Notify when batteries getting too low - Page 1
Forum section: electronics / beginners

author: fixit7
author_rank: Regular Contributor
timestamp: June 15, 2019, 01:58:25 am
content: I have 2 18650s powering my bike headlight.The light fails pretty suddenly when the voltage gets to about 2.5 volts.I was thinking of building a low current monitor the uses a momentary switch to check the voltage without having to take the batteries out of the holder.Are there other options?Thanks.

author: Audioguru
author_rank: Super Contributor
timestamp: June 15, 2019, 02:05:00 am
content: An 18650 battery cell is lithium. It is destroyed when its voltage is lower than 3V and yours are probably in series so if they produce only 2.5V then they have only 1.25V each.

author: fixit7
author_rank: Regular Contributor
timestamp: June 15, 2019, 02:33:34 am
content: No, that was voltage of INDIVIDUAL cells.You should not make a lot of assumptions.

author: SparkyFX
author_rank: Frequent Contributor
timestamp: June 15, 2019, 04:29:58 am
content: Search the net for "18650 discharge curve", you´ll find various examples for the capacity/voltage relationship. As these curves are rather flat and then go steep, lithium capacity sensing is either imprecise (given all other factors, like ambient temperature) or contains some form of computational model - probably too big of a deal for a bike lamp.You can of course test by adding additional load and if the lamp still works, the battery holds sufficient charge.Spare batteries or premature charging might be more practical in the end.

author: sleemanj
author_rank: Super Contributor
timestamp: June 15, 2019, 05:00:40 am
content: A simple cell monitor devicehttps://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20190614205829&SearchText=lipo+cell+monitorcycles through each series cell and displays it's voltage, typically handle 2-8 cells

author: mariush
author_rank: Super Contributor
timestamp: June 15, 2019, 05:52:52 am
content: A 6-8 pin microcontroller could measure voltage and optionally disable light when voltage goes below some threshold.. could have a tiny led blink when low.You could start with an Arduinno and learn some stuff...

author: Mr.B
author_rank: Unknown
timestamp: June 15, 2019, 06:01:37 am
content: Quote from: fixit7 on June 15, 2019, 02:33:34 amPlease don't be so rude.He was only offering accurate information about 18650 batteries.Even if you are talking about individual cells, 2.5v is to low for a LiPo battery.

author: soldar
author_rank: Super Contributor
timestamp: June 15, 2019, 06:44:16 am
content: Quote from: fixit7 on June 15, 2019, 01:58:25 amQuote from: fixit7 on June 15, 2019, 02:33:34 amNo need to be rude to someone trying to help you. Not only that but there is no assumption that is wrong. It is your phrasing that leads to the misunderstanding. As you do not specify otherwise it is correctly interpreted as 2.5 volts to the light.You should always show respect to those who are your superiors in age, authority and knowledge but especially when you are seeking their help.

author: fixit7
author_rank: Regular Contributor
timestamp: June 15, 2019, 01:29:09 pm
content: I was not trying to be rude.I try to give all relevant information that I can think of.But I sometimes or maybe often do not give all the information.I do the best I can.

author: fixit7
author_rank: Regular Contributor
timestamp: June 15, 2019, 01:39:23 pm
content: Quote from: sleemanj on June 15, 2019, 05:00:40 amThanks sleemanj.I would like to find a connector for that monitor.I saw one picture that showed 3 connectors going to the batteries.I can see 2 leads for the + and -, but do not understand what the 3rd connector is for ?https://www.aliexpress.com/item/DTXMX-RC-Cell-Checker-BB-ring-Lipo-battery-low-voltage-alarm-Voltage-Indicator-volt-meter-monitor/32843043047.html?spm=2114.search0104.3.35.24231be7aFCOWq&ws_ab_test=searchweb0_0,searchweb201602_10_10065_10130_10068_10547_319_10546_317_10548_10545_10696_10084_453_454_10083_10618_10307_537_536_10059_10884_10887_321_322_10103,searchweb201603_52,ppcSwitch_0&algo_expid=b4922bd1-c709-403c-9a99-2ad648d9c504-5&algo_pvid=b4922bd1-c709-403c-9a99-2ad648d9c504&transAbTest=ae803_5

author: fixit7
author_rank: Regular Contributor
timestamp: June 15, 2019, 01:41:10 pm
content: Quote from: mariush on June 15, 2019, 05:52:52 amThanks marjush.I will look for an Arduinno and learn more.Andy

author: sleemanj
author_rank: Super Contributor
timestamp: June 15, 2019, 01:52:16 pm
content: Quote from: fixit7 on June 15, 2019, 01:39:23 pmMost negative, next most negative, third most negative... most positive

author: TK
author_rank: Super Contributor
timestamp: June 15, 2019, 02:03:09 pm
content: There are ICs that monitor LiPo batteries like TI BQ2023:BQ2023 (NRND) Single-Wire Advanced Battery Monitor IC for Cellular and PDA Applications

author: fixit7
author_rank: Regular Contributor
timestamp: June 15, 2019, 03:22:31 pm
content: Quote from: sleemanj on June 15, 2019, 01:52:16 pmI do not understand your post.Andy

author: fixit7
author_rank: Regular Contributor
timestamp: June 15, 2019, 03:24:22 pm
content: Quote from: TK on June 15, 2019, 02:03:09 pmThanks TK.Andy

author: Zero999
author_rank: Super Contributor
timestamp: June 15, 2019, 03:47:37 pm
content: Quote from: fixit7 on June 15, 2019, 02:33:34 amThat shouldn't make any difference. If the voltage of one of the individual cells drops that low, it's likely it'll be damaged.People will make assumptions, in the absence of enough information.

author: Audioguru
author_rank: Super Contributor
timestamp: June 15, 2019, 05:46:59 pm
content: Since you said 2.5V then I thought it is too low for a white LED so your two cells must be in series and are producing a total of 2.5V.My radio controlled model airplanes use two Li-PO cells in series and the airplane warns me the voltage is low at 3.15V per cell by pulsing the main motor, then cutting that motor when the cells are 3.05V each.With the cells at 3.05V then there is enough power to control the servos for me to steer a glided-in landing. The manufacturer says that running the cells down to 3.15V frequently damages them, use a timer instead.At home I use the two-cells display thing that shows total voltage, then cell 1 voltage then cell 2 voltage. I use it when I discharge the cells down to 3.7V each (half a full charge) for winter storage.

author: sleemanj
author_rank: Super Contributor
timestamp: June 15, 2019, 10:30:21 pm
content: Quote from: fixit7 on June 15, 2019, 03:22:31 pm

author: fixit7
author_rank: Regular Contributor
timestamp: June 16, 2019, 01:11:15 am
content: Quote from: Zero999 on June 15, 2019, 03:47:37 pmDouble the voltage makes a lot of difference.The batteries charged up to full capacity.Your last statement is very humorous. You made my day. :-)Andy

author: fixit7
author_rank: Regular Contributor
timestamp: June 16, 2019, 01:12:48 am
content: Quote from: sleemanj on June 15, 2019, 10:30:21 pmThanks.Your artwork is impressive. :-)

author: Audioguru
author_rank: Super Contributor
timestamp: June 16, 2019, 02:46:48 am
content: My 2-cells Li-PO batteries have 3 wires so that they can be balance-charged and so the voltage tester can show the voltage of each cell and their total.

author: fixit7
author_rank: Regular Contributor
timestamp: June 16, 2019, 03:14:15 am
content: My 18650s are a different type as yours, right?SafetyMain article: Lithium-ion battery § SafetyApple iPhone 3GS's Lithium-ion battery, which has expanded due to a short circuit failure.LiPo cells are affected by the same problems as other lithium-ion cells. This means that overcharge, over-discharge, over-temperature, short circuit, crush and nail penetration may all result in a catastrophic failure, including the pouch rupturing, the electrolyte leaking, and fire.All Li-ion cells expand at high levels of state of charge (SOC) or over-charge, due to slight vaporisation of the electrolyte. This may result in delamination, and thus bad contact of the internal layers of the cell, which in turn brings diminished reliability and overall cycle life of the cell. This is very noticeable for LiPos, which can visibly inflate due to lack of a hard case to contain their expansion.For a comparison with LFP cells on this subject, please see LiFe cell safetyWeren't those banned on planes at one time? A little research.Q2. What kinds of batteries does the FAA allow in checked baggage (including gate-checked bags)?A2 . Except for spare (uninstalled) lithium metal and lithium-ion batteries, all the batteries allowed in carry-on baggage are alsoallowed in checked baggage. The batteries must be protected from damage and short circuit or installed in a device. Battery-powereddevices—particularly those with moving parts or those that could heat up—must be protected from accidental activation. Sparelithium metal and lithium ion/polymer batteries are prohibited in checked baggage—this includes external battery packs. Electroniccigarettes and vaporizers are also prohibited in checked baggage. “Checked baggage” includes bags checked at the gate or planeside.

author: sleemanj
author_rank: Super Contributor
timestamp: June 16, 2019, 05:53:41 am
content: For all intents and purposes "LiPo" and "Lithium-Ion" are the same thing, an 18650 can be thought of as equivalent to a LiPo which was rolled up and stuck inside a metal can, or conversely a "LiPo" can be thought of as an 18650 which was unrolled and put in a soft pouch.This is a simplification, but not much of one, from the user perspective, charging, discharging, protection, balancing, and electrical safety are the same if you are using a hard shell (18650...) or a soft shell (LiPo...) Lithium-Ion cell architecture.LiFePO4is different and must not be confused with Lithium-Ion, they have different characteristics.

author: fixit7
author_rank: Regular Contributor
timestamp: June 16, 2019, 01:23:57 pm
content: Quote from: sleemanj on June 16, 2019, 05:53:41 amIt comes down to what the user personal choice is.LiPo BatteriesProsMuch lighter weight, and can be made in almost any size or shape.Much higher capacities, allowing them to hold much more power.Much higher discharge rates, meaning they pack more punch.ConsMuch shorter lifespan; LiPos average only 150–250 cycles.The sensitive chemistry can lead to fire if the battery gets punctured.Need special care for charging, discharging, and storage.

author: Audioguru
author_rank: Super Contributor
timestamp: June 16, 2019, 01:47:26 pm
content: Quote from: fixit7 on June 16, 2019, 01:23:57 pmNo. Name-Brand Li-POs last for 500 cycles or more, the same as Lithium-Ion. Cheap Chinese ones last for 20 cycles or less.They have the same chemistry as Lithium-Ion so they need the same special care.

author: Zero999
author_rank: Super Contributor
timestamp: June 16, 2019, 07:40:27 pm
content: Quote from: fixit7 on June 16, 2019, 01:11:15 amGoogle lithium ion minimum voltage shows 2.8V is the absolute minimum discharge voltage, per cell, with 3V being the safe accepted minimum. Not only does discharging to lower voltage damage the cells, but greatly increases the risk of fire, during recharging.Good, I'm glad I amused you. In future provide as much information as you can, otherwise people will make silly assumptions, whether you like it or not.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 16, 2019, 10:20:14 pm
content: Your headlight very likely has some SMPS in it that keeps the output voltage constant and therefore the light output, untill your suddenly in the dark in the woods with a completely flat battery...For flashlights a more advanced version of SMPS is usually used.It keeps the light output constant untill aobut 70% to 80% of the battery is drained, and then starts decreasing the light output gradually.Alternatively you can replace the SMPS with a simple resistor.Total efficiency will be less, but light output will also decline gradually.Also, from the little I kno of Li-Ion (there are many different chemistries) is that is seems better to charge more often (when the battery is about half empty) then to drain them completely (which shortens their lifetime).