18650 Li-ion Charger vs BMS  - Page 1
Forum section: electronics / general

author: WannabeGeek
author_rank: Contributor
timestamp: June 02, 2025, 08:35:37 pm
content: I'm trying to charge a 3s 18650 Li-ion battery pack from my car battery. I have a 3s BMS board with balancing. I also have a buck converter with voltage and current regulation. The car battery provides upwards of 14v while running and more than enough amps. My plan was to have the buck converter set at 12.6v and I haven't really decided on the optimal amp value for the 3 18650 cells in series. My buck converter is rated at only 5 amps and the BMS is rated at 20 amps. The battery pack should be used to provide power to my dashcam in parking surveillance mode (which only records for 60 seconds at a time when motion is detected). The reason I want to do this is because I don't want to drain my car's battery when it is parked for a week at a time. The dashcam hardwire kit includes a module that is supposed to shut it off when the battery voltage goes below 12.4v. This will probably happen pretty quickly if it works correctly. I typically drive for 2 to 6 hours only on weekends. This might not even be long enough to keep the 18650 cells charged. On the other hand, I'm a little worried about overcharging them because the buck converter won't cut off regardless of the charge state of the cells. The BMS will need to do this. Is this a dangerous setup? Is there a better way to charge 18650 cells from a car battery?

author: thm_w
author_rank: Super Contributor
timestamp: June 02, 2025, 10:40:43 pm
content: The buck converter will not overcharge the battery when set to 12.6V, that is 4.2V per cell. I would personally drop that to 12V or so to increase the longevity of the cells though.For current, depends on what the cells are rated for (charge C rating) and if the buck regulator looks decent quality or not. If the buck regulator is no name, usually best to derate it and not run it at full current.Make sure you put a diode and fuse in series with the buck regulator to avoid backfeeding.Its not a dangerous setup if done correctly, but, personally I would just buy an off the shelf LiFePo4 12V battery and use that instead.In terms of battery life, you'll have to measure or look up the dashcam power draw. Then tell us what the battery capacity is, calculate its watt hours. Shutting down at 12.4V is obviously too high, so you'll either need to modify or change that, some have switches on the side to set the voltage cutoff. Test it and see what voltage it actually cuts off at as well.edit: you can delete your double post over herehttps://www.eevblog.com/forum/beginners/charger-vs-bms/msg5934174/#msg5934174

author: WannabeGeek
author_rank: Contributor
timestamp: June 03, 2025, 01:12:25 pm
content: The 18650 cells are still in transit. They should arrive today or tomorrow. After that, I will test them to see what the actual values are. They always seem to be rated higher than what they actually produce. I certainly won't be running the buck converter at the full 5 amps. The power is coming from the car's fuse box and it already has a 5 amp fuse on it. Adding a diode on the same wire seems like a good idea. Any suggestions about a suitable diode here? At the moment the dashcam is wired to the Acc of the car and also a separate, permanent 12v source from the battery. When the car is running, the dash cam takes the power from acc (which also comes from the battery) but, since it shuts off when the car ignition is turned off, I'm assuming there's a relay in play somewhere along the way. When the car is not running, the dash cam uses the other source which is always connected to the battery through the hotwire kit's battery shut off unit. This is switchable, but I have it on the highest setting because some dash cam owners were complaining about their batteries being dead when they tried to start the car. At the moment, I'm not using the parking mode so the dashcam is running only on the acc power when the car is running. As far as "off the shelf" battery packs designed for use in cars, they're really expensive for what they offer. They usually cost between 75 and 200 dollars and my dash cam was around 100 dollars. I also don't want to be unplugging the battery every week to charge it at home. This would defeat the purpose of the battery pack to power the dash cam parking mode. The DIY battery with buck converter and BMS will be under 20 dollars. In parking mode, the dash cam should draw 100 to 300 mA. The 18650 cells are supposed to be 2600 mAh each. 6 days unattended would either drain the battery pack or shut off in a couple of days max. I could do 6 cells and double the amps but I'm pretty sure I won't be able to charge that battery pack with my car's battery in the limited driving time I have. That's also why I would be inclined to leave the buck converter at 12.6 volts. Also, I'm pretty sure the BMS won't even shut off if the total voltage never reaches 4.2 volts per cell. I guess that's ok and the whole thing is a compromise anyway. I figure a few hours or days of parking mode with a dedicated battery that won't drain my car's battery is better than nothing. With a battery pack, the dash cam will be running solely on the acc. This will charge the battery pack and power the dash cam at the same time. The wire that is now connected to a permanent 12v from the car battery, will then be connected to a separate acc wire in the fuse box with the hardwire kit's shut off unit and the battery pack between the car's acc and the dash cam. The battery protection unit will be set to the lowest setting (11.8volts) which is still overkill because the battery pack would then be disconnected when the cells reach 3.9 volts each. If this unit makes parking mode impractical, I will probably just bypass it since the BMS should keep the batteries from being completely drained anyway. For the dash cam operation, it shouldn't make a difference but the car's battery won't be used at all while the car is parked. That's the theory anyway.

author: thm_w
author_rank: Super Contributor
timestamp: June 03, 2025, 10:33:59 pm
content: Any schottky 5A+ diode is fine.QuoteHere they are $20-50:https://www.amazon.com/XZNY-12V-LiFePO4-Lithium-Battery/dp/B0CQ534RXG. You would charge it the same way you charge the lithium pack you want to build, no need to remove it to charge. Either way the solution is the same, its just convenient to have a battery premade in an enclosure.QuoteSo ~200mA at 12V, 2.6Ah cell. If you use 4S1P your pack would last about 13 hours. 4S2P about 26 hours.Assuming you bypass the 11.8V cutout as you say.

author: WannabeGeek
author_rank: Contributor
timestamp: June 04, 2025, 01:21:38 am
content: Well, I didn't know those batteries even existed. It's an interesting idea, but it still doesn't solve the charging problem. I guess I would still use my buck converter or in that case use a more powerful one. Either way, it seems like it would never have enough time to charge that many cells with an average drive time of less than 6 hours a week. I know very little about LiFePo4 batteries compared to Li-ion batteries. I've heard they're safer but do they also charge faster? The dash cam manufacturer (Viofo) is also a little vague about the power draw in parking mode. I don't know if standby will use that much power or if they're just talking about when it's actually recording something whenever the sensor detects motion. I would probably need to test this in a real world situation to get the actual values. The battery from Amazon also weighs 1 Kg. The 6 18650 cells that arrived today weigh 273 grams. According to the calculator here:https://www.orbtronic.com/18650-li-ion-battery-charge-time-calculator-charging-chart/charging 3 18650 cells in series at .5 c (assuming 85% efficiency) would take around 2 hours and 21 minutes. That seems reasonable. If that is the case, perhaps I will use all 6 cells (parallel pairs of 2) and just charge them at around 2 amps. I was a little worried about the heat dissipation, which is why my initial idea was to only use 3 cells but the run time in that case might make the whole idea less worthwhile. My idea was to create the battery pack and wire it to the BMS. The pack would be heat shrinked together and I would velcro it under the fuse box where there is a bit of room. I don't really have a practical solution for a battery that weighs a Kilogram. I also have a question about why I should use a schottky diode as opposed to a silicon diode. Don't shottky diodes have more reverse current leakage? If the purpose is to prevent backfeeding wouldn't a silicon diode be better in this application?

author: thm_w
author_rank: Super Contributor
timestamp: June 04, 2025, 11:03:06 pm
content: lifepo4 is safer but whether or not it charges faster depends on the specific cell and how many cells are in parallel.For viofo you can find measurements online, 2-3W is about right from what I've seen. As you say, until you test in the real world you won't have the exact number.At 2A heat should not be an issue with any decent 18650.The small reverse leakage from a schottky diode is not a concern.The concern was just the voltage drop. If you do really always get 14V out, and the silicone diode drops it to 13.4V, then its probably a non-issue. But some alternators/charger circuitry is different than others, and might only output ~12.5V.

author: WannabeGeek
author_rank: Contributor
timestamp: June 05, 2025, 02:41:20 pm
content: OK. That makes sense. I just happen to have a 31DQ10 5A schottky diode. Do you think that would work? Do you see any issues with my buck converter or the BMS? If I don't find any major problems, I'll go ahead and build it and start testing before I install it in the car ;-)

author: thm_w
author_rank: Super Contributor
timestamp: June 05, 2025, 09:33:23 pm
content: Looks good to me.Balancing BMS which is good, I would still check cells are close in voltage prior to install.

author: MF-jockey
author_rank: Regular Contributor
timestamp: June 05, 2025, 11:16:21 pm
content: I have a similar buck converter. It has a resistor built into the negative path for current measurement. The negative side of the load must not reach the car's chassis, otherwise the current limiting won't work.I hope the cam has no electrical connection to the car chassis.

author: WannabeGeek
author_rank: Contributor
timestamp: June 06, 2025, 08:43:33 pm
content: I have the same resistor. The dashcam itself will have a fused connection to the cars acc for operation when the car is running. When the car is parked, it should be connected to the battery pack only. The car's battery will no longer provide power. There's a separate wire on the dashcam for parking mode which is connected to the battery pack through a seperate fuse to the fuse box. This same wire will provide power to charge the battery pack when the car is running.Can you explain to me why a .05 Ohm Resistor would negate the current limiting? All the negative connections are coming from the car's chassis which is also connected to the negative pole of the car's battery. The output of the buck converter will go directly to the BMS board (no direct connection to the car chassis) however, the input side will connect indirectly to ground. I'm not sure how isolated the input to the dash cam is. I guess there could possibly be a ground loop somewhere.

author: thm_w
author_rank: Super Contributor
timestamp: June 06, 2025, 09:24:30 pm
content: Its not an issue if you wire things as a normal electronics person would: two wires going in to the buck, two out, positive and ground. Some car guys might grab ground input and output from the chassis, which in this case would just be laziness. Running two wires is not hard.If you tied the input ground and output ground to the chassis, then you are bypassing the 0.05 ohm resistor, which is used to sense the current being delivered by the buck converter. The small voltage (1A * 0.05 = 50mV) is amplified to use for current limiting by maybe 20x or more.

author: MF-jockey
author_rank: Regular Contributor
timestamp: June 06, 2025, 10:41:56 pm
content: Only a portion of the current from the positive terminal of the switching regulator flows back through the sense resistor; the other portion flows through the other ground terminals. This results in a reduced voltage drop across the resistor, and the current regulator sets correspondingly higher values ​​to compensate.Only if all ground wires are solidly wired can the current be readjusted appropriately for the entire system. However, the reduced voltage drop makes the current regulator more susceptible to interference.

author: radiolistener
author_rank: Super Contributor
timestamp: June 06, 2025, 11:24:01 pm
content: Li-Ion are very sensitive to overvoltage, their limit is max 4.2 + 0.05V, so 4.26 is critical.DC/DC cannot provide so precise voltage, so it's bad idea to use it as a charger.But if you put proper Li-Ion charger circuit between DC-DC and Li-Ion battery it will be ok.

author: WannabeGeek
author_rank: Contributor
timestamp: June 07, 2025, 03:59:28 am
content: Well, that sounds like a different problem. So you're saying I need the car's battery connected to a fuse, a diode, a buck converter, a li-ion charging circuit and a BMS before the battery pack will charge properly? Shouldn't the BMS prevent the individual cells from being overcharged regardless of the input source? I thought that was the whole point of having a BMS board. Also, the buck converter seems like it has some sort of charging chip because there are LEDs indicating "charging" and "full." What's going on there?

author: MF-jockey
author_rank: Regular Contributor
timestamp: June 07, 2025, 12:17:16 pm
content: With a decent BMS that also interrupts charging at over 4.2V/cell, the buck converter should be sufficient as a charge controller. The only drawback is the wide adjustment range of the voltage trimmer. Vibrations could cause the wiper to move slightly, causing the voltage to change. I would set the charging voltage to 12.3V, which charges the batteries to about 90%. This also increases the possible number of cycles.

author: WannabeGeek
author_rank: Contributor
timestamp: June 07, 2025, 02:04:47 pm
content: That sounds reasonable. I don't really need to keep the batteries charged at 100%. Safety is the highest priority. The battery pack isn't that expensive but I don't want to start a lithium fire in my carAny suggestions on the charge current? I was thinking 2 Amps would be enough of a safety margin.

author: MF-jockey
author_rank: Regular Contributor
timestamp: June 07, 2025, 05:23:28 pm
content: Charging at 0.5C is usually a good choice, except at temperatures below 0°C. The battery's temperature range must be observed.The battery pack should be equipped with connectors to allow removal in extreme temperatures. In the passenger compartment of a car, temperatures can reach very high levels in direct sunlight. 60°C is not uncommon with inadequate sun protection.

author: WannabeGeek
author_rank: Contributor
timestamp: June 09, 2025, 09:17:43 am
content: I'm planning on mounting the battery pack under the fuse box, which is located on the driver side to the left of the clutch pedal under the instrument panel. It will never see direct sunlight but the car will still get hot in the summer and quite cold in the winter. It will be held in place with velcro and I have XT60H connectors for quick removal. The temperature ratings of the electronic components are well above and below the typical range here in Austria. If it gets really cold, I typically won't be driving the car. In extreme weather, it might reach the high end limit for charging the 18650 cells (0 to 45 degrees Celsius) and reach the low end for optimal charging on most days in January. The temperature in the car will typically be around 20c while driving. I have air conditioning ;-) It's just the first 5 or 10 minutes that may exceed the limits for the 18650 cells. I'm not sure how critical this is. I don't do much driving in January and February, so I'm planning to remove the battery pack and dash cam during this time.