Digital Oscilloscope for Testing & Debugging Astro Remote Controls

[Gina]

I've been thinking about charging the Li-Ion cells.  I have a separate charger and was going to swap the cells out for another pair of ready charged ones but with an ATMega328P there already I could build in a charger and provide an input to run from a separate PSU.

[Gina]

I've now finished the kit assembly and have it working   Currently running off my bench PSU at just over 9v. 

The oscilloscope seems to work well though certain facilities provided on full blown lab instruments are not provided.  I think this would only be a problem with more sophisticated waveform examination.  Having worked in a research laboratory in my career I have used oscilloscopes to their limits.  This little kit is not intended to match an oscilloscope costing thousands of pounds but AFAICT should be quite adequate for developing or testing astro kit.

This unit does do things that many oscilloscopes don't such as saving a waveform to memory for recall later - I haven't yet checked if this is saved if the 'scope is switched off.  It will also measure a number of characteristics of the waveform displayed.  I haven't yet been through all the features available from the menu - accessed from the push buttons on the right.

[Gina]

Now have the up converter in circuit supplied with 7.4v from bench PSU.  All working fine.  Oscilloscope power draw at 9v is 120mA and running off 7.4v power supply current is about 160mA.  With the rated capacity of 4AH charged to 90% and discharged to 20% for maximum life, a charge should last 70% of 4000/160 = 17.5 hours.

[Gina]

Battery boxes connected up and Li-Ion cells installed in them, with DMM as ammeter in circuit, showing 165mA and running the oscilloscope   This reminds me that I shall want an on/off switch in the power circuit.

[Gina]

I shall now need to look at the design and layout of the power supply board and thence to the case and external connectors and extra switches and LEDs.

[Gina]

I've been further researching best practice for charging Lithium Ion cells to obtain maximum life as opposed to maximum running time per charge.  With the 4AH cell I'm using I have plenty of capacity for running away from mains and long life is more important.  The way most laptop computers are used (also using Li-Ion batteries) where the computer is left on mains while in use and also charging the battery does not provide best life from the battery.  Apparently, it is best to charge this type of battery separately.

Quote

A portable device should be turned off during charge. This allows the battery to reach the set voltage threshold and current saturation point unhindered. A parasitic load confuses the charger by depressing the battery voltage and preventing the current in the saturation stage to drop low enough by drawing a leakage current. A battery may be fully charged, but the prevailing conditions will prompt a continued charge, causing stress.

It is not necessary to have the battery supplying the device when connected to a mains supply.  The PSU can power the device and charge the battery at the same time without the battery actually connected to the device.  The easiest way to achieve this would be with relays, I think.  I also think cells would be better charged as separate units rather than connected in series as a battery.  When in series with no connection to the junction between the cells there would be no way to ensure that charging of individual cells is switched off when the target voltage is reached.

All the above means that the charger is not as simple as first thought.  With two cells each needs to be charged as an individual cell with the charge current turned off when the cell voltage reaches 4.2v (or maybe lower for longer life).  The initial current inrush for a nearly discharged  cell must be limited to prevent overheating.

[Gina]

As an alternative to treating the cells separately I've come across what's called a "ballancer".

[Stub Mandrel]

Would your step-up board work with two cells in parallel? If so, you can parallel them for the same total capacity and charge them using these:

http://www.ebay.co.uk/itm/10Pcs-5V-Micro-USB-1A-TP4056-Lithium-Battery-Charging-Board-Power-Charger-Module-/281688636544?hash=item4195f37c80

Work out at 27p each, although one of my ten doesn't work. Just plug in a micro USB lead and wait until the green LED lights. The chip used also works with undervoltage cells giving them a slow current limited charge to recover them. Now I have everything from 3-big cell 12Ah packs down to little ones rescued from old mobile phones in my projects with minimal faff.

 

[Stub Mandrel]

Yep! Your module works down to 3V so all you need to do is carefully balance the voltages oin your lions (connect together using a 500R resistor and wait until voltages are the same, then wire together and treat as a single cell. You can do with with lead acid and Lions, but not Nicad or NIMH as they unbalance and mutually discharge.

[Stub Mandrel]

Some discussion of paralleling Lions here: http://electronics.stackexchange.com/questions/126673/charging-li-ion-batteries-in-parallel

[Gina]

Blow me down - yes, you're right the up converters work down to 3v - thank you Neil

I don't think I need to use two cells in parallel - I think one would do - I can't see an occasion where I would want to use the oscilloscope continuously away from mains power for more than a few hours.  At 3.0v the up converter draws about 0.38A and at 3.5v the current goes down to 0.32A.  At 3.7v the current is just 0.3A - the current is inversely proportional to the voltage as the efficiency is high and the power constant.  Even at 50% capacity usage the cell would last 6 hours on a charge.

This is the oscilloscope running off 3.0v 

[Stub Mandrel]

I'm convinced. I've ordered 3 similar 'replaces 6006' boards from China at 99p each(!) that also have a USB socket

[Gina]

With one cell the charge indicator is much simplified.

[Gina]

Possible charger circuit.  First LM317T acts as a current limiter and second controls the voltage.

[Stub Mandrel]

Sorry to keep doing this... but, if you use a chip like the one in the link below it will pre-condition and recover the cell if you accidentally discharge it too far. This approach does work, I have rescued 'dead' cells that a normal charger refused to charge with one of these and afterwards they charged normally. I would be wary of anything for charging Lion that doesn't limit the charge to a trickle at low voltages.

https://datasheets.maximintegrated.com/en/ds/MAX1811.pdf

[Gina]

That's alright Neil - I'm glad of any advice   I've already gone off the circuit above anyway and been designing a different one.

Seems to me there are 5 possible operating modes :-

1. Off.
2. Running off battery.
3. Running off external supply.
4. Running off external supply and charging battery.
5. Charging only.

These can probably be covered by a couple of switches but I'll sort that out tomorrow.

[MarkyD]

I would just get one of these and strip it out for the board - easier than faffing about with creating the circuits. The Imax B6 are pretty good chargers for the price and commonly used in the field at flying clubs.

http://www.ebay.co.uk/itm/like/161822491265?lpid=122&chn=ps&googleloc=1007044&poi=&campaignid=620865095&device=c&adgroupid=27378760866&rlsatarget=pla-181484337186&adtype=pla&crdt=0

[MarkyD]

Or see if you can get hold of one of these

https://www.adafruit.com/product/259

Multi balance charger using the above

https://learn.adafruit.com/multi-cell-lipo-charging/simple-balance-charger

[Gina]

Thanks Mark   Several ideas then for ready made chargers from China.  I'll give this some thought...  Maybe see if I can find similar from a UK source and available in a few days rather than several weeks.   Meanwhile I have several things to sort out but I was hoping this would be a project I could complete fairly quickly.  I have many ongoing projects and one that could be done and dusted quickly would make a nice change.  I'm feeling the need to actually finish a project even a small one but everything I take on seems to take longer than expected...

[MarkyD]

1 minute ago, Gina said:

Thanks Mark   Several ideas then for ready made chargers from China.  I'll give this some thought...  Maybe see if I can find similar from a UK source and available in a few days rather than several weeks.   Meanwhile I have several things to sort out but I was hoping this would be a project I could complete fairly quickly.  I have many ongoing projects and one that could be done and dusted quickly would make a nice change.  I'm feeling the need to actually finish a project even a small one but everything I take on seems to take longer than expected...

I know how you feel.  I have 6 planes in partially built yet to complete but other kits arrive before I finish, The dew controller programming is yet to be finalized and tidied up but it works as is and another Nano project has arrived at the forefront - a Bee Hive monitor for my new Hive

[MarkyD]

Found a UK supplier for the charger modules - £7.20 each

http://www.hobbytronics.co.uk/batteries/usb-lipo-charger

[Gina]

Here's an idea for running both oscilloscope and charger off a 9v supply.  Or, in fact, any supply between 8v and 12v but NOT the 13.8v supply in the observatory - that would need another voltage regulator/dropper to reduce the supply to the oscilloscope to the 8-12v range.  The instructions say never go above 12v on the supply or damage will be caused.

[Gina]

Been looking at the circuit of the charger above and the -ve input and output are common and labelled Vss (or Gnd).  OTOH the up converter does have a voltage difference between the -ve input and the -ve output - I measured this at 0.75v with the up converter in use.

I'm wondering actually if it would be better to run the oscilloscope off the battery even indoors on the work bench.  It would mean that earth loops could be avoided with one common earth/ground at the 'scope signal input.  The battery could be left to recharge overnight especially if only using 500mA charging current maximum rather than something like 0.5C.  500mA represents about 0.1C and recharging will take 8 - 10 hours.

[Gina]

Here is another idea for the power circuit.  The charger could be run off a 5v supply or USB, the battery/cell switched to the charger or the up converter to drive the oscilloscope circuit.  Maybe centre off - so we have CHARGE - OFF - ON.  The standard concentric power socket that is supplied with the kit could be used to supply the oscilloscope from an external 8-12v PSU - the contact would disconnect the Gnd from the -ve output of the up converter when a plug is inserted.  The Gnd contact is not connected on the PCB and I could solder a wire to that pad to connect to the -ve o/p of the up converter.  The +ve from the up converter would be connected to the +ve input.

[Alien 13]

I pretty much run all my projects off 5V USB now with converters, the local discount shop has 11000 mAh phone battery banks for £8 or the smaller 4800 mAh for £5 but it means the kit can be plugged into a variety of sources  even my microscope runs off one.

Alan