I need to design a ni-cad battery charger to charge 18 volt power tool batterys from a 12dc van ciggeratte lighter,my problem is working out what constant current is neededor charge voltage, and can it be the same if a different battery is plugged in.is it just a case of giving a percentage of the ma/h rating of the battery as a constant current for a given time. be great if anyone has an answer,i can design the circiut if i just knew what current or voltage to give/or how to wor it out.

 

usually I use my nimh charger to charge my ni-cad batteries which is done in about 3 hours - the nimh charger usually charge at about 160mAh or so - so you can aim for that.

 

i'd probably do a 555 chopper to a step-up transformer to get about 20v dc on the out, then simply use some form of current limiting resistor (100ohm 5watt) in series to the battery. simple/basic charger. as the voltage climbs on the battery the current will go down. when battery voltage reaches 18v the 2v diff and resistor will be like a trickle charge. if you get fancy you could build in a "off" circuit when a certain low current is reached.

i dunno, just a idea.

 

thanks dc kid, so you say use a 20 v dc out square wave peak and constant current , im not sure what the constant current should be for a set number of cells,is there a basic output i can use to charge different batterys,thanks for your help

 

if the supply voltage is greater than battery voltage there will be a ingres of current (energy). different types of batteries like to be charged at different rates (current). i guess you could build the supply greater than your greatest battery voltage and then use some form of switch to switch between various resistors to change the ingres current depending on what battery is being charged, etc. there wont be a constant current unless you include a current source circuit, etc.

for under 12v i guess another method is to use 555 with varying PWM to charge battery. changing the PWM changes the rate at which battery charges, etc. but since you need to charge 18v you'll need to use some form of voltage step-up.

 

cheers dc kid, this is what i will do then , ill have a 19-20 volt output, and ill set the output current to a constant, ,when setting the constant current is there a rule , ie, if the battery is 1200 ma/h is there a rule for designing what constant current i should push through, thanks for your help, i think once i have the constant current i can get to the drawing board.

 

different types of battery chemistries like different charge rates. search the Wiki and Google and AAC for this info. there are also many schematics out there for battery chargers. also check the National and TI sites and search for battery charging application notes.

the charger i mention is a simple charger i use to keep a small car battery on my tractor charged through the winter. but its much simpler as the source is a 14vdc 1amp wall wart and i just have a 15kuF cap to make the dc flat and a inline 100ohm 10watt resistor. because of the voltage diff a little bit of current flows into the battery keeping it charged through the cold months. works like a champ.

there is a science behind charging batteries of different chemistries.....

 

You need to know what type of charging is recommended for that particular type of battery: trickle charge, quick charge, fast charge, etc. That will determine the charging current and time required.

I did a quick charger circuit for NiCd batteries several years ago. I used a buck converter to provide a constant current under software control, using the pulse charge method. The battery pack was evaluated before charging by loading it and checking the battery voltage after a couple of seconds delay. If the battery voltage dropped by some threshold amount under that load, a quick charge cycle was initiated during which the battery was charged for some time period at C/3 and then allowed to "rest" for a short time before the voltage was checked. Because of the switchable load, the software could also optionally provide a "burp" pulse. The charge/rest/burp/check cycle period was on the order of 1 second.

The voltage profile was checked for the characteristic "dip" of a charging NiCd or "flattop" of a NiMH. Charging could also be terminated due to timeout, temperature rise or cell overvoltage.

This approach allows a very versatile charger, and since it is a regulated current source the number of cells could be variable, with an upper limit determined by supply voltage.

Admittedly, that may be a little more complicated than what you want, but you need some "smarts" in the charger if you wish to use any of the quicker-charging methods. Otherwise, use a resistor to trickle charge for 14-16 hours...

 

for most (if not all) chemical batteries the slow "trickle" charging method will allow max battery service life. OP never mentioned how fast the battery needs to be charged. the reference to the cigarette lighter as source and power tools does seem to indicate that perhaps it will be used for worksite use.

 

You can find the recommended charging profiles on many of the RC model sites. Here is one site with useful information:

http://www.powerstream.com/NiCd.htm

Have you considered buying a commercial unit? Many of them run off of 12V and will charge batteries of greater voltage. I use the Triton, which will charge up to a 24-cell (28V) set. You can fast charge, slow charge, cycle, and discharge. An LCD display lets you know the charged capacity. The commercial units offer multiple battery chemistries, so when you replace the NiCd with A123 etc., you can keep the same charger. The Triton is now available as the Triton2. New units are about $120 and used (ebay) are about half of that. John

 

thanks for your help guys,i will look into the web site given for info on the type of current ect the nicad cell needs.