How to isolate battery charger from battery. - Page 1
Forum section: electronics / projects

author: Zipdox
author_rank: Regular Contributor
timestamp: April 19, 2022, 10:36:31 am
content: I'm working on a portable speaker. This speaker will contain a LiPo battery with a BMS, as well as a constant-current buck converter for charging the battery. The problem is, the buck converter bleeds 13mA from the battery on standby (10mA after removing the LED). I tried mitigating this by using a MOSFET to connect the battery to the buck converter while charging, but the problem is that the MOSFET body diode still allows current to flow. I've attached a picture of the circuit, with the BMS removed as it isn't relevant.I could use a relay to connect the battery, but I'd rather keep it solid state. Using a schottky diode is possible, but not ideal because with a charging current of 5A, it'd still dissipate quite a bit of heat due to the voltage drop. The voltage drop would also reduce the charging voltage to the battery. Increasing the buck converter output voltage doesn't seem smart as the forward voltage of a diode isn't entirely constant, but dependent on the current. What options do I have?

author: moffy
author_rank: Super Contributor
timestamp: April 19, 2022, 01:02:49 pm
content: There is an AC switch using two MOSFETs back to back so that the body diodes block one another. I drew it up in LTSpice and ran the simulation just to demonstrate. The two traces are:1. Current through R12. Voltage driving the gates.You could use one of those photovoltaic couplers to drive the gates. This is simply how some of the AC SSR work.

author: Zipdox
author_rank: Regular Contributor
timestamp: April 19, 2022, 03:32:45 pm
content: I'm not sure how this would apply to my circuit. Could you make it clear in your schematic?

author: moffy
author_rank: Super Contributor
timestamp: April 19, 2022, 04:29:38 pm
content: You would use something like:https://au.mouser.com/datasheet/2/196/Infineon-PVI1050N-DS-v01_00-EN-1512038.pdfphotovoltaic optocoupler with its output being V2, and the input LED energised when power is applied to the input of your buck regulator. This is all only illustrative, you would need to choose your devices properly. Consider R1 as being the output of your buck regulator, I just used it to show that the switch will stop the battery discharging when off.If you want more of an explanation, then perhaps some others could help, or you could just Google "AC Solid State Relays", there will be two common types:1. Using a triac as the AC switch, it can only switch AC no DC as it will stay on permanently with DC(holding current).2. Using the back to back MOSFETs as the switch, it can switch both AC and DC. It will typically use a photovoltaic optocoupler as the isolating element.

author: Zipdox
author_rank: Regular Contributor
timestamp: April 19, 2022, 06:28:57 pm
content: I read something about using a MOSFET to pass current in the non-conventional direction (called third-quadrant operation). Would this be a possible solution?

author: moffy
author_rank: Super Contributor
timestamp: April 19, 2022, 10:21:00 pm
content: Perhaps you are thinking of the back to back MOSFETs, I am illustrating as one of the MOSFETs is passing the current in the reverse direction to the other? If for example M1 is passing current from D to S then M2 is passing current from S to D.

author: inse
author_rank: Super Contributor
timestamp: April 20, 2022, 01:09:11 am
content: For the beauty of simplicity, I would stick to the relay solution..

author: moffy
author_rank: Super Contributor
timestamp: April 20, 2022, 03:26:35 am
content: Quote from: inse on April 20, 2022, 01:09:11 amI like the simplicity of a relay also, very robust.

author: Zipdox
author_rank: Regular Contributor
timestamp: April 20, 2022, 09:53:05 am
content: I'll try a MOSFET before I go back to a relay.

author: Terry Bites
author_rank: Super Contributor
timestamp: April 20, 2022, 03:54:22 pm
content: Use a different converter. Some of buck converters draw uA or less in shutdown. Orders of magnitude less than the internal leakage of your battery.

author: Zipdox
author_rank: Regular Contributor
timestamp: April 20, 2022, 05:55:12 pm
content: Quote from: Terry Bites on April 20, 2022, 03:54:22 pmEasier said than done. You know any in particular? I need it to convert 19V to 16.8V for 4S at 5A current limit.

author: Zipdox
author_rank: Regular Contributor
timestamp: April 20, 2022, 06:33:23 pm
content: Ok this is what I have in mind now.

author: Zipdox
author_rank: Regular Contributor
timestamp: April 20, 2022, 08:34:56 pm
content: Ok I built it and it seems to work. Only a small issue. The buck converter passes current from the output to the input side, so it never turns off.This was easy to fix though, as the DC jack I'm using has a pin that gets pulled to GND when the connector is unplugged. I used this to pull the optocoupler input low like this. The only catch is that you have to unplug the charger from the device, not just from the wall.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 21, 2022, 01:17:24 am
content: Quote from: Zipdox on April 20, 2022, 08:34:56 pmAdd a zener in series with the optoisolator input to set a minimum voltage needed to activate the circuit.

author: tooki
author_rank: Super Contributor
timestamp: April 21, 2022, 05:04:57 pm
content: Quote from: Zipdox on April 19, 2022, 10:36:31 amJust to be sure, you aren’t planning to use a simple CC buck as your charger, are you? That’s a bad idea. Just use a charger ICBy using a 1S or 2S battery followed a boost converter, rather than a 4S battery, you massively increase your range of battery charger ICs.In the Bluetooth speaker project I’m just finishing up, I use a TI BQ25895 charger IC, which has a software-controlled “shipping mode” where the main switching mosfet is turned off, reducing the standby current draw to just 12 μA. Shipping mode is exited by either: pulling a dedicated pin low for 2 seconds*; disconnecting and reconnecting a battery pack, if using a removable pack; connecting the charger’s power supply; or via software.The “Vsys” output of the charger IC then goes to various boost converters to generate the voltages needed. The power amp supply has a fairly beefy boost converter that is configured to accept a 0-3.3V control voltage, which sets the boosted output voltage between 5 and 24V. (The control voltage is generated by the MCU’s DAC based simply on the volume setting (with bass boost gain added).) With the control voltage pin floating, the output voltage is around 17V.*the same pin also has a force-reset function when held low for about 15 seconds, enabling recovery from a microcontroller hiccup, etc.Unsolicited LFMF/learn from my experience:*TI’s WEBENCH power designer is tremendously useful for designing DC-DC converters. You enter your parameters and it gives you a list of candidate parts with complete designs including schematic, values, simulations, and PCB layout. (And you can change values and support components and re-simulate.)*TI also has a separate, downloadable Java program called Power Stage Designer that has lots of handy tools, including one for designing the resistor networks for control voltages.*Even if you don’t splurge on TI’s crazy overpriced USB-I2C interface, you can still use their Battery Management Studio (BQ Studio) software to both edit and parse their IC’s register values. Without their interface, you just have to copy the hex values by hand.*Monolithic Power Systems makes a bunch of charger ICs that are suspiciously similar to many TI parts. I haven’t used their chips so I can’t vouch for them, but their development tools are literally less than 10% as expensive as TI’s.*Other manufacturers also have simulators and such, but so far, none I’ve found has beenquiteas comprehensive as TI’s.

author: Zipdox
author_rank: Regular Contributor
timestamp: April 21, 2022, 10:35:51 pm
content: It's a constant voltage constant current buck. I'm using a 4S battery and I couldn't find any premade charging modules for 4S. I'm not intending to design a whole custom charging circuit for this project. I'm using a 4S battery precisely to omit the need for a boost converter.

author: Zipdox
author_rank: Regular Contributor
timestamp: April 21, 2022, 10:39:13 pm
content: Quote from: NiHaoMike on April 21, 2022, 01:17:24 amThat's not a bad idea, but then I'd need to get my hands on a higly specific zener. I'm thinking around 17.5V is a good idea?

author: tooki
author_rank: Super Contributor
timestamp: April 22, 2022, 04:56:19 am
content: Quote from: Zipdox on April 21, 2022, 10:35:51 pmSo you’re not using a LiIon charger IC at all? That’s… unwise.4S charger ICs and modules most certainly exist. Don’t forget that you’ll need a balancer circuit too.

author: Zipdox
author_rank: Regular Contributor
timestamp: April 22, 2022, 10:51:13 am
content: I'm already using a BMS. I think you missed that in my original post. I also said I omitted it from my circuit for simplicity sake. The buck converter shouldn't output anything over 16.8V, but if it does, the BMS has overvoltage and overcurrent protection.

author: tooki
author_rank: Super Contributor
timestamp: April 22, 2022, 11:47:45 am
content: No, I saw that you have a BMS. But a BMS’s job isn’t to control charging, that’s the charger’s job. The BMS’s protection is not intended to be the sole protection, it’s only the “emergency brake” so to speak.Also, you know you can’t just keep trickle charging the battery in the CV phase forever, right?? Your approach doesn’t have any charge termination mechanism, never mind one as well defined as what lithium should have.

author: Zipdox
author_rank: Regular Contributor
timestamp: April 22, 2022, 12:24:12 pm
content: Quote from: tooki on April 22, 2022, 11:47:45 amYou're probably right. But this isn't a device that'll be charged as often as a smartphone. I probably won't use it that often.I couldn't find any other charge modules online, only BMSs.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 22, 2022, 01:13:36 pm
content: Set it to 16.4V for 4.1V/cell.

author: tooki
author_rank: Super Contributor
timestamp: April 22, 2022, 07:12:46 pm
content: Quote from: Zipdox on April 22, 2022, 12:24:12 pmOK. But for your own safety, consider using 18650s instead of LiPo: the cylindrical cells are more forgiving of overcharging and other abuse than LiPo.

author: Zipdox
author_rank: Regular Contributor
timestamp: May 11, 2022, 08:39:49 am
content: Quote from: tooki on April 22, 2022, 07:12:46 pmI don't have that option. I already designed my entire enclosure around a hard case LiPo.