DC Power Issue understanding

[pmsherman]

larryandamy gave you one of the best responses to help you isolate the problem. There are a number of factors that are causing the problem you're having and I suspect that the alarm is a combination of more than one, which will make it very difficult to troubleshoot. The immediate problems that come to mind are:
1. Battery is not fully charged - possible but likely a minor contribution to the problem
2. Battery output is effected by temperature, when cold, batteries exhibit less capacity
3. Batteries have internal resistance. The higher the current load on them, the lower the output voltage from them. Adding the CPAP and furnace motor current draws is likely a major contributor to the problem.
4. Voltage drop in the wiring from the battery to (in this case) the propane detector.

In my 2011 Georgetown, the propane detector is wired directly to the house battery, giving it the maximum possible voltage. Your rig may not have heavy enough wire from the battery and the CO detector is connected at the distribution panel where the fuses are located. In this case, voltage drop in the wire from its resistance will exacerbate the problem, as mentioned by larryandamy in their prior post.

I solved the idiot light charge level problem by installing a digital voltmeter (~$10) that reads down to 1/100 of a volt in my Georgetown. I have a set of 2/0 wires from the kitchen area to the battery and just attached the meter to them. These heavy wires feed an inverter that's never used when we're not travelling so they made a convenient attach point for the meter. I also recalibrated the purchased voltmeter using my calibrated meter because very few of these low cost meters are accurately calibrated when manufactured.

Phil

[Herk7769]

A thought occurred to me while I was in the Dr. waiting room. One of the advantages of two 12 volt deep cycle batteries (vs two 6 volt batteries) is the ability to share the load across two electron generators with relatively thinner plates; thus much less Peukert Effect (the reduction in apparent capacity under load) on the battery system.

At lower amperage draws on equal AH systems, 2 6 volt deep cycle batteries will see nearly the same performance as a pair of 12 volt deep cycle batteries.

However at higher amperage demands, the ability to share the amperage load between the two 12 volt batteries reduces current demand from each battery; thus lower capacity penalty.

In the example below with a dual 12 volt 100AH setup the 35 amp at 12 volts (420 Watts) demand would be shared equally (with proper interbattery wiring) at 17.5 amps each. The Peukert Effect penalty would be reduced from 50% of rated total capacity to 35%. Since only the voltage and not the generation capability is doubled by putting two 6 volt batteries in series; each 6 volt battery "sees" the full 35 amp demand and the full 50% penalty.

[Tom48]

Originally Posted by Edmund

I know there is simple answer, but I have been RVing for over twenty years and can't figure out what the acronym CRAP refers to, other than something my dog does.
Thanks.

CPAP Continuous Positive Airway Pressure

BiPAP Bi level Positive Airway Pressure

Breathing machine for those of us with a bit of Sleep Apnea.

To keep us from stopping our breathing 10-30 times an hour and causing our self brain and heart damage

[Tom48]

Originally Posted by Appalachian Ed

I forgot to mention that I do have 2 Honda 2000W generators and run them about 2-4 hours when boondocking with the 30 amp plug connected to charge and to run the coffee post etc...

I just went through the menus on my CPAP and found an option to set the humidity level to off. I will try it tonight. Not sue how much I'll need the furnace tonight though. We pulled off in the Mojave around Barstow CA to sleep.

Thanks again. I really enjoy and learn so much from this community.

Where you at and were are you going. We are in Chino Area about 90 miles South of Barstow. I have a bad itch to work on an RV since we sold our TH a month ago and still have not gotten delivery of our new FR TH. I also have a friend who is something of an automotive electrical GURU who lives about 40 miles down the 15 from you who would always be up for a challenge. Pm me is you are frustrated enough to reach out for help.

Tom

[ScottBrownstein]

Originally Posted by Herk7769

A thought occurred to me while I was in the Dr. waiting room..

That is a really interesting thought. Have to mull that over but it seems to indicate that any installation that has multiple parallel batteries would derive a benefit from a lower Peukert penalty by keeping the load current on each individual battery to a minimum.

Thought provoking...but...that just seems to simple!

[Herk7769]

Originally Posted by ScottBrownstein

That is a really interesting thought. Have to mull that over but it seems to indicate that any installation that has multiple parallel batteries would derive a benefit from a lower Peukert penalty by keeping the load current on each individual battery to a minimum.

Thought provoking...but...that just seems to simple!

There has to be a reason for the popularity of 12 volt deep cycle batteries where weight is a consideration. If you had room for 4 batteries and wired as indicated, the current demand per battery would be 1/4 of total load, while the 4 6 volt batteries would see 1/2 of the load per 12 volt cell.

When the PE in the previous example is taken into account, the longevity differential of the bank would be huge.

35 amp load @ 12 volts across 4 12 volt batteries would see only 8.75 amps per battery and the 100AH battery capacity would only be reduced by 15% or 85AH per battery. The longevity of the bank would be 340AH.

35 amp load @ 12 volts across 2 12 volt (2 6 volt in series) batteries would see 17.5 amps per set and the 200AH battery set capacity would be reduced by 35% or 130AH per battery set available. The longevity of the bank would be 260AH.

[Herk7769]

Devil's advocate: The opposite view

Assumptions:

All Batteries (6 and 12 volt) are physically the same size and have the same internal construction. What varies is the thickness of the plates for more storage in 6 volt batteries. The 6 volt batteries have plates twice as thick for twice the storage, but there is only room in the container for 3 cells (not 6 like a 12 volt battery).

The electrons needed to produce current demand are only "available" at the surface of the plate and it takes "time" for electrons held deeply in the lead to migrate to the surface. The thicker the plates the more storage the battery has, but the less able that battery is to generate large currents.

Peukert Effect is the capacity penalty incurred when deep cycle battery thick plates are tasked to give up more electrons than they can immediately generate (without recovery time). Voltage (and available capacity) drops due to the increase in internal resistance when you try.

In a parallel circuit, the current is equally split through all branches provided the resistance is equal in all branches.

In a series circuit, the current is the same through both resistances, the only thing that could vary is the voltage drop across each resistor. The voltage (and available capacity) drop is identical if both resistances are the same.

Did I miss anything?

[szabla]

In a parallel circuit, the current is equally split through all branches provided the resistance is equal in all branches

On paper that is true, but in the real world I have never seen a system where each cell had the same internal resistance or even end up with the same specific gravity. That is why we work in acceptable tolerances. Even the cables act as shunts and the batteries at the end of the lines have a slight current loss.
To most lay people they will never see or recognize this.

[B and B]

Originally Posted by Herk7769

A thought occurred to me while I was in the Dr. waiting room. One of the advantages of two 12 volt deep cycle batteries (vs two 6 volt batteries) is the ability to share the load across two electron generators with relatively thinner plates; thus much less Peukert Effect (the reduction in apparent capacity under load) on the battery system.

At lower amperage draws on equal AH systems, 2 6 volt deep cycle batteries will see nearly the same performance as a pair of 12 volt deep cycle batteries.

However at higher amperage demands, the ability to share the amperage load between the two 12 volt batteries reduces current demand from each battery; thus lower capacity penalty.

In the example below with a dual 12 volt 100AH setup the 35 amp at 12 volts (420 Watts) demand would be shared equally (with proper interbattery wiring) at 17.5 amps each. The Peukert Effect penalty would be reduced from 50% of rated total capacity to 35%. Since only the voltage and not the generation capability is doubled by putting two 6 volt batteries in series; each 6 volt battery "sees" the full 35 amp demand and the full 50% penalty.

Herk makes valid point here, Good work.

[B and B]

I may have missed this somewhere, I aplogize if so.


In series 6 volt string if one cell has bad plates this may very contribute to voltage loss quickly when a load is applied causing low voltage. It could be as simple as a bad battery with 1 cell bad.

[ScottBrownstein]

Originally Posted by Herk7769

Devil's advocate: The opposite view...

At first I excitedly thought we were on to something, but now I am not so sure. Here are my assumptions. 12 volt system has 4 batteries with 100 AH capacity each, 6 volt system has 2 sets of 2 6 volt batteries, each with 200 AH capacity. These do not correspond exactly to any battery specs, but they are useful for a "gedanken" experiment.

Using these numbers and Chris Gibson's Smartgauge Excel spreadsheet, we get the following results:

12 volt capacity at 8.75 amps each is 87.43 AH or a total bank capacity of 349.72

6 volt capacity at 17.5 amps is 174.86, bank capacity is...wildly, exactly the same...349.72!

Now how could this be? Each 6 volt battery has twice the AH rating as the 12s and also exactly twice their 20 hour rate amperage. Peukert doesn't care about voltage as it is an electron, electrolyte and plate phenomenon.

Simply stated, when we double the current through the two 6 volt pairs, we also double their AH capacity and their 20 hour rate and it seems to all work out to exactly the same number!

Thoughts? Comments? Seems straightforward, did I go wrong somewhere?

[ScottBrownstein]

Originally Posted by B and B

I may have missed this somewhere, I aplogize if so.
In series 6 volt string...

Basic battery failure mechanism is pretty much always a bad cell and it all goes downhill from there. 12 volt units have the same kind of cells and are susceptible to the same failure mechanism.

[Herk7769]

Originally Posted by ScottBrownstein

At first I excitedly thought we were on to something, but now I am not so sure. Here are my assumptions. 12 volt system has 4 batteries with 100 AH capacity each, 6 volt system has 2 sets of 2 6 volt batteries, each with 200 AH capacity. These do not correspond exactly to any battery specs, but they are useful for a "gedanken" experiment.

Using these numbers and Chris Gibson's Smartgauge Excel spreadsheet, we get the following results:

12 volt capacity at 8.75 amps each is 87.43 AH or a total bank capacity of 349.72

6 volt capacity at 17.5 amps is 174.86, bank capacity is...wildly, exactly the same...349.72!

Now how could this be? Each 6 volt battery has twice the AH rating as the 12s and also exactly twice their 20 hour rate amperage. Peukert doesn't care about voltage as it is an electron, electrolyte and plate phenomenon.

Simply stated, when we double the current through the two 6 volt pairs, we also double their AH capacity and their 20 hour rate and it seems to all work out to exactly the same number!

Thoughts? Comments? Seems straightforward, did I go wrong somewhere?

That is what I said in the assumptions. At 5 amps draw (where the AH definition comes from), there is no difference between 2 100AH 12 volt batteries and 2 6 volt 200AH batteries. The difference only becomes apparent when the Peukert Effect is considered at higher amperage demand on the system.

https://en.wikipedia.org/wiki/Peukert%27s_law

In that article is a link to Chris Gibson's Peukert calculator. The forum software will not allow the upload of xls (Excel) documents.

If you use the exponent for lead acid flooded cell batteries (1.44), the adjusted AH at the various amps we discussed seems to validate the 12 volt argument.

[Herk7769]

Here is a great article that clarifies the effect during charging as well.

SmartGauge Electronics - Peukert and charging

I was surprised to learn that the effect also takes into account the resistance of the electrolyte as well as the plate and structure resistance.

[Herk7769]

Originally Posted by ScottBrownstein

Simply stated, when we double the current through the two 6 volt pairs, we also double their AH capacity and their 20 hour rate and it seems to all work out to exactly the same number!

Thoughts? Comments? Seems straightforward, did I go wrong somewhere?

I think we are confusing the issue with 4 12 volt batteries. If we use 2 12 volt batteries and 2 6 volt batteries, it will make it easier for my poor brain.

Lets look at that 35 amps through the 2 12 volts (17.5 amps) and 35 amps through the 2 6 volts (but in series).

How does that work out in your spreadsheet?

The only way I see them coming out the same is if the 35 amp current is split between the 2 6 volt batteries so each never sees the full 35 amps (just 17.5 like the 12 volt ones). That just flies in the face of how the internal resistance associated with the two batteries is effected by the 35 amps flowing through the circuit.

Using the 5 amp discharge rate, a 200 AH battery lasts 40 hours, just like the 2 12's. It is only when 35 amps flow through that "giant battery pair" that PE seems to swing the scale.

[ScottBrownstein]

Originally Posted by Herk7769

If you use the exponent for lead acid flooded cell batteries (1.44), the adjusted AH at the various amps we discussed seems to validate the 12 volt argument.

That is exactly the calculator that I used to derive the results that I posted. As an aside, however, the Peukert coefficients are not anywhere near 1.45.
Trojan publishes their values as:

6 Volt
T105 = 1.24
T-125 = 1.23
T-145 = 1.25
L-16H = 1.79

12 Volt
SCS-150 = 1.29 (group 24)
SCS-200 = 1.22 (group 27)
SCS-225 = 1.24 (group 31)

Mathematically it seems to me that your hypothesis doesn't hold since when you halve the load current, you also halve the 20 hour discharge amperage as well so the benefit is cancelled out by the lower AH rating and 20 hour current value. My calculations were for 4 12 volt batteries vs 4 6 volt batteries.

[ScottBrownstein]

Originally Posted by Herk7769

I think we are confusing the issue

OK, 2 12 volt batteries, as you say, will cut the current in half to 17.5 amps. But, the 20 hour rating of the two 6 volt batteries in series will be twice that of the 12 volt batteries so it will be the same percentage of the 20 hour rate in both cases. There may be small differences in the Peukert coefficient for the batteries in question, but those differences will be relative small.

Thought you were really on to something here, but whenever you take out a 12 volt unit you replace it with two 6 volt units that generally have twice the AH rating so I think we are chasing our tail here.

[Herk7769]

Originally Posted by ScottBrownstein

OK, 2 12 volt batteries, as you say, will cut the current in half to 17.5 amps. But, the 20 hour rating of the two 6 volt batteries in series will be twice that of the 12 volt batteries so it will be the same percentage of the 20 hour rate in both cases. There may be small differences in the Peukert coefficient for the batteries in question, but those differences will be relative small.

Thought you were really on to something here, but whenever you take out a 12 volt unit you replace it with two 6 volt units that generally have twice the AH rating so I think we are chasing our tail here.

Thanks, I knew it was too simple to be true.

[zuterbru]

Here is a question, what gauge wire did you use to connect the batteries together to get 12V? Too small a gauge will cause a voltage drop.

All the suggestions are good, but you might need to get bigger batteries. I love love love AGM batteries! They last forever and have never failed me, and you never have to check the level on them or add water. I have 2 of them and they work great. Sounds like you are having to much draw at one time and the batteries you have can't keep up. Check the terminals on the batteries, make sure they are clean. Might still want to clean them up with a wire brush.

[Tom48]

Originally Posted by zuterbru

Here is a question, what gauge wire did you use to connect the batteries together to get 12V? Too small a gauge will cause a voltage drop.

All the suggestions are good, but you might need to get bigger batteries. I love love love AGM batteries! They last forever and have never failed me, and you never have to check the level on them or add water. I have 2 of them and they work great. Sounds like you are having to much draw at one time and the batteries you have can't keep up. Check the terminals on the batteries, make sure they are clean. Might still want to clean them up with a wire brush.

And find the point that the negative bolts to the frame and pull that off too. Clean it up with that wire brush and put it back with care to be sure you have a really solid ground. The whole system comes down to that one point and mine looked great, but the lights dimmed every time the water pump came on and finally after testing my batteries until I was blue in the face I just happened on to hidden rust under that Ground Connection that was the cause of all sorts of symptoms.