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Charging a 9v battery from a car alternator.

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czwalga00gt

New Member
So I race rally cars, we have intercoms in our helmets. The new intercom I have is powered off of a 9v battery. This is due to the intercom picking up interference from the noisy alternator if it's powered directly.

The intercom CAN be powered by the 12v+ from the car; just has more noise in the system. My question is if I install a 9v rechargeable in the system. Then wire the 12vdc from the car on a switch in parallel with the 9v. Would this charge the battery when we're not using the intercoms? Or is there more to charging a battery than that.
 

JimB

Super Moderator
Most Helpful Member
Just connecting the 9v battery in parallel with the 12v supply would probably have the little 9v on the point of exploding very quickly!:eek:

You need at least a resistor to limit the charging current to the 9v battery.
I suggest that you use a 180 or 200 Ohm resistor in series with the connection from the12v to the 9v, to limit the charge current to about 20mA.

Or, a better idea would be to forget the 9v battery and to filter the 12v supply to the intercom in order to remove the alternator noise.
A nice big iron cored inductor in series with the supply from 12v and a capacitor to 0v would probably be the way to go.

JimB
 

czwalga00gt

New Member
Just connecting the 9v battery in parallel with the 12v supply would probably have the little 9v on the point of exploding very quickly!:eek:

You need at least a resistor to limit the charging current to the 9v battery.
I suggest that you use a 180 or 200 Ohm resistor in series with the connection from the12v to the 9v, to limit the charge current to about 20mA.

Or, a better idea would be to forget the 9v battery and to filter the 12v supply to the intercom in order to remove the alternator noise.
A nice big iron cored inductor in series with the supply from 12v and a capacitor to 0v would probably be the way to go.

JimB


Thanks! I'll probably go with the resistor route. There is noise suppression built into the intercom. There's no way to filter out everything though; it varies from car to car. Only easy way is to just eliminate it from the system, which is precisely why the went the battery route. I'm just looking for an easy way to charge it without having to swap it in and out.

Maybe i'll look for a DC to DC battery charger if they exist. 12v to 9v.
 

audioguru

Well-Known Member
Most Helpful Member
A 9V rechargeable battery is an old Ni-cad that can be trickle charged with a low current. Or it could be a newer Ni-MH that must NEVER be trickle charged. A Ni-MH battery is charged by a battery charger IC or circuit that detects a full charge then disconnects the charger. A simple resistor from 12V will seriously overcharge a battery and destroy it or shorten its life.

Read the fine print because a 9V rechargeable battery can have 6 cells and actually be 7.2V or it can have 7 cells and actually be 8.4V.
 

dr pepper

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Most Helpful Member
The problem is as soon as you connect the intercom to the cars power your subject to noise problems again.
Why is the alty noisy?, theres no particular reason for it to be any noisier than a stock one, unless your running a dozen cibie oscars.
A dc to dc converter would isolate the intercom from the car and give you extra noise immunity, and you can probably get a 9v o/p one or ones thats adjustable.
Car radios often have a big fat choke in the + wire to the set, if your into cars then you'll know what I mean, if you can rob one of these from an old radio and put it in the + line to the intercom that'll help.
 
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