# Help me build a simple nicad charger

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#### sv_clintrace999

##### New Member
i have a 7 600ma NICAD batteries and want to build just a simple nicad charger for it, a charger that i can leave for lets say 8 hours. I have a 1amp spare nintendo adapter which is 12Vac but the DC part is 16vdc and i want it to become a charger for my nicad following the diagram below, now my question is that:

Can i limit the current through a series resistor by 200ma? what would be the value then of my R3?
By how much voltage should i adjust for an 8.4v?

I understand that maybe at least 10% of C should be charged to the battery to have a charged it slowly but if i will limit the current to 200ma then how many hours can i expect my battery to be removed?

Any help would be very much appreciated!

Mabuhay!!!

Clint

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#### audioguru

##### Well-Known Member
You should read about Ni-Cad battery cells from The Battery University or from the tutorial at Energizer battery company's site.
1) A Ni-Cad cell is 1.2V before it is charged and when it is not discharged too much. The voltage is 1.4V to 1.6V when fully charged. Your 7 cells will be 9.8V to 11.2V when fully charged. The actual voltage depends on the charging current.
2) You must limit the current when it is charging then turn off the charging when fully charged.
3) You must detect that a battery is already charged and prevent overcharging it.

Use a battery charger IC that detects when the battery is fully charged then shuts off. Most semiconductor manufacturers make the IC especially Maxim.

#### MikeMl

##### Well-Known Member
You have 600mAh Nicads. The recommended charge rate is the 0.1C rate, 0.1*600mA, or 60mA. To fully charge these, you put in 40% more than the capacity, or 0.84mAh, which means 60mA for 14h. You need a constant-current charger for this. The circuit you have posted is not a constant-current charger suitable for Nicads; it is a current-limited constant-voltage charger which is better suited for charging lead-acid batteries, either flooded or SLAs.

For Nicads, you use constant-current charging that is time limited (use a programmable timer to terminate the charging). Faster charging above a 0.3C rate requires monitoring the temperature of the Nicads, and terminating the charging when the temperature of the batteries rises above a limit specified by the maker.

Here is a schematic of an LM317 used as a constant current source suitable for charging Nicads.
(taken right off the spec sheet). To get 60mA, R1 should be 1.25/0.06=20Ω. Since at full charge, your Nicad pack will be approximately 1.4v/cell*7 cells ≈ 10v, and the dropout voltage of the 317 is ≈ 2V, you need to start with a 12Vmin d.c. supply.

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#### sv_clintrace999

##### New Member
thank you Audioguru for the information,
Thank you Mikemi this would be of great help, the last thing is that if the voltage that was being set exceeds 12v lets say 15v, would that damage the cells? does the voltage doesnt affect the battery as long as the current was set correctly?

##### Banned
Yes, it's a bout current with batteries not voltage. Though I'm going to guess if the voltage gets above some higher limit it would indicate a fault.

#### MikeMl

##### Well-Known Member
...the last thing is that if the voltage that was being set exceeds 12v lets say 15v, would that damage the cells?
Yes. If the cell voltage ever climbs that high, you have cooked the batteries. Let me try say it again! You turn off the charger based on TIME, not the voltage reached!!!!

does the voltage doesn`t affect the battery as long as the current was set correctly?
As long as you turn off the charger after 14 hours!!!!

#### audioguru

##### Well-Known Member
Energizer does not make toxic Ni-Cads anymore. They import Japanese Ni-MH cells with 4 times the capacity now.
In their Ni-Cad applications manual they show over-charging at 1/20th the mAh rating for 2 years. The first discharge had a 65% capacity and a lower voltage. The second discharge had a 95% capacity and normal voltage.

#### sv_clintrace999

##### New Member
thank you again gentlemen for these helpful knowledge and for the information sir audioguru. again i highly appreciated these prompt reply of yours thank you very much again

Maraming Salamat,

Clint (Davao City,Philippines)

#### Boncuk

##### New Member
Check out the MAX712/713. They have everything necessary to charge batteries from 1 to 16 cells.

Boncuk

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