All car batteries have Reserve Capacity ratings instead of Ah ratings for deep-cycle batteries. Using this data to test the health of the car battery a good way to estimate life left?

Let say I have a car battery with an RC of 80 minutes with a constant current load of 25A I could, in theory, see if how long it takes until it reaches 10.5v. Based on the real data vs label calculate the loss of capacity in a used car battery (State of Health)?

That could be a very easy way to test. Just curious I had a thought about this.


  • \$\begingroup\$ You can estimate how much charge you have left based on the voltage alone, that is how a lot of battery gauge chips work. I recommend looking into them too if you want to build a project that has battery charge tracking. \$\endgroup\$
    – Mu3
    Apr 5, 2019 at 7:49
  • 1
    \$\begingroup\$ @Egor Tamarin This will only give information about the SOC (State of Charge), not about the SOH (State of Health). \$\endgroup\$
    – jusaca
    Apr 5, 2019 at 8:44
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    \$\begingroup\$ @jusaca then I misunderstood the question. Perhaps it should be edited? "Life left" can refer to both charge and health. Maybe OP can also clarify a bit. \$\endgroup\$
    – Mu3
    Apr 5, 2019 at 9:02
  • \$\begingroup\$ You are right it wasn't clear. Let me edit the question. I'm talking about testing SOH on a car battery how much capacity has deteriorated. \$\endgroup\$
    – ELCouz
    Apr 5, 2019 at 11:04
  • \$\begingroup\$ Constant current load and measure the time until you hit a pre-defined undervoltage limit? \$\endgroup\$
    – winny
    Apr 5, 2019 at 11:07

2 Answers 2


The Battery Council International (BCI) created a group of standardized specifications for lead-acid batteries including Reserve Capacity. This particular test gave consumers a useful comparison for a lead-acid battery. Measuring the full discharge in minutes is the reserve time a consumer could expect with a new battery after an alternator failure and a standard 25A constant drain.

Although I was not aware of this standard test until this question, I agree it is quicker, reliable and more accurate than leaving all your vehicle lights on and hoping it will start in the morning.

Although it may not be the criteria used for warranty purposes or factory acceptance testing, I agree it is an important consumer test for State of Health (SOH). The ratio to the original battery spec or similar replacements is a valid metric.


  1. It demands both a good short term 30 second crank (CA or CCA) and long term C20 max capacity (20hr CC) performance which uses all the storage capacitance and not just the short term charge without needing a precision piece of specialized test equipment or knowledge of the 20h battery capacity to choose the right CC current level.
  2. It can be used as a ratio of the Mfg spec for a new battery.

Reserve Capacity, RC is not a test you want to do often, nor is it practical for manufacturers to spend this much time but it is relatively easy to perform.

Here are two different priced batteries comparing their specs from one supplier for Absorbed Glass Mat (AGM) 12V batteries.

RC: RESERVE CAPACITY in minutes for a fully-charged battery to supply 25 amps at 80°F (28'C), until the battery is discharged down to 10.5 volts (1.75V/cell)

AGM compare:  Batt A (D27F) Batt B (D51)
-----------   ----------    -----------
RC: (25A)*    140 minutes.  66 minutes

*RC equiv.*   58Ah          27.5Ah  *calculated

C20 Capacity: 66 Ah         38 Ah
CCA:(30s)     830 Amp       450 Amp 
CA: (30s)     1025 Amp      575 Amp 

*CA equiv.*   8.5 Ah        4.8 Ah  *calculated

charge cycles 300+          300+ 
Weight:       53.2 lb       26 lb 
Ri:           0.0025 Ohms   0.0046 Ohms
Warranty:     12 mo.        12 mo.
Series w/o BMS 1 mo.        1 mo. 

RC: how long a fully-charged battery can deliver 25 amps of current in an 80°F environment before the battery is discharged down to 10.5 volts (1.75V/cell)

Reserve capacity, RC is the number of minutes a battery can maintain a useful voltage with a constant 25A drain from a full charge.

AMP-HOUR Rating. This is battery capacity and is measured by the 20-hour constant current product from 100% to 0% in 20 Hrs.

CCA is the maximum amperes sustained for 30 sec. when 12.5V battery with Sp. Gr.= 1.255 - 1.260 is loaded down to 7.5V at 0°F. This applies to starter batteries.

(Thanks for the question, I learnt something new)

  • \$\begingroup\$ Thanks for this answer. I know my question was a bit of crap shot since this is something unheard of in the battery testing world. I wanted to build a battery tester that could test my cars battery effectively. Even if this take more than 2 hours. I want real world data not some microload 2 sec battery testing. I just wanted to be sure RC was a good way to test against for cars. \$\endgroup\$
    – ELCouz
    Apr 6, 2019 at 16:07
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    \$\begingroup\$ One could also make up another version use ESR and RC combined and call it RC50 with 50% DoD using the ESR= ΔV / 25A and a threshold for 50% SOC at ambient temp that is verified by comparison to 100% DoD \$\endgroup\$ Apr 6, 2019 at 16:16
  • \$\begingroup\$ Also note the Ah calculated from RC supplies most of the C20 energy while CCA or a 1A pulse test is superficial or short term only. \$\endgroup\$ Apr 6, 2019 at 16:24
  • \$\begingroup\$ you didn't say what the ratio of RC to spec was nor the battery age \$\endgroup\$ Apr 7, 2019 at 3:21

I think there is a reason why Ah or RC are not the only relevant/documented characteristics of a battery. Cold Cranking Current is another one and there may be more. If one of those relevant factors goes too much out of spec, the battery is dead. Thus the capacity (Ah or RC) is at most one indicator of a battery's health. At least the ESR/CCC should be considered too. - Unless there is some known relationship between e.g. CCC and RC, of which I'm unaware (- due to lack of research on my side).

Based on the real data vs label calculate the loss of capacity in a used car battery (State of Health)?

One should probably not take the value from the label but the one measured when the battery is new. I can imagine that not all batteries which leave the factory meet the value on the label...


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