DOD vs PSOC cycling

[Seminole Wind]

i would like to explore the benefits/drawbacks we encounter as we are working around different depth of discharge targets and partial state of charge cycling. it is interesting to follow these numbers and see what is going on. i am using rough and rounded figures for ease of math. if something is way off, please point out and explain the correction. but a couple % points one way or another have been forsaken for clarity and to be able to see the relationship. i based my charge efficiency on what i see on my digital battery meter installed on the system

primarily flood lead acid battery tech for this thread please

so we all pretty much agree with 50% dod is a decent soft target

we also know that once you charge back up to around 20% dod (80% state of charge/SOC) give or take the charging process not only slows down but it is less efficient. ie you have to put considerably more than 1 amp hour into the battery to gain 1 amp hour stored

for those of use that have larger solar arrays that can fully recharge daily to  a true 100% that last 20% of inefficient charging does not really matter. but for those starting out with a smaller system, where is the sweet spot between a little less battery life but having more usable power

for discussion, lets use a hypothetical set up. a 200 amp hour battery bank, flooded lead acid and 200 watts of solar and lets use the .3 times the watts to estimate the amp hours sent to the batteries each day nominally. that would be 60 amp hours of charging

to keep from getting complicated, lets say we are only discharging at night after the sun goes down. so there is not simultaneous charge/discharge

if we start with full 100% batteries and discharge by 60 amp hours on night one. this will basically bring us down to 30% depth of discharge (70% soc). this is a very healthy (for the battery) level, they would last a long time

now on day one we get 60 amp hours of charge. the first 20 amp hours is pretty efficient, bringing the batteries up to 20%dod (80% soc) now the remaining 40 amp hours really looses on the efficiency. very reasonable to only get 50% efficiency and that would leave us at 10% dod (90% soc) this partial state of charge is hard on batteries. but it can be costly to get that last few % to reach 100% if you cant do it with solar, that is where a generator/charger or running your engine once a week or so comes into  play

now on night 2 we draw down another 60 amp hours, bust since we started from a 10% dod (90% soc) we come down to 40% dod (60% soc)
day 2 charge 60 amp hours it now takes 40 amp hours to get back 20% dod (80% soc) then the remaining charging would only get us up to 15% dod (85% soc) 

night 3 draws out 60 amp hours bringing the battery bank down to 45% dod (55% soc) 
day 3 charges 60 amp hours back to the batteries but it takes 50 amp hours of efficient charging to get to 20% dod (80% soc) and the last 10 amp hours would only gain us another 2.5% bringing the batteries to 17.5% dod (82.5% soc)

you can see after a few more days on this cycle we get darn close to cycling from 50% dod/soc and 20% dod (80% soc) by staying above the 50% we are good for battery life, but because we are not getting charged to 100% we are hurting battery life. but for each amp hour we charger, we get an amp hour stored ( or close) that we can use. once a week we could run and engine or generator/charger to top off the batteries

but if we use the engine/generator/charger to charge the last 20% we need to run the charger twice as long. 

so by this example, we can see that if you are going to run an engine/generator/charger it is best to do it first thing in the morning when the batteries are low so that the engine runtime can be kept low saving gas.

for example if you had an alternator on your car that could put out an extra 30 amps. you could run it for about 2 hours in order to get the 60 amp hours needed to bring the batteries to 20% dod (80% soc) if you started charging like that at 8 in the morning then you could shut it down around 10am and that is about when the power starts coming on strong from the solar. you might even get the full 60 amp hours from the solar and even if it was only 33% efficient you could bring the bank to a full 100% state of charge 0% depth of discharge

with smaller solar this is probably approaching the best bang for the buck as far as battery life/ usable power and cost of charging

if you tried to bring the bank to full charge each day with the small solar you would only be able to use about 30 amp hours or so because most of the recharge cycle would be in that very inefficient last 20%

of course different battery chemistry's ( MUCH MORE EXPENSIVE) can alleviate much of this problem

so, what say ye? if you had the example battery bank/solar charging input what sort of cycle would you target?

 

[tripper]

When my system was smaller, I really didn't think about it much. All I could really do about the situation was try and conserve energy when I couldn't get them to charge enough.

If it was still on bulk charge by 4pm, I would know to use the absolute minimum energy. If the system was on float mode prior to darkness I would use what I liked once it hit float. If cloudy/rain was in the forecast next day I would try to conserve also. No theory/math/SOC meters etc. could do anything about it when you just do not have enough solar.

 

[Trebor English]

Rather than 1 watt of solar per amp hour of battery I have 1.33.  Specifically I have 100 watts of solar and a 75 amp hour battery.  

Actually the ratio of solar watts to battery amp hours isn't as important as the ratio of solar watt hours to energy used.   Daytime energy use bypasses the battery losses but it does subtract from the energy available for charging.  

In addition to the inefficiency of having to put in more amp hours than you get out, the voltage matters.  One amp hour at 14.4 volts is 10% more watt hours than the first amp hour at 13 volts.  

There are too many variables for a simple rule of thumb to take care of.  Lattitude, seasons, time of day of energy use, near by tree height are just a few.  However, the example given shows the beginning of the spiral vortex down into the swirling drain of capacity loss.  As the state of charge is lower at the end of each day the amount of lead sulfate that remains increases each day.  The sulfate hardens.  Did you ever make candy?  Granular sugar heated, melted, and cooled forms a hard solid.  The lead sulfate hardens.  Then that sulfur is not returned to the sulfuric acid.  As the plate material available gets tied up and as the electrolyte weakens the capacity vanishes.  When the 200 amp hour (new) battery bank looses 25% of the capacity it becomes a 150 amp hour battery.  Now on day 127 the 60 amp hour discharge is 40% of the new actual reduced capacity not 30% of the prior 200 amp hours.  Now on day 128 instead of starting 10% low it is more, maybe 12% low.  40% plus 12% gets you under 50%.  On day 129 another 10% to 12% down plus 40% gets you to the 40% state of charge.  Now to charge the battery  the bulk mode generator run time will be starting from a 30% state of charge but only charging a 150 amp hour battery.  The absorb mode finishing will go quicker with 200 watts of panel and only 150 amp hours of battery.  The charge controller sees 14.4 volts for one hour (or whatever it takes) and goes to float mode.  All is well.  It looks like the system is performing well until the capacity loss makes the daily use unsustainable.  The inverter beeps and shuts down.  The first symptom is the inverter beeps and shuts down.  By then your battery capacity has been reduced to less than what you use daily.  Until then it looks like it is charging better and better.

I don't know how much you can take from a battery or how long you can wait to put it back.  The one to one rule of thumb depends on fully recharging most days.  The battery is big enough to be used lightly most days with some reserve for deeply overcast and rainy days.  The original post mentiond days 1 to 3 and a couple more.  I don't know about your batteries but I wouldn't expect a regular 5 day charge cycle to work well.  

Another way to look at it is this.  If you only recharge your 200 amp hour battery to 90%, your capacity will soon be only 180 amp hours.  That 10% you regularly left as lead sulfate becomes permanent.  Two months later, repeat.  180 - 18 = 162  

What I have done is used flooded lead acid batteries and a $10 hydrometer.  I have been using the same battery almost 2 years.  Without an actual capacity test I don't really know what the capacity is.  The hydrometer sees the sulfuric acid as dense so I don't have a bunch of permanent lead sulfate.

 

[John61CT]

but for each amp hour we charger, we get an amp hour stored ( or close) that we can use.

Completely unrealistic goal, especially with lead batts.

Don't worry about efficiency, to overcome the trailing amps / PSOC issue, focus on getting long enough Hold Absorb time.

Yes, if you need to supplement your solar with dino juice, that must be done before the solar day starts.

But 2-3 times a week is enough if getting to 100% Full is inconvenient.

Much less often is OK with Firefly Oasis, uniquely so.

And the whole issue goes away with LFP.

 

[Weight]

yes. what john said. But LiFePo opens other more expensive issues.

 

[frater secessus]

so by this example, we can see that if you are going to run an engine/generator/charger it is best to do it first thing in the morning when the batteries are low so that the engine runtime can be kept low saving gas.
...
with smaller solar this is probably approaching the best bang for the buck as far as battery life/ usable power and cost of charging

Agreed.  When I started working on a alternator+solar rvwiki article I was thinking about it from a total cost point of view.  Driving early in the morning can get alternator-charged house banks up from 12.1v to 13.8v faster than typical solar installs. From there the solar still has to achieve and hold absorption but about 2/3rds of the heavy lifting is already done. 

I should point out that there are counterarguments that alternator charging is overrated in the public's understanding.  I agree with that in some ways, because Joe Sixpack seems to believe that alternator charging = full charging for lead acid batteries.  It almost never is, but combined with a bit of solar it can do a lot of good.  

a 200 amp hour battery bank, flooded lead acid and 200 watts of solar
...
so, what say ye? if you had the example battery bank/solar charging input what sort of cycle would you target?

200Ah:200W would be happy in areas with a sunlight falling on them.  That same setup with alternator charging would be very happy.
In either case discharging to 50% DoD.
Because I planned for almost-exclusive off-grid camping I set it up this way:

• overpaneled system (220Ah:570w)
• voltage sensing relay for alt charging
• shore power port feeding DIY converter in those rare cases I have access to shore power.   This also allows for the use of a single coil tabletop electric stove (and small electric heater) to save propane.
• 12v circuits divided into critical and noncritical.  The critical stuff shuts down at 50% DoD.  Noncritical stuff is shut down when voltage drops below 12.7v or similar, so it basically only runs those loads when there is excess power.  Yes, I am a dork.

Most of the time I boondock the system is still holding 12.7v in the morning when I wake up. Sometimes 12.6v and rarely 12.5v.  And that's running a roof vent, 12v compressor fridge, and mifi + other small loads all night.

 

[John61CT]

The term "over panelled" should not IMO be used wrt the ratio between solar wattage and storage capacity.

It should specifically reference the relationship between panel and controller output.

 

[frater secessus]

I do make the distinction between the two types on the RVwiki overpaneling page, and I list the panel:controller relationship as the dominant meaning. So I agree substantially.