How to determine max healthy intermittent load for battery bank?

jdobush

Hi all,
I'm putting the finishing touches on designing a new battery bank for my existing off grid system. Without getting crazy into details - my previous battery bank was both too small and with very short lived AGM batteries. I'm spending the time now doing many different calculations of my loads and system, and making sure depth of discharge can stay within a 30% window the majority of the time. 

My question here is - how do you determine the maximum intermittent (but regular) load of a given battery bank? This is one calculation I can't find a concrete answer for just opinions. 
Case in point - I run my well pump off of my setup. Some days it doesn't run at all. Other days when we are active, it can run 8 or 10 times through out the day. Each time only runs for 2 minutes roughly. (draws 1200 watts running average +greater startup) Usually these greater use days total maybe 0.25 to 0.5 kwh per day. That alone is next to nothing, but most battery bank calculators don't ask anything other than daily watt hours to calculate depth of discharge. 

My batteries (whats left of them) say maximum 500amps intermittent discharge (crazy but no other guideline). And my wiring and inverter are well within range. So what is your chosen maximum (for a couple minutes) draw from your battery bank? A 20 hour rate? 10 hour? 1 hour? How does this ultimately affect the life of your batteries?

Thanks! Jim

mcgivor

Welcometo the forum Jim

Lead acid batteries can support extremely high current demands but suffer voltage drop, the larger the nominal battery  capacity the lower the voltage drop. Since neither the battery capacity not the nominal voltage is supplied it's not possible for others to make helpful  suggestions. 

To get some idea of suitably measure the battery voltage at pump startup and during running, the initial surge on startup will drop the voltage to its lowest value, then voltage should recover to a higher value. For example if the nominal voltage is 12V, the battery is fully charged resting at 12.5V,  when the pump starts the voltage may initially  drop to 11.5V then return to 12.2V, if it can maintain that value during the 2 minutes run time, then return to 12.3-12.4V the battery capacity would be sufficient, if values are significantly lower more capacity may be needed.

For offgrid the 20 hour rate is commonly used this is because the window of charging opportunity is limited to ~4 hours , the actual amount of energy used by the pump running for a few minutes is relitivly low, 2 minutes as a decimal of an hour is 0.033, so 1200W × 0.033 = 39.6 Wh, a 12V 100Ah battery has 1,200 Wh (600Wh at 50% maximum recommend discharge)  so theroitically the pump could run ~15 times for 2 minutes before it is discharged to 50%.

jdobush

Thanks for the reply. Apologies, I hadn't included my current bank size which is 24v 400ah AGM. Currently planning on moving to 24v 600ah, which will better support my usage overall.

The voltage drop you mentioned is some of my concern. I'm currently hitting my low voltage cutoff (10.5v) every once in a while during that startup surge, after discharging overnight. (again, these batteries are seriously sulfated and won't hold more than a surface charge, its why I'm replacing them) I measured the voltage drop the other day when close to fully charged and had some minimal solar still coming in hence the high starting voltage - starting V 26.5v, initial spike 25.18v, returning to 25.5 ish but dropping steadily. I know I can't compare my current numbers to a new bank, since this bank at one time for a year or two handled it just fine.

So that's all well and good being able to watch the voltage drop - but I'm trying to figure this out for a new theoretical battery system. Is it possible to calculate what the voltage drop should be for a given load? Or other ways to plan out a max load? Maybe it is just opinion based, but I know it would be crazy to hit this bank with 500 amps occasionally - so where's the line? Is it really just hardware limitations?

Marc Kurth

This data is for Concorde AGM batteries. It will allow you to project battery voltage vs. SOC vs. Load. You should get the data for your specific brand because AGM batteries because the numbers could be drastically different.

Dave Angelini

Since you are redesigning, you might consider the current pumping scheme. Really a much better strategy (in many ways) offgrid and in winter where we concentrate, is running this well load near noon and using a smaller softstart pump for house supply from a 300 gallon or larger house tank. Winter is where it all happens offgrid!
.

jdobush

Marc Kurth said:

This data is for Concorde AGM batteries. It will allow you to project battery voltage vs. SOC vs. Load. You should get the data for your specific brand because AGM batteries because the numbers could be drastically different.

Thanks - this is kind of what I had in mind, but I've yet to find it for my current or new batteries, I will go back to looking now that I have an example of what to look for. (this charts drops shown aren't as bad as I feared)

Still curious as to how regularly hitting for instance that 1 hr or 8 hr rate affects the overall life of a lead acid battery. I have to believe there is some loss of life/efficiency long term more than just the immediate peukerts effect.

Edit now that I think about it - I guess I'm asking more on the terms that a starting battery with normally deal with. But considering 50ish amps from a bank compared to a couple hundred from a starter seems fairly insignificant.

jdobush

Dave Angelini said:

Since you are redesigning, you might consider the current pumping scheme. Really a much better strategy (in many ways) offgrid and in winter where we concentrate, is running this well load near noon and using a smaller softstart pump for house supply from a 300 gallon or larger house tank. Winter is where it all happens offgrid!
.

Right, I know there are plenty better options out there, but for now I feel it's more responsible to use what I have until the end of it's life. I'd love to work in a rain water collection system as well if i go to a tank storage system.

Dave Angelini

18,000 gallons of rain in the tanks, almost full!