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Electronics/Battery conversion: Follow up question



In the past I have build simple IC devices from scratch and I am quite comfortable with basic electronic repair.

I have a TDK Bluetooth boom box that has a failed battery. It has developed a memory and will not charge properly. I am going to replace it with a Lithium Ion. I have a few questions. I plan on using a rechargable battery from a drill set I have. I will either disassemble the device and make an external lead for the new battery that connects to the internal battery inputs or use the DC power input port to supply the power via the new battery (an easier solution if it will work). The power adapter is an AC to DC 18v 2amp output. I am not sure of the internal battery's specs. They battery I plan to use is an 18v but only 1.5 amps. Is that an issue when using it to power the adapter port? If so can it be overcome with a minor modification. Alternatly, should I just forgo trying to use the input power port and connect directly with the internal battery leads with the new battery?

Can you also comment on the following... I assume the AC/DC power adapter is inputting different power specs due to the fact that it is also being asked to recharge the battery.

Thank you very much.

ANSWER: I need more information to know what you are working with, electronically speaking. Can you give me the model number for the boom box or at least its internal battery type, kind and voltage rating - of the battery not the terminals of the pod transformer.

Some internal battery charging circuits are ac, depending on the battery diode to provide a pulsing dc charge current. Others have full wave rectification and filter to provide a more constant dc.  

There are many things to consider when switching to different battery; if it is close in electrical qualities it may be nearly plug compatible, but not close you may need to do some engineering or make a different battery selection.

I await your further information.

---------- FOLLOW-UP ----------

QUESTION: Thank you ZZ,
I have two last questions, and based upon your advice, I will either convert the battery type or replace with original factor equipment (OEM).

The unit we are discussing is a TDK A73 boom box. The battery specs are BYD H-AA2000mAh 7.2V M29, NiMH.

Can you tell me what circuitry I need to make the conversion from the a Li-ion 18v 27Wh rechargeable battery to meet the power requirements of the above listed existing battery.

Thank you again.


The simple approach is to put a resistor in series with the 18v battery and the battery in the boombox.  If the battery stays in the set it will act as a voltage regulator to keep the charging correct.  If you must remove the factory battery and connect power to the 18v unit you will find it very difficult as the series dropping resistor will get hot if not sufficient size or power rating.  Also, about half the power supplied from the 18v battery will be wasted in heating up the series resistor.  Not a good solution at all.

There are ways of converting the 18v down to 7.2 but it requires an ac/dc converter - which will eat up lots of power to run the converter.  But, what you need is to convert the 18v dc to ac then from ac to the new dc voltage of 7.2 volts.  Not an elegant solution either.

I think you should just purchase a battery with the proper voltage range to avoid all the business outlined above.

Here's one and you can find many others by online searching:

Hope this helps you get the right solution.  


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Electronics questions about AC, DC and digital theory.


Graduate electrical engineer with over 40 years in electronic design, manufacturing, project organization and patent review. Experience in fields of industrial and consumer electronics (audio, video, acoustics, etc.)

IEEE (Institute of Electrical and Electronics Engineers); Senior Life member AES (Audio Engineering Society), Fellow Life member

BSEE University of North Dakota

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