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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?

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.

Hans
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    you can draw maximum 110 A without discharging it in less than 1 hour..Did you check if it gets very hot at 110 A ? – tobalt Nov 26 '21 at 17:38
  • Define safe. Safe for the user? Safe when it comes to internal chemistry and how it affects the lifetime of the battery? – winny Nov 26 '21 at 17:49
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    The car battery is used for engine cranking, not for cycling. But you could cycle it for 20-30% of its capacity, so I would say 20-30A for 1h Perhaps you could find some good points at the battery university web site. – Marko Buršič Nov 26 '21 at 17:55
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    You certainly cannot draw 110 A for 1 hour. That 110 Ah is probably the 20 hour rate. Higher rates mean lower capacity. Every time you draw 100% of the capacity, you hammer its lifetime. Limit your draw to 50% capacity to get the 'expected' life. Marko's figures sound pessimistic, but I bet they're fairly close to the truth, if you want to use the battery more than handful of times. – Neil_UK Nov 26 '21 at 18:06
  • @Tobalt, Marko, Neil: Thank you for your answers. 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's 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 asking. – Hans Nov 26 '21 at 19:29
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    PS: Also please answer as an official answer so I may mark the best answer as such. – Hans Nov 26 '21 at 19:33
  • What is the amount of current you want to draw for an hour? Then perhaps we could say if there is likely to be a problem. – Andrew Morton Nov 26 '21 at 19:45
  • @Andrew_Morton: Well, If I can draw 2500W continuously until the battery is down to 60% capacity, that would be great. – Hans Nov 26 '21 at 23:05
  • Does anyone have a guess? – Hans Nov 27 '21 at 10:33
  • @Hans Please see [What is a safe max. discharge rate for a 12V lead acid battery?](https://electronics.stackexchange.com/q/130580/36731) and consider that you are proposing to draw over 170 A from the battery - and it could be prudent to add a cooling fan to it as well. – Andrew Morton Nov 27 '21 at 11:36

1 Answers1

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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.

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:
enter image description here

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:
enter image description here

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:

enter image description here

EJE
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