It seems to me that the current of the charging circuit would gradually decrease as the battery reaches full charge. My initial observations done by plugging an ammeter into a charging circuit seem to support this. Is this generally correct? Is this how an automatic trickle charger "knows" to stop charging a wet cell battery? Or does it instead go by the voltage of the charging circuit?

  • \$\begingroup\$ To put it simply, as the state of charge of the battery increases, its own voltage will rise. This means the voltage difference between your charger, which is locked at a constant voltage, and the battery, will be diminished. A lower voltage differential between your charger and the battery will result in lower current transfer between the two. \$\endgroup\$
    – schizoid04
    Jun 26, 2017 at 6:23

2 Answers 2


Well, basically at any given moment, the battery acts like it has a secret, internal open circuit voltage (Voc) and a series resistance (Rs). The charger outputs Vin. Then the charge current, Ic, is: Ic = (Vin-Voc)/Rs. As the battery gets charged, Voc goes up, so Ic goes down. It may well be that Voc is effected by other things besides state of charge. But this is an OK first approximation to help you understand how the current is set.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ A lot of chargers don't act like a constant voltage source. They themselves have a series resistance or at least, voltage goes down as current goes up. \$\endgroup\$
    – user57037
    Jun 26, 2017 at 6:26
  • \$\begingroup\$ What's a series resistance? How does it differ from a simple resistor? The comment seems inverted. Shouldn't it be "The current drops as the voltage rises."? Also, I think you mean "affected" rather than "effected". \$\endgroup\$ Jun 28, 2017 at 6:53
  • \$\begingroup\$ The comment is referring to the output voltage of the charger. As the current flowing from the charger into the battery goes up, the output voltage of the charger goes down. Just forget about the comment. Focus on the answer. Rs is an internal resistance of the battery. It is part of the battery. \$\endgroup\$
    – user57037
    Jun 28, 2017 at 8:14
  • \$\begingroup\$ Thanks. Do you happen to know whether a battery charger monitors this current when "deciding" to stop charging the battery? At this point I would guess that it does. \$\endgroup\$ Jun 28, 2017 at 8:40
  • \$\begingroup\$ it really depends on the charger. Smart chargers monitor the current, and also elapsed time. And they use different voltages at different phases of the charge cycle. \$\endgroup\$
    – user57037
    Jun 28, 2017 at 18:00

Your observations are correct.

It does not 'go by the voltage of the charging circuit'. Instead, the amount of current transferred - and the direction the current flows - is largely determined by the voltage difference between the two sources, and the resistance between them.

In general, your trickle charger does not 'know' to stop charging the battery at any given point in time.

Think of it more like this: The trickle charger has a constant output at a given voltage (Think of this as Vin in mkeith's answer's example). This will always be true.

Your battery, at the time you begin charging it, has a specific voltage - in @mkeith's example, Voc.

As this battery is plugged into the trickle charger, the difference in voltage, along with the resistance of the connection will determine the amount of current that's transferred to the battery.

This resistance will likely stay the same as the battery is charged; however, as the battery gains charge, the difference between the battery and the trickle charger's independent voltages will decrease.

If you're familiar with Ohm's Law, you'll then understand that as a direct result of this, the amount of current that transfers will drop.

  • \$\begingroup\$ I believe there is a product ("Battery Tender" or whatnot) that stops charging a wet cell battery when it determines the battery is charged. Hooking a wet cell battery to a constant voltage source indefinitely apparently damages the battery, so the charging must be monitored by some kind of "smart" circuit that cuts the voltage at a certain point. I now believe that point is when the charging current drops below a certain level. It's interesting that the damage can occur even with the current dropping proportional to the voltage difference. Perhaps someone can explain this. \$\endgroup\$ Jun 28, 2017 at 7:03
  • \$\begingroup\$ @TomRussell can you provide a source or reference to where you've heard that a constant voltage damages a battery? \$\endgroup\$
    – schizoid04
    Jun 28, 2017 at 14:16
  • \$\begingroup\$ @Schiznoid04 Not an authoritative one. After doing some more research, I actually now think it's probably just the outgassing and resulting dehydration that causes any reported damage to wet cell batteries undergoing long-term constant charging. Thanks for pointing out my bad assumption! \$\endgroup\$ Jun 29, 2017 at 8:53
  • \$\begingroup\$ @TomRussell Yes; I'm under the understanding that keeping a float charge and not exceeding the appropriate (temperature adjusted) float charge voltage should be safe; I believe this is usually about 13.8V, 0.003V higher for every degree celsius colder than 25C and 0.003V lower for every degree celsius warmer than 25C in the ambient temperature. \$\endgroup\$
    – schizoid04
    Jun 29, 2017 at 12:30
  • 1
    \$\begingroup\$ lead chemistry batteries are often floated at a reduced voltage. Other chemistries are not usually floated. Lithium ion batteries must not be floated or trickle-charged indefinitely. At least not floated at their full charge voltage of 4.2V. \$\endgroup\$
    – user57037
    Jun 30, 2017 at 4:49

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