RegGuheert
Well-known member
Lead sulfate can harden in as little as 24 hours in hot conditions. I have demonstrated that the LEAF does not return the battery to a full charge for as long as 21 days. Sulfation is not an all-or-nothing issue. It is a gradual degradation process by which a battery dies a slow death. The kind of charging done by the LEAF's charger greatly increases the rate of sulfation.lorenfb said:Yes, I agree. But the key issue is what the minimum driving/charging-voltage/frequency is to avoid this problematic sulfate mode. It appears that your hypothesis is that there isn't any, for which as yet no data have been presented.RegGuheert said:Unfortunately, once lead sulfate has hardened from a soft paste to hard crystals, the capacity that it represents is lost. Normal charging will not ever recover that capacity.
That's hardly relevant here. The LEAF does not fully charge the 12V battery and then maintain that battery at a float (maintenance) voltage continuously. Rather, it discharges the battery over time and never fully recharges it. (Except in rather cold weather, it can't after three weeks without a full charge, even if it tries to.)lorenfb said:Furthermore, the data I presented up-stream, hardly robust, over the years indicates that battery maintenance charging between 13.0 and 13.5 will keep a 12V lead-acid in a reliable SOC, i.e. with near output Ahr and cold cranking amps as original for over five years.
The lead sulfate forms during the discharge reaction. That is why holding a FULLY-CHARGED battery at a float (maintenance) voltage prevents sulfation. The real question is how long does it take for lead sulfate to harden. The answer is that it depends on the temperature:lorenfb said:If data are available indicating the rate of the formation of sulfate over time and charging voltage, please provide it. Ideally one might expect a family of curves where the X-axis is time and the Y-axis is percent sulfate formation with each curve representing the maintenance charging voltage.
Lead-acid battery sulfation is a well-known phenomenon and proper charging techniques to minimize the hardening of lead sulfate are well-understood. The LEAF battery charger violates many of these well-known principles and the result is premature failure of the 12V battery due to sulfation.Rolls Batteries said:Causes of battery sulfation:
- Batteries sit too long between charges. As little as 24 hours in hot weather and several days in cooler weather.
- Battery is stored without some type of energy input.
- Undercharging of a battery to only 90% of capacity will allow sulfation of the battery using the 10% of battery chemistry not reactivated by not completing the charging cycle.
- Low electrolyte level - battery plates exposed to air will immediately sulfate.
- Incorrect charging levels and settings. Rolls recommends a 3-phase charge cycle (Bulk, Absorption & Float). See State of Charge & charging information.
- The longer a battery sits and is not re-charged the more damaging sulfation build up there may be on the plates.
Studies have noted that nearly half of the L-16 battery capacity can be lost if the regulation voltage is too low and the time between finish-charges is too long.
In normal use, battery plates are getting sulfated all the time. When a battery is being discharged the lead active material on the plates will react with the sulfate from the electrolyte forming a lead sulfate on the plates. When there is no lead active material and or sulfate from the electrolyte remaining the battery then is completely discharged. After a battery reaches this state, it must be recharged. During recharge, the lead sulfate is reconverted into lead active material and the sulfate returned to the electrolyte.
When the sulfate is removed from the electrolyte the specific gravity is reduced and the reverse takes place when the sulfate is returned to the electrolyte. This is why the state of charge can be determined with the use of a hydrometer.
If a battery is left standing in a discharged condition the lead sulfate will become hard and have a high electrical resistance. This is what is normally called a sulfated battery. The lead sulfate may become so hard that normal recharging will not break it down. Most charging sources, engine alternators and battery chargers, are voltage regulated. Their charging current is controlled by the battery's state of charge. During charging, battery voltage rises until it meets the charger's regulated voltage, lowering the current output along the way.
When hard sulfate is present, the battery shows a false voltage, higher than it's true voltage, fooling the voltage regulator into thinking that the battery is fully charged. This causes the charger to prematurely lower it's current output, leaving the battery discharged. Charging at a higher than normal voltage and low current may be necessary to break down the hardened sulfate.
Hardened sulfate also forms in a battery that is constantly being cycled in the middle of its capacity range (somewhere between 80% charged and 80% discharged), and is never recharged to 100%. Over time, a portion of the plate's active materials turns into hard sulfate. If the battery is continually cycled in this manner, it will lose more and more of its capacity until it no longer has enough capacity to perform the task for which it was intended. An equalizing charge, applied routinely every three to four weeks, should prevent the sulfate from hardening.
In both cases, the fact that the battery "won't take a charge" is a result of improper charging procedures which allowed the sulfate to harden. In most instances, it is possible to salvage a battery with hardened sulfate. The battery should be charged from an outside source at 2.6 to 2.7 - volts per cell and a low current rate (approximately 5 Amps for small batteries and 10-Amps for larger ones) until the specific gravity of the electrolyte starts to rise. (This indicates that the sulfate is breaking down.) Be careful not to let the internal temperature of the battery rise above 125° F. If it does, turn the charger off and let the battery cool. Then, continue charging until each cell in the battery is brought up to full charge (nominal 1.265 specific gravity or higher).This time needed to complete this recharge depends on how long the battery has been discharged and how hard the sulfate has become.