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Building a Bosch 12V Battery Eliminator

Nick Marques

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Hi all,


Not sure if anyone here is into tinkering with things to make them better, but I own a lot of Bosch 12V power tools. I love them for the combination of price/performance. I currently own the PS20, PS21, PS41, PS60, and FL12. I also own a lot of Bosch 18V tools.


I set out to create a battery eliminator for the 12V battery. There are a few times where I am doing a lot of use and don't want to keep swapping or rotating batteries. So, I got some 12V barrel plugs and jacks, removed the cells from an old battery. I wired up the plug to a 12V / 3A AC-DC adapter. It measures about 12.25V on my Fluke meter. 


I originally came up with the idea for my FL12 work light (which is FANTASTIC), it does go through batteries quickly as there is no variable settings. It's full bright, all the time. The adapter battery works 100% the FL12 flash light, but on a PS20, it pulses for a split second then dies... and it keeps doing this with consecutive trigger presses until nothing happens -- The adapter I am using goes into a "shutoff" or "protect" mode. If I remove it from AC, wait 10 seconds, plug it back in.. it works again.


Any idea why the PS20 won't just spin forever? There's only 12V contacts inside the tool, no temp or other wires connected. 


Does the AC-DC adapter need to be under 12V? I know the batteries are technically 10.8V but the battery I measured is around 11.5V. 


Do I need an adapter with more current?


You can see the adapter I made here.. nothing special, just a DC jack at the bottom. 



BTW- I am an audio visual engineer, but my knowledge of how batteries work and power tools "think" is not my strong suite. 

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First off welcome to the forum Nick!

Are your talking about making the Bosch cordless tools into corded tools? Also the FL12 has a low setting. When you press the power on button, keep pressing it and it will cycle down to a lower power level!

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There are multiple tabs on the batteries. Not exactly sure which ones do what. You would need to dig up a schemagic from Bosch on them. I would venture to guess that your adapter is putting out the proper voltage but not the required amps needed. 


I know it is fun to tinker like this, but a word of caution for you. Posting things like this on a public forum could get your warranties voided if Bosch saw this. Not saying it's very likely but usually tampering like that does void manufacturers warranties. 

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i dont know but why do you wanna change a cordless tool/ lamp into corded? doesnt make sense to me.

ok the 12V tools are tiny and handy but thats what there are build for....

i get a heck of runtime out of my batteries.

the FL12 has a great runtime - thats what bosch says:

  • Great runtime - 6 hours of runtime with a 2 Ah battery and 12 hours of runtime with a 4 Ah battery, runtime is doubled when the dim function is used
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Thanks all...


I've never heard of a manufacturer pro-actively voiding warranties just because they saw a forum post... For starters, the battery is what is modified... not the tool. Yes, the fine print may say that using non-OEM batteries may void warranty but in my vast experience with warranties (and voiding them), it generally only applies if they can prove that is the cause of the problem. It's like using non-OEM brake pads on your car, it CAN void the warranty IF they prove the non-OEM pads caused a pre-mature caliper failure. 


So, if my issue with the motorized tools is amperage, I wonder what amount is required? What kind of current do the 10.8V batteries provide? 


Oh and thanks for the bit on FL12 dim mode. Mine did not come with a manual even though it was new with a sealed box. 

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  • 1 month later...

I can pretty much assure you that your poweradapter is way to weak.

First off, the mentioned problems is just what any well designed AC-DC inverter is supposed to do when you overload it. So thats the first clue.

Secondly, 3 Amps won't get you anywhere near what you need. I have measured the peak current draw when testing an old medium sized (30 nm) 12V NIMH drill I took appart. And it peaked at around 50A (sometimes more). Would guess your smaller drill easily could at least pull 25A or more Peak.

If I were to guess, I would esimate you should at least have 10-15 A continuous supply current, and something around 30A or more in peak current. 

Thirdly: High power Li-Ion batteries can deliver at least 6-7C continuous, and probably deliver 10-15C peak.

(Ah x C = A) Meaning a 2Ah battery could deliver peak 20-30A and 12-14A continuous.

Off course, this is rough estimates, but it was not far off in my experience. Tried to use the drill earlier mentioned with a 15 Ampere Power-supply, and most times it would trigger the over-current and be reset. A solution was to connect a large capacitor in parallell with the output. But whenever i power the system down and turned it on again the next day, the surge current in the capacitor would trip the PSU again. So I needed to reset and power it up again a couple of times until the capacitor was charged enough (so the surge current would be low enough).

This could probably be solved with a smaller Cap. Which I did not bother getting. But a larger PSU did the trick. Good luck. ;)

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