Cheapest way to detect 0°C (+/-5°C) to limit lipo charge current when <0°C - Page 1
Forum section: electronics / projects

author: AloyseTech
author_rank: Regular Contributor
timestamp: December 11, 2019, 01:31:47 am
content: Hi,I'm designing a device running on a Lipo battery. The device temperature can be from -30 to +50°C. I need to ensure the charge current of the lipo is limited to 15mA or less when the temperature is below 0°C, but it is mandatory to have an higher charging current (50mA at least) when the temperature is higher than 0°C.The cost of the solution is very important and must be very low. The charger is a HX4054A (cheap charger similar to MCP73831) : ~ $0.02 including attached passives.Is there a better option than using a cheap analog comparator (~$0.05)? Maybe the good thermistor could be used as the Rprog directly, but it would require a higher beta coefficient than standard 3000-5000, right?

author: jbb
author_rank: Super Contributor
timestamp: December 11, 2019, 01:47:08 am
content: I would suggest getting a charger with a temperature sensor channel built in. Might cost a few more cents, but will be smaller and likely more reliable. And provide high temperature shutoff too, which I highly recommend.How is the battery attached? A battery with a 3 wire (+, - and NTC) lead can lose accuracy due to voltage drop in the -be lead. This is usually only a factor at high temperatures.If the NTC is on your PCB near the LiPo, I suggest doing some lab testing to make sure the NTC temperature is close to the LiPo temperature. Heat flows along PCBs and through case plastics can be surprising.Maybe you could use a temperature switch IC? You might be able to find a cheap one with a preset 0C trip point and open drain output or something.

author: AloyseTech
author_rank: Regular Contributor
timestamp: December 11, 2019, 01:04:52 pm
content: Using a most advanced charger IC with temperature management is possible, but I've not been able to find a cheap solution.I forgot to mention that it is mandatory that the battery charges below 0°C, the charger must not shutoff. Most charger with temperature management will simply shutoff if not between 0°C and 125°C.My battery pack is a 2 wires lipo, the temperature sensor must be on the PCB. Lab measurement to initially trim the system is possible but a per-product calibration must be avoided.Also, I already have an over current protection by design.Do you have some cheap temperature switch reference ? I could not find any really cheap one...

author: xani
author_rank: Frequent Contributor
timestamp: December 11, 2019, 01:37:48 pm
content: resistor/thermistor divider driving a transistor working as cheap comparator ? You probably won't find anything cheaper unless it is a specialized chip

author: AloyseTech
author_rank: Regular Contributor
timestamp: December 11, 2019, 02:34:27 pm
content: Quote from: xani on December 11, 2019, 01:37:48 pmYou're probably right. The issue with this solution is that I have to find a transistor with a well characterized threshold. Would you have some part number in mind?

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 11, 2019, 02:58:09 pm
content: It should be doable with something like the LM26LV. Not expensive, but nowhere near $0.02.I'm afraid all very cheap solutions will be pretty inaccurate. What's the tolerance on the 0°C threshold?

author: perdrix
author_rank: Frequent Contributor
timestamp: December 11, 2019, 03:56:33 pm
content: What about a simple bimetal thermostat? Or have they got expensive now they're not used so much?David

author: AloyseTech
author_rank: Regular Contributor
timestamp: December 11, 2019, 05:35:39 pm
content: Quote from: SiliconWizard on December 11, 2019, 02:58:09 pmThe TMP390 is cheaper I guess, but way over the budget.Quote from: SiliconWizard on December 11, 2019, 02:58:09 pmThe tolerance on the 0°C threshold is around +/-5°C. Lower would be better but not mandatory.Quote from: perdrix on December 11, 2019, 03:56:33 pmI'm afraid their are not that cheap and take a lot of place. I've not enough room in the product to fit this kind of thermostat...Anyone know a N mosfet with a stable and characterized Vgs threshold ?

author: Zero999
author_rank: Super Contributor
timestamp: December 14, 2019, 06:46:09 pm
content: You won't get good accuracy at such a low price point. There are various cheap ways to sense temperature, a diode is probably the lowest cost one I can think of, but a comparator is still required to sense the threshold.

author: splin
author_rank: Frequent Contributor
timestamp: December 19, 2019, 02:40:33 am
content: As suggested, an NTC using a bipolar transistor as a voltage reference + comparator is probably the cheapest solution but then you have the problem of the variability of Vbe v Ic between individual transistors. This isn't usually specified but the BC549 is one that does (min, typ, max @ IC = 2mA): 580, 660, 700mV; you could design to 640mV +/-60mV or approx +/-10% which is approximately +/- 2.5C for a typical NTC so you might just be able to make it work with all the other tolerances.At least you don't have to worry about the -2mV/C temperature dependence of Vbe as you are only concerned with operation at 0C. In reality though I would expect that the Vbe spread would be much lower than the barn door specs - the manufacturers aren't going to provide tight specs for parameters that are of little interest to the vast majority of customers. So you could do your own batch testing and characterisation to give you enough confidence that this solution will work.A downside is that you have to prevent the purchasing function from sourcing 'equivalent' transistors from the cheapest supplier, but the chances are that Vbe will differ markedly between different manufacturers of transistors with similar part numbers.For more precision you could use a TL431 type shunt regulator. These have a 2.5V reference and can be configured as a comparator. They are cheap < $0.01 for Chinese versions and around $0.03 for Diodes Inc's AS431.That said, I'm concerned about your requirement to charge LIPOs below 0C. Conventional wisdom is that this is a no-no as it can lead to Lithium plating of the anode damaging the battery and creating safety concerns. There are some suggestions that charging at -30C is possible providing you limit charge current to less than 0.02C but I haven't seen anything authoritive - Battery Unversity doesn't count as 'authoritive'. Your requirement is to reduce charge from 50mA to 15mA below 0C - does this mean 50mA is only (50/15*0.02) = .07C? That seems somewhat low (15 hours) but not unreasonable. Surely it would be better to use an NTC to reduce charging current gradually below 0C rather than one of two charge rates? This would at least remove the need for a comparator and reference voltage.More to the point, does the LIPO manufacturer approve your 'down to -30C' charging scheme, especially given the huge 5C tolerance on the 0C threshold? Your very low cost requirement implies large volume production so liability issues must be a big concern. At the very least you might want to ensure the tolerance is -0C to +5C.This response to sub-zero charging on stack exchange sems to cover the issue. Apologies if you know all this...https://electronics.stackexchange.com/questions/263036/why-charging-li-ion-batteries-in-cold-temperatures-would-harm-them

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 19, 2019, 03:02:23 am
content: I'm always pretty sad (when not concerned) when I see engineers designing devices arounds Li-ion (/LiPo) batteries with the "cheapest" solution as a goal. How many times will we have to say that those batteries are not toys and that shit still happens when you mishandle them?It's pretty impossible to get even +/-5°C reliably with a diode or bipolar transistor IMO unless you select them one by one. Your bet could be to add means of trimming the threshold on the board, which would not necessarily be very stable over time and would add significant production cost...If something like a TMP390 is still too expensive, there may be some chinese ICs that are equivalent for a fraction of the cost. That'll take some time to find, but I'd tend to favor that if you really have to go low cost.

author: thm_w
author_rank: Super Contributor
timestamp: December 20, 2019, 12:01:06 am
content: Quote from: AloyseTech on December 11, 2019, 01:04:52 pmWhat is the point of protecting the cell from starting charging below 0C, but not protecting it from starting at 1C and having it drop to -10C? In either case it will suffer.https://electronics.stackexchange.com/questions/263036/why-charging-li-ion-batteries-in-cold-temperatures-would-harm-them