Lithium charging IC with low ground pin current - Page 1
Forum section: electronics / projects

author: jmaja
author_rank: Frequent Contributor
timestamp: April 14, 2017, 07:11:16 am
content: I'm trying to find a charging IC for a low power system with a small solar panel. The system will consume <100 uA hopefullty. The solar panel will output up to 50 mA, but I want to have it running at well below 1 mA while keeping the battery full. There will be a ~50 mA LiPo battery.All the charging IC I have found so far have at least a few hundred uA gound pin current while charging, most have more than 1 mA. Why is that? I can find a LDO with a few uA ground pin current.Could I use a 4.2 V LDO for charging? Just disable it from the mcu when Vbat has been 4.2 V for some time. Is there a risk for overcharging? How much charged will the battery be, if I just disable it once 4.2 V is reached? The charging current wil be less than 20 mA most often, thus 0.4C. There shouldn't be any need to limit the charging current, since the panel can't output more than 1C.Or could I do even without the LDO? Just disconnect the solar panel with a FET when 4.2 V is reached. The panel will ouput up to 5.5 V in bright sunlight.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 14, 2017, 07:46:19 am
content: Quote from: jmaja on April 14, 2017, 07:11:16 amIf you have an MCU running, capable of measuring the cell voltage, and the panel cannot produce current exceeding the cell max charge rating, then there's nothing wrong in doing it all by yourself using this extremely simple algorithm:If the cell voltage is between, say, 3.0 and 4.1V, enable the FET; if the V>4.2V, disable the FET. Prevent charging if V<3V.You should also check the temperature (using integrated sensor in the MCU, if available?) to prevent charging at low temperatures (<0 deg C).Of course, this does not utilize optimum operating point of the cell, but neither does the linear li-ion charging regulator.

author: jmaja
author_rank: Frequent Contributor
timestamp: April 14, 2017, 07:53:18 am
content: Why charging needs to be prevented below 3V and 0C?The charging IC's seem to apply smaller current, if V<3V. Just preventing would mean the system is dead for good.

author: mtdoc
author_rank: Super Contributor
timestamp: April 14, 2017, 07:54:16 am
content: I recently had to swap out the lithium charge IC on my Chinese AD584LH voltage standard. It uses a small UN8HX lithium charge IC. Looking into it, I couldn't find a proper datasheet on these. Google turned upthis spec sheetwhich doesn't have much detail. But a little more digging led me tothis pagewhich with the help of google translate indicates a standby current of 45 uA. I haven't measured it.I picked up 10 of themfrom AliExpress for $1.53 with free shipping to the U.S..

author: danmcb
author_rank: Regular Contributor
timestamp: April 14, 2017, 08:01:42 am
content: check the data sheet for the cell. There are a number of things you should do:- CICV (constant current then constant voltage) charge- end of charge when current falls below some threshold, or time out- disable charge on hi or lo temp- charge at low current if cell voltage is too low until voltage gets up- fast cut off if you go over voltage for some reason (e.g. hardware failure, this is important, this can cause a lithium fire)for 4 or less cells LT1510 is pretty ok - simple and inexpensive and does most of this. Read the data sheet carefully thouigh, especially the temperature rated stuff.

author: jmaja
author_rank: Frequent Contributor
timestamp: April 14, 2017, 08:27:44 am
content: LT1510 is totally useless for this applications. Doesn't even work at 1 cell voltages I use and has 3 mA supply current while I would like to have less than 100 uA.I found the reasons for not charging below 0C. How do the solar cell + lithium powered equipment permanently istalled outdoors deal with that? They can't just hope for the temperature to rise above 0C before the battery dies.It seems that low charging current is OK below 0C. Two FET's with and without a resistor to limit the charging current?

author: Siwastaja
author_rank: Super Contributor
timestamp: April 14, 2017, 08:46:03 am
content: Quote from: jmaja on April 14, 2017, 07:53:18 amOverdischarged cell has increased leakage due to copper dissolution, which also increases the risk of internal short; a safety issue. This is why overdischarged cell is reported faulty and replaced, and charging must be denied. There are corner cases where the cells can be "revived" using a specific algorithm, specified by the cell manufacturer (but don't blindly depend on their specifications, either!), but you should not implement it, because it requires understanding, complicates things; and finally, doesn't make much sense; just fix the actual problem of destroying the cells by overdischarge in the first place. Anyway, the reviving algorithm can't do wonders. If your load switching is faulty, the chances are high that you are overdischarging the cell beyond any revival.Some commercial IC's, even from big brand name players, implement dangerous, faulty "revive" algorithms due to the simple incompetence of their designers. These IC's are broken-by-design and dangerous, and require real understanding to utilize despite their faults. By implementing your own checks, you can get the right thing done. This is why I try to avoid commercial li-ion related IC's, most often they are utter jokes, and expensive on the top of it!Yes, li-ion battery is dead for good if overdischarged. Hence, you need a low-voltage cutoff in the load, trivial to implement, but often overlooked. If you have the MCU, just go to some 1µA power-off sleep state when you estimate the cell to be at, like, 5% SoC. Proper cells have practically zero self discharge near 0%, so there is no problem letting an empty cell sit even for a long time. The issue is always within trivial mistakes in electronics, causing considerable leakage currents, hundreds of µA or more.Charging at low temperatures (as well as currents too high) increases lithium plating on the negative electrode, also a safety risk in extreme cases. Seldom a real safety issue, but damages the cell anyway (decreases the capacity). You can charge at lower temperatures by derating the charge current, but it's easiest to implement a on/off 0 degC limit.All in all, this is extremely simple:Drop all loads when empty, for example, at 3V. Design it so that leakage current is negligible, in the range of few µA. This way you'll never destroy a cell.Start charging only when both voltage and temperature are inside valid range. For example, between 3.0V and 4.1V, and between 0 degC and 50 degC. (I suggest 4.1V instead of 4.2V to add some hysteresis; there is noneedto charge when the cell is already nearly full, even though it would be completelyvalidto do so.)Charge cutoff current is noncritical; watch the voltage. You can float charge indefinitely at slightly reduced voltage, such as 4.15V. Or you can terminate it at whatever current you like. Or you can terminate mid charge, no need to do full charging. Note that with high charging currents, if you suddenly stop without tapering the current down, you are not going to get near 100%.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 14, 2017, 08:58:23 am
content: QuoteDoes the ground current actually matter? Does it actually matter if it will only start charging once the cell is putting out a couple of mA?If you want to squeeze the most out of the solar cell you should probably be using a boost converter.The reason chargers aren't optimised for ground current is that for almost all applications, it simply doesn't matter whether it loses 100uA or 10mA.

author: jmaja
author_rank: Frequent Contributor
timestamp: April 14, 2017, 09:36:06 am
content: Quote from: mikeselectricstuff on April 14, 2017, 08:58:23 amIt matters a lot, if the panel sees only little light for a longer time. With 1 mA wasted there is no charging at all at 1 mA while there could be 900 uA charging, which would allow 9x time in the dark.If you can relay on getting so much light that you will get 10 mA or even 50 mA, loosing 1 mA doesn't matter. The 50 mAh battery will get fully charged anyway after just a few hours.Also it would be nice to be able use a smaller solar panel, which may only give a few mA at most.

author: jmaja
author_rank: Frequent Contributor
timestamp: April 14, 2017, 09:50:47 am
content: Thanks Siwastaja for a clear explanation!I'm just testing a battery I happen to have. Charging at 1C leaves the battery voltage to around 3.7 V, if stopped when 4.2 V is reached. That would lead to starting the charging again. With 0.2-0.5 C, which is more realistic for this solar panel, the battery seems to get over 4 V.Certainly there needs to be some hysterisis with the no charge and system stop voltages. If they are both 3 V, there is no recovery from getting there. So maybe stop all activity (expect battery monitoring) at 3.1 V and give up, if battery is below 2.9 V.What are the safety concerns with a 50 mAh LiPo, which has internal protection systems, with this setup? Are there any real concerns, if I mess up with the software etc? This is not a handheld device, but may sometimes be close to the user.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 14, 2017, 10:45:06 pm
content: Quote from: jmaja on April 14, 2017, 09:36:06 amBut what is the difference in light level between 1mA and 2mA output, as a proportion of the total range of likely light levels?Does operation in a slightly darker environment actually matter? Is it worth spending money to improve>?There will always be situations where it won't work, so how important is it for your application to need a little more light to charge?

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 15, 2017, 01:13:20 am
content: Have you already looked at the LTC4070? It is specifically designed for small solar powered devices like wireless sensors.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 15, 2017, 08:16:36 am
content: Quote from: NiHaoMike on April 15, 2017, 01:13:20 amProbably also worth looking at energy harvesting chips from other suppliers - TI definitely does at least one, don't recall the number.However these dedicated chips tend to mostly score where the supply voltage is very low, where it's hared to do anything with off-the-shelf parts.If the solar cell's output capability inherently keeps charge current below the limit for the cell, then a simple voltage cutoff may be all you need.

author: jmaja
author_rank: Frequent Contributor
timestamp: April 15, 2017, 08:32:01 am
content: Quote from: NiHaoMike on April 15, 2017, 01:13:20 amNo, I didn't find that when searching lithium charging IC from their site. Strange! So that is basicilly just a zener diode, but limited to 50 mA. The panel I plan to use could potentially output just slightly more, which could be a problem, but there was an option to use a FET to increase this current.There is no option to stop charging below 0C. How does the high temperature float reduction work with a fully charged battery? Wouldn't there be a high discharging current, if the battery gets hot e.g. from sunlight and LTC4070 lowers the volatage from 4.2 V to 3.8 V? Or does the open circuit voltage drop that much with increasing temperature.

author: jmaja
author_rank: Frequent Contributor
timestamp: April 15, 2017, 08:39:00 am
content: Quote from: mikeselectricstuff on April 14, 2017, 10:45:06 pmI don't know. If there is an IC with low ground pin current, it is not necessarily more expensive. At least indoors the 100 uA - 1 mA is quite typical, probably also in daylight when the panel is facing away from the sun, which will be 50% of the time in this application. All that would use the battery, while it could use the panel and even charge a bit.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 16, 2017, 02:44:32 pm
content: Quote from: jmaja on April 14, 2017, 09:50:47 amBased on the huge amount of voltage sag you are seeing, sounds like this cell is really not suitable for charging at 1C.Only quality cells specified by their (proper) manufacturers for 1C charge are capable of such high charging currents, and these are typically "power" type cells. 1C is a lot of charging current.Yours is most likely some random crap, so be prepared for any kind of problems. Such a small cell won't hold a lot of combustion energy, but I wouldn't charge it at more than 0.3-0.4C, at least for battery life.