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RV Battery Power Management

How long can you run lights, TV, furnace water pump etc., before battery recharging is needed? Getting the most from your batteries is a mix of conservation, intelligent charging and proper care.

Follow the steps below to determine how many days you can rally or boondock before cranking up the generator. It's best to do this test when you have an electrical hookup available so you can recharge the batteries as soon as possible.

  1. Start with fully charge batteries.
  2. Switch the refrigerator to run on propane.
  3. Turn off unnecessary 12 volt appliances.
  4. Record the time.
  5. Shut off converter/charger (or unplug your RV if there is no way to switch off the converter/charger).
  6. Run your 12 volt appliances and lights as you normally do.
  7. Occasionally check the battery voltage with a digital volt meter and record the time when meter drops to each % of charge. See the char below. Don't go below the 50% level in this test
  8. When you reach the voltage that approximates 50% of charge (on chart ), turn your battery charger back on.
  9. Check the time.
        

You now know how long you can run the RV, in "normal" use, without charging, before a "deep cycle" condition.

Of course this time isn't exact and can change. The furnace will rum more often in winter than in the other seasons. You use the lights more in the winter when days are shorter and skies are overcast. So, the more you run your RV on 12-volts the better you'll be at estimating the time.

NOTE: When the battery is under load, but not being charged, your volt meter will read lower than actual battery state. For example: If TV and lights are on, the meter might read 12.4. Don't panic. Turn heavy loads like those off. Watch meter. If batteries aren't bad, the voltage should increase and then stabilize. That's the point where you take your reading. (An easy, quick way to observe this is to watch the meter while you run enough water to make the pump start. You'll see a drastic drop in voltage. Shut water off. When pump stops you'll see meter reading start to creep back up.)

Don't be alarmed if your 105AH battery, when voltage indicates it's at 50% of capacity, has given you a lot less than the 50AH you expect. Most batteries are seriously overrated by the manufacturer, seldom yielding 80% of their stated rating.

% Of Charge

Standard Battery
Typical Specific Gravity
(After Temperature Correction)

Standard Battery
Equivalent
Resting Volts

Gel-Cell Battery
Equivalent
Resting Volts

100%
1.260 (auto) to 1.280 (industrial) 12.60-12.75 12.90-13.00

95%
1.255 12.60-12.70 12.80

90%
1.250 (Resting S.G. for standard RV battery.) 12.60-12.65 12.70

85%
1.245 (Same as above. No point being too picky.) 12.60  

80%
1.235-1.240 (We try not to discharge below this point.) 12.50-12.55 12.60

75%
1.225-1.230 (1.230=minimum SG for a charged battery.) 12.50  

70%
1.220 (Anything below 1.220 is "poorly" charged.) 12.45 12.50

65%
1.215 12.40  

60%
1.205 12.35 12.40

55%
1.200 12.30  

50%
1.190-1.195 (Try to never discharge below this point.) 12.25 12.35

45%
1.185 12.20  

40%
1.180 12.15-12.20 12.25

25%
1.160-1.170 (Dangerously low; battery being damaged.) 12.10-12.15  

20%
1.150 (Cells die soon at this point.) 11.80-12.00 12.15

You may also want to read: RV Battery Use, Care & Maintenance

This info is updated/adapted from phred's poop sheets.

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