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Lead acid battery - how long can it be left discharged?

GhostLoveScore

Nov 27, 2016
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So the situation is this: Recently we experienced frequent power outages. Shortest ones were for 2, 3 hours, longest for about 24 hours. I have 100Ah AGM lead acid battery that powers inverter to provide power for light, computer and TV. I was wondering how long can the battery stay discharged, let's say that I depleted it in 10 hours and the power will not get back for another 12 hours. From what I understand I should charge it immediately, but that's not possible sometimes.

What about the different situation, if I have LEDs that use 10W of power. They could be powered from that battery for 100 hours. How will that affect the battery when it's let's say 50% depleted and because the LEDs draw relatively small current it will spend a lot of time slowly discharging. Will that damage the battery?

Actually, what damages the battery, the time spent on low voltage or time spent on low voltage while nothing draws the current from the battery? To clarify, is it the same for the battery if it sits on 11V doing nothing or if it sits on 11V slowly discharging 1A into LEDs?
 

Harald Kapp

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Battery University has some useful information on all types of batteries, including lead acid.
 

kellys_eye

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Lead acid batteries are like muscles - they need exercise to be kept in top condition. Chemical depositions build on the plates over time (unused) but they can be recovered by various methods.

Simplest way to maintain them is to use them. Otherwise why own them?
 

GhostLoveScore

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Thanks, but I think I was misunderstood. I'll try to simplify my questions:

1: I use battery for 5 hours and deplete it 100%, then it stays depleted for 10 hours
2: I use battery for 15 hours and deplete it 100% then I charge it immediately

What is the difference between those two situations? Is #1 more harmful to battery?
 

Harald Kapp

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Leaving the battery uncharged is more harmful than immediate recharge.
 

GhostLoveScore

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Leaving the battery uncharged is more harmful than immediate recharge.
I understand that.

let me ask it this way:
If you knew power will be off for 24 hours, would you discharge it slowly over 24 hours so that when power comes back you are still using the battery but it's 90% depleted OR

would you use it for 10 hours until it's 90% depleted and then leave it for 14 hours until power comes back.
 

Harald Kapp

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Because a fully depleted battery will not last as long as a 90 % depleted battery, see battery university.
 

GhostLoveScore

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Because a fully depleted battery will not last as long as a 90 % depleted battery, see battery university.

You misunderstood me again, in both cases battery will be 90% depleted, meaning I would use 90 Ah from the battery. (100Ah is it's capacity).
 

(*steve*)

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Depending on the type of battery, discharge more than 50% may be a bad thing.

The best solution to the above problem is the one which leaved the most charge in the battery the longest.

In addition, a slower discharge will allow you to get more energy out of the battery.
 

GhostLoveScore

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Depending on the type of battery, discharge more than 50% may be a bad thing.

The best solution to the above problem is the one which leaved the most charge in the battery the longest.

In addition, a slower discharge will allow you to get more energy out of the battery.

It's deep cycle AGM battery. It should be fine with 50% discharge.
Of course I understand that if I discharge it more, the battery will last shorter amount of cycles.
 
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kellys_eye

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Discharge even a deep cycle battery past 75% is dodgy as far as life span is concerned.
 

kellys_eye

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Thanks, but I think I was misunderstood. I'll try to simplify my questions:

1: I use battery for 5 hours and deplete it 100%, then it stays depleted for 10 hours
2: I use battery for 15 hours and deplete it 100% then I charge it immediately

What is the difference between those two situations? Is #1 more harmful to battery?

Nothing worth worrying over imho.

Leaving a battery discharged for 10 hours isn't a 'killer' situation and likely even difficult to discern without specialist equipment - assuming it is DISCONNECTED from the load when discharged. Leaving it connected is a definite no-no.
 

duke37

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A lead/acid battery contains sulphuric acid which combines to the plates when discharged. After time, this lead suphate becomes stabilised and is more difficult to dissociate into lead and sulphuric acid so capacity is lost.
I do not think it matters how the battery is discharged.
Keep the battery charged to reduce this effect to a minimum.

Auto batteries have alloyed plates for strength which complcates things.
 

Arouse1973

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Nothing worth worrying over imho.

Leaving a battery discharged for 10 hours isn't a 'killer' situation and likely even difficult to discern without specialist equipment - assuming it is DISCONNECTED from the load when discharged. Leaving it connected is a definite no-no.

Yeah I agree.Also I would have thought your circuit would stop working by the time the battery is 80% discharged.
 

siggy

Mar 3, 2012
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leaving a battery discharged is bad - period whether partial or total

If this is the case invest in a deep cycle battery

If you’re using the battery to keep vital circuits functioning then I would build a battery bank

Work out maximum current draw including in rush current of all devices factor in maximum current at 120% of maximum and connect batteries in series and parallel to provide 26hours of current at 120% maximum load
 

GhostLoveScore

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I do have deep cycle battery. 100Ah capacity 12V. Enough to keep my computer running for at least 6, 7 hours. If I'm being optimistic then 10 hours.
 

dave9

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If you care about battery life you won't discharge it "too" low.

It's like asking is it worse to shoot myself in the foot or in the ear. You know it's harmful so just don't do it. If your power needs exceed your battery capacity then you increase your battery capacity. That's the accepted trade-off for using cheap AH lead acid cell batteries. If you don't want that trade-off then you buy batteries with chemistry that is more tolerant of very deep discharge.

However, 11V is not deep discharge. You should read up on battery curves and just find the cycle life vs voltage you find acceptable and put in a low battery cutoff circuit to accomplish that goal. It's always a trade off with lead acid, whether # of cells to attain capacity and/or the design to accommodate deeper discharge.

Ultimately if the load is just 1A LEDs, a huge SLA battery to keep discharge level low is probably a misallocation of money spent on the wrong batteries.
 
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