×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

softball question here

softball question here

softball question here

(OP)
So I have an old stereo with an internal battery that runs on 7v 1.2 amps and its without a powercord. just so happens this radio is great for the beach because it has an internal battery, so I would like to rig up a charger to use in my truck. I know my truck delivers 12v 10amps max. If i include a fuse at say 2.0 amps for safety and a small capacity resistor to limit the current to 1.2 amps, should i be ok powering my stereo at 1.2 amps? With respect to the battery charge, is the only thing important the amperage and not the voltage?

I'm not a electrical guy, clearly. thanks in adveace.

RE: softball question here

Hooking a circuit designed for 7 volts to a 12 volt battery will NOT be good move. NO, voltage is actually more important than amperage in this situation. The 10 amp rating simply means that the maximum current that you can expect to draw is 10 amps.

Think of the wire coming from the battery as being a water pipe. Voltage is the pressure and Amperage is the rate of flow. Now think what happend when opening a facet a small amount. Only a certain amount of water will flow out but the pressure will remain basically the same. It's the same with an electrical circuit, the Voltage will be what it is, but the flow of current will depend on what it's hooked to. Lower the resistance (the valve being opened to allow more water) the higher the amount of current that will flow (the more water that will pass through the pipe).

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

RE: softball question here

With a resistor to limit current the voltage will be determined by the battery you are charging. You have to monitor the battery voltage because once it is fully charged here will still be a charging current which will over charge (destroy) your battery. Basically the same as any cheap Chinese rechargeable battery set-up.

RE: softball question here

State of charge is almost always measured through voltage, since it's essentially impossible to measure the amount of charge inside the battery. A nominal 7V battery probably gets overcharged at around 9V. While the truck battery is nominally 12V, the truck charging system typically delivers 14V or so for charging, and the truck battery will typically max out at about 13.5V. Therefore, if the truck's charger is ever active while your system is connected up, it'll most likely damage the system, even with a current limiter.

If you still wish to go this route, you should get a charger specifically for this battery and use it to buffer the voltage from the truck and prevent damage to your battery and system.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers

RE: softball question here

Some truck batteries now go up to over 14 Volts. Don't forget load dump events when the alternator output may peak at around 100 Volts.
First find out the voltage that your battery needs to charge. Then buy an integrated circuit voltage regulator. Don't forget that the regulator may be subject to 100 Volt transients.
Others here may provide more specifics on selecting and wiring a voltage regulator circuit.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: softball question here

Anything with an internal rechargeable battery should have a socket for charging the internal rechargeable battery. Beside the power input socket, or in the manual, it should say what voltage, current and polarity is required. Is this the 7 volts at 1.2 amps you mentioned?

If so, then you can maybe (still?) purchase adjustable DC regulators that plug into a cigarette lighter socket (universal DC car adapter, example image embedded below) and slide the many-position slider switch over to the nearest voltage to 7 volts. 1.2 amps might be a bit high; most of these top out at 1 A...

If you can't find one, then you'll need to arrange a 7 volt PS. eBay has quite a selection of DC-DC converters (easily meeting these specs), but they would require some wiring and packaging.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources