[Al Skierkiewicz]

Matt,
Interesting data and a great visual on how the battery charger works. As you will note in all the batteries, the charging voltage is a minimum when the charge current is at maximum. This is a great display of how the battery changes impedance with the charge. I am guessing you were monitoring the charge current with an ammeter inserted into the lead between battery and charger. Although the current appears to be pure DC it is in fact pulsing. This accounts for the slight differences between batteries for specific currents at specific voltages. It also accounts for the apparant high terminal voltage near the end of charge as the battery impedance gets higher, the voltage drop inside the charger become less, so the peak voltage rises to near 14 volts. (13.8 volt theoretical for a lead acid battery) The phenomena you noticed when the LED changes state is the portion in the charge curve when the charger begins a "delta" check on the battery. This alternating cycle is the charger taking a look at the difference in battery voltage under a slight load. The charger samples the battery voltage as it looks for a reduction in change over time. As a lead acid cell nears full charge it will continue to draw current but the terminal voltage rises at a different rate. As you can see in the graphs, the sudden upturn in charging voltage signals that the battery is nearing full charge, yet it continues to draw current. At this point, most of the charging current is being turned into heat as little or no additional chemical reaction can occur. Of particular interest is the fact that the battery is listed as a 12 volt battery yet produces 14 volts at full charge. This is phenomena is known as "surface charge" and it will disappear after a short period of time or under load. Although this testing shows that all your batteries are taking a charge at roughly the same current, it does not give an indication of how the battery acts in discharge on a robot. Load testing vs. time is a good indicator of the condition of your batteries in practice. Thanks for all the info. Can I use this in a presentation some time?