Home

Vehicle and Rig Battery Trivia

Over the years I have become frustrated with the confusing battery rating systems and an inability to get straight answers from vendors about lead-acid storage batteries.  The material presented below was accumulated over a period of time from many sources, and has simplified dealing with vehicle and rig batteries for us.

Battery Ratings

There are a dozen or more vehicle battery rating methodologies.  The Society of Automotive Engineers (SAE) has established two ratings for domestic made batteries - Reserve Capacity (RC) and Cold Cranking Amperes (CCA).  The Cranking Amps (CA) rating is also still commonly used.  Industrial batteries often specify Ampere Hours (AH) and marine batteries may carry a Marine Cranking Amps (MCA) rating.  Definitions of these more common rating systems are given below:


RC - Rating in minutes a battery will carry a 25 amp load at 80F and maintain a minimum terminal voltage of 10.5 volts.

CCA - Rating in amps which a new, fully charged battery at 0F can continuously deliver for 30 seconds and maintain a terminal voltage equal to or higher than 1.20 volts per cell.

CA - Rating in amps which a new, fully charged battery at 32F can continuously deliver for 30 seconds and maintain a terminal voltage equal to or higher than 1.20 volts per cell.

AH - (@ 20 hours) a battery having a 100 AH rating must carry a 5 amp load for 20 hours and maintain a terminal voltage of 10.5 volts at 80F (100 20 = 5 amps).

MCA - Rating in amps which a new, fully charged battery at 30F can continuously deliver for 30 seconds and maintain a terminal voltage equal to or higher than 1.20 volts per cell.

Ratings Conversions:      RC = 1.75 x AH          AH = (RC 2) + 15.5

Opinion

Modern "maintenance free" batteries do not tolerate abuse as well as older style batteries.  If you are experiencing shorter battery life on your oilfield equipment than you think reasonable, try an older style "maintenance" type battery (one where you actually have to check the fluid level).  If you routinely run down batteries and charge them in the shop (as on old rigs with non-functional charging systems), consider a marine / RV battery; they will take abuse that will kill automotive maintenance free batteries in a fraction of the time.  Finally, if you like maintenance free batteries, consider something other than the maximum CCA rated battery available in a given package size.  To get 1000 CCA out of a small automotive battery, manufacturers use a variety of tricks, like making the plates very thin.  This results in a more fragile battery.  The same package size in a 650 or 700 CCA rating is likely to be a tougher battery and to last longer in oilfield service.

CHARGING CHEAT SHEET

Charge State

    Charge    Specific Gravity (Sg)      12v          6v       Freeze
     100%        1.265 - 1.285       12.6 - 12.68     6.3      -71F
      75%        1.220 - 1.225       12.4 - 12.45     6.2      -33F
      50%        1.175 - 1.190       12.2 - 12.24     6.1      -15F
      25%        1.140 - 1.155       12.0 - 12.06     6.0      + 4F
     Dead        1.110 - 1.120       11.8

Charging Time

1. Down and Dirty Charge Rate Method - Chargers with Ammeters

Charge the battery until the charge rate (as indicated by the charger ammeter) has dropped and stabilized at 1/3 to 1/2 of the charger's full output rating.  For example, a 10 amp charger will stabilize at 3 to 5 amps.  This is an inaccurate method and care must be taken not to overcharge.


2. Down and Dirty Bubble Method

When the battery fluid is gently bubbling, the battery should be considered charged.  This is a very inaccurate measure of charge state and care must be taken not to overcharge.


3. Voltage Method

Charge the battery until an accurate digital voltmeter indicates the following end of charge voltage:

     Temperature         Conventional           Maintenance Free
        95F              14.2 volts               14.5 volts
        75F              14.4 volts               14.7 volts
         0F              15.1 volts               15.4 volts

4. Specific Gravity Method - Not for Maintenance Free Batteries

Charge the battery until reaching the 100% charge specific gravity (Sg) shown in the Charge State table above.  Disconnect the charger before testing.  Older batteries or batteries with sulfated plates will reach full charge at a lower Sg; continued charging may result in damage.


5. Calculation Method

Determine the percent of charge needed (use either a hydrometer or voltmeter, and refer to the Charge State table above).  An approximate charge time in hours, "Time", can be calculated as follows:


                      Time = .667 (RC) (Charge Needed)
                                  Charger Rating

                                       or

                      Time = 1.6 (AH) (Charge Needed)
                                  Charger Rating

| Home | PBS Tech & Tips | AnaLog Tech & Tips | Surface Hardware |

Last 10-20-10