Calculating ARB or any fridge run time, math help.

[richardalan1975]

I posted this in a FB group and didnt get a response. I need some help calculating the run time of ARB or any fridge. I thought our battery was failing, I remember running nearly two days before needing to charge. The math to support the above isn't adding up, please help me determine an approximate time before the fridge cuts off. I have a PC1100 Odyssey 45AH. I am seeing about .90 as the current draw per hour opening it two times during the the last 6 hour test. The fridge suspends drawing power from the battery at 10.1v on the low setting. Am I really looking at a half a day? On the monitor as of late I am seeing about 80% before I have to start recharging the battery to turn the fridge back on. Any input would be much appreciated.

The battery is a managed by a REDARC 1225D. The REDARC, AUX battery and fridge connect are within inches of each other. The fridge sits on top of the space where everything is stored. Give it to me straight DOC, am I really looking at half a day? Can you show me the math, I understand there are variables unknown. Just a best guess under ideal conditions.

 

[DRAX]

In a perfect world, meaning no efficiency losses in the electrical system and a battery that can actually supply 45AH that would translate into 50 hours of runtime if the fridge averages 0.9A/hr. Having the fridge pull the battery all the way down to 10.1v is going to severely shorten the life of the battery because you're using a starter battery as a deep-cycle battery. Granted, that 10.1v is the cut-off while under load. What is the battery voltage with no load after the fridge cuts off?

Is the battery ONLY powering the fridge?
How old is the battery?
How frequently do you wait until close to 0% SoC to recharge it?

Odyssey says that battery is good for 400 cycles at 80% discharge. Going beyond 80% discharge will greatly reduce the number of cycles before the battery is toast.

A comparable battery in terms of capacity but actually meant to be used the way you're using it would be a Dakota 54Ah deep cycle - https://dakotalithium.com/product/dakota-lithium-12v-54ah-deep-cycle-marine-trolling-motor-battery/

11 year warranty and maintains 80% of capacity after 2,000 cycles. Starter batteries used in deep cycle operation just don't last very long. The only time I would use a starter battery as a power source is if it were permanently connected to solar or was otherwise never allowed to get below 60% SoC.

Your math isn't wrong (45Ah / 0.9A/hr = 50 hours), I just think your battery may have reached the end of life. It will still charge fully and power things, but the capacity is no longer there and that is why your runtime is not what it used to be.

 

[richardalan1975]

In a perfect world, meaning no efficiency losses in the electrical system and a battery that can actually supply 45AH that would translate into 50 hours of runtime if the fridge averages 0.9A/hr. Having the fridge pull the battery all the way down to 10.1v is going to severely shorten the life of the battery because you're using a starter battery as a deep-cycle battery. Granted, that 10.1v is the cut-off while under load. What is the battery voltage with no load after the fridge cuts off?

Is the battery ONLY powering the fridge?
How old is the battery?
How frequently do you wait until close to 0% SoC to recharge it?

Odyssey says that battery is good for 400 cycles at 80% discharge. Going beyond 80% discharge will greatly reduce the number of cycles before the battery is toast.

A comparable battery in terms of capacity but actually meant to be used the way you're using it would be a Dakota 54Ah deep cycle - https://dakotalithium.com/product/dakota-lithium-12v-54ah-deep-cycle-marine-trolling-motor-battery/

11 year warranty and maintains 80% of capacity after 2,000 cycles. Starter batteries used in deep cycle operation just don't last very long. The only time I would use a starter battery as a power source is if it were permanently connected to solar or was otherwise never allowed to get below 60% SoC.

Your math isn't wrong (45Ah / 0.9A/hr = 50 hours), I just think your battery may have reached the end of life. It will still charge fully and power things, but the capacity is no longer there and that is why your runtime is not what it used to be.

I can not thank you enough for chiming in.

For the purpose of this experiment the battery is only powering the fridge. An on board air compressor, rear tail gate light and a few DC ports are also tied into the blue sea block.
The battery was purchased from AAL and installed 16 months ago.
The battery is rarely drawn past 80%. The fridge has a cut off I do not see how to bypass at 10.1 and I think the REDARC will stop looking for it at 8.1. I am cherry but I guess voltage doesnt simply follow AMP hours down to zero :) You can LOL at that... I do faintly recall early on the SOC dropping down lower than 80% and the fridge still running.
The fridge is acting odd at times as if its being underpowered.

To say this back to you, I should be able to get to a SOC of 70 or 60 or even 50 and still meet the 10.1v to keep the fridge running with the right battery?

My current battery charges back to 100% so quickly recently, one of the initial reasons I was thinking it was the battery. When the fridge gets to 80% SOC the voltage is around 10.5.

Thank you for recommending a replacement, any reason the REDARC can not manage your suggested battery? I need to look closer at the specs to see if it will fit my space.

 

[DRAX]

I can not thank you enough for chiming in.

For the purpose of this experiment the battery is only powering the fridge. An on board air compressor, rear tail gate light and a few DC ports are also tied into the blue sea block.
The battery was purchased from AAL and installed 16 months ago.
The battery is rarely drawn past 80%. The fridge has a cut off I do not see how to bypass at 10.1 and I think the REDARC will stop looking for it at 8.1. I am cherry but I guess voltage doesnt simply follow AMP hours down to zero :) You can LOL at that... I do faintly recall early on the SOC dropping down lower than 80% and the fridge still running.
The fridge is acting odd at times as if its being underpowered.

To say this back to you, I should be able to get to a SOC of 70 or 60 or even 50 and still meet the 10.1v to keep the fridge running with the right battery?

My current battery charges back to 100% so quickly recently, one of the initial reasons I was thinking it was the battery. When the fridge gets to 80% SOC the voltage is around 10.5.

Thank you for recommending a replacement, any reason the REDARC can not manage your suggested battery? I need to look closer at the specs to see if it will fit my space.

So, there are a lot of different tests and calculations you can do to determine SoC and capacity of a battery. SoC is related to voltage but any load on the battery must be part of the equation. To make this simple, we'll assume there is 0 load on the battery (fully disconnected). If the battery reads 11.7-11.9v with no load then it is depleted. That's generally 10% SoC. Below 11.7v and you risk killing the battery or severely shortening its life. 10.1v under load isn't necessarily a problem because the load affects the SoC calculation, so to keep it simple I say just check the voltage with no load. Also, if the battery was being charged make sure a minimal load is put on the battery for a few minutes to remove the surface charge, then take the voltage reading when there's no load. The surface charge will give a false SoC.

You also say the battery charges back to 100% very quickly, this is another sign that the battery is near end-of-life. When you think about charging a battery think about refilling a water tank. All else being equal, a larger tank is going to take longer to fill than a smaller one. So when a battery's capacity has been greatly diminished due to use you basically have a smaller tank. It charges faster because it has less capacity. Less capacity means it's depleted faster with no change in load/discharge rate.

You also mention the fridge acts underpowered, that could also be a sign of a shorting or damaged cell. These are also common as a battery is consumed and is nearing EOL. Most of my failing batteries have actually been due to a dead or shorted cell that can be intermittent.

That REDARC will work just fine with the Dakota battery, just make sure you configure it for the Lithium charge profile.

Back to your question about voltage with the fridge running, what you need to know is the draw when running and not the hourly average. For argument's sake let's say it's drawing 60w @ 12v. That's 5A or a C-rating of roughly 0.11 for that 45Ah battery. An AGM with a C-rate discharge of 0.10 will be at roughly 10% SoC when the battery reads 11.2-11.3v under load. If you're seeing 10.1v with the fridge running then that is absolutely 0% SoC as 0.1C @ 10.2v = 0% SoC. Permanent battery damage is very likely.

Again, in a perfect world. I would also be checking for voltage drop between the battery and fridge. Even though they're close, the size and length of wires will affect voltage drop. If the fridge is showing the same voltage as the battery then great, but if you're seeing 10.1v at the fridge and 11v at the battery then that's some pretty major voltage loss for such a short run.

Not trying to make it overly complicated or anything, there are just a lot of variables. Based on what I'm seeing here, if you're seeing 10.1v at the battery with only the fridge running then you're literally killing the battery or it's already mostly dead., unfortunately. The battery may never have the same capacity again which is what you're experiencing now.

I wouldn't let the battery drop below 11v with the fridge running or 12v with no load.

 

[1Louder]

Based on experience you need a larger battery. There is no exact science to this. Buy a Watt Meter from Amazon and put it inline with fridge and your battery. Check it every hour to see how much power it consumes. The problem is there are too many variables for consistent data. How full is your fridge? What temperature? How hot is it inside your vehicle..... on and on.

I personally would recommend a battery around 95 AH unless you are going the lithium route. That is based on 8 years of experience running a variety of fridges. Solar to keep the battery charged whenever possible as well. I have 100 watts on my vehicle roof. It works great in the southwest. Portable options are great as well.

 

[richardalan1975]

Thank you both for continuing to guide me. I have very limited space, the suggested battery is about 1.5" wider and may not fit. Going to open up the area soon and have a look at my options. I am now convinced the battery is going bad and I am not in the best battery for my application. I can see looking back to a few months after the install the battery reaching 60 & 70 SOC over 12Ah used and the voltgage holding steady at 12.43. Today I can not even get to 85% without being in the 10's for voltage. Under no load the voltage currently is 12.10 after fulling charging it. Attaching an image of my space.

 

[diabetiktaco]

Based on experience you need a larger battery. There is no exact science to this. Buy a Watt Meter from Amazon and put it inline with fridge and your battery. Check it every hour to see how much power it consumes. The problem is there are too many variables for consistent data. How full is your fridge? What temperature? How hot is it inside your vehicle..... on and on.

I personally would recommend a battery around 95 AH unless you are going the lithium route. That is based on 8 years of experience running a variety of fridges. Solar to keep the battery charged whenever possible as well. I have 100 watts on my vehicle roof. It works great in the southwest. Portable options are great as well.

I agree. A 45AH battery is small. I have a single 80AH battery running multiple things but also powered by solar. Works well for me but I'm probably pushing the limits with that.

 

[LanceMagnum]

Looks to me like you could upgrade to 200 or more AH of lithium with just a little re-arranging of the items in your storage area. I've got 200 in mine and since it's LiFePo4 I can run down to 20% SOC if needed. Let me know if you want any details. I'm running a Manager 30 but maybe the 1225D has enough protection circuitry to run raw cells.

 

[diabetiktaco]

Looks to me like you could upgrade to 200 or more AH of lithium with just a little re-arranging of the items in your storage area. I've got 200 in mine and since it's LiFePo4 I can run down to 20% SOC if needed. Let me know if you want any details. I'm running a Manager 30 but maybe the 1225D has enough protection circuitry to run raw cells.

View attachment 203622

That's only SLIGHTLY more expensive.

 

[richardalan1975]

Looks to me like you could upgrade to 200 or more AH of lithium with just a little re-arranging of the items in your storage area. I've got 200 in mine and since it's LiFePo4 I can run down to 20% SOC if needed. Let me know if you want any details. I'm running a Manager 30 but maybe the 1225D has enough protection circuitry to run raw cells.

View attachment 203622

I would love some details! If I could get to 100AH that would be great. Any chance you have a link to your complete build?

 

[LanceMagnum]

I do have a build thread on another forum, it's a portal where people talk about expedition.

Thread is called 'yet another white rubicon', I'll try putting the link here:

Yet Another White Rubicon - JLUR Build

There are many, but this one is mine. Here's a pic from one of the first outings in the White Mountains National Forest - not as easy to find secluded primitive camping as in many other national forests, but if you study the maps there are a few places:

expeditionportal.com

Many details are not in the post about the battery system but I'm happy to answer any questions.

 

[richardalan1975]

I do have a build thread on another forum, it's a portal where people talk about expedition.

Thread is called 'yet another white rubicon', I'll try putting the link here:

Yet Another White Rubicon - JLUR Build

There are many, but this one is mine. Here's a pic from one of the first outings in the White Mountains National Forest - not as easy to find secluded primitive camping as in many other national forests, but if you study the maps there are a few places:

expeditionportal.com

Many details are not in the post about the battery system but I'm happy to answer any questions.

Amazing, I skimmed through it and will sit down tonight when I settle to read it throuighly for ideas. I envy the easy access to your AUX battteries and REDARC.

This is me Redarc, Fridge, AUX Battery, Monitor, Platform & More - In One Spot - JLUR

I certainly could do some rearranging in the rear cubby, do you have a battery recomendation? I need it to work with the 1225D.

 

[LanceMagnum]

Nice! We have very similar approaches to our builds.

As far as a specific battery I don't have a recommendation, but here's some details about the process I went through.

There's lots of advice on diysolarforum, here's the topic for building your own bank and buying raw cells:

DIY LiFePO4 Battery Banks

Advanced user discussion on BMS/ Cell Configurations/ Temperature Regulation/ Cell Sources and more

diysolarforum.com

They have some specific vendors mentioned, and I used BLS:

BLS-lithium iron phosphate LiFePO4 battery ,LTO Battery and systems

BLS focus on design and development of lithium iron phosphate LiFePO4 battery ,LTO Battery and system,and developed battery self-heating scheme and load-weight scheme.These products are widely used on AGV, AMR, UPS, Golf Trolley Cart, Yacht, Fishing Boat, Motorhome, Auxiliary Power Supply System...

blslifepo4battery.com

As long as you get 'grade a' cells you should be good.

As far as the design and if it will work with your 1225D, it comes down to the protections. I am not using a BMS like you'd find on many DIY packs. I rely on the Manager 30 for temp protection so it does not charge below freezing - that's probably the most important one to confirm the 1225D supports if you go BMS-less. I also use an active balancer from Electric Car Parts:

Affordable Electric Vehicle Batteries & Components from Electric Car Parts Company

Electric Car Parts Company is your one-stop-shop for affordable electric vehicle parts and components including batteries. Shop our selection and order now!

www.electriccarpartscompany.com

It keeps the cells very balanced, just a few mV difference. Then the rest is just good fuses and a disconnect switch. The Manger 30 has other protection but I think they are more on the input side of things (over/under voltage, for example).

For the physical installation I have a piece of dense foam (the packing material from the shipping box for the cells), about 1' or so, underneath the cells to provide a little cushion, then use some tie-down straps and brackets like you'd find on a marine battery box.


I use jack nuts to hold things in place, they work well with the plastic of the cargo tub, but you have that nice bracket in yours so that looks more solid.

 

[richardalan1975]

Nice! We have very similar approaches to our builds.

As far as a specific battery I don't have a recommendation, but here's some details about the process I went through.

There's lots of advice on diysolarforum, here's the topic for building your own bank and buying raw cells:

DIY LiFePO4 Battery Banks

Advanced user discussion on BMS/ Cell Configurations/ Temperature Regulation/ Cell Sources and more

diysolarforum.com

They have some specific vendors mentioned, and I used BLS:

BLS-lithium iron phosphate LiFePO4 battery ,LTO Battery and systems

BLS focus on design and development of lithium iron phosphate LiFePO4 battery ,LTO Battery and system,and developed battery self-heating scheme and load-weight scheme.These products are widely used on AGV, AMR, UPS, Golf Trolley Cart, Yacht, Fishing Boat, Motorhome, Auxiliary Power Supply System...

blslifepo4battery.com

As long as you get 'grade a' cells you should be good.

As far as the design and if it will work with your 1225D, it comes down to the protections. I am not using a BMS like you'd find on many DIY packs. I rely on the Manager 30 for temp protection so it does not charge below freezing - that's probably the most important one to confirm the 1225D supports if you go BMS-less. I also use an active balancer from Electric Car Parts:

Affordable Electric Vehicle Batteries & Components from Electric Car Parts Company

Electric Car Parts Company is your one-stop-shop for affordable electric vehicle parts and components including batteries. Shop our selection and order now!

www.electriccarpartscompany.com

It keeps the cells very balanced, just a few mV difference. Then the rest is just good fuses and a disconnect switch. The Manger 30 has other protection but I think they are more on the input side of things (over/under voltage, for example).

For the physical installation I have a piece of dense foam (the packing material from the shipping box for the cells), about 1' or so, underneath the cells to provide a little cushion, then use some tie-down straps and brackets like you'd find on a marine battery box.
View attachment 203667View attachment 203668

I use jack nuts to hold things in place, they work well with the plastic of the cargo tub, but you have that nice bracket in yours so that looks more solid.

Tons of info there, I very much appreicate the post. I need to find something a little simplier, ideally a battery with the BMS included. I am really having a hard time finding something that will fit the space. How does everyone feel about using two idential LIFEPO4 batteries each with their own BMS together as one? After several hours of searching this is the best option I have found for the space. 12V 35Ah Lithium Battery for Renewable Energy and Mobility Applications | RELiON Moving a few things around I feel I could fit two of these and still have a litte space on all sides of the battery.

Has anyone had a custom battery made? If so by who? I have a seen companies that have the capabilities but not looked closely.

 

[smritte]

Unless I missed it, only @1Louder hit it correctly.
One if my fridges draws the same as yours (on paper). It can draw as much as 6 amps continually if its in a hot environment. I just had this conversation with a friend and his 50 (ish) qt Dometic. We calculated on a recent trip his fridge drew over 5 amps continually. It was in the vehicle with the freezer section turned down a bit and it was warm out. As was posted, you really need something to test the draw in your common environment. I find for mine my 120 watt solar keeps up in the warm times. When its cooler out my average draw is under an amp.

 

[1Louder]

Unless I missed it, only @1Louder hit it correctly.
One if my fridges draws the same as yours (on paper). It can draw as much as 6 amps continually if its in a hot environment. I just had this conversation with a friend and his 50 (ish) qt Dometic. We calculated on a recent trip his fridge drew over 5 amps continually. It was in the vehicle with the freezer section turned down a bit and it was warm out. As was posted, you really need something to test the draw in your common environment. I find for mine my 120 watt solar keeps up in the warm times. When its cooler out my average draw is under an amp.

I did not mention that years ago I tried to run a fridge off of a 2nd battery that was around 40-50AH. It did not work well.

 

[richardalan1975]

I got lost, wanted to circle back. Ive learned I was in the wrong battery type. Thank you. As a band-aid to the band-aid I borrowed a deep cycle AGM.



We spent a week at Ricketts Glen it was amazing. We hiked the Falls Trail with KIDS, turning back was not an option. Scarey crazy and just crazy. If you haven't hiked this you should.

The actual band-aid is a cheap, you get what you pay for Life4P04 on Amazon. Our space is limited and we needed to move quickly. We are about to pull out for 30+ days on the road mostly cheap hotels/motels and some camp grounds.