# Full Charge Indicator

Can someone explain or give an outline of how the indicators on chargers work?

I have a charger for a device, which displays a red light when connected and not fully charged, but after it is charged it becomes green. I am guessing it has to be with a voltage checker (maybe a transistor), but can someone explain how they work?

• Varies based on battery type and the actual type of device you are using. Please update with that information. – Passerby Sep 23 '13 at 1:40

## 2 Answers

I'm assuming that you mean a battery charger, and a lead-acid charger in particular. A typical scenario is that the charger applies a constant current to the battery until the voltage rises to the desired level, maybe 14.4V for a 12V battery. The charger then switches to constant-voltage charging at this voltage until the current falls to some small value. At that point the red light goes out, the green light goes on, and the charger switches to a lower "float" voltage.

The circuitry involved is generally much more than a transistor. To do this right you need a good voltage reference, a circuit that converts the charging current to a voltage, some comparators, and some flip-flops to keep track of the current charging mode.

• Why was this downvoted ? It is somewhat useful and correct enough as far as it goes. Well worth an upvote. – Russell McMahon Sep 23 '13 at 1:21
• @RussellMcMahon probably has to do with the assumption of 1, a stand alone battery charger, and 2, that said battery is a lead-acid, instead of any other type (ni-cad, nimh, lipo, etc). From how I read OP's question, it seems to be more about cell phones/tablet or any other consumer device, which are mostly li-ion, but the question is very vague. The question deserves a downvote, not the answer, atleast not an uncommented downvote. +1 just for that. – Passerby Sep 23 '13 at 1:39
• @Passerby Thanks for the +1. I knew when I wrote the answer that it had little chance to be what the OP really wanted to know, and I wanted to show by example what happens when you ask poor questions. I fear that this site is starting to look like Yahoo! Answers. On the other hand, I think my answer did address the general function of a battery charger and the purpose of the red and green indicators, – Joe Hass Sep 23 '13 at 2:29

Too complex for a simple answer.
The answer varies with battery chemistry.
The charger manufacturer chooses the method right for the cell concerned and implements electronics to suit.

Some batteries can be charged to a setpoint voltage.
eg LiFePO4 at high charge rates.

Some, like LiIon, and LiFePO4 at lower charge rates [and lead-acid in some cases (as Joe Hass says)] are 1st charged to a set voltage at constant current and then at constant voltage until current reduces to some % of max charge rate.

Some (eg NimH, NiCd) use detection of a DROP in terminal voltage at a given current (negative delta V) as full charge is reached, or rate of temperature rise (delta T).