Understanding Charger Circuits and Diode Voltage Drop

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Discussion Overview

The discussion revolves around charger circuits and the voltage drop across diodes when charging batteries, particularly focusing on lithium-ion batteries. Participants explore the implications of diode voltage drop, the necessity of charger circuits to prevent overcharging, and the specific charging requirements for different battery chemistries.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning
  • Experimental/applied

Main Points Raised

  • Ramone questions whether current would stop flowing into a battery when its voltage reaches 4.2V, given the supply voltage equals the battery voltage plus the diode voltage drop.
  • Some participants note that while there are chargers that operate similarly to Ramone's description, they may require resistive current limiting, which can lead to prolonged charging times.
  • It is mentioned that diode voltage is dependent on current, and at low currents, the battery could still be overcharged if not monitored properly.
  • Some participants emphasize the sensitivity of certain battery chemistries to overcharging, suggesting that precise voltage regulation is necessary to prevent damage.
  • There is a discussion about the different charging methods required for various battery types, with a focus on lithium-ion cells needing a two-step charging process.
  • Participants express the importance of understanding specific battery requirements to avoid damage during charging.
  • One participant mentions the availability of standard circuits for lithium battery charging, directing others to search for relevant ICs.

Areas of Agreement / Disagreement

Participants generally agree on the need for careful regulation in battery charging to prevent overcharging, particularly for lithium-ion batteries. However, there are multiple competing views regarding the specifics of charging methods and the implications of diode voltage drop, leaving the discussion unresolved in certain aspects.

Contextual Notes

Participants highlight the importance of temperature dependence and other variables that affect charging, indicating that assumptions about voltage levels and current may not hold universally across different scenarios.

Who May Find This Useful

This discussion may be useful for hobbyists and engineers interested in battery charging circuits, particularly those working with lithium-ion batteries and seeking to understand the implications of diode voltage drop and charging methodologies.

ramonegumpert
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Dear Experts

I just connect a diode to a battery rated at 4.2 volts.

The diode takes about 0.8volts ( i thought should be 0.7v but measurement says 0.8v) and the battery is currently at 3.12v.

The supply voltage is 5v dc.

After some time, if the battery reaches 4.2v, would current stop flowing into the battery given that the supply voltage = battery + diode voltage ?

If so, why do we need a charger circuit to limit the voltage level of the battery from overcharging?

Thanks for reading.

Best regards
Ramone
 
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There are battery chargers that work exactly like that.

There are problems with them, though. They have to have some resistive current limiting for when the battery is very flat and this means that as the battery approaches the supply voltage the charging current becomes very small.
So, it may take a very long time to fully charge the battery.

A charger that charges at a fairly constant rate and then switches off will charge much more quickly than the type described here.

Incidentally, diode voltages are not independent of current. A diode will conduct with around 0.5 volts on it. The current may be small, but if the battery was sensitive to overcharging, it could eventually get overcharged by this small current. At this low current, the voltage across the battery in your example would be 4.5 volts and this may be enough to damage it.
 
ramonegumpert said:
If so, why do we need a charger circuit to limit the voltage level of the battery from overcharging?
Hello again, RG. :wink: Did you get your solar charger sorted out?

Some battery chemistries are very sensitive to overcharging. A difference of 0.1V may mean the difference between the cell being fully charged and it being dangerously overcharged. So you need the charging circuit to switch off at a very precise point. Further, the voltage of the cell may be temperature dependent, so the point at which charging should be terminated needs to take temperature into account. Also, the voltage delivered by a basic charger may change according to the size of the cell, or the mains voltage, etc., so some sort of regulation needs to be built into the charging/protection circuit associated with each individual cell to ensure optimal charging (and maximizing the life) of the cell.
 
You can damage batteries by charging them the 'wrong way'. Some are best with 'constant voltage' charging and others with 'constant current' charging. The Lead Acid car battery is pretty good natured and a simple transformer-rectifier system is fine but most other types are much more fussy..
Take advice about the requirements of the specific battery you need to charge or you may spoil it (or yourself).
 
sophiecentaur said:
You can damage batteries by charging them the 'wrong way'. Some are best with 'constant voltage' charging and others with 'constant current' charging. [...] Take advice about the requirements of the specific battery you need to charge or you may spoil it (or yourself).
If I'm not mistaken, the OP is using lithium ion cells which needs to be charged in two steps; constant current until the cell reaches 4.2V followed by constant voltage until the charge current drop to around 5% of the max current. If charging to full capacity is not a concern the second step can be omitted. I agree that OP should take care and avoid the potential hazard of overcharging these type of cells, but one can only assume he is aware of the safety issues. Right?

http://www.youtube.com/watch?v=SMy2_qNO2Y0#t=1m55s
 
Dear Vk6kro

Thank you for enlightening.
:)

Best regards
RG
 
Dear NascentOxygen

I stopped working on the solar charger for a while. Its a weekend hobby.

Thanks for your tips.
:)
 
Thanks Sophiecentaur and Gnurf :)

I have been aware that its dangerous to overcharge lithium batteries.

Now i will try make a circuit to ensure precise charging .

Is there standard circuit for this purpose?

Thanks.
 

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