.

When the open circuit voltage gets significantly lower at the point where 50% of my battery's rated Ah has been discharged, I'll know it's time to do reverse alchemy again and convert gold into lead.

[Sunking]

Backwoods I think I gota tip you may want to consider. I know you have the skills and equipment to do this. Best of all it gives you some real data to determine battery health. You can look inside so to speak. What I am talking about is the battery Internal Resistance (Ri) using the Delta Voltage/Current measurement. Ri will tell you when your battery is toast. When Ri doubles, you are at 80% rated capacity and time for new batteries.

Really simple to do. With the batteries charged and rested (room temp), apply a light load like 1 to 5 amps and measure the voltage on the battery Temp Post. Example you measure 6.3 volts @ 2 amps. Call the voltage V1 and current I1.

Now repeat the same test except now you as much current as you can get, say 200 amps. The higher the current, the more accurate result you get. So lets say you get 200 amps @ 6.0 volts. Call the voltage V2, and current I2.

Ri = (V1 - V2) / (I2 - I1)

Ri = (6.3 - 6) / (200 - 2)

Ri = 0.3 / 198

Ri = .0015 Ohms

It works for any kind of battery, and Ri will tell you a whole lot about the battery. Example how much current can you take and keep voltage sag to 3% or less. .03 x 6.3 = .189 volts. So .189 volts / .0015 Ohms = 126 amps. I am not making these numbers up. They are your real number, look at the cut sheet. Ri = 1.5 milliohms.

You can also use the same concept to measure the whole plant resistance including cables. If the batteries are 1.5 x 8 batteries = 12 milliohms. Cable connection resistance should be 25 micro-ohms of less and there would be 20 connection x 25 micro-ohms = 500 micro-Ohms = 12 = 12.5 milli-ohms. total resistance. Makes it real easy to know if you have a problem and how to find it.