Help with Battery pack and related charger IC - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: July 09, 2020, 04:32:28 pm
content: Hi,I spent few hours researching but could not find a battery solution and matching charging IC that I feel comfortable about.The project is for 80 off units of a portable instrument.I am looking for both of the following:1) a battery (pack?) with:- voltage anywhere from around 3.6 to around 8V (I can work around it with switching regulators to get the 5V and 3v3V I need on the board)- 2.5Ah minimum but ideally as high as possible (preferred 3.5Ah)- max discharge current 300mA- preferably (but NOT an essential requirement!) a chemistry that is safe/allowed to be transported by air as part of the product (i.e. permanently locked inside the product enclosure)2) related charger IC:- suitable for that battery specific pack/chemistry- standalone and as simple as possible. No need to have any microcontroller control. Only a mean for the Micro to read the battery level,but even that not that important.- the input(only when charging the battery)is a wall adaptor and I can choose any voltage accordingly to whatever is required by the IC/battery. It does not need to be a USB charger NOR a 5V charger. Whatever input voltage the IC accepts I can find a wall-adaptor to work with it.- I guess for safety it might need some monitoring and cutoff perhaps?- OPTIONAL (would be nicebut no need to have) is means for the charger IC to allow to also supply the system (i.e. feeding whatever the incoming DC voltage is to my onboard switching regulators)I appreciate any parts suggestions because so far it has been a frustrating process finding them.Much appreciated!Many thanks

author: tooki
author_rank: Super Contributor
timestamp: July 09, 2020, 06:32:49 pm
content: As someone who is working on a project where I had to select those things, too, you’ll have to provide a lot more requirements with which to narrow it down. Does it need processor control? Or is “standalone” operation needed? Do you need it to be “smart”? What’s it going to be powered by? Does it need to know how to talk USB? Do you need a built in boost converter to provide a guaranteed minimum system voltage?

author: Martinn
author_rank: Frequent Contributor
timestamp: July 09, 2020, 06:35:12 pm
content: What's wrong with any decent 18650 Li-Ion cell? If you absolutely need 3.6 V min, you'll need 2S as minimum voltage is around 2.5 - 3.0 V.Maybe you should explain your application and whether it's one piece DIY only or some small series production.

author: ricko_uk
author_rank: Super Contributor
timestamp: July 09, 2020, 07:37:25 pm
content: Thank you Martinn and Tooki, much appreciated!!I updated the original post with the additional details you asked, copied here too:2) related charger IC:- suitable for that battery specific pack/chemistry- standalone and as simple as possible. No need to have any microcontroller control. Only a mean for the Micro to read the battery level, but even that not that important.- the input (only when charging the battery) is a wall adaptor and I can choose any voltage accordingly to whatever is required by the IC/battery. It does not need to be a USB charger NOR a 5V charger. Whatever input voltage the IC accepts I can find a wall-adaptor to work with it.- I guess for safety it might need some monitoring and cutoff perhaps?- OPTIONAL (would be nice but no need to have) is means for the charger IC to allow to also supply the system (i.e. feeding whatever the incoming DC voltage is to my onboard switching regulators)

author: Martinn
author_rank: Frequent Contributor
timestamp: July 11, 2020, 09:12:26 am
content: Assuming 100 pcs and a certain level of reliability, I'd use a top brand standard pack like this onehttps://www.amazon.de/ANSMANN-Li-Ion-Akkupack-7-4V-2-6Ah/dp/B073P9XNCQ.You could also have one custom made, but that's unlikely to be any different.Charger: Li-Ion has been around for decades, there are hundreds of choices. For simplicity go to ti.com and select one. For 2s and standalone, switch mode, you gethttps://www.ti.com/power-management/battery-management/charger-ics/products.html#p1152=2;2&p338=Li-Ion/Li-Polymer&p273=Switch-Mode%20Boost;Switch-Mode%20Buck;Switch-Mode%20Buck-boost&p1341=Standalone;Standalone%20(RC-Settable)&sort=p1130;ascPick one... BQ24133 might be OK.If you don't want to design your own, adafruit has lots of charger modules, although you'll end up with 1s probably.You'll also find that there are arbitrarily cheap batteries and chargers (on ebay, Ali, DX...). I'd say you get what you pay for. Top brands (like cells from Panasonic - from official sources, otherwise chances are good they are fake) are a safe bet, but expensive. Anything cheaper might be OK or might be junk and/or fake. Your risk.

author: ocset
author_rank: Super Contributor
timestamp: July 11, 2020, 09:54:09 am
content: Attached is a charger solution for you..

author: tooki
author_rank: Super Contributor
timestamp: July 11, 2020, 12:53:39 pm
content: Quote from: ricko_uk on July 09, 2020, 07:37:25 pmWell, those features take you a bit beyond a super basic charger, actually!a) Givenb) OKc) Battery charger ICs don't actually do that. As it turns out, accurate battery charge level calculation ishard, so it's the purview of dedicated "fuel gauge" ICs. The vast majority of these are designed for big-development-budget large-volume gadgets whose engineers can devote a lot of time to carefully characterizing both the battery and the load to produce a complex (~200 parameters, IIRC!) profile that's uploaded into the fuel gauge. A small selection are "easy" models that just require a few basic parameters (capacity, voltage ranges, etc.), in exchange for reduced precision. Also, most of these are little tiny BGAs poorly suited to manual assembly.d) OKe) The whole point of a battery charger IC is to charge a battery correctly, requiring it to perform monitoring and act accordingly, starting, stopping, or changing charging as needed. On the other hand, if you're talking about pure safety cutoffs (severe undervoltage, overvoltage, or short-circuit), that's the responsibility of a battery protection IC, another discrete device.f) TI calls that "power path management" or something. It's a common feature.Take a look at the solution I decided on for our gadget. It's essentially borrowed from TI'sTIDA-01182reference design, copying U1 (BQ25895 charger IC) and U5 (protection IC) directly, but replacing U4 (fuel gauge) with the BQ27441-G1, which is one of the "easy" chips in a somewhat less hateful SON package, and for which I found an Arduino library to make coding easier. We'll be using a single-cell (well, multiple in parallel) 18650 Li-Ion, which gets boosted to a 4.5V system voltage which then feeds various point-of-load DC-DC converters to produce 1.8V, 3.3V, ±5V, 7.5V, and dynamic 6-18V.The BQ29895 gives us the flexibility to use any input from 3.9-14V (it's optimized for 9-12), including USB, will power the load from the DC supply but augment it with the battery if needed (so you can use a wimpier AC adapter), and it lets you program its own minimum output voltage from 3.5-4.5V, so that you don't need your downstream DC-DC converters to support wide input voltages. (Simpler chargers have the load in parallel with the battery, meaning the load must be able to work from both the battery's minimum voltage (typically 2.5-2.9V for a single Li-Ion) all the way up to the battery's maximum voltage while charging (typically 4.2V for a single Li-Ion). The "NVDC" system used here means that the output will go to the higher of a. the minimum system voltage you program and b. the current battery voltage, eliminating all the really low voltages.) It's fully MCU-controllable, but also supports resistor dividers to configure current limits for standalone operation. It also supports a "ship mode" where the battery is practically completely disconnected to prevent deep discharging for storage, obviating the need for a physical cutoff switch like in treez' design. (One can easily use this feature for an essentially "true power off".)If your load is small enough, you could just use the 3.5V system output voltage and a small LDO linear regulator to drop it down to 3.3V.I just received the PCBs on Thursday and the parts will arrive Monday, I'll let you know how it works out!

author: ricko_uk
author_rank: Super Contributor
timestamp: July 11, 2020, 11:24:47 pm
content: Thank you Tooki, Martinn and Treez. Much appreciated feedback!!Tooki,what I am not clear about that IC is the output. Do I have to connect the rest of my system (i.e. my other regulators) to the positive from the battery or only after the inductor connected to the SW pin of the IC?If the latter, does the IC then switches automatically between the battery and the DC input to provide the output voltage?Many thanks again

author: tooki
author_rank: Super Contributor
timestamp: July 11, 2020, 11:46:24 pm
content: Look at the schematic and/or block diagram of the reference design, and read the datasheet: all clearly show the load (e.g. downstream regulators) connected to the SYS output of the BQ25895.I already answered the question about input switching. Re-read my reply above.