I have been asked this several times, and wondered myself about it, particularly whether or not it is large enough to be usable for anything and thus needed in capacity vs use estimates.
I never really could figure out how you would really know when you were discharging surface charge vs into "real" capacity after you unplugged. I still don't know how you would do that accurately, but I did stumble on some information while doing a drawdown/recharge test the last few days.
The test was just to set a baseline rested voltage for our batteries after being pulled down 50%. It turned out to be 12.36 volts at 77*, and that is a bit higher than expected, so that is good. This is the number we will use in future drawdown tests to estimate how much capacity we have lost over time.
The test was just discharge, and then an auto recharge to 2 amps battery current, which is just a bit lower than the Lifeline .5% recommendation of 2.2 amps. This is not totally full on our batteries, however, as they will go to about 1.4 amps. But it adds another 4 hours to the charge cycle to get there, so we choose not to leave them at 14.3v that long as the float charge will also get the batteries to the totally full level, albeit taking 48+ hours to do. If on the road doing a periodic shore charge to full, we would leave them on longer, and at a higher voltage to help recover from a week or so of mid SOC cycling.
Where the surface charge came into all this is when the charger went to float, the amp hour counter had come back right to zero, which is unusual. It normally would be considerably higher than that at +20 or more. I just let it go to see how it would read after the long float for comparison, as in the past we would get to about +40ah after a 50% down recharge. I looked after about 12 hours and the gauge read MINUS 8 amps which really seemed strange, since it was on float charge. The amps were reading -.6 amps, which is about the base amps for the van, so it was still discharging to run things. Voltage was nearly down to the 13.1 volt float voltage at 13.12.
What this seems to be showing is that there were over 8AH of power stored in the surface charge between the 14.3v absorption voltage and the 13.1v float charge. That would be just under 2% of our 440AH bank capacity. I didn't expect that the surface charge would be anywhere near that large. If 12.8v is really the right voltage for full, there would be even more available in the last .3v of drop to the 12.8v.
This would also contribute to why the drawdown test looked very good, as the batteries were just off a totally full charge, so they would have that extra 8-10AH of surface charge power in them. I would bet the manufacturers use it to beef up their capacity tests as high as possible. By using a higher charge voltage and beginning the test right away, they could get higher capacities by a bit.
While this is a relatively small amount of power, especially as a %, but it is pretty interesting to see it as large as it is. For us, it would run our frig about 8 hours
How big is surface charge?
Linear Mode
