|May 30, 2013, 08:07 AM|
Best-yet Auto Disconnector for LiPo Storage/Discharge
What is it?
This is a switch that will permanently disconnect a LiPo pack from a load when the cell or pack voltage reaches a set level. It is designed for discharging batteries to storage level but can also be used when testing where you want the load disconnected when you hit the end to avoid over-discharging the pack.
Got 6 fully charged packs and flying was cancelled? No problem, just use a parallel charge board. Connect a simple auto light globe load and they will gently discharge to storage and stop.
Because the switching is done by a cheap heavy-duty auto relay, it is just a mechanical switch. It does not affect any test circuit, and can switch as much current as the relay is rated for.
I posted the original version of this as a construction article using a CellLog 8S LiPo monitor back in 2009 as an “Improved simple auto disconnector for battery charging/testing”.
It was based on an even earlier version I made using a Maxpro LCD6S LiPo monitor.
It seems to have been useful and quite a lot of people built this – in fact it has even appeared on different forums as build threads with other folks getting the credit.
A couple of things bugged me about the design. It used two relays to enable permanent cutoff (clumsy) and required a separate power source, or multiple connectors, for the relay voltage (a bit of a PIA in practice).
This one solves both of those problems and makes a very rugged self-contained cutoff. It is completely powered by the pack itself. To drain a LiPo to storage charge, just plug the disconnector with a load attached into your pack, push the button, and come back when the lights go out.
UPDATE: There is an alternative version that uses a Solid State Relay that I have detailed in Post 41. Thanks to Ohmic for the idea. It is even simpler and is a neat solution - but it will not switch as much current. See second picture for what it looks like.
|May 30, 2013, 08:07 AM|
What do you need to build it?
What do you need?
You don’t need any electronics skill. If you can solder a wire to a battery connector you can build this thing. If you do have electronic skills just jump ahead to the schematic. This circuit is embarrassingly simple and obvious.
You do need 6 components (plus any wire and your favorite connectors).
See the first picture.
1) A CellLog 8M LiPo monitor. A number of vendors sell this popular and sophisticated little LiPo monitor including ProgressiveRC and Hobby King. It’s about $18. This clever gadget does all the hard work. It will monitor your cells and has an alarm output which can be set to trigger when a number of conditions are met. In my case when the first cell hits my chosen low voltage limit. You don’t need the more expensive 8S version, which includes a PC connection – although that and a PowerLog will also work if you already have one.
2) A DC voltage convertor. You need a DC Converter Buck Boost Voltage Regulator. There is now a huge range of ready-made voltage convertors on eBay and elsewhere. Most either reduce or boost the input voltage. A Buck Boost convertor does both. Effectively it accepts any voltage in its input range and can be adjusted to put out any voltage within its output range. Commonly this is 3-35V to 1.2-30V. There are a number that will do the job but I got this $12 one [See second picture] because it comes encapsulated in a strong metal box. They are designed to be soldered on to PC controller boards for solar power systems.
See third picture with connections labelled.
3) A standard auto horn/light relay. This is a 12V relay with heavy-duty contacts. Commonly they will switch 40 or 60 Amps over the DC voltage range we want which is more than adequate, but 150Amp ones are easy to find if you want to use this for high current testing. They may come with 4 or 5 contacts. The 5th is not required and can be removed to make it easier to assemble if you prefer by bending it back and forward a few times with pliers. Make sure you break off the correct one! $4-$20 depending on how big it is and where you buy it.
See fourth and fifth pictures.
4) A small push-button switch. It needs to be the very common type where two terminals are connected when you push the button and release when you let it go. If it has a third terminal that is connected when the push button is released just ignore it. $2 or so. See sixth picture.
5) Two diodes. These need to be the larger kind of power diode capable of an Amp - not a signal diode. The tiny diodes you find in electronic circuits are not up to it. Any electronics shop will have them for a few cents or you can rat them from an old transformer style plug pack or many discarded electronic devices that have a built in power supply. They are commonly a small black cylinder with a wire sticking out each end a white band round one. The most common type is called an IN4004, IN4007 etc. but any power diode will work in this circuit.
|May 30, 2013, 08:08 AM|
How it works
The circuit shows how it works.
I have attached a conventional circuit, but I know some people who might want to build this aren’t comfortable with electronic schematics so I have also done a “practical” diagram that tries to represent the real thing. They are functionally the same.
1 Without power applied the output alarm transistor switch on the CellLog is open and A and B are not connected. No current can flow from the LiPo to the load as the relay is open. When you plug in the balance connector to the CellLog it powers up. The alarm switch will then close because we set it as NC (Normally Closed) in the alarm menu. The relay still does not close since the only voltage is at C and the push button is open.
2 When you push the button the Voltage Convertor is powered and immediately puts out 12V. Diode 2 stops any heavy current flowing thorough the push button to the load and destroying the switch contacts. The 12V from the convertor pulls in the relay and connects the LiPo to the load.
3 Now Diode 2 passes current from the output of the relay to the voltage convertor, which keeps running even when you release the push button.
4 The relay stays closed until the CellLog alarm output triggers and opens. A and B are disconnected, the relay drops out and cannot reconnect.
5 Diode1 is to protect the CellLog from a high voltage spike when the relay disconnects. It has no other function – but it needs to be there.
In practice just plug the device with load attached into your LiPo pack power and balance leads and push the button. The pack will discharge to whatever trigger you have set on the CellLog. If you need to stop for any reason pull out the balance connector from the CellLog and everything resets.
This original design tends to exhibit "cycling" behaviour as it approaches the storage voltage cut off. The load disconnects then reconnects with slower frequency until it eventually disconnects all together. After some discussion on the thread, the conclusion is that the capacitors in the Buck/Boost converter store enough energy that the relay re-connects when the CellLog switches OFF then ON.
Some people like this behaviour - including me for a light globe load. It effectively ramps down the voltage to the set storage voltage independently of the pack size and load size. However it is not so good for solid state loads with fan cooling as the fans get a workout with frequent stops and starts.
It is extremely simple to make the disconnector stop cycling. Just connect a resistor across the output of the converter to bleed off the energy when the load is disconnected. A 200 Ohm 1 watt works well but it is probably not be all that critical. If you are running cooling fans directly off the converter that should be enough load anyway and nothing additional is needed.
See the revised circuit.
|May 30, 2013, 08:09 AM|
Building and Testing
The layout and construction are not critical. The easiest way to build it is to solder your standard connectors and 12g wire to the relay connections but I wanted as compact and convenient a unit as possible so I soldered XT60 connectors straight on to the relay. First step was to connect Diode 1 across the coil terminals, then attach the XT60s and wire up the push button switch.
See first picture. (Ignore where the yellow wire is soldered - I attached it to the wrong relay pin and forgot to take a new picture when I fixed it!)
I added wires to the input pins of the voltage convertor, bent them down below the case and then hot glued the relay to the top.
See second picture.
Apply a LiPo pack voltage to the inputs – anything from 2S to 8S and adjust the little brass pot knob on the voltage convertor to get 12V on the output. The CellLog was attached with 3M Outdoor Double Sided tape to the side of the relay/convertor assembly so that the power connector for the battery and the balance connector wire are adjacent and then I finished the wiring.
See third and fourth pictures.
I then used some masking tape and Blu-Tack to build a dam wall around the whole thing and poured in some 5 minute epoxy to encase the lot (making sure the pushbutton was kept clear). It ends up as very rugged and compact unit.
See last picture.
This circuit is so simple it should work first time.
The only critical thing is that the two diodes MUST be installed the right way round. Trace the banded ends to make sure Diode 1 is connected to the output of the convertor and Diode 2 is connected to the input with its other end going to the switched relay contact and load + input.
If you have Diode 1 reversed the CellLog alarm output will probably be destroyed when you push the button, if you have Diode 2 reversed the push button itself will be damaged first time you switch a load.
A simple test is to leave the CellLog and load off, connect the CellLog alarm wires temporarily with a bit of thin wire. Push the button and the relay should click. This confirms Diode 1 is correct. Let the button go and the relay should not click but stay connected. This confirms Diode 2 is correct. Remove the temporary link. Now it is good to go.
Connect the CellLog and a LiPo pack balance lead and set the alarms as required.
NOTE: This is intended to be used as a discharge cut out - then disconnected after a reasonable time. The CellLog takes a very small amount of current from the first two cells through the balance connector to power itself. You shouldn't ever leave a CellLog connected permanently to a LiPo as it will very slowly discharge the first two cells.
In the next post are my instructions from the original thread for setting any type of alarm if you aren’t familiar with the CellLog alarm menus.
I have added a couple of pictures of the modified version with a 200 Ohm bleed resistor added. I put a switch in line so I can choose to have the disconnector cycle down to the set voltage or cut off as soon as it reaches it.
|May 30, 2013, 08:09 AM|
Configuring the CellLog
The Cellog has a fairly comprehensive menu structure and is navigated with only 3 buttons so can be a bit confusing. Here's step by step instructions for setting it up for this project.
Connect the balance connector of the battery to the CellLog.
NOTE: Pin 1 MUST be connected to the -ve of the battery. Normally this is a BLACK wire on a balance connector. Pin 1 is clearly marked on the CellLog case but it is hard to see because it is a black molding.
The CellLog comes up after 2 initialization screens with the last monitoring screen you were using so will depend on what you did last. The steps below should work regardless of what screen you start from.
The CellLog has only 3 buttons and all normal functions are achieved by Pressing or Holding one button at a time only. Press means to push the button and let it go immediately. Hold means to press the button and hold it for more than 3 secs until the screen changes. The 3 buttons are TYPE, HOLD and MENU, which are also the functions for UP, DOWN and ENTER. Which function depends on what you are doing but it is fairly obvious.
Note that the label on the button HOLD refers to a function in the CellLog not the action of Holding.
Worth remembering is that in most cases Holding the MENU (Enter) button takes you back one screen or step or to the MAIN MENU.
Setting the correct output for the alarm circuit From the normal monitoring screen:
1. Hold the MENU (Enter) button till the MAIN MENU screen appears
2. Press the HOLD (Down) button once to reach System
3. Press the MENU (Enter) button
4. Press the HOLD (Down) button 2 times to reach ALM Output
5. Press the MENU (Enter) button
6. Press the HOLD (Down) button once to reach NC
7. Press the MENU (Enter) button to select NC
8. Hold the MENU (Enter) button 3 times till the normal monitoring screen appears
Setting the alarm voltage and trigger function From the normal monitoring screen:
1. Hold the TYPE (Up) button till the SELECT TYPE screen appears
2. Press the HOLD (Down) button as many times as necessary to reach the type of battery you are testing. Default (no press) is LiPo, and there is LiIo and LiFe plus 5 “User n” labels. You can set separate alarm voltage conditions for all 8. They can all be renamed to anything you want for convenience. It’s a bit tedious but works –see the manual page 10. For example you could set up a set of alarm conditions named “Standard” for bench testing.
3. Press the MENU (Enter) button to select the label
4. Press the HOLD (Down) button 2 times to reach Alarm
5. Press the MENU (Enter) button
6. Press the TYPE (Up) and HOLD (Down) button as required to reach Cell Voltage, Pack Voltage or Time Over. Press MENU (Enter) to select the one you want to set.
a. You can set an upper voltage limit HV a lower voltage limit LV or a maximum cell voltage difference ΔV for the cells. Pressing MENU (Enter) moves from one to another. TYPE (Up) and HOLD (Down) increases or decreases the value. Holding the MENU (Enter) button returns to the previous screen.
b. You can set an upper voltage limit HV a lower voltage limit LV or a maximum pack voltage difference ΔV for the whole battery.
c. You can set a Time Over alarm to limit the total time of the test.
7. Hold the MENU (Enter) button as many times as required till the SELECT TYPE screen appears. Choose the battery type you are configuring and press the MENU (Enter) button.
8. Press the HOLD (Down) button twice, then the MENU (Enter) button to get to the ALARM SETTING screen.
9. Press the HOLD (Down) button three times, then the MENU (Enter) button to get to the ALARM TRIGGER screen.
10. Press the TYPE (Up) and HOLD (Down) button as required then press the MENU (Enter) button to toggle on or off which alarm conditions you want to disconnect the battery. You can choose none (doesn’t make much sense though!) or all four if you like.
Hold the MENU (Enter) button as many times as needed until you get back to the normal monitoring screen, or just unplug the CellLog – it’s quicker.
|May 30, 2013, 08:54 AM|
Question. I see that the main charge/discharge leads on the battery are disconnected to stop the discharge. But the CellLog is still connected. Won't it still draw some current from the battery? I know it probably won't be much but any current over a long period of time could still kill the battery.
|May 30, 2013, 09:03 AM|
True Glen. The CellLog takes power from the first two cells. The current is small however - about 20-30 mA from memory. You should not leave this connected for a very long time to the pack after the discharge finishes. The pack would gradually unbalance as the first two cells go down.
I didn't design this for long term "set and forget" but as an automatic disconnector for discharging to storage and when high current testing. That's what everyone seems to use them for. Basically discharging packs to storage before you put them away.
|May 30, 2013, 11:05 AM|
I just wanted that point made so that some of the new folks would understand.
It looks like a great circuit. Thanks for the good work.
|May 30, 2013, 12:26 PM|
Nicely done; though I still use the original circuit ( http://www.marcee.org/Articles/BatteyPackDischarger.htm ).
Do all types w/it: NiXx, LiPo, A123, Pb, etc.
Eve did a spreadsheet to size resistors for other relays; and configured one that works for S400 & S500 ECL:
|May 30, 2013, 12:53 PM|
Joined Oct 2004
Nicely done John! Fantastically executed and documented.
I may be somewhat of the odd man out but I have a particular fondness for the original version that does not latch off when the lowest cell hits cutoff the first time. I simply let it click away until the lights stop flashing and my packs are always at the same voltage regardless of # of packs in parallel or load resistance used (similar to a CC/CV mechanism).
The DC-DC converter is a nice touch for those who use 4S and higher cell count packs. Since I use only 2S and 3S packs, I simply wired the relay coil and main discharge path in parallel. Will have to order up a few of the DC-DC converters. I've built several of these for friends and some that use higher cell count packs can certainly benefit from the same convenience of a single power connection.
|May 30, 2013, 06:19 PM|
Someone PM'd me and said they had trouble finding the same voltage convertor I used.
I didn't include an eBay link in the original posts because they will change over time but here is one supplier who is currently selling it for $12.30 with free shipping. Any other buck/boost convertor with similar specifications will work but most are on open PC boards and less rugged and neat.
A search that includes
3-35V to 1.2-30V DC Converter Circuit Buck Boost Voltage Regulator
should also turn them up.
|May 31, 2013, 02:07 AM|
|Jun 01, 2013, 10:48 PM|
Joined Jan 2002
Hi John, Have you considered using a solid state relay (SSR) instead of the auto relay? Perhaps this would simplify the design even further.
Since many of the SSR's I've seen operate on 3-32v DC and I've seen some that can switch as much as 5-110v DC, I would think using one of these SSR's would eliminate the for the Buck/Boost converter.
Here's once example I'm referring to rated for 80A:
It seems attractive, especially for $12.91 USD delivered to the USA.
Here's another similar SSR rated for 40A:
Input : DC 3-32V;Output : DC 5-110V
$12.22 USD delivered to the USA.
I realize you're not in the USA, but just using my pricing as an example.
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