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12v transformer to charge a 1.2v 40mAh Ni-MH - Charge 4mA 14h

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dickspanner

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Hi,

I am getting into electronics slowly and enjoying it.

I could do with a little help please

I am building a project where I want to charge a battery mentioned above from a 12v car battery.

Does anybody know of a design that already exists to do this task using stuff you can buy from Maplin or similar.

Cheers

Dickspanner
UK Novice
 
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40mAh is awfully small for a battery isn't it? If it is not for an application that is super important (as I'm assuming it's not) then you can probably get away with simply charging it with a trickle charger of sorts. If that is not what you are looking for then an IC that is purpose built for charging batteries. The MAX712/ MAX713 (which are nearly the same part) can charge a wide variety of cells based on programed conditions (don't worry, the programing is simply connecting pins to different points in the circuit). They can both easily be set to charge your battery.
 
For a simple (near) constant current charge, simply place a resistor in series with the batteries. The resistor value would (Vb - 1.2V) / 4mA where Vb is the car battery voltage (12.6V nominal or about 14V when the engine is on).
 
Ok,

Unless my maths is rubbish (which it could possible be) I get 12v - 1.2v / 0.004A = 2700ohm resitor? Maybe boost to a 3200ohm resistor when battery at 14v with engine on etc.

Or am I being simple?

Again what may be a stupid question.

Would it be a simple case of putting the resistor in line from the positive from the battery (car) to the negative to the small battery? nad negative from 12v to positive of 1.2v battery. Sorry if that is a really stupid question but I have no idea.
 
No, the two batteries must be connected with the same polarity, other wise your circuit is doing nothing. Other than that, yes it is as simple as connecting a resistor in series with the two batteries.
 
cheers for your help in this matter.

Another simple question.

If the battery is fully charger in 14hrs I presume I would have to have some form of knock off charge system in place or if the timer that the battery is powering is operational at all time would I just leave the connection on all the time?

If I did this how would I work out what power drain this would have on the battery?

It is part of a solar design so I am trying to work out what drain will be on my battery so I can spec up the panel to recharge the 12v battery during the sun gain hours etc.

cheers

dickspanner
 
A battery disconnect is useful to prevent the battery from being damaged from over voltage. If your battery is simply being charged by a solar cell, I can't see the battery being damaged by any over voltage so a disconnect is not necessary.

You would find the battery run time by the mAh rating. A battery that is rated for 40mAh can deliver 40mA of current for 1 hour.
 
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Would it be a simple case of putting the resistor in line from the positive from the battery (car) to the negative to the small battery? nad negative from 12v to positive of 1.2v battery.................
To charge a battery you need to put charge back into it, the reverse direction from a normal discharge, thus you have to connect positive to positive. If you connect positive to negative you would discharge the battery.
 
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