"Surface charge": How significant is it?

[Nehmo]

The term"surface charge" is usually applied to lead-acid car batteries, but I've found it used for other chemistry including Li-poly. It seems to mean a condition in which a battery appears to have full charged voltage, but under load, soon the v drops to reveal the true charge condition of the battery.
Battery U has something on it:
"Lead acid batteries are sluggish and cannot convert lead sulfate to lead and lead dioxide quickly enough during charge. As a result, most of the charge activities occur on the plate surfaces. This induces a higher state-of-charge on the outside than in the inner plate. A battery with surface charge has a slightly elevated voltage. To normalize the condition, switch on electrical loads to remove about one percent of the battery’s capacity, or allow the battery to rest for a few hours. Surface charge is not a battery defect but a reversible condition resulting from charging."

Is surface charge real? Is the "one percent" amount from Battery U a reliable figure? Is it a phenomenon limited to some chemistries but not others?

 

[amberwolf]

Some info in previous discussions:
http://www.endless-sphere.com/forums/search.php?keywords=%22surface+charge%22&terms=all&author=&sc=1&sf=all&sk=t&sd=d&sr=posts&st=0&ch=300&t=0&submit=Search

 

[John in CR]

IME Lifepo4 has the most prominent surface charge. I charge 23s to 83V and bleep the throttle for a second and the voltage drops and stays below 80V. It unlike lipo lifepo4 doesn't have any useful energy near top of charge voltage even at a light current load. Once you hit the plateau though it keeps a flatter voltage than anything else.

 

[dogman dan]

I consider the surface charge to be that which disappears immediately with the discharge of 1 wh or less. Or that which will self discharge overnight, in the case of an older battery.

So my new ping battery charged to 60v, and with the first 50 feet would drop to 56v, the true 3.5v fully charged voltage.

As the battery aged, the full charge voltage changed, but the charger and bms did not. So much later when the pack would only hold 54v, the surface charge had increased.

Then, there is the first few wh of the discharge curve from full charge. As John says, the voltage drops quick at first for lifepo4, then reaches the plateau of the discharge curve. So voltage drops a lot in the first mile, then settles in for the next 10 miles. For my pingbatteries, most of that discharge happens between 52 and 54v, for a 48v pack.

Lico is different. Less overcharged to balance it, there is less surface charge to start with. And the discharge curve is less flat. But even so, you do still see a bit more voltage drop in the first mile than in the second. As it ages, you will start to see that overnight self discharge, even when not overcharged like typical with lifepo4. Losing .05v overnight per cell on used lico packs is not unusual.