Understanding Charger Circuits and Diode Voltage Drop

[ramonegumpert]

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

 

[vk6kro]

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.

 

[NascentOxygen]

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

Hello again, RG. 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.

 

[sophiecentaur]

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).

 

[gnurf]

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

 

[ramonegumpert]

Dear Vk6kro

Thank you for enlightening.
:)

Best regards
RG

 

[ramonegumpert]

Dear NascentOxygen

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

Thanks for your tips.
:)

 

[ramonegumpert]

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.

 

[jim hardy]

Is there standard circuit for this purpose?

Lots of them, search on Li Battery Charger IC.

here's one from Fairchild with brief explanation of approach:

http://www.fairchildsemi.com/an/AN/AN-9721.pdf [ QUOTE]