Battery voltage and surface charge

Often battery voltage is the only method a person has available to see where their battery lies, in terms of state of charge, and while this is not a metric to ignore, it is also very misleading.

Especially when there has been a charging source employed, and one which is capable of relatively large currents like an alternator. Many people drive somewhere, look and see voltages well above 13, and declare their battery as fully charged. The following link shows how misleading battery voltage can be in terms of state of charge.

http://www.pbase.com/mainecruising/battery_state_of_charge

Keep in mind he is only applying 15 amps for short durations and the surface charge voltage lingers for many many hours, indicating a fully charged battery, where in reality the battery is only at 50%.

Battery experts such as dealers and service personnel test battery using a load test. The battery is fully charged. A heavy load (how many amps?) is put across the battery for (how many seconds?). The voltage across the battery is then read.
Sorry I could not readily locate the figures. But to agree with S/W the surface charge must be removed.

Does this 'fool' solar chargers? i noticed when i charge for just a few hours at work sometimes it will blink the battery light as if it was full even after i unhook the charger. i usually just turn something on or the fridge kicks/on but I was curious if i didnt do that would the solar not charge come daylight?

It could easily fool the solar charger if the charger is unplugged before the battery is fully charged. The solar would have seen the charging voltages from the charger and backed off the current, going 'open circuit' when it noticed that battery voltage did not drop. Unplug the charger and the solar figures the batteries were already full and only produces enough current to hold it into float.

Where as 10 or 15 amps might have been needed to hold the batteries at 14.4, only 2 amps might be required to hold the batteries at 13.2, so the solar can meet this, and stays in float mode, when the batteries still want more time and the higher amperage required to hold 14.4, or if trojans t105's, 14.8v.

Get a hydrometer and see how your flooded batteries are doing when you suspect one or another charging source is in 'float ' mode, yet you suspect the batteries need more time in the mid to high 14's and those higher charging currents required to hold those voltages.

The Solar charge controller is programmed as if it is the only charging source, so other charging sources can trick it into early float voltage, resulting in chronically undercharged batteries caused by premature float voltages to be triggered.

Really the hydrometer is the best tool available to know what the batteries are doing in relation to the charging sources applied.

if you can bring the batteries up to the high 14's and it only requires 1/5 to 3 amps to hold a pair of healthy gC batts in the mid to high 14's, then you are in the 95% range or so.

If 10 amps are still required to hold mid to high 14's then the GC batteries are in the 80 to 85% range.

Again the digital clamp on meter, in conjunction with a hydrometer, can tell you things no voltmeter alone ever could, if you are interested in getting the most from your batteries. If not, then they are only batteries, when they no longer meet your needs, then replace, and don;t think about what might have been, if you had gone through all the effort required to make sure they reach full charge regularly and as often as possible.