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Open Roads Forum  >  Tech Issues

 > Which 12.0 volt?

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TomG2

Central Illinois

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Posted: 04/15/19 02:43pm Link  |  Quote  |  Print  |  Notify Moderator

"Everybody" says to not drain 12 volt batteries below 12.0 volts. Operating a small refrigerator drops the apparent voltage to 11.9 volts which recovers to 12.5 volts between cycles. Too low or can I wait until the idle voltage drops to 12.0 volts?

KD4UPL

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Posted: 04/15/19 03:33pm Link  |  Quote  |  Print  |  Notify Moderator

"not going below 12.0 volts" is the resting voltage. You can only accurately measure it when the batteries are neither being discharged or charged and have had some time to sit there unused.

time2roll

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Posted: 04/15/19 03:46pm Link  |  Quote  |  Print  |  Notify Moderator

I think you need a second battery. How long was the fridge operating before you get to 11.9 volts?


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TomG2

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Posted: 04/15/19 03:51pm Link  |  Quote  |  Print  |  Notify Moderator

time2roll wrote:

I think you need a second battery. How long was the fridge operating before you get to 11.9 volts?


Never actually got below 12.2 volts but I used 11.9 to keep it simple. This was with a little 35 ah agm battery that I was evaluating. My posting was to verify that people use resting voltage and not loaded voltage to determine cutoff point.

BFL13

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Posted: 04/15/19 03:53pm Link  |  Quote  |  Print  |  Notify Moderator

TomG2 wrote:

"Everybody" says to not drain 12 volt batteries below 12.0 volts. Operating a small refrigerator drops the apparent voltage to 11.9 volts which recovers to 12.5 volts between cycles. Too low or can I wait until the idle voltage drops to 12.0 volts?


12v resting is about 40% SOC. You can do that a lot. No big deal.

While loaded as with an inverter running a 120v fridge, you might see 11 volts if the battery is getting low. Inverter will alarm and let you know it! when inverter sees 10.5 volts it will quit and battery voltage will go back up.

Idea is to have enough battery on the inverter (and fat enough wires) so you don't get the inverter alarm before your next battery recharge.


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time2roll

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Posted: 04/15/19 04:06pm Link  |  Quote  |  Print  |  Notify Moderator

TomG2 wrote:

time2roll wrote:

I think you need a second battery. How long was the fridge operating before you get to 11.9 volts?


Never actually got below 12.2 volts but I used 11.9 to keep it simple. This was with a little 35 ah agm battery that I was evaluating. My posting was to verify that people use resting voltage and not loaded voltage to determine cutoff point.
Yes that battery seems too small for the load.

Resting voltage is difficult to obtain in an RV as there is always something on.

[image]

I think most use the loaded chart but still 12.0 to 12.2 is the general recommended minimum.

[image]

The 12 Volt Side of Life

TomG2

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Posted: 04/15/19 04:10pm Link  |  Quote  |  Print  |  Notify Moderator

time2roll wrote:

Yes that battery seems too small for the load.

Resting voltage is difficult to obtain in an RV as there is always something on.


To keep things simple, the refrigerator was connected directly to the battery. 0.3 voltage drop does not seem excessive to me for running a compressor style 12 volt refrigerator. Either way, I will be using a larger battery in actual use.

naturist

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Posted: 04/15/19 08:09pm Link  |  Quote  |  Print  |  Notify Moderator

KD4UPL wrote:

"not going below 12.0 volts" is the resting voltage. You can only accurately measure it when the batteries are neither being discharged or charged and have had some time to sit there unused.


This.

All batteries show lower voltages when under a load, with the extent of the lowering being a function of both the size of the load and the chemistry of the particular battery. This is normal, and NOT a sign the battery is discharged to any particular degree.





TomG2

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Posted: 04/15/19 08:26pm Link  |  Quote  |  Print  |  Notify Moderator

Thanks to all. I was pretty sure people everyone based their cutoff on resting voltage but it never hurts to ask for opinions. I will be doing a lot of testing on my new Alpincool C15 refrigerator/freezer. So far the results have been better than hoped for with maintaining over 12 hours of 35 degree F storage with one 35 ah battery and a voltage drop of less than 0.4 volts.

landyacht318

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Posted: 04/15/19 10:12pm Link  |  Quote  |  Print  |  Notify Moderator

Thats a lot of voltage sag powering a DC fridge depending on the actual wattage it draws. Mine can go as high as 6.5 amps if I allow the compressor to run at 3500rpm. I only need 2000 rpm and there is a brief 55 watt start up surge and basically 32 watts running.

Either the battery is small, or sulfated or your fridge is drawing way more than 65 watts is my internet diagnosis.

---edit

Just saw a 35Ah battery in previous post.

See how long it lasts before it shuts down from teh low voltage cut out, so you know how long you can run it, assumnig you can keep the battery relatively healthy.

Keeping it healthy is all about the recharging, do not worry so much about teh 50% level, its not like 49% and BOOM!

Recharging that 35AH AGM should get about 12 amps until voltage rises to 14.5 to 14.9v, hold that voltage for 4 more hours. Amperage should taper to about 0.2 to 0.3 amps by the end. if you can recharge this battery like above I'd expect 200+ deep cycles running just the fridge until it shuts itself off on the lowest battery protection setting.
If your are going to expect 400+ cycles then do no draw it much below 50%.

The 50% rule is a bit nanny driven, especially on AGMs. Work the battery hard, just recharge it at the right amp rate to true full charge promptly and it will work well even discharged to 10.8v each cycle

* This post was edited 04/15/19 10:24pm by landyacht318 *

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