Safe Continuous Discharge Rate of a Car Battery

Question

I have a car battery (Varta Silver Line of Products) that's rated at 110Ah, and 900A Cold Cranking Amps.

The marketing of the battery from different sources says "huge reserve capacity" but never quotes the actual reserve capacity value (not even the product specifications sheet from Varta does).

What I'm wondering:

How much is the maximum amount of amps I can probably safely* draw from the battery continuously for 1 hour?

*By "safely" I mean: such as doesn't involve an immediate hazard for the user (me) and won't shorten the battery's useful lifespan by >15-25% of it's expected useful life in the car in which it sits.

In lack of detailed data I expect the answer to be a general estimate of course (which is also why my question has a fairly lose definition of the term "safely" for the purpose of this question).

I have a practical application at hand which I'd just like to move on with, so all I care about is a "rule of thumb" answer/solution to what is the maximum I can probably draw, it doesn't have to be ideal or perfect for the battery in the very long term as long as the effect in negligeable <1-2 years of use with a weekly 1h continuous draw at that amperage.

Any educated guess from someone who has good general knowledge of car batteries?

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EDIT: I realize my question was misleading in the way in was formulated. I do know that discharging a car battery below 50% damages it (some people say being gentle towards a car acid battery is never discharging it <80% charge). But that fact is not what I'm wondering about so please factor that out of your answer (i.e. take my question as "What is the maximum rate at which I can safely continuously draw from the battery until it reaches ~60% charge level?").

I apologize for not thinking about discharge level while initially asking.

Answer 1

Your Varta battery is a Sealed Lead Acid, AGM type. I could not find useful information on Varta's website, so I will refer to other manufacturers that provided data. I tried to find from YUASA brand, but available data was not clearly stated for AGM type, but GS Battery provided more detailed information on a single place (to enable valid comparisons). I will use this GS data to support the following answers along my post:

1. How much is the maximum Continuous (sustained) discharging current? Spoiler: \$ C_{cont} = 330 A \$
2. How much is the maximum Short-term discharging current? There two values - for 5s and 30s; see ahead.
3. How much does the Discharging current influence the total energy delivered?
4. How much does the Termination voltage affect the energy delivered by the battery?
5. How long does the battery last, as a function of Depth of Discharge?

So, let's start.

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Rated battery capacity - usually stated for 20 hour discharge = 0.05C or C/20.
In your case, \$ C_{batt} = 110 Ah \$.

Discharging current - There are two numbers, usually as multiple of nominal capacity C:

• Continuous discharge can go up to 3C and some models accept up to 6C.
  In your case, \$ C_{cont} = 330 A \$
• Short term discharges of 5 seconds can go as high as 15C.
  In your case, \$ C_{max 5s} = 1650 A \$, which is 180% of stated CCA = 900A.

For \$ C_{max 5s}\$ please observe there is no Termination/Minimum voltage requirement, just an absolute value that seems to be related to the structural integrity and safety of the battery if the maximum time is respected.

• Also keep in mind that "Cold Cranking Amp - CCA" has a different definition, by SAE J537 Jun 1994 American Standard. CCA measures the output amp of a 12V battery for 30 seconds while maintaining 7.2V at 0°F (-18°C). So, both \$ C_{max 5s}\$ (5 seconds) and \$ CCA \$ (30 seconds) are coherent to my eyes.

Termination voltage for discharge: Depending of actual discharge rate C, final voltage can be further reduced. Following table shows this:

Capacity per Discharge rate - Total energy delivered by battery is reduced as the discharging rate increases, even considering the lower termination voltage as mentioned - see:

As can be seen:

• At 0.05C discharge, the total capacity drained from that battery has 60 "units" (100% of C, i.e. the rated capacity) with a termination voltage of 10.5V. Markings in blue.
• At 1C discharge, that battery provides less, 42 units or 70% of rated C, even terminating at 9.0V. Markings in green.
• At maximum sustained 3C discharge, just 25 units or 42% of C is delivered, before reaching 8.1V. Lower markings in purple.
• But if the same voltage of 10.5V (as in 0.05C) is used for termination, then the user can only extract 18 units or just 30% of C. Upper markings in purple.

Service Life of battery - Total number of discharge cycles will depend on the Depth of Discharge (DoD) and how soon you recharge it. Usually it uses the reference of reduced capacity to 50% of rated C. It can be seen that for 30% DoD, battery withstands 1200 cycles (green lines) to reduce its capacity from 100% C to 50% C (blue lines).
However, it just lives for 400 cycles (orange) for 50% DoD and 200 cycles (red line) for 100% DoD - as follows: