When an SLA battery is under load, they say not to go below a certain value to avoid wear. So, don't go under 30% State of Charge (SoC), which is also a certain Open Circuit Voltage (Voc), say 11.8V .
So, here both values are linked, one value means the other, independent of the load rate.

Under load, however, you can't just guess the Voc . So ok, no problem, we have graphics of voltages and state of charging with different load rates (ignoring existing wear, let's keep it simple).
So far, so good.

But here comes my problem ➝➝ Although I can't find much documentation on this, there are two opinions!

  1. You can't go under a certain State of Charge (SoC)
  2. You can't go under an absolute voltage (V, not Voc!!).

I made two graphics to make this clear. Only the colors are different. Yellow and red mean: avoid (resp. at all cost) to avoid fast wear. Soc V So, 2 possibilities:

  1. (Top)
    Best not to go under 30% state of charge. The voltage under load depends on the load (current).
    E.g. on a load of C/3, you can go to somewhere between 10.3-10.7V.
  2. (Bottom)
    Best not to go under a certain absolute voltage.
    E.g. on a load of C/3, you can go to 11.8V, but you would still have 95% state of charge left!
    And it gets even weirder. Some say that voltage should be the Voc that correspond with the 30% Soc. Others just take a random voltage.

Me, personnally, I bet on number one, and number two is from people that were a bit confused. Although this is how not too intelligent controllers's cut off is set.
But can someone with deep knowledge confirm or correct me? Possibly with some explanation. 🙏🙏 Thank you. 🙏🙏


2 Answers 2


Well, you'll get as many references as opinions.

Safest: ask a manufacturer.

On the practical side of things, the voltage limit sucks because that necessarily prohibits high currents at low SoC -- yet you may need that for a UPS, or cranking an engine, etc. Note that internal resistance Ri goes up with discharge as well.

But maybe that kind of operation wears the battery extra, so it's best avoided.

As far as the chemistry goes, consider that the electrodes are made of spongy material, which greatly increases the surface area and thus capacity, but also greatly increases the distance ions need to travel through the electrolyte to reach any given point on that surface. Which means much more voltage drop through those pores (at high current density), and so they'll always lag behind the SoC of the outer/direct facing surfaces.

Perhaps the answer is both, because the outer surfaces could be utterly depleted (0% SoC) while the pores are still just waking up. Thus, wear proceeds inhomogeneously over the total electrode surface, and capacity drops as a result.

Capacity loss occurs (as far as I know), primarily because of formation of insulating PbSO4 crystals, and mechanically due to flakes/segments falling off or otherwise losing electrical contact (which might perhaps become broken by formation of such crystals, or by corrosion of the metal; note that material isn't plated back perfectly in place, with some deformation occurring over cycling, and this also limits battery life even under the most gentle use).

The porous structure also determines the dis/charge curve with respect to time; ionic diffusion is already a significant effect (giving a ~sqrt(f) impedance curve), but the distribution of pore sizes/lengths also gives an equivalent diffusion effect, and both effects stack to give quite long time constants -- hence the hours or even days cycle times, like for the float charge cycle having a long tail of current, dropping off slowly while held at fixed voltage.

As for references, let's see what the Google has to offer today...

You're welcome to play along, critically evaluating the veracity of these links.

Suggests AND condition, i.e. limit both voltage and SoC. Seems to be a user. At a glance, no references linked. Is this a primary source? Probably not.

Here's a shockingly familiar plot... no citations though. On the upside, it seems Richard was a clever guy: http://www.omagdigital.com/publication/?i=394240&article_id=2743496 unfortunately, we can't know what references he was pulling from.

12V Sealed lead acid battery level sensing
Links a datasheet -- primary manufacturer data -- though that doesn't include what cycle life the indicated cutoff point should give. Disregarding the lack of cycle life data, this gives a current-dependent cutoff, which roughly corresponds to an awareness of internal resistance: evidently about 0.6V/20A = 0.03Ω (taking the maximum current from each range).


One of the best resources about charging and discharging SLA batteries is from the manufacturer Powersonic:


They show discharge curves to as low as 9 volts under various loads, and 7.5 volts for very heavy discharge rate, which can be as high as 10C. That seems to correspond to the internal resistance effect, and according to that technical reference:

"It is important to note that deep discharging a battery at high rates for short periods is not nearly as severe as discharging a battery at low rates for long periods of time."

Battery University is another good resource, with articles such as the following: https://batteryuniversity.com/article/bu-501-basics-about-discharging

There are also variations due to temperature and other factors that affect the SOC, and some batteries are intended for occasional high discharge applications, like car starting, while others are optimized for deep discharge at moderate rates. All lead-acid batteries are subject to Peukert's Law, which explains why a 100 A-h battery will provide that capacity at 1/20 C ( 20 hours at 5 amps ), but only about 50 A-h at 1C ( 1/2 hour at 100 amps ).

The discharge curves shown by the OP are for relatively low discharge rates, at which it seems reasonable to base the safe level on SOC no lower than about 15-30% under load. When the load is removed, the voltage will recover within 15 minutes or so, where that voltage will be a good metric for SOC.


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