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Guest Yevgenip

Simple battery charger

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Guest Yevgenip

I see many battery Charger circuits, any they are quite complicated. I mean, why can't you just connect the battery to a voltage source and charge it? What is so complicated about rechargable batteries?

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Hi Yevgeni,
Of course a battery charger must be designed for the chemistry of the rechargable battery because a lead-acid, Ni-Cad, Ni-MH or Lithium batteries are all different.

Most rechargable batteries need current-limited charging because they have such a low internal resistance. They would blow-up if charged from a voltage source, and also would the wires and the source.

Rechargable batteries need a circuit to prevent overcharging which produces heat.
Quick-charge circuits have redundant sensors in case one fails, to prevent overcharging and the resulting explosion or fire.

I don't think you want to be splattered with acid!
I don't think you want to see a lithium battery burn!
I don't think you want to be near a Ni-Cad or Ni-MH cell that is frying and about to go BANG!

Before battery charger ICs were invented, circuits were very complicated. The cheap new ICs protect you and the battery and are fairly simple to use. ;D

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Before battery charger ICs were invented, circuits were very complicated. The cheap new ICs protect you and the battery and are fairly simple to use. ;D
Hi
Audioguru
I want to use a small rechargeable battery in l.e.d.-torch. Capacity say 3.6v,60mAh. Which one is good? NiCd, NiMh, LedAcid or Li-Io? Can u suggest me a charger ckt or atleast charger IC for that?
Aakaash

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I want to use a small rechargeable battery in l.e.d.-torch. Capacity say 3.6v,60mAh. Which one is good? NiCd, NiMh, LedAcid or Li-Io? Can u suggest me a charger ckt or atleast charger IC for that?

I have never seen a rechargable battery as small as 60mA/hrs, it would be a button size battery. A fairly small AAA Ni-MH cell is rated at 850mA/hrs.

Only 3.6V is barely enough for a white LED. You would need a DC to DC converter circuit to stepup the voltage of a weakening battery and to limit the current when the battery is new. Cheap white LED keychain torches use 3 button size alkaline cells and simply use their internal resistance to limit the current. You can see them dim as the battery voltage runs down.
Ni-Cads are obsolete, LeadAcid is too big and heavy and Li-Ion is dangerous if the charger goes wrong. Ni-MH cells would be good but I don't know any that are smaller than AAA size. Hobby shops might have some for model airplanes.

www.maxim-ic.com have some good battery charger ICs. ;D

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Hi All,
I think the voltage rating and the charging current to be applied is written on the battery.It is recomended that charging current should not be  exceeded.
Why not design a constant current source,so that the charging current does not exceed?Why people go for constant voltage source?
A constant current source can be designed with a vlotage divider bias with zener between base and ground and battery in collector load.Again Why go for complex circuits?
-Shekhar

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Hi Shekhar,
Some battery charger circuits use a constant current source, but it is recommended for all batteries that they are not overcharged. How are you going to detect it without using a battery charger IC that is made to do the job correctly?

The voltage rating of the charger is not usually written on the battery. Just the battery's nominal voltage when loaded. The voltage of a rechargable battery is higher at the end of a full charge. (The voltage of Ni-Cad and Ni-MH cells actually drops a little near full charge)

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"Optimum" battery charging is far more complicated!!!! Each different chemistry will have it's own optimum charging profile with different charge rates to specific voltage levels, dwell times and float charge levels. Optimum charging is NEVER linear and can have multiple voltage level dwell times.

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