Battery operated lunchbox - Page 1
Forum section: electronics / projects

author: ianov
author_rank: Newbie
timestamp: May 16, 2023, 09:46:23 pm
content: Hey everyone,I am currently planning to start a small project to create a battery-heated lunchbox for myself.I have been looking everywhere for one but seems that Luncheze is the only reliable one out there, but to ship it to the UK would cost over $250 and that is simply way too much for a lunch box.Therefore I have gathered some components and wanted to see if anyone here had a similar idea before and would have some recommendations.Thus far I have the following components to start off with -Cheap amazon $20 electric heated lunch box with 1 switch, as basic as it gets18650 battery pack 7.4v 8000mahPTC heating elementsPeltier heating elementsI think the PTC would do great, and just needs a basic circuit with a voltage regulator to not over discharge the battery.Can you recommend a basic circuit design? I haven't done one since university(17 years ago)Many thanks in advance

author: ebastler
author_rank: Super Contributor
timestamp: May 17, 2023, 09:37:18 am
content: Energy content of your proposed battery seems too low. 7.4V * 0.8 Ah amounts to 21 kJ, and you need 4 kJ to heat 1 kg of water by 1°C. Hence, even under the unrealistic assumption that you can fully discharge the battery and have no heat loss to the environment, you could heat 1 kg of food by 5°C, or say 250 g of food by 20°C. If you start at room temperature, the end result is not what one would consider hot food.If you want to move forward with this, I would suggest that you start with buying the $20 lunch box, connect it to an external power supply and Ampere meter, and measure the total power * time it takes to heat up your food as desired.Personally I would be reluctant to build something like this, because of safety concerns. I assume you will be taking this to sites which are outside of your "jurisdiction" (say to work or university); you will probably want the timer-controlled, unattended heating like in the LunchEaze; and both the heater and the battery could become fire hazards if something goes wrong.Edit: You will probably want to put in refrigerated food in the morning, to ensure food safety, which makes the numbers worse.Edit: I see that the LunchEaze has 10x the battery capacity, 8.4V * 8 Ah. That's more like it.

author: eTobey
author_rank: Super Contributor
timestamp: May 17, 2023, 12:08:12 pm
content: You could build a heat/cool device! It takes the energy from some soda, and puts in in your food. Peltier would be the way to go then. And when your at it, build in a pump, so that you could go around and serve some beverages ;-). You would be the hero at your place!But get better (more capacity) batterys, and some more of them!

author: ajb
author_rank: Super Contributor
timestamp: May 17, 2023, 03:02:00 pm
content: Quote from: eTobey on May 17, 2023, 12:08:12 pmThe problem with peltier coolers is the efficiency drops dramatically with ΔT. With everything starting at the same temperature you'll initially get decent heat flux, but getting a meal up to ~50C while keeping the drink at ~5C will be challenging. Pushing a TEC too hard eventually just starts driving the cold side temperature up as well as the hot side, and at that point it's easier to just use a heater. You can get better performance at high ΔT by putting multiple TECs in series, but that comes with tradeoffs since the stages closer to the hot side have to move all of the waste heat from the stages on the colder side. Plus there are other challenges, like thermal coupling to the cold side and getting good thermal insulation around the TEC heat path, and it will be difficult to regulate the temperature of both cold and hot sides, especially if the thermal mass on the two sides varies significantly.

author: eTobey
author_rank: Super Contributor
timestamp: May 17, 2023, 04:15:25 pm
content: Quote from: ajb on May 17, 2023, 03:02:00 pmI see no problem. We would have an efficiency of 100% if we can get all the heat to the food. With a peltier we would even add some percent there!

author: ebastler
author_rank: Super Contributor
timestamp: May 17, 2023, 04:33:19 pm
content: Quote from: eTobey on May 17, 2023, 04:15:25 pmYou are overlooking the fact that the Peltier elements are far from ideal heat pumps. As ajb explained, initially (while the ΔT between the hot and the cold side is small or even negative) you will indeed have an efficiency above 1. But as the ΔT reaches some 10s of degrees, the efficiency drops dramatically, well below 1 and eventually approaching 0. You will need multi-stage (stacked) Peltiers to obtain larger ΔT, but these are expensive and less efficient from the start.It's the same problem as the "retrofitting heat pumps to heat old buildings" discussion which has been in the news in Germany for the past several months: If the ΔT between the outside temperature and the hot water circulating in the heaters becomes too high, efficiency drops severely. And with the little cheap-and-cheerful Peltier elements, it drops much more than with a well-optimized large heat pump.

author: eTobey
author_rank: Super Contributor
timestamp: May 17, 2023, 06:37:28 pm
content: Quote from: ebastler on May 17, 2023, 04:33:19 pmYou both are overlooking the fact, that i dont care about beeing efficient at getting heat away from something, but rather want to get heat into something. The cooling of a beverage would be just an additional bonus.Stop parroting what is said about peltier in other appliances and get into my idea. If you dont understand something i say, ask.

author: ajb
author_rank: Super Contributor
timestamp: May 17, 2023, 08:05:20 pm
content: Quote from: eTobey on May 17, 2023, 06:37:28 pmOkay then. Feel free to design a TEC based solution and show your work. To be clear, neither ebastler nor I said it wouldn't work, just that TECs are far from magical heat movers and they have (very well documented) fundamental characteristics that make them challenging in high ΔT applications like this. You have to account for those downsides. A TEC-based solution might even be able to get more heat into the food per joule of battery energy, but if that results in a bulkier, harder to manufacture, harder to use, or more expensive end product then is it really a better solution?Oh and BTW, I've personally designed TEC-based thermal control systems, so what I'm "parroting" are very well documented performance characteristics of TECs that I have used in my design work and directly observed in testing.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 17, 2023, 10:10:44 pm
content: Dead battery, no lunch for you!

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 18, 2023, 02:14:48 am
content: Instead of heating it up when needed, what about just have it stay hot for hours? In other words, an insulated container. Adding some wax packs can extend the time it stays hot.Presumably, it could be a portable slow cooker if the wax packs are chosen to release heat at cooking temperatures.

author: eTobey
author_rank: Super Contributor
timestamp: May 18, 2023, 04:54:06 am
content: Quote from: NiHaoMike on May 18, 2023, 02:14:48 amIts also possible to get used to eating it cold, or just heated to 30-40 degrees. It cant get easier than that, and its cheaper too!

author: ianov
author_rank: Newbie
timestamp: May 18, 2023, 06:12:56 pm
content: Thanks everyone for replying on the matter, yes as was mentioned byebastler, I will probably end up with a similar concept as Luncheze, but maybe with some improvements, will be using similar 8.4v 8Ah battery or maybe 10Ah.The challenge right now is to see what works better in terms of heating elements -I currently have the following options to go with -- Peltiers- PTC's- Polyimide Film elementFrom experience, I know Peltiers don't last long and efficiency-wise, probably not the best, but PTC heating elements will probably work the best?what do you think?

author: ebastler
author_rank: Super Contributor
timestamp: May 18, 2023, 06:22:21 pm
content: My first choice would be to use the heater which is already installed in the wired lunch box you plan to upgrade. Keep it simple; no change to that box, its sealing and fire safety. (Provided it has those in the first place, at a $20 price point...)The battery pack and control electronics could be a flat base that attaches to the lunch box proper with Velcro or such. When you get home, take the two units apart, drop the lunch box in the sink for cleaning, while connecting the battery pack to a charger elsewhere.

author: eTobey
author_rank: Super Contributor
timestamp: May 19, 2023, 10:04:34 am
content: Quote from: ianov on May 18, 2023, 06:12:56 pmWell, you wouldnt use thoes that often, would you? Of course, you shouldnt drop those.But dont get fooled with the efficiency! You will get that energy, you dumped from your battery, plus what you transferred throught the Peltier. That would be (yes it sounds unbelievable for some) over 100%! The only downside would be, if it is done heating, you should eat it immediately, as the heat would go out through the peltier.

author: ebastler
author_rank: Super Contributor
timestamp: May 19, 2023, 10:27:43 am
content: Quote from: eTobey on May 19, 2023, 10:04:34 amThe caveat highlighted in bold typeface also appliesduringthe operation of the device, of course:As the temperature differential between the hot and cold sides increases, more and more heat is "leaking" out towards the cold side. Hence during the latter part of the heating cycle, only part of the electrical energy you put in ends up in the food, dragging the efficiency below 100%.A simple resistive heater makes life easier in that respect, since you can thermally insulate it (and the food container) from the outside world.

author: ajb
author_rank: Super Contributor
timestamp: May 19, 2023, 02:58:19 pm
content: Quote from: ebastler on May 18, 2023, 06:22:21 pmAgree with ebastler on this. Regardless of what sort of heater you use, the electronics aren't the hard part of this project. The hard part is getting good insulation and good coupling of heat source to food while keeping it all sealed, food safe, and easy to clean. A demountable battery pack is a great idea as well.Back to the TEC subject, there's a serious fundamental limitation to the idea of cooling a drink while heating the food, which is that you can only get so much heat out of the drink. If you have the same amount and they have similar heat capacities, then you can only increase the food temp by about the same amount you decrease the drink temp. The TEC contributes additional heat via its power dissipation, but in any case the final temps you can achieve are highly dependent on the starting temperatures and thermal masses on the two sides. Using a TEC between the food and the outside world means you have way more heat capacity on the cold side, but also means performance is highly dependent on the ambient environment, and you will have to deal with condensation on the cold side. You would get the benefit of being able to actively cool the food as well as heat it, but on the other hand you may NEED to actively cool it to compensate for the heat that would otherwise leak through the TEC and heat exchanger versus if you just used a heater and could maintain better insulation all around--that's the sort of tradeoff you have to consider here, not just with the TEC itself, but with the effects that has on the overall system design.

author: eTobey
author_rank: Super Contributor
timestamp: May 19, 2023, 07:19:18 pm
content: Quote from: ebastler on May 19, 2023, 10:27:43 amDo you got some links for me so i can educate myself?

author: ebastler
author_rank: Super Contributor
timestamp: May 19, 2023, 07:42:14 pm
content: Quote from: eTobey on May 19, 2023, 07:19:18 pmReferences which discuss Peltier elements specifically as aheaterare hard to find -- presumably because the cost-benefit ratio is not great, so it's rarely done. But every chart which shows the efficiency of Peltiers as acoolershows that the efficiency (coefficient of performance) becomesnegativeonce ΔT exceeds a threshold:https://www.meerstetter.ch/customer-center/compendium/70-peltier-elements#COPIn that regime there is "negative cooling", i.e. heat transfer to the cold side. The Meerstetter reference also spells that out:QuoteFor theheateruse case, that heat energy is not going to the hot side where you want it, but is lost to the cold side. So your assumption that "at least the electrical power sent in will go to the hot (food) side, and then some" does not hold in this regime.

author: eTobey
author_rank: Super Contributor
timestamp: May 21, 2023, 06:02:34 pm
content: Quote from: ebastler on May 19, 2023, 07:42:14 pmSo why is it then getting cold in the first place?

author: ebastler
author_rank: Super Contributor
timestamp: May 21, 2023, 09:33:19 pm
content: Quote from: eTobey on May 21, 2023, 06:02:34 pmWrong question. Initially, while ΔT is still small, the Peltier acts as an effective heat pump. Nobody is disputing that, and it is also shown in the graph I shared.But you got me anyway. The question "How is it gettingthatcold, to enter the regime where heat leakage to the cold side exceeds cooling?" is a very valid one -- and of course the answer is "it isnotgetting there." If one starts with a small ΔT, the system will cool, becoming gradually less effective at cooling until it saturates at a minimum temperature of the cold side. At that point, the heat loss towards the cold side equals the heat that still gets pumped from cold to hot side, so the heating efficiency becomes 1.So in an otherwise perfect system, one starts with a heating efficiency well above 1, and it gradually drops to 1 over time. But it can't drop below 1, unless you already start with a very cold drink.Nevertheless, I still think the practical problems will imply rather limited rewards and a lot of headaches:(a) The need to contact the Peltier with both, the hot and cold side makes it much more difficult to achieve good passive insulation of your food container, meaning heat losses which bypass the Peltier. It also makes it tricky to design a sealed, easy-to-clean enclosure. And it may bring condensation on the cold side, which may leak into the enclosure and isolation.(b) The cold side may get very cold faster than one likes. Already pointed out by ajb above: If there is too little liquid to be cooled, or a weak coupling from a radiator to the environmental air, ΔT gets large quite soon in the process, and the efficiency collapses towards 1.So my preference is still with the "keep it simple and robust" resistive heater.

author: ianov
author_rank: Newbie
timestamp: August 29, 2023, 10:41:40 pm
content: Hey everyone,So I ended up trying a quick idea and seeing if it works that way, so I bought this lunch heater box from amazon -https://www.amazon.co.uk/dp/B0BWDLYZJH?ref=ppx_yo2ov_dt_b_product_details&th=1Then I bought the following battery to power it with -https://www.vladsmall.com/product/12v-20000mah-li-ion-rechargeable-battery-pack-charger/12 V 20000 mAh Li-ion battery pack, which appears to be 6x 18650s.When I plug it in, the heater lunch box works and heats up the food inside very well indeed and lasts for around 20-25mins, but the battery heats up significantly, and though it's enough to heat up my food, I'm worried if it's a bad idea to keep using this method or not.The battery specs are - battery discharge requirements:Input/output: two 20# line, 150 mm longMaximum instantaneous current: 10 AMaximum working current: 5 AThe lunch box has two PTC heater elements, which look identical to this one for example -https://www.amazon.co.uk/gp/product/B07DWY753N/ref=ox_sc_saved_title_2?smid=A1I5R2VMZGY1HD&th=1Can anyone help or explain if this is safe to continue or maybe introduce some current draw limits? or maybe a more efficient or better alternative?

author: ajb
author_rank: Super Contributor
timestamp: August 30, 2023, 12:57:55 am
content: That lunchbox claims to consume 100W, which at 12V means 8.3A. So not a great fit for a battery back that claims 5A max continuous. I would also be concerned about the lunchbox overdischarging the pack, guessing that it doesn't have any sort of UVLO.

author: ianov
author_rank: Newbie
timestamp: August 30, 2023, 12:04:11 pm
content: Is there a way to limit those PTC elements in the lunchbox drawing so much current?

author: ebastler
author_rank: Super Contributor
timestamp: August 30, 2023, 01:28:27 pm
content: Quote from: ianov on August 30, 2023, 12:04:11 pmIf they are currently wired in parallel, you could connect them in series instead.

author: ianov
author_rank: Newbie
timestamp: August 30, 2023, 03:49:59 pm
content: Sure I will check if they are, and if they are already wired in series? what could be my other options to limit to a specified current draw in order to make this safe and usable?

author: ElectronicsHobbyist
author_rank: Contributor
timestamp: August 30, 2023, 04:17:02 pm
content: You could use a PWM controller. Either with a fixed Pulse width or you could even adjust it automatically using a microcontroller and therefore measure and regulate the current or power going to the heater.