LED Light Panel Build, Part 2: The LiPo Battery

For Part 2, I’ll be going over the aspects of the lithium polymer (or LiPo for short) battery setup that I’m using for portable setups (such as the LED light panels), sans AC power. LiPo battery were used in this project, specifically for their long-life spans and price points.

DSC_0102The reason why LiPo batteries are so popular is that if you properly maintain them, they can be recharged several times without degrading the battery over time. I won’t go over the basics of LiPo batteries (a Google search will yield a lot more information). Instead, I’ll be focusing on the basics of the specific type of LiPo I used within these projects. I snagged a few ZIPPY Flightmax 8000mAh 4S1P 30C off HobbyKing. If you happen to find a comparable brand, that’s fine. As long as it’s a 4 cell LiPo battery, rated for at least 8000mAh. If you’re curious about learning more about the battery terminologies, such as 4S1P, you can go here and play around with the settings on the Script Asylum website.

A 4S1P battery is essentially for 4 battery cells in series (hence the 4S). A 4S2P battery would be 2 parallel (hence the 2P) connections of the 4 battery cells in series. I personally wasn’t interested in the 2P versions. Although they do yield slightly more current capacity (8400mAh vs 8000mAh), I prefer the 4S1P for its low profile and weight factors.

The LiPo battery protection.

One could hook up a LiPo positive and negative terminals directly to their equipment or circuit, but the problem with that is the stock LiPo batteries don’t have a battery protection circuit. A battery protection is a must-have to prevent over-discharging the battery below a nominal voltage level. Discharging a battery past its nominal voltage will damage the LiPo battery. A cheap LiPo battery alarm can be used, as a quick solution to alert users when one of the cells have dropped below a preset voltage. However, a better solution would be to create a circuit that automatically shuts off power from the LiPo battery when one of the cells have dropped below a voltage level. Matt from DIY Perks has a great, basic circuit layout that I used as a stepping stone to work off of. The image below is my version of circuit diagram that I’ve adapted from DIY Perks, to fit the 4S1P LiPo batteries.

DIY-Battery-Protection-Circuit
Figure Alpha – LiPo Battery Protection Circuit

The LiPo battery alarm.

IMG_1029The first step is to purchase a bunch of cheap LiPo battery alarms. The more, the better, because I have noticed that the voltage read outs on these may be off by 0.2-0.5 V, which may be a big deal if you’re discharging the battery down to as close to the recommended minimum level of power (around 3.2V/cell). To weed out the alarms with poor voltage read outs, I HIGHLY recommend measuring the voltages of each cell off the balance connector. I used a digital multimeter to check the voltage between the 1st (black) and 2nd wire of the balance connector. This is the 1st cell. Then I checked the 2nd and 3rd wire. This is the 2nd cell. So on and so forth. Record what the values are for each cell. For instance, the voltages may be 3.8V, 3.81V, 3.79V, and 3.82V for 1st, 2nd, 3rd, and 4th cells, respectively.

Next, attach the LiPo balance connector to the LiPo battery alarm, as shown below. The seven-segment displays will display the voltage read out. Make sure, as the display cycles through all the cells, that the values roughly match up to what was recorded. The image below shows how you should connector your balance connector to the LiPo battery alarm.

DIY-Battery-Protection-Circuit-3

Once you’ve weeded out the LiPo battery alarms that are way out of range from what you recorded, it’s time hack these good alarms. You can click here to check out the step-by-step that DIY Perks has out on his channel on dismantling the alarm speakers from the battery alarm testers. Once the two black alarms are removed, I attached a couple of pigtails to the top left and bottom right solder pads (see left image). These represent the +5V and ground to set off a dual latching relay.

DIY-Battery-Protection-Circuit-2On the schematic I provided above (see Figure Alpha), J1 and J2 are both connections made from the LiPo battery alarm to the battery protection circuit (see the zoomed in picture on the right). Since these LiPo battery alarms are so dirt cheap, it makes more sense to grab these off-the-shelf parts and modify them to our needs. As shown on the right, the yellow and blue wires off the top of the LiPo battery alarm will go to a relay. I made a small connector to connect between the 9-pins on the LiPo battery alarm to the rest of the circuit.

FullSizeRender

The LiPo battery connections.

DIY-Battery-Protection-Circuit-3I used male pin connectors to interface the female connector on the LiPo battery. The LiPo battery connections are pretty straight forward (2 pins, one for positive and one for negative). You will probably notice that the color coding on the LiPo battery balance connector (see below) is different from the color coding I used over my schematics. The reason why I used a different color coding from the LiPo battery balance connector was because the connectors that I purchased off of eBay were a different color than the LiPo battery balance connector. So to avoid any confusion, the following table will hopefully clear this up:

Pins Schematic Colors LiPo Balance Connector
1 Black Black
2 Red Light Blue
3 White/Gray White
4 Yellow Yellow
5 Orange Red

DIY-Battery-Protection-Circuit-4I spliced the connections on the LiPo battery and balance connector, since the ones from the battery were going straight to the board. As explained in the DIY Perks video, this allows us charge the battery, without have to remove it from the circuit, every time when we charge the battery. It’s a bit extra work, but well worth the effort.

IMG_1071As you can see on the image on the left, the LiPo battery leads were shortened significantly to fit inside the battery case. To do this properly, without shorting the leads of the battery, make sure you work on one wire at a time. Whatever you do, DO NOT cut the positive and negative wires simultaneously! You will end up arcing whatever metallic cutting implement you are using, most likely damaging the tool and yourself.

Instead, work on one battery lead wire at a time. I suggest working on the negative lead first. Shorten it, and then solder a wire from the lead to the board. Use shrink wrap and hot glue to “cap off” the end of the exposed wire. Then move onto the positive wire.

The XT-60 connector I used, at the top of the left image, was the female connector. I decided to provide some protection to those terminals by using the end of a male connector, and putting shrink wrap and hot glue around the solderable end. This provides a protective “cap” on the exposed terminal.

The 6-pole switch.

The crazy 6 pole switch (S1 on the schematic), or the technical term, 6PDT latching switch. 6PDT stands for 6-pole, double throw, meaning there are 6 set of “switches” and 2 throws, or connections that the switch can make contact with. The latching switch is a switch that can stay continuously stay on when pressed, or completely off when pressed again. This is the basis of a light switch or power button. In this case, the 6-pole switch acts as the power-on button.

Below, I’ve shown what happens when the 6PDT latching switch is “on”. The circuit between balance connector and the 9-pin LiPo voltage indicator/low voltage alarm is completed, turning on the LiPo battery alarm. At this point, you should the seven segment displays cycling through each cell, displaying the voltage of each cell, as well as the overall voltage. Likewise, when the 6PDT latching switch is “off”, then you break the path between the balance connector and LiPo battery alarm, which effectively turns off the LiPo battery alarm.

DIY-Battery-Protection-Circuit-5

The relay circuit.

Without the relay circuit, the 6PDT latching switch only turns on the LiPo battery alarm. You could wire the LiPo battery’s positive terminal to a switch, and forget about adding the relay circuit. But you would still have to monitor the LiPo battery alarm, making sure that the battery doesn’t drop below 3.2V/cell. And you would have that switch on the LiPo battery itself, which is inconvenient, especially if you end up forgetting to turn off the battery. And if you end up leaving the battery hooked up to your circuit, draining the battery well past the 3.2V/cell minimum, then you’ve ruined your LiPo battery pack.

This is where the relay circuit shines. The work done previously in removing the alarm speakers on the LiPo battery alarm will also pay off in the relay circuit. As the 6PDT latching switch is turned “on”, the relay doesn’t get set, because of the momentary switch (S2 on the schematic). S2 is a normally-open (NO), momentary switch which only closes the circuit for as long as the switch is pressed. Once you let go of the switch, the circuit is broken, which opens the circuit again.

The specific relay chosen was a Panasonic DSP1A-L2-DC5V. It’s a dual coil, latching relay. A lot of relays require constant power applied to the coils on the relay, to either keep a circuit open or closed. This type of relay doesn’t require that, and that’s why we use a momentary switch to energize the coil. As shown below, when S2 momentarily closes the circuit, the reset coil on the relay is energized. D1 is a diode that drops the voltage from Cell 2, from approximately 7V to a manageable 6V (it takes 5V to energize the coils on either the reset or set coil). When the reset coil becomes energized, the open switch in the relay will close the circuit, which allows for the negative terminal on the output power to power your electronics. When you release your finger off the S2 switch, the relay will continue to latch (thus, keeping our electronics powered), unless the 6PDT latching switch shuts off power, or our LiPo battery alarm triggers the set coil on the relay.

DIY-Battery-Protection-Circuit-6

Manually shutting off power to battery pack.

This can be done by switching our 6PDT latching switch, to “off”. To release the contacts within the relay, voltage across the relay coil may be applied in reverse. This is where the cleverness of the S1 switch comes in (I have to say that it’s pretty genius of Matt, from DIY Perks, to come up with this!). The last set of “throws” on the 6PDT latching switch, has its pole wired to ground, off one end of the relay contacts. This should be the opposite relay contact of the LiPo battery ground, since we don’t want to continuously draw power off the battery.

DIY-Battery-Protection-Circuit-7So what happens here is that we use the line off the very last cell of the balance connector (in this case, cell 4) and the battery ground to reverse the voltage across the reset coil on the dual coil relay. Remember how when we turned on the LiPo battery alarm, via 6PDT latching switch, and pushed down on the momentary switch (S2) to allow current to flow from the bottom to the top of the reset coil of the relay? Now we are simply reversing the voltage across the coil, allowing the current to move in the opposite direction. However, we cannot just apply power from the last cell of the balance connector. A resistor is required to step the voltage down to 5V from nominal 14.8V off the 5th pin (last cell) of the balance connector. I’ve also added some basic math to help those determine how to size their R2 resistors in other configurations, below.

DIY-Battery-Protection-Circuit-8

In the above equation, there are a few known values. If you’re using the Panasonic DSP1A-L2-DC5V, the datasheet will provide RCOIL (the coil’s resistance) and VCOILMAX (the maximum voltage that can be applied to the coil), which are 83Ω (ohms) and 6.5V, respectively. The maximum voltage for the LiPo (VLIPOMAX), in this case, is approximately 16.8V. If another dual-coil latching relay is used instead of the Panasonic one, refer to the datasheet for that component and look for the resistance of the coil and maximum voltage that can be applied to the coil.

The other great thing about the relay circuit is that once the reverse voltage is applied, as the power to the battery pack is shut off, when the contacts release, it creates an open circuit. Although it may seem crude, to have two switches that allow you to turn on power, you don’t risk a constant low current draw that will ultimately drain your LiPo battery past its minimum voltage.

Lastly, the auto shut-off circuit.

The auto shut-off circuit takes advantage of the LiPo battery alarm’s low voltage indicator. Typically, if we kept the speaker alarms on the the LiPo battery alarm, a loud noise would sound off when the battery drops past the threshold. The threshold can be set by the small pushbutton switch on top of the LiPo battery alarm.

IMG_1075
If the circuit is powered up, and the reset coil is energized to close the contact on the relay, then when the LiPo battery alarm is set off (due to the voltage of our LiPo battery dropping below the minimum allowed voltage), the set coil is energized on the relay. When the set coil is energized, the contact on the relay opens. One important thing to note is the orientation of how voltage is being applied to the reset and set coils. Make sure the reset coil has the positive voltage and ground connections aligned similarly to that of the LiPo battery alarm voltage pads. If the coils are not both positive on one side and negative on the other (don’t worry about the manual shut-off circuit with R2, because we want that specific one to be reverse voltage applied to the reset coil), the relay won’t work correctly.DIY-Battery-Protection-Circuit-9

And that’s pretty much it. This was a really long article that I put up. And these types of articles require a lot of work, especially when providing colored diagrams to illustrate what is happening within the circuit. If you like this, let me know. If the work is worth it for everyone to see how something works (especially with electronics), then I’ll try to put up more detailed content like this in the future. If you’re confused with anything, let me know and I’ll try to clean up the descriptions a bit more.


Drill Press Table

I acquired a free drill press a long time ago. She still works pretty decently. It’s a vintage, standing drill press, in case you couldn’t tell. She’s seen better days. These older drill presses have a stacked set of pulleys that need to be fiddled with when changing the RPMs, unlike the new-fangled drill presses that can be controlled the RPMs by dialing the speed, digitally.

The problem with the drill press was that it had such a tiny work surface to work off of. It’s a small circular surface that doesn’t hold much. So, after looking at existing designs on the internet, I decided to build a decent, but larger table for my drill press! The most important thing I wanted was the ability to have a solid fence that had slots built into it for stop-blocks, for repeatable drilling. I also wanted to still be able to change the height of the pedestal, by extending the handle outwards.

The table top.

I sort of went a wee bit fancy with the look of the table. I glued up 2 sheets of 1/2″ Baltic birch plywood, and glued 1/2″ thick walnut pieces to cover up the plywood edge. I like the dark and light contrast between the Baltic birch and walnut. The surface has 2 dados cut for a couple of blue aluminum t-tracks for the t-bolts to follow on.

IMG_1039

I wanted to make the table top removable, in case I wanted to use the small table top that the drill press came with. So I had 4 holes drilled into the pedestal, where I attached bolts from the table, through the drill press stock table, to some 5-starred knobs. As you can see in the image below, the stock table is TINY!

IMG_1053

The sacrificial boards.

I wanted to make some sacrificial boards that would eliminate tear out on the back of a workpiece, when drilling. I also didn’t want to mar up my nice Baltic birch ply-top. I’ve seen so many different drill press table designs that have the sacrificial MDF cutouts aligned perfectly centered to the drill bit, which always struck me as odd. It’s always a large, square MDF piece that gets chewed up pretty quickly, since you’re making the same holes in the same spot over and over again.

I’m not sure where I stole this idea from, but an off-centered sacrificial piece was a better alternative! And it’s even more brilliant when the sacrificial board is a circle, because if you needed to drill in a fresh section, you could easily just spin the sacrificial board to a clean spot and start drilling! You can see below, how the sacrificial board gets a little more use out of its life…and yes, the sacrificial board has seen better days.

IMG_1046

Making these sacrificial boards are easy as well. Using a circle jig that I attach to my router, I can make a ton of spares with such ease!

IMG_1049

The fence.

I wanted a sturdy fence that would always stay straight and wouldn’t move with the seasons. So MDF and solid wood fences were out. I could’ve done a plywood fence, but I wanted something that was sturdy and could take a beating. So I settled on extruded aluminum. I managed to snag some 80/20 aluminum off eBay for a good price with shipping. I also knew that if I was going to use some 80/20 aluminum, I’ll need some plates and bracket pieces that 80/20 manufactures as well. So, I grabbed 2 L-brackets, some fasteners, and a mating plate, that you see at the end of the fence, below.

IMG_1040

IMG_1042I love 80/20 aluminum. They may be pricey, but they have all sorts of crazy fasteners for any type of ridiculous build you can imagine! No, unfortunately, I’m not sponsored by 80/20, but if they’re reading this…hint, hint!

IMG_1043So as you can see with the left and right pics, I used some aluminum plates and brackets to create the fence. With the aluminum plate, I fashioned the main fence to attach to a small piece of extruded aluminum that is dead flush against the one side of the table. I used a speed square to make sure that the fence was perpendicular to that one edge of the table. Behind the fence, I have a small t-bolt attached through an L-bracket, fastened with a knob. When I use the fence, since I know it is always perpendicular to my sides, I’ll make sure the one side is tight against the table, and then I’ll screw down the knobs.

The t-bolts ride in the blue aluminum track. Instead of screwing the aluminum track directly to the plywood, I used some t-nuts on the underside of the plywood table. This was done because I was concerned about regular screws tearing out of the plywood if I over-tightened the knobs on the L-brackets. On the underside, I recessed the t-nuts a bit, so they wouldn’t protrude. I used some flat-head bolts to fasten the aluminum track to the t-nuts.

IMG_1052

I also made some stop-blocks for the fence as well. They were made out of gluing 2 1/2″ pieces of Baltic birch plywood, and a hole drilled through for a t-bolt to ride the 80/20 extrusion fence. A simple 5-starred knob tightens them in place. They come in handy when you’re mass-drilling multiple parts over and over again.

IMG_1054

 The height adjustment.

And lastly, we have the height adjustment. Nothing too crazy. I had some spare pieces of maple that I used. I managed to find a rod coupler, with one end fastened to the stud of the gear that allows vertical movement on the drill press. The other end was fastened to a thick rod of steel. I sort of finagled the angle and the hole location for the rod to sit in the maple. Nothing too fancy. Once I got the right angle, I fastened it to the underside of the drill press table.

Oh, I found a really sweet water pump handle off eBay and decided to use that as my crank handle. The old handle was flimsy and made out of plastic. This beast is cast-iron, and works like a charm!

IMG_1044

The future.

I think I may revisit this project later in the future. I don’t like how the chuck of the drill press doesn’t clear the fence. I may split the fence in the middle and add a vacuum feature that will still provide clearance for the chuck. The steel rod that extends the height adjustment crank needs to be longer. I screwed up on the measurement and prematurely cut the steel rod a little short and it now interferes with the side of the fence. I think with the next steel rod, I’ll most likely apply some wood wax to the surface to minimize surface rust and to help “lube” up the holes where the rod sits in.