Amps used per unit ov voltage drop

Discussion in 'General Electronics Chat' started by ger32, Dec 5, 2010.

  1. ger32

    Thread Starter New Member

    Jul 5, 2010
    In a 44 amp, deep cycle AGM battery,
    how many amps have been used from
    full charge (12.5+) down to 12.0 amps?
    11.0 amps?
    10.5 amps (or dead)?
    Or any size battery, say a Series 27 at
    92 amps.
    The battery I have in mind is the
    Interstate SLA1161
  2. SgtWookie


    Jul 17, 2007
    A fully charged battery of that type at 25°C/77°F will be ~12.8V.
    At that same temperature, it will have 70% charge remaining at ~12.4v, and 13.2AH of it's capacity will have been used.

    As charge efficiency goes (around 75%), it will take roughly 17.6AH worth of charging to bring it back to full charge.

    See post #38 in the Tips 'n' Tricks sticky thread. Discharging these cells deeply will lead to a short service life.
  3. nuckollsr


    Dec 17, 2009
    In a 44 amp, deep cycle AGM battery, how many amps have been used from full charge (12.5+) down to 12.0 amps? 11.0 amps? 10.5 amps (or dead)? Or any size battery, say a Series 27 at 92 amps. The battery I have in mind is the Interstate SLA1161

    It is unfortunate that the common vernacular for battery capacity speaks in terms of ampere-hours. To state that any given battery is a "44 ampere-hour" device is meaningless without also citing the rate in amps (or the interval of time) that the discharge test occurs. I'll refer you to an exemplar data sheet on a sealed lead acid battery:

    In particular, check out the plots in the figure titled "Duration of Discharge vs. Discharge Current". EVERY battery of any technology produces such a plot. Further, be aware that this family of plots assumes some nominal battery temperature . . . typically room temp or about 20C. Cool the battery off and the curves move down. Warm it up and the curves will move up. Note that the battery in the data sheet is "rated" as a 33 a.h. device. In the curve I cited above, and the data table on the left we see that the 33 a.h. capacity is realized if the discharge is carried out at 1.65 amps over a 20 Hour period of time.

    If you discharge it at a 20 amp rate, the battery is used up after 1 hour. Hence, the useful capacity is now only 20 a.h. That 20 hour figure is NOT necessarily a standard for how YOUR battery was rated. You need to pull the manufacturer's data sheets to see how the 44 a.h. number was deduced.

    The reason for this is that no battery is a perfect energy delivery system. Every battery has a component of internal resistance. This means that as you deliver energy to some external load, there is a proportional LOSS of energy internal to the battery which goes to warming the battery up and is lost to the user.

    Measuring the open circuit terminal voltage of a battery offers only a rough estimate of energy remaining. That's because whoever went to the trouble to produce a voltage vs. capacity chart for HIS battery has no idea how aggressively you intent do discharge YOUR battery.

    If it is important that you know how much energy you've used and how much remains, there's no simple means by which one converts the battery's terminal voltage into a really useful number.

    There are sophisticated instruments that can be used to monitor and characterize your battery's performance under the conditions that you use it. It calculates watt-seconds of energy taken in during charge, watt-seconds of energy taken out during discharge, and step changes in supply voltage as current is varied.

    After a few charge/discharge cycles, the instrument knows the battery's internal resistance and chemical capacity. With proper massaging of the data, it can drive a display that announces remaining capacity . . . in WATT-SECONDS or WATT-HOURS, etc.

    Unfortunately, Ampere-Hours is an almost meaningless number.

    If you'd like to study battery physics in more detail, you can acquire one of these slick test tools:

    You can study the effects of discharging a battery under a variety of conditions and see how wildly the USEFUL capacity changes with load.

    Having said all this, there is ONE useful number that applies to all 12v lead-acid batteries. While loaded to any particular value, the battery is very close to end of charge life when the voltage drops below 11.0 volts. You can deduce this for yourself by simple observation. Note the slope of battery voltage in the discharge plots. At 11.0 volts the nose dive for voltage is steep and getting steeper.

    Check out the offerings at The Battery University which you can access at:

    This website is the brainchild of Mr. Battery himself. If there is something about batteries that Isador Buchmann doesn't know, it probably doesn't

    Good luck!
    ger32 likes this.
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009

    The "20 Hour Rate" is the standard test for determining amp hours. It is a measurement of what the maximum current draw is for the battery to be dead in 20 hours.

    In the case above, the 33 AH battery, divided by 20 Hours, tells you the draw was 1.65A

    If a battery is rated for 400AH, the draw was 20Amps, and so on.

    It is very difficult to state exactly how long a battery will last in a given application. It may use large pulses of current once every half hour, the "SWAG" would be adding together the time of all pulses at the max current, and assume that is worse case. It will last longer than that, of course, but you generally never want to run a lead acid battery totally dead.

    For smaller cells (Non-Lead Acid), the Amp Hour rating is determined by a continual discharge at 0.1C, or 10% of the capacity, so a 2 Amp Hour AA Rechargeable under testing would have a draw of 200mA until it was dead (roughly 10 hours).
    ger32 likes this.
  5. timrobbins

    Active Member

    Aug 29, 2009
    Anything from the web needs to be taken with a grain of salt, even Mr Battery - as the website is commercially biased. General and commonly available information and experience is great, but anything relating to a commercial service or product or technology is just a commercial view.

    Ciao, Tim