Battery SOC question

[happyvibe]

I was looking at the picture and it looks like two 6v batteries . They look like they are sitting the same way in the tray. And it looks like the two negatives and two positives are hooked up together. If this is the case you have a 6v battery not 12. Is there something I am missing with the way the batteries are hooked up?

[hclarkx]

Originally Posted by Presidio

We have been boondocking here for 3 days when this started. Before that, we were connected to the campground power (for a few months) and fully charged the batteries all the time. I even equalized the batteries a few days before we left ( I do that every 3 months or so)

So the batteries got down to 10.5V -- fully discharged -- over three days. I need to go back and review what you've written about those days....



And, it sure seems an SOC monitor drift is not the issue.

[757driver]

Presidio,

Let’s look at this from another angle. What Magnum inverter/charger do you have? Under the set menu what values are entered for each menu selection?

[hclarkx]

Presidio,


I just noticed, it's been about six months since your original post. How have the batteries and charging system and SOC monitor behaved since? Has the problem reoccurred? Are you now using solar?

[RV Vagabond]

Originally Posted by hclarkx

BTW, lead-acid batteries live longest if kept at or near 100% SOC. I.e., charging as often as is practicable and getting them to 100% will give longest life.

What good are batteries with a really long life if you cannot use them? I am not going to turn all my electrical devices off because my batteries are down to 80%. You will get about the same amount of use out of batteries regardless how low you discharge them. Battery life is not measured in cycles or years, but in the number of hours you have used them.

[SCVJeff]

Originally Posted by RV Vagabond

What good are batteries with a really long life if you cannot use them? I am not going to turn all my electrical devices off because my batteries are down to 80%. You will get about the same amount of use out of batteries regardless how low you discharge them. Battery life is not measured in cycles or years, but in the number of hours you have used them.

No, they ARE measured in cycles (go look at the cut sheets). Its well known that killing the batteries will reduce their life. And since they are measured in aggregated discharge cycles, getting them back up to 100% obviously stretches the advertised cycle life. As far as hours is concerned, that number is a longivety rating at a given current with a new battery. They obviously can’t spec anything other than new since they have no idea how they have been used at any given time..

[twinboat]

Originally Posted by SCVJeff

No, they ARE measured in cycles (go look at the cut sheets). Its well known that killing the batteries will reduce their life. And since they are measured in aggregated discharge cycles, getting them back up to 100% obviously stretches the advertised cycle life. As far as hours is concerned, that number is a longivety rating at a given current with a new battery. They obviously can’t spec anything other than new since they have no idea how they have been used at any given time..

You are correct on battery life being measured in cycles but his point about only drawing down to 80% holds true.

Carrying enough battery capacity to only use 20% is like stopping for gas when the gauge gets to 3/4 tank.

If you need a specific amount of energy to get thru each night, and only want to use 20% of your capacity, you will need about 3 times as many batteries in your bank as drawing down to 40% capacity.

Sure 3 times the batteries will last 3 times longer but then they will cost 3 times as much to replace them.

Its a pay me now or pay me later scenario.

[hclarkx]

Originally Posted by RV Vagabond

What good are batteries with a really long life if you cannot use them? I am not going to turn all my electrical devices off because my batteries are down to 80%. You will get about the same amount of use out of batteries regardless how low you discharge them. Battery life is not measured in cycles or years, but in the number of hours you have used them.

Your statement about measuring life is not correct. Lead acid life can indeed be measured in cycles. If you let it sit at 100% charge it will last pretty much forever. If you discharge it to 20% daily then recharge to 100%, you will be lucky to get a year out of it. In fact, if you are going to discharge daily and want much more than a year of life, you should limit discharges to 50% or less. So, depth of discharge and frequency of discharge are the key factors affecting battery life.

I should have emphasized the last sentence in my post about getting lead-acid batteries back to 100%. You can use your lead-acid battery, but if you want it to last a long time, you need to do three things ...

1) Return the battery to 100% charge when charging. I.e., don't do partial charges if you can avoid it.

2) Don't let the battery sit any longer than necessary at less than 100% charge. I.e., re-charge to 100% as often as is practical and float it there when not in use. I.e., if you don't re-charge while camping, re-charge as soon as you get home.

3) Don't discharge any deeper than necessary. You can take a lead-acid battery down to 20% but it will live a lot longer if you can re-charge after 50%. I.e., get enough capacity so that you only discharge to 50% (or even less) if you want longer life. You will pay more for higher capacity, but in the long run it will be cheaper because you will not be discharging so deeply and will get much longer life.

BTW, I would not call any lead-acid battery a "long life" battery unless it sits always at 100%, say in a backup situation or you don't discharge more than 10 or 20%. If you want long life, say 10 years or more, then you want a LFP battery.

[hclarkx]

Originally Posted by twinboat

You are correct on battery life being measured in cycles but his point about only drawing down to 80% holds true.

Carrying enough battery capacity to only use 20% is like stopping for gas when the gauge gets to 3/4 tank.

If you need a specific amount of energy to get thru each night, and only want to use 20% of your capacity, you will need about 3 times as many batteries in your bank as drawing down to 40% capacity.

Sure 3 times the batteries will last 3 times longer but then they will cost 3 times as much to replace them.

Its a pay me now or pay me later scenario.

Correct for the most part, but I would quibble with three times the battery capacity lasting three times longer. My sense is that reducing the regular depth of discharge by 2/3 (say, from 60% to 20%) would increase life a lot more than by a factor of three.

The likely correct and well used "50%" rule of thumb suggests this. I.e., below 50% you lose life at a non-linear or exponentially increasing rate. Likewise, discharging less gets much longer life, approaching a very long life if kept close to 100%.

[twinboat]

Originally Posted by hclarkx

Correct for the most part, but I would quibble with three times the battery capacity lasting three times longer. My sense is that reducing the regular depth of discharge by 2/3 (say, from 60% to 20%) would increase life a lot more than by a factor of three.

The likely correct and well used "50%" rule of thumb suggests this. I.e., below 50% you lose life at a non-linear or exponentially increasing rate. Likewise, discharging less gets much longer life, approaching a very long life if kept close to 100%.

Actually I've found many Depth of Discharge charts are linear or close to it, between 20% to 80% DOD.

https://www.solar-electric.com/learn...ttery-faq.html

[CountryFit]

Notice the vertical coordinate isn't linear scaled.

Take the right chart (or bottom depending on if you are using a phone or laptop) as an example -
when discharge 80%, the life expectancy is about 400 cycles.
When discharge 20%, the life expectancy is about 2800 cycles.
Significant, isn't it? It's self explanatory.

[SCVJeff]

Originally Posted by twinboat

You are correct on battery life being measured in cycles but his point about only drawing down to 80% holds true.

Carrying enough battery capacity to only use 20% is like stopping for gas when the gauge gets to 3/4 tank.

If you need a specific amount of energy to get thru each night, and only want to use 20% of your capacity, you will need about 3 times as many batteries in your bank as drawing down to 40% capacity.

Sure 3 times the batteries will last 3 times longer but then they will cost 3 times as much to replace them.

Its a pay me now or pay me later scenario.

That I clearly get.. I’m the poster child for high drawdown usually running till 2AM before the interter is shut down for the night (cept DW seems to use the TV as a night light). I’m typically at 55-60% by the time I roll out of bed on a typical Quartzsite trip. This running 4ea. T-145’s @ 520AH. Draw: 9Hrs/day @~18-20AH/ night from usually recovered batteries.

As for the BMK, theres a rather detailed writeup in the Magnum manual about what it does and how it calculates SOC. This is all I use anymore because my batteries are never at “rest” long enough so I can’t accurately measure them unless I either kill the inverter (as in remove power completely) and all other 12v loads, and/or the solar. Of the few times I have, it tracks SOC good enough to know where I am. And No, you don’t need to return the batteries to 100% every time (although thats always nice). The BMK will do its own SOC calibration once it is. Thats why it says “Thinkn’” when powered up. Cuz it’s still stupid.. If you're anal enough to calculate with a sustained and calibrated load, you can enter that parameter yourself. Or simply run Auto and call it a day.

[RV Vagabond]

Originally Posted by SCVJeff

No, they ARE measured in cycles (go look at the cut sheets). Its well known that killing the batteries will reduce their life. And since they are measured in aggregated discharge cycles, getting them back up to 100% obviously stretches the advertised cycle life. As far as hours is concerned, that number is a longivety rating at a given current with a new battery. They obviously can’t spec anything other than new since they have no idea how they have been used at any given time..

What is a cycle?

Quite often over a 24 hour period, my battery bank SOC % will fluctuate between 30% and 70%. Is that a cycle? Half a cycle? 4/10th of a cycle?

I may wake up with them at 30%, charge them up to 70%, down to 40% back up to 50% then again wake up to 30%. How do you translate that into cycles?

[Smitty77]

My DW says I think in a different way.... (Assume this not meant as a positive statement on her part!)

I consider 'a cycle' to be a discharged down to 50% SOC, then back to 100% SOC.

I'm one that admits that I added extra battery AH capacity, to live on the top 25% SOC of the battery bank. From an overnight usage perspective. (I'm usually between 75-80% SOC in the AM, after completing our running our coffee pot.)

I did this for the mentioned reason to reduce the life cycle usage of the battery bank. The goal was to get longer life out of the bank. So if I'm only using about 50% of the average normal 50% SOC drain & refill of the battery - I was hoping to yield between 25-33% longer life. (Would be very pleased for it to be double the normal life, but other factors besides pure life cycles can impact battery life.)

The other reason for putting in a larger bank, is for extra contingency to gap me thru shady days. Good example is while at Gros Ventre, in a partially shaded site, I've gone three days before reaching 50% SOC in the AM. (The solar, 1200W, even with partial shading. Was able to put some power back into bank each day. So that is why it was the AM of the third day before I was at about 52% SOC after the coffee was brewed.) I then fired up the generator, and between the generator and solar was at 100% SOC before the end of daylight hours. (I try to make noise whenever one else is doing so. So the generator ran for abut 3 1/2 in the morning, and another 2 1/2 hours or so in the evening.)

If you have a monitor that keeps track of total AH's Out/In, you can do the math on where you are in the overall Life Cycles of your battery bank. This way you don't need to do the 1/4 or 1/3 or full 'cycle usage' dance. Just look at the total AH's OUT/IN, and do the math for the full life cycle of your bank of battery per it's DOD rating.

Best of luck to you, and have some fun,
Smitty