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Old 03-20-2024, 06:56 AM   #1
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Battery math question

Fair warning. I failed 7th grade math. So... this is all in prep for choosing new batteries. I'm going with two 6 volt golf cart batteries.

I'm trying to figure out this amp hours stuff. I've read a bunch of threads and found that I'm still confused. I asked an AI to figure it out. It probably gave me a good answer but it assumed a 120 volt system.

I have a 12 volt refrigerator. The label says it's power rating is 13.5 amps. The AI told me to convert the amps to watts but then reversed it in the final formula making it more or less 13.5xhours, with the hours being relative since the 13.5 is the max power draw so I would estimate more like 8 hours being 16 hours of functionality. I have found AI to be a fun toy but completely unreliable so I'd like a human answer.
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Old 03-20-2024, 07:28 AM   #2
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I have lithium batteries that have bluetooth and show me exactly how many amps are getting discharged.... my fridge 10 cubic foot , uses about 5 amps
-----------------------------------------------------------------
lead acid.... you can buy a amp meter (cheapest option)
or a shunt .... electronic calculator for a battery it does the math for you
--------------------------------------------------------------------------

In real world.... Get the fridge COLD at home or campground using 120v shorepower and then switch to battery.... you won't see the fridge use 13.5 amps from the battery

I could see a discharge of about 5 amps when the compressor was running
it was running only close to half the time

Your 2 x 6 volts commonly comes in 225 amp hours (or 110ah)
when you join them in series to get 12v you only have 225 ah max
but for lead acid type batteries you can't go below 50-80% of the capacity (ah)
Most people use the 50% rule ...
so your true usable capacity is about 110ah for a 225 battery.

5 amps continuous draw battery will last you around 20 hours 110 / 5 = 22
fridge only on 50% of the time you could almost double the time
Depends how often you open/close the fridge
how warm stuff is you put inside
how warm the temperature outside the fridge...
-------------------------------------------------
When you drive ... your alternator should replace the power your fridge uses
maybe even put some charge back into the battery
---------------------------------------------------------------
add 1 or 2 solar panels and controller ... to keep fridge and battery happy

-------------------------------------------------------------------------------
Lithium batteries 200ah cost is always dropping got mine for $400
I can run fridge for about 60 hours , so can spend a weekend as long as I am careful with other power usage.
switching to lithium is not hard . Is safe... lifepo4 style for RV's are NOT prone to catching fire like electric cars and scooters
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Old 03-20-2024, 07:47 AM   #3
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Quote:
Originally Posted by Aussieguy View Post
I have lithium batteries that have bluetooth and show me exactly how many amps are getting discharged.... my fridge 10 cubic foot , uses about 5 amps
-----------------------------------------------------------------
lead acid.... you can buy a amp meter (cheapest option)
or a shunt .... electronic calculator for a battery it does the math for you
--------------------------------------------------------------------------

In real world.... Get the fridge COLD at home or campground using 120v shorepower and then switch to battery.... you won't see the fridge use 13.5 amps from the battery

I could see a discharge of about 5 amps when the compressor was running
it was running only close to half the time

Your 2 x 6 volts commonly comes in 225 amp hours (or 110ah)
when you join them in series to get 12v you only have 225 ah max
but for lead acid type batteries you can't go below 50-80% of the capacity (ah)
Most people use the 50% rule ...
so your true usable capacity is about 110ah for a 225 battery.

5 amps continuous draw battery will last you around 20 hours 110 / 5 = 22
fridge only on 50% of the time you could almost double the time
Depends how often you open/close the fridge
how warm stuff is you put inside
how warm the temperature outside the fridge...
-------------------------------------------------
When you drive ... your alternator should replace the power your fridge uses
maybe even put some charge back into the battery
---------------------------------------------------------------
add 1 or 2 solar panels and controller ... to keep fridge and battery happy

-------------------------------------------------------------------------------
Lithium batteries 200ah cost is always dropping got mine for $400
I can run fridge for about 60 hours , so can spend a weekend as long as I am careful with other power usage.
switching to lithium is not hard . Is safe... lifepo4 style for RV's are NOT prone to catching fire like electric cars and scooters
So in summary, if an appliance uses one amp while in operation for one hour it is said to have used one amp hour. With something like a refrigerator, you will need real world data to account for erratic electrical demands from the appliance.

With a larger appliance like a refrigerator, you could turn off all other appliances and measure the depletion of your battery for a 24 hour period (one day). Or, you could probably just Google the data on the internet.

In all cases, the usage would only be an approximation.
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Old 03-20-2024, 07:55 AM   #4
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I agree with going lithium, I have 2 - 100AH on order and would not consider lead acid in a camper at this point. I don't think it is really needed but I got ones with low temperature charge protection just in case.

To get maximum capacity you will need a lithium charger, converter or solar. But your current converter will get you to 80-90% charge which isn't bad.

The 2 6volt batteries would probably be ok but they won't last as long and you need to check/add distilled water regularly
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Old 03-20-2024, 08:08 AM   #5
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Since your fridge draws 13.5 amps. Just multiply 13.5 x the number of hours you want your fridge to run on battery.


If you want a full 24 hours of fridge run time 13.5 x 24 = 324 amp hours of battery. Round that up to 330 to account for assorted losses. 48 hours wold require 660aH. 12 hours would require 165aH and so on.
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Old 03-20-2024, 08:40 AM   #6
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Hopefully you have verified that 13.5 figure by reading the fridge's user manual (download it if you don't have it), if you don't have the factory specs you may be chasing your tail...
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Old 03-20-2024, 08:49 AM   #7
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I too am thinking about lifepo4 batteries but will probably ride my AGMs until they need to be replaced. That said our real life experience is.

In 80ish degree daytime ambient temperature with a furrion 10 sqft fridge seeing a 6 hour run time per day. Fridge control set on 2. We can run indefinitely “5 day test” while camping and using stuff normally. With 300 aH of AGM on 350 watts of solar on nice sunny days

I have vented the cabinet the fridge is in to let it work more efficiently. This cut the run time by 50%
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Old 03-20-2024, 08:59 AM   #8
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Hopefully you have verified that 13.5 figure by reading the fridge's user manual (download it if you don't have it), if you don't have the factory specs you may be chasing your tail...
You will only see that 13.5 on startup of a warm fridge on 12.0 volts. Once the fridge is down to temperature the draw will go down quickly as the system finds its balance. Also pretty much every battery system will consistently run above 12 volts reducing the actual amperage draw.

That 13.5 is an absolute worst case number. You will never see it unless there is something wrong
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Old 03-20-2024, 09:04 AM   #9
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13.5 amp continuous draw seems very high to me but... Any idea the cycle time?

13.5 amp draw for 1 hour is 13.5 amphours. 5th Grade math. A single Group 24 battery typically says it has a 75 amphour capacity so in a fantasy world that single battery can run the refrigerator less than 6 hours. In that same fantasy world a 225 amphour battery bank will do it for 16 hours. Neither of these is acceptable to me.

What's the fantasy? Amp hour ratings on batteries are a lie. Maybe technically correct but once the battery voltage gets down to a certain point nothing will run.

-- Chuck
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Old 03-20-2024, 09:51 AM   #10
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13.5 amp continuous draw seems very high to me but... Any idea the cycle time?

13.5 amp draw for 1 hour is 13.5 amphours. 5th Grade math. A single Group 24 battery typically says it has a 75 amphour capacity so in a fantasy world that single battery can run the refrigerator less than 6 hours. In that same fantasy world a 225 amphour battery bank will do it for 16 hours. Neither of these is acceptable to me.

What's the fantasy? Amp hour ratings on batteries are a lie. Maybe technically correct but once the battery voltage gets down to a certain point nothing will run.

-- Chuck
Maybe you should read the previous posts and your questions may be answered!!

Maybe things would be more acceptable ………..to you……..
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Old 03-20-2024, 09:56 AM   #11
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If you decide to use flooded lead acid batteries, you will need double the amount of ampere hours than I stated up in post #5. FLA batteries can only be discharged to 50% of their rated capacity before battery damage occurs. This isn't an issue with lithium.
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Old 03-20-2024, 06:38 PM   #12
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You will only see that 13.5 on startup of a warm fridge on 12.0 volts. Once the fridge is down to temperature the draw will go down quickly as the system finds its balance. Also pretty much every battery system will consistently run above 12 volts reducing the actual amperage draw.

That 13.5 is an absolute worst case number. You will never see it unless there is something wrong

I mentioned the manual because 13.5 amps seems unusually high, Our 10 cubic foot is 11 at startup and max cooling. Besides the current draw at max and average, the manual may state battery life based on size and temp assumptions.
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Old 03-21-2024, 10:37 AM   #13
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The real question

My opinion is that it doesn't matter what the start/run amps are. The bottom line is; how much does it use per day. Follow up question is how much battery capacity is needed.

My latest test I ran my 2-way (LP vs 120VAC) for 24 hours using my inverter and battery bank. It used 390Ah of my capacity. I wouldn't normally do that but it does let me know that I could if I needed perhaps during a long ferry ride where no LP is allowed.

A compressor refrigerator runs with about 75-100Ah per day. My 300W of solar can replace 100Ah on a decent day.
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Old 03-21-2024, 04:30 PM   #14
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The 50% thing is a myth.

when you join them in series to get 12v you only have 225 ah max, True !

but for lead acid type batteries you can't go below 50-80% of the capacity (ah) False!

Why not ? How can the battery industry get away with selling products that only produce 1/2 the listed energy ?

Batteries lose capacity thru use. You can us 100% of its capacity, but to leave a bit of cushion, don't go below 20% capacity. Most battery capacity graphs will list the voltage at 20%.
Using the 50% myth, you will be throwing out batteries with 1/2 of there capacity left.
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Old 03-21-2024, 06:40 PM   #15
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@Slammer, what type of camping are you doing/going to do?
What kind of camper?

I know I used approximately 75AH over maybe 12-13 hours running a heater and the refrigerator in colder temps. Does not tell you squat though.
It does say the refrigerator was not running 13.5 amps all the time or my batteries would have been dead. I have 150 amp hours of batteries on board, AGM. Figuring 50% run time for a refrigerator is usually a safe calculation.

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The 50% thing is a myth.
Agree. They make is sound like you kill your battery going below 50%. Anybody ever leave your lights on in your car and run down the battery completely? Charges up fine and continues to hold a charge.
Sure you may only get four or five years out of a battery instead of seven if you run down excessively or below the 50% all the time but going below 50% does not kill a battery. Just shortens the life.
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Old 03-21-2024, 07:37 PM   #16
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IN SHORT
don't try to cool down the HOT fridge using battery power
that's what shorepower 120v +the converter is for.

MY simple test
amp draw when cold + compressor running was 5 amps (MEASURED with battery BMS)

Run TIME was 55 hours on 200ah + had at least another 4-5 hours left when I stopped the test

whether it is lithium or lead acid ..... fridge don't care what battery type supplied it

MY results from a 200ah battery bank ..............
60 hours x 5 amps = 300ah
the cycle of on/off of the fridge allowed battery to run 50% more time
Room temperature was 93degrees during hot part of the day ... forgot to turn on air conditioner

--------------------------------------------------------------------------------------
you can buy a shunt or ammeter to measure amps
turn on your different loads ... and work out estimate run TIME for that load, example:
5 amp fridge x 12hours a day run time = 60ah
8 amps CPAP x 8 hours night run time = 64ah
2 amps phone charge x 1 hour = 2ah

TOTAL = 126ah battery get 200ah battery bank to be safe
Factor in if you will be driving or have some solar

OR get a good multimeter that can read DC amps using a clamp
Multimeter will serve you well for other ....... troubleshooting and testing in a RV
don't leave home without one!

You can run down a lead acid battery ... and shorten it's lifespan
You can also abuse a Lifepo4 and shorten it's lifespan
Up to you...

Price of lifepo4 + it's benefits makes it hard to justify sticking with lead acid.
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Old 04-05-2024, 07:52 PM   #17
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I installed this battery monitor right in the plastic lid of my battery box. Super simple. Less than 1/2 hour to install. It will tell you what you need to know for sure.
https://www.amazon.com/dp/B07FGFFHC6/
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