Trebor English
Well-known member
In the absence of THE charging enthusiast it seems that full charging hasn't been encouraged enough. With that thought it seemed appropriate to do something about it. Here's my attempt.
During the day with solar does the battery get fully charged most days? I mean fully charged as the absorption voltage, 14.4 or 14.8 or whatever your battery spec says, is held for hours until the charge current accepted by the battery bank drops to 1% of C or 1/2% of C?
The reason I asked is because lead acid batteries don't like partial state of charge cycling. Lithiums don't seem to mind it but lead acids get a capacity loss. If you only recharge to 95% that becomes the new full.
Think of a dry sponge standing on edge. As you spritz it on one side with a spray mist squirt bottle it absorbs the water. That's like a lead plate taking up sulfate ions from the sulfuric acid becoming lead sulfate as a battery discharges. At first the outer surface gets moist then the moisture gets deeper as you spritz it more. When you charge the battery it is like blowing it with a hair dryer. Quickly the surface dries but the drying process slows as the remaining moisture is deeper inside the sponge. While holding 14.4 volts the charging current tapers away as the remaining sulfate ions are harder to get to. Eventually it dries. If you moisten it 60% full of water, 60% discharged, and then dry it to 95% dry, 5% still wet, there is some moisture remaining. The remaining moisture is deep. Then if you moisten 10% and remove 10%, that buried 5% is still there, untouched. Lead sulfate hardens and cannot be removed. Think of mold growing in your sponge. Once the mold grows and takes up that deeply buried 5% moisture it is stuck as the mold will hold the moisture even as the surrounding sponge gets dry. That moldy 5% of the sponge no longer has the capacity to take more water or give back the water. As you apply charging voltage, blow with the hair dryer, you can only reach the surface directly. The deeper the moisture the slower the drying. The older the moisture the more it is held by the mold.
Your 100 amp hour battery is now a 95 amp hour battery. Repeat monthly and the once proud 100 amp battery is only 95 - 4.75 = 90.25 amp hour. Next month it is only 85.7375 amp hour.
With AGM and gel batteries you cannot use a hydrometer to measure the electrolyte to determine how much sulfur is in the sulfuric acid and by deduction how much is still buried in the plates. The only thing you can do is fully charge the battery, as described above, and reset your coulomb counter battery monitor. When you set up your battery monitor one of the things you told it was your battery capacity. If you were to take 60 amp hours from a 100 amp hour battery that's 60%. If you take 60 amp hours from a 4 month old 81.45 amp hour battery that's almost 74% not 60%. The situation spirals.
What you see is the charging puts in amp hours and the battery voltage goes up and everything is good. As your capacity vanishes everything is fine until the fridge beeps and gives a low battery voltage error code. Then you are puzzled. The solar is charging and the battery is full by 10AM. It used to take until noon but now it works better than ever, it is full by 10. The green light on the charge controller says the battery is full and the voltmeter says 14.4 then float mode, 13.8, so nothing is wrong. But the fridge won't run past 3 AM. The battery is now only a 50 amp hour battery. It charges in half the time. The battery monitor still says it has 100 amp hours in it because it is full. It can only give 20 amp hours before it's down to 11.9 volts at a 5 amp load.
This is, in my opinion, the main benefit of LiFePo4 batteries. They can go 10 years cycling in the 20% to 80% range with no need to ever get to 100%
With flooded lead acid batteries you can see the problem developing. Specific gravity drops and water consumption rises. You can do something about it. You can discharge to 10.5 volts or beyond, leave stuff on till it's dead. Get that sponge wet all the way through then recharge using an elevated absorption voltage, maybe up to 16 volts, and get back some of the lost capacity. While doing that don't overheat the battery and keep adding distilled water. It will take a lot. It won't get back all the capacity and it won't speed up charging like when it was new but it can help. It's better to avoid the problem with better charging in the first place. Seriously, monitor the temperature of the battery and don't overheat it.
I hope the sponge analogy helps.
During the day with solar does the battery get fully charged most days? I mean fully charged as the absorption voltage, 14.4 or 14.8 or whatever your battery spec says, is held for hours until the charge current accepted by the battery bank drops to 1% of C or 1/2% of C?
The reason I asked is because lead acid batteries don't like partial state of charge cycling. Lithiums don't seem to mind it but lead acids get a capacity loss. If you only recharge to 95% that becomes the new full.
Think of a dry sponge standing on edge. As you spritz it on one side with a spray mist squirt bottle it absorbs the water. That's like a lead plate taking up sulfate ions from the sulfuric acid becoming lead sulfate as a battery discharges. At first the outer surface gets moist then the moisture gets deeper as you spritz it more. When you charge the battery it is like blowing it with a hair dryer. Quickly the surface dries but the drying process slows as the remaining moisture is deeper inside the sponge. While holding 14.4 volts the charging current tapers away as the remaining sulfate ions are harder to get to. Eventually it dries. If you moisten it 60% full of water, 60% discharged, and then dry it to 95% dry, 5% still wet, there is some moisture remaining. The remaining moisture is deep. Then if you moisten 10% and remove 10%, that buried 5% is still there, untouched. Lead sulfate hardens and cannot be removed. Think of mold growing in your sponge. Once the mold grows and takes up that deeply buried 5% moisture it is stuck as the mold will hold the moisture even as the surrounding sponge gets dry. That moldy 5% of the sponge no longer has the capacity to take more water or give back the water. As you apply charging voltage, blow with the hair dryer, you can only reach the surface directly. The deeper the moisture the slower the drying. The older the moisture the more it is held by the mold.
Your 100 amp hour battery is now a 95 amp hour battery. Repeat monthly and the once proud 100 amp battery is only 95 - 4.75 = 90.25 amp hour. Next month it is only 85.7375 amp hour.
With AGM and gel batteries you cannot use a hydrometer to measure the electrolyte to determine how much sulfur is in the sulfuric acid and by deduction how much is still buried in the plates. The only thing you can do is fully charge the battery, as described above, and reset your coulomb counter battery monitor. When you set up your battery monitor one of the things you told it was your battery capacity. If you were to take 60 amp hours from a 100 amp hour battery that's 60%. If you take 60 amp hours from a 4 month old 81.45 amp hour battery that's almost 74% not 60%. The situation spirals.
What you see is the charging puts in amp hours and the battery voltage goes up and everything is good. As your capacity vanishes everything is fine until the fridge beeps and gives a low battery voltage error code. Then you are puzzled. The solar is charging and the battery is full by 10AM. It used to take until noon but now it works better than ever, it is full by 10. The green light on the charge controller says the battery is full and the voltmeter says 14.4 then float mode, 13.8, so nothing is wrong. But the fridge won't run past 3 AM. The battery is now only a 50 amp hour battery. It charges in half the time. The battery monitor still says it has 100 amp hours in it because it is full. It can only give 20 amp hours before it's down to 11.9 volts at a 5 amp load.
This is, in my opinion, the main benefit of LiFePo4 batteries. They can go 10 years cycling in the 20% to 80% range with no need to ever get to 100%
With flooded lead acid batteries you can see the problem developing. Specific gravity drops and water consumption rises. You can do something about it. You can discharge to 10.5 volts or beyond, leave stuff on till it's dead. Get that sponge wet all the way through then recharge using an elevated absorption voltage, maybe up to 16 volts, and get back some of the lost capacity. While doing that don't overheat the battery and keep adding distilled water. It will take a lot. It won't get back all the capacity and it won't speed up charging like when it was new but it can help. It's better to avoid the problem with better charging in the first place. Seriously, monitor the temperature of the battery and don't overheat it.
I hope the sponge analogy helps.
