Danny Fenton looks at charging and temperature management of LiPo batteries.
In this second instalment Danny finishes off his LiPo Charging Case and converts a fridge for LiPo storage!
STORAGE CHARGE
It is recommended that your packs are not kept fully charged for more than a day or so. If they are to be stored any longer the consensus is to discharge the packs to a storage level, which is around 3.8V.
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For discharging a large supply battery is not needed. For this I have a 4S A123 pack that I use to power the chargers in the case. Discharging can be slow as the chargers will often only discharge at a couple of amps. The idea is, after a flight, and if you are not flying again, discharge the pack.
I also use two URUAV dischargers. These are dirt cheap and will slowly bring a pack down to storage voltage of around 3.8V per cell eventually (and I do mean eventually!) I use these for my indoor 1S packs. I dread to think how long a fully charged 5S-5000 would take!
There are other dischargers around, and this M600 10A charger is particularly good, but it will not really do 10A. But it is a viable option for bringing your packs down to a storage voltage. The unit uses the pack you are discharging to power itself so is very neat.
I have been trying to convince clubmate and fellow RCM&E scribbler, Chris Bott to develop a high current discharger and he has been beavering away on a prototype. I am sure we will see something from his workshop soon.
FRIDGE CONVERSION
You will recall at the start of this article I mentioned that the storage temperature of LiPos has an impact on their longevity. My main problem was a garage that often plunged to almost freezing in the winter. So, a temperature stabilised box or vault was needed. To that end I had a plan…
I had been on the lookout for a smallish fridge for some time, but I really could not find anything that fitted the bill. Initially I thought that a broken fridge would be fine, but after some more thought the cooling function would be useful in the summer, So I felt easier about purchasing this new table-top mains fridge from Argos online. It came with free delivery and was £70.
When I started planning the interior layout my thoughts were for some shelves for individual batteries. However, this really did not work as the layout inside isn’t a box, there is an intruding section where the pump is installed behind and outside of the chamber. Some thinking had to take place…
The inside of the door was intruding into the fridge considerably and I decided that the milk holder etc. was superfluous, so I proceeded to hack my brand-new fridge to bits! It came apart easily and still needs a plastic sheet to tidy the edges up, but for now is fine. There is still a considerable thickness of foam to insulate the door. The seal was refitted as it also contains the magnetic strip that holds the door shut. It was looking promising.
I store my packs in army surplus ammo boxes and I thought, ‘I wonder if they will fit in the fridge?’ And they did, one along the bottom and two above. All seven of my 5S packs are in one box. The other boxes contain odd sizes for control line and smaller R/C models.
As you can see there is not much space left inside, with three boxes in the vault, so some careful thought had to be given to the heater. The box you can see tucked in, top right in the accompanying picture is an old remote weather station unit. Usually these sit in your garden and report things like temperature and humidity back to a base unit. In this case the unit is feeding back the internal temperature of the vault. This is a just a temporary measure as I have another way of controlling the environment.
More of that shortly.
I fitted a cheap battery driven temperature panel with a probe, which gives humidity and temperature. The idea is to mount this in the front door, with the probe inside. This gives a quick visual check for when you walk past the unit in the garage.
If you recall I was thinking about a heater. My original idea was to fit a cylindrical shed heater type unit. However, one of those was never going to fit if I wanted to store all of my ammo cases – there simply wasn’t room!
Browsing the ‘tinternet I stumbled across thermal heater pads for reptile tanks. They come in a variety of sizes and the one I chose is only 10 watts, so it might take some time to heat the vault. But once at the desired temperature it should work.
I marked up the position of the temp panel and chain drilled the fridge… err… vault door. This took a little longer than I expected as the edges had to be tidied with hand files; power tools, even the Dremel, were hard pushed to deal with the steel. A hole was made through the foam to allow the probe to be positioned inside the door.
TEMPERATURE CONTROL
The final stage was deciding how to control the temperature. The heater pad is supplied with a simple rheostat to alter the current. I wanted to set the temperature and allow electronics to deal with all of that. I also thought it would be neat to be able to monitor the temperature and even set alarms, so that I would know when my valuable batteries’ environment was outside the set parameters.
The solution was very novel and, although it was a bit temperamental at first, a firmware update after talking to the designer had it working reliably.
So, what did I use? The answer was a ‘Shelly’ unit. These are gadgets designed by a clever chap called Dimitar and he sells them via the web; if you are curious just google ‘Shelly’. These devices are fitted around your home to do all sorts of automation, from controlling garage doors to changing lighting. Chris Bott has loads of these controlling lighting around his home. They can be connected to your home network and can even be controlled by voice via Amazon Alexa.
I kept it fairly simple and added two optional temp probes to a Shelly 1P unit and fitted them in the back of the vault. The Shelly was programmed to apply full power (a whopping 10W) to the heater pad until the vault internal air temp reached 20 degrees, detected by the internal probe. At that point the power was removed. As the temperature dropped the power was restored again at 19.8 degrees. This made allowance for the hysteresis in temperature caused by the heater pad warming and cooling down quite slowly. The resultant swing was 19.3 degrees (min) to 20.3 degrees (max) and had a duty cycle of about 50% on. This was with the ambient temperature quite low, at sub 8 degrees C.
An alarm is also set via the Shelly cloud to go off if the internal temp hits 19 degrees. This would indicate that the software had missed a trigger. I added another probe to the rear of the vault to give the ambient temperature of the garage, but only for interest.
Initially, about once a day, the alarm would go off on my mobile and I would know that a trigger had been missed. However, the alarm also re-triggered the heater, so no intervention was needed on my part.
As I alluded to earlier, a quick chat with the Shelly designer, explaining how I was using his device, resulted in a hasty firmware update. It has not missed a beat since, fingers crossed.
My packs will now sit at 20 degrees no matter what the temperature is in the garage. I will fit a second Shelly to the vault when it gets warmer; this one will be triggered by a temperature greater than 20 degrees and turn the fridge pump on to cool the contents.
George suggests that if your batteries are cold when you want to go flying, let them acclimatise to room temperature for a couple of hours before charging. And do not let them freeze! In my case I should be able to charge them almost immediately, should a flying opportunity arise at short notice.
As always if you want to drop me an e-mail, I can be reached at [email protected]
