Portable device with Li-Ion batteries - Page 1
Forum section: electronics / projects

author: c64
author_rank: Frequent Contributor
timestamp: October 06, 2020, 01:54:40 am
content: I'm designing a portable device. It must be running from DC adapter 12..20v or batteries (I think ~ 4 of 18650 will be enough). When DC adapter plugged in, it must charge the battery. Also I need to know at any given moment, how much charge is left.Voltages required by device are 12v and 3.3vAnyone can recommend an easy way to implement this?Also, do I need protected cells or not, and there to source them?

author: jbb
author_rank: Super Contributor
timestamp: October 06, 2020, 02:03:15 am
content: Honestly, battery powered devices suck. Charging in device sucks. Battery gauging sucks. Battery shipping sucks. Prepare for aggravation.With that said, let’s see what we can do. Some questions...- What temperature range do you need the device to run at, charge at?- How long do you need the device to run? How many hours on a charge? How many times would you recharge it?- What is your peak and average current draw at 3.3V?- Could you run at 3.0V or 2.5V instead?- What is your peak and average current at 12V?- Do you really need a steady 12V? Would 10V - 17V be acceptable?- Do you need the 12V rail live all the time, or can it be switched off when not in use?

author: c64
author_rank: Frequent Contributor
timestamp: October 06, 2020, 02:47:48 am
content: Device will be operating in "active use" and "standby" modes.- Operating / charging temperature 5-40°C- How long do you need the device to run? Permanently on DC, on battery 2-5 hours in active use, 12-24 hours in standby mode.- How many times would you recharge it? 1000 would be nice.- What is your peak and average current draw at 3.3V? 10-50mA in standby, ~1.5A average, peak 3A- Could you run at 3.0V or 2.5V instead? Probably not. Would it make my life easier?- What is your peak and average current at 12V? None in standby, 0.5A average, peak ~2A- Do you really need a steady 12V? 12V ±5%- Do you need the 12V rail live all the time, or can it be switched off when not in use? Not needed in standby

author: c64
author_rank: Frequent Contributor
timestamp: October 06, 2020, 02:50:01 am
content: Quote from: jbb on October 06, 2020, 02:03:15 am

author: james_s
author_rank: Super Contributor
timestamp: October 06, 2020, 04:45:18 am
content: Battery powered devices work very well, laptops have solved this problem decades ago. There are BMS ICs specifically designed to do what you want. There are also BMS boards available from China, many of which work ok but I don't know if I'd use something like that in a production device.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 06, 2020, 04:51:47 am
content: I have both wired and wireless vacum cleaner and hand drill i always reach for wireless version they are very handy and quick to reach and start operation, unlike their wired siblings, need to pull wire and find mains socket, bad luck if in remote area.. i only wish designer will make batteries easily replaced (in dedicated openable compartment) if damaged.. my hand drill is 5x 18650 lion 20v, my vacum is like 6 or 7x 18650, need full dismantling to reach batteries, thats bad. well every cordless devices are bad just because of this... some even nastier using stupid ic that kills itself or everything once batteries removed and replaced with new set..esp expensive brand like dslr and laptops..

author: james_s
author_rank: Super Contributor
timestamp: October 06, 2020, 05:16:35 am
content: I have given away my wired drill, handheld vacuum cleaner and string trimmer as I have replaced them with cordless models, the brushless motors with lithium batteries are in many cases more powerful than the mains powered devices I replaced. Next on the list is my leaf blower, I'm looking at the Makita brushless electric model to replace my corded one so I don't have to worry about tripping over a cord while I'm up on the roof.

author: jbb
author_rank: Super Contributor
timestamp: October 07, 2020, 02:38:17 am
content: All right, sorry for delay. On phone, sorry for poor formatting.——Let’s start with energy and power:- max power is 40W (9.9W for 3V3, 24W for 12V, assume 85% efficiency for power converters)- average active power is 12W (5W for 3V3, 6W for 12V, 90% efficiency)- for 5 hours operation, need 60Wh useable- don’t go over 100Wh nameplate or it will be harder to ship (cargo aircraft rules applied by courier companies)For LiIon (guesstimate!)- assume we reduce capacity by 10% by dropping charge V a little and increasing end of discharge V a bit- assume battery degrades by 30% by ageing- want around 95Wh nameplateFor LiFePO4 (guesstimate!)- assume we reduce capacity by 10% by dropping charge V a little and increasing end of discharge V a bit- assume battery degrades by 15% by ageing (they don’t age as fast)- want around 80Wh nameplate——Now let’s look at peak current. Obviously that will depend on the voltage, so let’s look at some options. We will use the minimum cell voltage because that’s when the current is highest. We calculated a peak power of 40W earlier.LiIonStop at 3V / cell.1 Series: 3V. 13.3A, manageable2S: 6V. 6.7A, much easier3S: 9V. 4.4A, no problem4S: 12V. 3.3A, no problemLiFePO4Stop at 2.7V / cell.1S: 2.7V. 14.8A, manageable but voltage drop a real issue2S: 5.4V. 7.4A, much easier3S: 8.1V. 4.9A, no problem4S: 10.8V. 3.7A, no problemAll right, it looks like 1S is a lot of current. Also, you would need a buck-boost converter to make the 3.3V rail.4S LiIoncould be interesting: it’s minimum voltage is 12V, so you could use a buck converter to derive the 12V output (and switch it off too).Battery voltage range: 14.8V nominal, 12.0 - 16.8V.Target capacity: 6.4AhCould do 2 Parallel (4S2P) 3000mAh cells for 88.8Wh nameplate rating3S LiFePO4could be interesting too: it’s always below 12V so you could make 12V output with a boost converter (and output switch).Battery voltage range: 9.9V nominal, 8.1 - 10.8V.Target capacity: 7.4AhCould do 5 Parallel (3S5P) 1500mAh cells for 81Wh nameplate rating——The simplest battery to manage would be 1 in Series, ie 1S. But I think the high currents that result would cause their own troubles (especially the 12V boost converter!)For 2S and more, we need a protector circuit to monitor each cell string voltage for safety. We might also need to do balancing, which is debatable. Would it be possible to open the equipment up for service occasionally?

author: c64
author_rank: Frequent Contributor
timestamp: October 08, 2020, 02:29:48 am
content: Quote from: jbb on October 07, 2020, 02:38:17 amYes, opening the equipment is no problem.Where is also space and weight constraints, so I think 6 cells is the maximum I can afford. 3s2p I guess? Reduced time is not a big problem, even if it works for only 2 hours it's enough.

author: c64
author_rank: Frequent Contributor
timestamp: October 08, 2020, 02:43:04 am
content: Quote from: james_s on October 06, 2020, 04:45:18 amCan you give any examples? Anything from your own experience?

author: james_s
author_rank: Super Contributor
timestamp: October 08, 2020, 02:58:13 am
content: Quote from: c64 on October 08, 2020, 02:43:04 amI've never designed a BMS system myself, but TI makes a selection of ICs, the sort of thing commonly used in laptops and such.https://www.ti.com/power-management/battery-management/overview.html

author: tooki
author_rank: Super Contributor
timestamp: October 08, 2020, 01:39:05 pm
content: Quote from: c64 on October 06, 2020, 01:54:40 amI’ve spent much of this year on a project involving precisely this. It’s not trivial to do right.1. Battery charge status is HARD to get right. This is accomplished by “fuel gauge” ICs. Most are designed such as to require a complex battery and load characterization process to produce a profile loaded into the gauge IC. There are a handful of “easy” models that are happy with a few key parameters (like battery capacity) and are easy to get going, but it won’t be as accurate. I forget the part number we’re using.2. You’ll want to use a proper charger IC. In my current project at work, we’re using the BQ29895, which might well fit your needs. Bear in mind that proper configuration is a must, otherwise you get only basic low-speed charging. A PC-based I2C interface like an Aardvark is super helpful for learning how to interface with the charger and fuel gauge ICs.3. In the same project, we’re also using the TPS61178 to produce 12V (variable). Might work for you. We use a few others to produce the low-current 3.3V, 7.5V, and ±5V rails. For standby capability, you’ll want to make sure you use a DC-DC converter with “enable” input for your 12V rail, so that your microcontroller can easily turn the 12V rail off and on. (We are using this capability to sequence the power rails, since all of them turning on at once can draw too much current for a moment, causing the BQ25895 to go into protection and shut down.)3. It’s your choice whether to use protected or unprotected cells. If you go with unprotected cells, then you MUST provide the protection yourself. We went that route: the protection IC (and its related MOSFETs and passives) is on the charger PCB the batteries are mounted on. If you go with protected cells (which you definitely want to do if the batteries are easily user-accessible by nontechnical users, especially if they’re 18650s or similar with exposed contacts), then you can dispense with a protection IC. Note that you can run protected cells in a circuit with its own protection; the redundant protection won’t hurt anything other than the budget.

author: tooki
author_rank: Super Contributor
timestamp: October 08, 2020, 01:46:04 pm
content: FWIW, we based our battery infrastructure on the one in this TI reference design, just swapping out a different fuel gauge IC:https://www.ti.com/tool/TIDA-01182Studying TI’s reference designs and white papers can give you a LOT of insight. As for DC-DC converters, their Webench power designer tool is enormously useful: you plug in some basic parameters and it spits out designs for you, complete with simulations, editable circuit diagrams, and PCB layouts. This tool saved us an enormous amount of work and uncertainty.

author: tooki
author_rank: Super Contributor
timestamp: October 08, 2020, 02:15:30 pm
content: P.S. I just checked our Digi-Key and JLCPBC order histories:The fuel gauge IC we chose is the BQ27441. (We needed an “easy” one in a non-BGA package, since beginning electronics apprentices need to be able to assemble it.)For the protection IC we used the BQ29705 (different versions of the BQ2970 family are preprogrammed with different protection parameters).The ±5V rails are generated by a TPS65133.The 3.3V rail is generated by a TPS63021.The 7.5V rail is generated by a TPS43061.