AGM battery Lowest voltage

kazpsp · February 2020

Hi guys

I recently installed my first on-off grid solar system in my apartment, it has been working great altho I think I could optimize it a bit for power savings.

My setup is as follows:
4KvA Axpert inverter
12 - 12v AGM batteries (3 series of 48v)
15 - 300W solar panels

My question for now is how low can I discharge my batteries before switching to the grid.

I currently have it set to 47v on the inverter and once it switches to the grid it bounces back to 48v when the load is off.

Can i go lower than that? or is that the sweet spot

Thanks all

mcgivor · February 2020

The voltage would largely dependent on loads, and battery capacity,. Setting it too high may cause the voltage to sag momentarily when a larger load such as a motor starting for example, the use of hysteresis will prevent this to a large degree.
Without knowing the battery capacity or loads it is not possible to quote a definitive value, some experimenting to see what the resting voltage is after loads are disconnect would be of more value as this would reveal more accurately the true state of charge.

kazpsp · February 2020

Sorry I missed some data.

Each battery has 114aH Capacity and my consumption is pretty steady through the whole day, 1.5Kva (1 small air conditioner some led lighting and a couple of computers/tvs)

currently the resting voltage is sitting at 48v

Thanks

mcgivor · February 2020

The resting voltage should be taken at around 4-6 hours after the loads are disconnected, if it's taken immediately the reading would be lower, 48V would be somewhere around the 50% state of charge if taken immediately. This is probably the most cost effective use of a deep cycle battery from an energy cycle point of view, going lower would reduce the life expectancy.

The thing to remember is if they are discharged to this level it's important that they receive a full charge as soon as possible, keeping them in a partial state of charge will lead to sulfation and premature failure. Using batteries is in most cases significantly more expensive than grid power, if the system is a backup UPS type, it makes sense to cycle them due to the fact that if you're not using them, you're loosing them, they do die within a given number of years even if never cycled.

Being unsure what you are attempting to achieve, using the solar array to support loads as much as possible, would make the most sense, this is a ballancing act. Speaking of ballancing acts, having 3 parallel strings of batteries can lead to charging/dischargeing imbalances especially if connected incorrectly, having a DC clamp on ammeter would be a valuable tool to have.

One consideration would be to cycle the batteries every second day this would allow them to recover fully at least 50% of the time, just a thought as there is limited information.

Marc Kurth · February 2020

Depending upon the battery brand, your battery bank may seriously degrade in about 1.5 years when cycled to 50% daily. Lower cost AGM's will provide around 500 charge/discharge cycles at 50% DOD - under ideal conditions. I use 70% of the factory cycle ratings for real-life systems, Even the best AGMs will only go out to 2-3 times that under the same conditions.

In my opinion, when a system requires daily cycling of AGM batteries, the battery bank needs to be sized for shallow cycles in order to get out to 7-10 years. Any system that I am involved in sizing, is based on a 20% to 35% average daily depth of discharge, depending upon several other details.

Additionally, most AGM designs NEED a long float time to stay in balance.

Marc

softdown · February 2020

Marc Kurth said:

Depending upon the battery brand, your battery bank may seriously degrade in about 1.5 years when cycled to 50% daily. Lower cost AGM's will provide around 500 charge/discharge cycles at 50% DOD - under ideal conditions. I use 70% of the factory cycle ratings for real-life systems, Even the best AGMs will only go out to 2-3 times that under the same conditions.
In my opinion, when a system requires daily cycling of AGM batteries, the battery bank needs to be sized for shallow cycles in order to get out to 7-10 years. Any system that I am involved in sizing, is based on a 20% to 35% average daily depth of discharge, depending upon several other details.
Additionally, most AGM designs NEED a long float time to stay in balance.
Marc

What do you like for absorb? I'm using 3 hours here. Float starts ~noonish with decent weather. Can vary significantly with winter clouds.

Turns out my darned router used significant energy. It sends power to the "parabolic" dish on the roof. Ever heard of that?

Marc Kurth · February 2020

For most AGM designs, I am a strong proponent of staying in Absorb until end amps are reached, with a limiting timer overlayed.

Concorde Sun Xtender specifies Less Than 0.5% of the C/24 capacity at the Absorb voltage. (0.5 amps per 100)

They offer these absorb times "as a starting point."

30% or less DOD = 2 hours

30% to 50% DOD = 3 hours

50% or deeper = 4 hours

In practice, Fullriver exhibits the same extremely low current draw at 100% SOC, but I have never gotten a firm statement in writing from them.

Marc

mike95490 · February 2020

softdown said:

What do you like for absorb? I'm using 3 hours here. Float starts ~noonish with decent weather. Can vary significantly with winter clouds.

Turns out my darned router used significant energy. It sends power to the "parabolic" dish on the roof. Ever heard of that?

You cannot simply set a timer for AGM. You have to either measure amps in/out and add 10% to replenish the battery, or measure end amps to determine when they are full. It's real easy to overcharge AGM and ruin them in weeks.

All sat dish antennas have a LNA at the focal point and it needs power, supplied via the coax cable. Often the modem consumes 50-75 watts when turned on. Some LNA's have a heater to keep the parts at a stable temperature for better performance.,

Marc Kurth · February 2020

Mike,

I agree that a "timer only" will most often either undercharge or overcharge batteries. I would like to add some comments to yours.

In my experience with the AGM batteries that I deal with, an hour or so of extra Absorb time each day takes years to cause any real drying out. In fact, I would rather see an hour of excess absorb as opposed to dropping into Float too early each day. Bumping up against the "end amps" of .003C to .005C is a gentle overcharge.

I raise this only because with the internet megaphone, people have become so afraid of overcharging their AGM batteries, that they go too far toward undercharging. Using broad brush strokes, sulfation from undercharging will ruin our AGMs much, much faster than a little extra Absorb time.

I will respectfully suggest that the right way is "end amps" with a safety timer overlayed.

Counting amps in/out will develop compounding errors because battery efficiency varies. In the case of the products that I deal with, between 90% and 98% depending upon the cycle depth, temperature, charge rate, plate size, and age. I have seen too battery banks destroyed because a constant efficiency value was used to determine the correct absorb time. Envision the excess time in Absorb, when a controller is trying to shove another 2% to 8% into a battery bank that is only drawing 0.3%C to 0.5%C because the battery bank is already fully charged. This example is about the only time that I see overcharging.

It works both ways of course!

Marc

softdown · February 2020

I'm not going to try to outsmart the electronics in the FM80. Modified the standard parameters and I watch the voltage a few dozen times/day. Perhaps I should go from 3 hours absorb to 2 hours absorb. BB recently suggested 4 hours of absorb - late last summer. I doubt that it makes too much difference. Better to overcharge than undercharge. We can all agree on that.

Interesting thing about the thousands of hobbies, professions and interests. The pros that are carving out a living in that niche usually have a few hundred must do's in order to stay alive. Because nobody has anything else going on. Yea - I know already. Everything will soon go up in a gigantic ball of flame!

We recently had a pro lecturing that we simply can't have batteries in our house. Did he also exclude the garage? Because he had been in several structures with hydrogen gas. How had he managed to defeat the odds and stay alive we may ask.

If I fine tuned my settings every time it was suggested, I would probably have moved my bed next to the controller for convenience. And would be able to program the CC blindfolded.

mcgivor · February 2020

Using end amps may be the best method, however shouldn't one need to consider applied loads, particularly constant ones which will offset the end amps value causing the controller to never actually achieve the determined setpoint?

mike95490 · February 2020

Thank you Marc, for the amplification of the different charging protocols, and I accept your comments as valuable.

The difficulty of proper charging ( and my region's 5-12 days of clouds in winter ) led me to the NiFe batteries, lossy, but resistant to abuse.

The Classic paired with the Whiz-Bang Jr, is the definitive way to measure end amps at the battery, IMHO, other methods are a guessing game.

Marc Kurth · February 2020

.

mcgivor said:

Using end amps may be the best method, however shouldn't one need to consider applied loads, particularly constant ones which will offset the end amps value causing the controller to never actually achieve the determined setpoint?

Yes, Sir.

Location of the shunt is critical in order to measure battery current only, without including any loads. You are right, end amps could not work if loads are included.

Marc

AGM battery Lowest voltage

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