# Milwaukee battery tap for 12v power supply



## raylo32 (Nov 25, 2006)

We just got an inflatable kayak and I also bought a nice 2 stage electric air compressor for it. Compressor comes with a cigarette lighter plug, of course, which is great some times. But in many cases it will more convenient to carry the kayak in its handy case down to the water and set it up there. So I made a plug to tap a Milwaukee M12 battery to run the compressor wherever. This will also work with your typical automotive 12v tire compressors. And it is also good anytime you need a 12v power supply in the shop or wherever. It does work with the little CP series batteries for the kayak that gets at most 6 psi, but for car tires and higher pressures and longer run times it is better to use the XC type that has more capacity and is able to provide higher current. You can get the 3D print file and instructions how to put it together here:

Milwaukee 12V battery tap by raylo32 - Thingiverse


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## chandler48 (Jun 5, 2017)

Will it hold a charge long enough to complete the job, or do you need to use more than one??


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## raylo32 (Nov 25, 2006)

Yes, the little CP can do the job. But it is marginal for multiples or to then deflate. The CP battery is 3S1P (3 cells in series only one set of 3). So the higher current needed by the pump will cause the voltage to sag even though the battery is not yet discharged. The pump shuts off when is senses low voltage even intermittently like that. The CP simply aren't designed for continuous high current discharge.

The larger 4 AH XC battery is 3S2P, IOW it has another set of 3 cells in parallel so can flow at least twice the current without sagging. Probably fill and deflate the thing 10 times.

I have yet another alternative in the works for Makita 18v batteries via a dock like this and a DC/DC step down transformer. One of those batteries will give even more capability. I'll post that up when complete.



chandler48 said:


> Will it hold a charge long enough to complete the job, or do you need to use more than one??


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## quatsch (Feb 4, 2021)

They usually don't parallel cells unless they use [steering] diodes, one per cell, to prevent one battery from overcharging the other.

Some of these compressors exceed the 10A available from the vehicle power port, even though the plug is designed to mate with the cigarette lighter plug.

I have the same setup except I take the compressor current directly from the car battery with large alligator clips to bypass the car's fuse. 
I had a heckuva time soldering to that chromed socket.


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## raylo32 (Nov 25, 2006)

I haven't taken these particular batteries apart but most all of them are made with standard 18650 size li-ion cells. You can pretty much see how they fit in the tube shaped parts of the case. The square packs use them also. Just like generic e-bike batteries and even cars. Except for small differences in the subtype of cells the only way to get higher amps and higher watt-hours is to parallel them. The only way to get higher voltages is to put more in series. The convention is XsXp. That's how all these things work. The cells balance when they charge.



quatsch said:


> They usually don't parallel cells unless they use [steering] diodes, one per cell, to prevent one battery from overcharging the other.


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## chandler48 (Jun 5, 2017)

I had a Ryobi battery go prematurely south once, and being the inquisitive type, I just took it apart. I found it wasn't so much the batteries inside as it was the manner in which they were connected. Flat copper busses, aluminum foil thick that burned when too much torque was applied to the tool.

I just desoldered them and replaced them with insulated wire of a heavier gauge. Battery worked just fine after that.


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## raylo32 (Nov 25, 2006)

Here is a pic of the insides of the small EGO 56V battery. It is 14S1P. You can see the 2 sets of 7 batteries all in series that gets you to 56V. Their next bigger battery (5AH) is the same except it is 2P, i.e. 28 cells total connected up as 2 sets of 14 in parallel. They have an even bigger 7.5AH one that is 3P... 3 sets of 14.


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## raylo32 (Nov 25, 2006)

Good job. The EGO battery above has a dead BMS unfortunately. I am pretty sure the cells are still good.



chandler48 said:


> I had a Ryobi battery go prematurely south once, and being the inquisitive type, I just took it apart. I found it wasn't so much the batteries inside as it was the manner in which they were connected. Flat copper busses, aluminum foil thick that burned when too much torque was applied to the tool.
> 
> I just desoldered them and replaced them with insulated wire of a heavier gauge. Battery worked just fine after that.


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## quatsch (Feb 4, 2021)

I saw many hits about paralleling batteries with no mention of diodes.
I saw only one hit with a diagram of current steering diodes used in the sense that I mean.

Maybe the cells are matched very closely for voltage and internal resistance. 
Maybe the internal resistance of a partly discharged cell is high enough to prevent excessive intercell currents.



> Flat copper busses, aluminum foil thick that burned when too much torque was applied to the tool.


So the foil acted as an internal fuse.


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## raylo32 (Nov 25, 2006)

I don't build or really work on batteries (much) and don't know everything about any particular one. But the battery BMS's are getting smarter, the ones in my e-bike batteries have bluetooth so you can monitor parameters on your phone and they have low voltage cutoffs to prevent overly discharging the batteries. And yes, good batteries are made with high grade matched cells.

They usually have small additional wires to each cell that are used to balance the cells during charge monitored and managed by the BMS. The main current is pushed through each parallel set charging those batteries in series with the balancing wires trickling extra juice as needed to even out the cells that need it... and/or maybe taking away some as needed here and there. And some newer batteries and tools are designed to prevent what happened to Larry like Makita's Star Safety System and others.

If you have hobby LiPo batteries the balancing also happens during charging and you connect up the separate small connector to a balancing board plugged into the charger. This does the balancing as described above. The balancing boards are specific to the number of cells in a battery and match the number of wires to the cells in the battery's balancing connector.




quatsch said:


> I saw many hits about paralleling batteries with no mention of diodes.
> I saw only one hit with a diagram of current steering diodes used in the sense that I mean.
> 
> Maybe the cells are matched very closely for voltage and internal resistance.
> ...


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## quatsch (Feb 4, 2021)

All of my tools are corded. 

If it works well enough & long enough, it's a success.


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## raylo32 (Nov 25, 2006)

I'm sorry! ;-)



quatsch said:


> All of my tools are corded.


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## quatsch (Feb 4, 2021)

Thanks for your condolence.


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## raylo32 (Nov 25, 2006)

Took the inflatable kayak for its maiden voyage. Easily filled all 3 air chambers... and deflated them after... with the one M12 XC battery. Probably could have done it all again a couple more times. Here's a short video of pumping up the floor chamber to 4.5 psi. The pump shifts from the blower to the 2nd stage somewhere around 1 psi. Much better inflating this in the grass near the water than in the gravel parking lot tied to the car's electrical.


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