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Power a 9 volt radio from a Makita 18 volt battery

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Makdab

New Member
Hi. I've got a broken cordless drill and would like to hack the battery connector on to my 9 volt radio so I can use Makita 18 volt 3ah to 5ah li-ion batteries.
Can anyone advise me of a suitable circuit or even a ready made part if it's small.
I would like it to be as efficient as possible.

Thanks for any help :)
 

spec

Well-Known Member
Most Helpful Member
Thanks for the fast reply!
How accurate does the amp output need to be? The radios adapter is 1.5a
The amps do not count, so long as you meet the minimum amps. What does count is the volts. So, for example if you made a 9V 1.5A power supply or a 9V 100A power supply, the radio would be quite happy with both. There are a stack of cheap ready-made switching regulators on ebay that would do your job well.
http://www.ebay.co.uk/itm/Adjustabl...120602?hash=item1a1ec1175a:g:cDwAAOSwMmBVmGbJ

http://www.ebay.co.uk/itm/LM2596-Step-Down-Power-Supply-Converter-Module-DC-4-40V-to-1-3-37V-LED-Voltmeter/282091101161?_trksid=p2047675.c100005.m1851&_trkparms=aid=222007&algo=SIC.MBE&ao=2&asc=38661&meid=bdace459d3734a96b2763cab42bb4529&pid=100005&rk=2&rkt=6&sd=112185120602

http://www.ebay.co.uk/itm/DC-DC-Buc...d0a3151&pid=100005&rk=4&rkt=6&sd=142083272112

The only problem you may have is radiated noise from a switching regulator, so you may have to put the regulator in a metal container to screen it. And you may have to add decoupling capacitors across the 9V line.

spec
 
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audioguru

Well-Known Member
Most Helpful Member
The drill has a circuit that shuts off the battery if its voltage drops to about 3.0V to 3.2V per cell which all Lithium-Ion circuits MUST HAVE. Your radio and the buck converter do not have this very important circuit.
If a Lithium-Ion battery is discharged too low and the buck converter and radio will do that then the battery is ruined and might explode or catch on fire if you try charging it.
 

spec

Well-Known Member
Most Helpful Member
The drill has a circuit that shuts off the battery if its voltage drops to about 3.0V to 3.2V per cell which all Lithium-Ion circuits MUST HAVE. Your radio and the buck converter do not have this very important circuit.
If a Lithium-Ion battery is discharged too low and the buck converter and radio will do that then the battery is ruined and might explode or catch on fire if you try charging it.
Yes, it would be wise to fit a cut off circuit between the battery and the buck inverter. Not sure about the pyrotechnics though.:)

Here is a link to a suitable battery cut-off circuit that can be adjusted for an 18V battery pack: http://www.electro-tech-online.com/threads/schematic-for-a-48vdc-battery-low-volt-disconnect.149413/ (Post #2)

spec
 

dr pepper

Well-Known Member
Most Helpful Member
If you want something simple and not wonderfully efficient, you could use a 7808 or 7809 voltage regulator (so long as the radio doesnt use more than 1 amp), you'd need a block of ally as a heatsink to dump the waste heat.
 

Makdab

New Member
Hello again.
I have ordered a buck converter for my 18v/radio splice project but I'm curious about the cut off circuit.
The suggested protection boards have multiple connections for each cell. I believe Makita batteries have this circuit already and the newer tools have a third prong which controls isolation. I presume it's the tool circuit I would need to mimic but as my old drill only has 2 prongs it's going to be tricky.

I'm wondering if I could have a simple cut out for the overall pack instead?
This may not be ideal but I have been using three 3ah batteries on these non protecting tools every week for 10 years, 6 of which I was running them flat before I knew better. They all still hold a charge and I still use them in less hungry tools.
Could I have a circuit that cuts below a certain voltage running off the 2 terminals?
 

MikeMl

Well-Known Member
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Is the radio AM or FM? Those stupid ebay buck converters generate so much RFI that they will make the AM band unusable; not so much on the FM band, but you might hear some crap there, too.
 

Makdab

New Member
The radio is dab/FM and I usually use dab.

I think Makita 18v 3ah pacs contain 10x Sony lithium 18650 3.7v 1500mah.
I'm a fair noob at electronics but keen to have a go. Would you recommend a beginner attempts this?
What voltage would you guess it should cut at?
 

spec

Well-Known Member
Most Helpful Member
I'm a fair noob at electronics but keen to have a go. Would you recommend a beginner attempts this?
Reluctantly I would say not to go for this project; it is a bit much to start with- sorry

I think Makita 18v 3ah pacs contain 10x Sony lithium 18650 3.7v 1500mah.
What voltage would you guess it should cut at?
say 5 x 3.1V= 15.5V

spec
 

Les Jones

Well-Known Member
Most Helpful Member
Hi spec,
I think most so called 18 volts Li ion tools use 5 cells. The sales department will always find a way to make the numbers look as big as possible. So they base it on 3.7 volts per cell. I've just measured the voltage of the battery from my new Aldi 18 volt drill and it measures about 15.1 volts. I then thought what do the state of charge LEDs show. The were all lit showing fully charged. I ran the drill for several seconds and it still showed fully charged. I then measured the battery voltage again. It showed about 20.1 volts. I left it for about 10 minutes and tested it again. Back to 15.1 volts again. I repeated these tests with the same results. I can only think that is the cell balancing circuit balancing the cells after use.

Les.
 

Makdab

New Member
Reluctantly I would say not to go for this project; it is a bit much to start with- sorry

say 5 x 3.1V= 15.5V

spec
I did do some basic acid etched circuits at school and do the odd repair job with my soldering kit these days. I'll study your schematic a bit more and see if I build up the courage ☺
 

spec

Well-Known Member
Most Helpful Member
Hi spec,
I think most so called 18 volts Li ion tools use 5 cells. The sales department will always find a way to make the numbers look as big as possible. So they base it on 3.7 volts per cell. I've just measured the voltage of the battery from my new Aldi 18 volt drill and it measures about 15.1 volts. I then thought what do the state of charge LEDs show. The were all lit showing fully charged. I ran the drill for several seconds and it still showed fully charged. I then measured the battery voltage again. It showed about 20.1 volts. I left it for about 10 minutes and tested it again. Back to 15.1 volts again. I repeated these tests with the same results. I can only think that is the cell balancing circuit balancing the cells after use.

Les.
Hmm, the plot thickens, but at least the cut-off circuit can be adjusted to more or less any voltage.

spec
 

spec

Well-Known Member
Most Helpful Member
I did do some basic acid etched circuits at school and do the odd repair job with my soldering kit these days. I'll study your schematic a bit more and see if I build up the courage ☺
Don't let me discourage you. I am sure anyone with a bit of savvy could build the cut-off circuit but if you are inexperienced and do not have the kit a sharp learning curve would be involved, especially as the opamp is only available in surface mount and is very small and difficult to solder into a circuit.

spec
 
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