Desigining (part selection) 3S battery charger - CC/CV step-down buck converter. - Page 1
Forum section: electronics / projects

author: Oxmstr
author_rank: Contributor
timestamp: March 31, 2023, 12:10:57 pm
content: I'm a hobbyist, so my electrical engineering skills are not top notch.I'm service tech and I'm working on modifying portable coolers (thermoelectric and compressor based) where we add batteries to said coolers to make them work without being connected to wall outlet.Currently we are making our own battery packs, and our problem is charging the batteries safely while inside the cooler. At the moment we are using these cheap - around 5$ per piece "XL4016 300W step-down CC/CV converters"that are quite bad in terms of efficency (80%) and power usage when running on battery (the battery self discharges in a week when connected to the DC-DC converter by it's own - 133 Wh), so I'm willing to spend a lot more for higher quality.I'd like to design my own battery charger / step-down converter that is more efficent, and doesn't discharge the battery when unpluged.Current system looks like this:My biggest hurdle is searching for appropriate DC-DC controller or dedicated 3S battery charger IC that has all the features that I'd like to have. Most of DC-DC controllers and battery ICs don't have proper parameters on LCSC, so searching for one is complicated. I've checked out the TI WEBENCH, but it's desings don't include current limiting chips.My design criteria are:BOM price: around 20$ - 30$ from LCSC at ~20 pcs pricing (for assembly from JLCPCB)Low power draw when disconnected from power supply.Input voltage: 13 V - 35 V (up to 24 V for charging from battery in truck, so 33 V max nominal)Output voltage for battery charging: 12.6 V (3S Li-Ion configuration)0 volts on input side, when battery is connected, but charger is disconnectedOutput current: 0A - 6 A / 0A - 12 A, selectable either via potentiometer or jumpers (could incorporate 2 different versions, as we usually need 70 watts of power max, but for some coolers 100 watts is needed. Not sure how to design this kind of different versions; additional MOSFETs/diodes/etc or different coil?)Reverse voltage protectionFan connection for 12V computer fan cooling the power supply. The cooler uses it's fan that's used to cool down the peltier and also it's power supply. But when we connect power to charge the battery and not turn on the appliance, the power supply is without forced air cooling. Would need somekind of ORing circuit that could force the fan to spin when either cooler is on or it's being chargedSuper optional: coulomb counter / gas/fuel gauge with display; while looking for said chips they are super expensive and mostly for 1S battery configuration. But customers are often confused when they plug in the charger and it's showing 100% SOC when in reality it's 80% (CV charging mode). I think that up to + 10$ bom price for this would be ok.Is there a chance that my project could be made?What parts should I be looking at?I think I will manage the PCB layout, just my knowledge in power supplies field is vastly insuficient to be able to design the whole system by myself and it work.Thank you.

author: thm_w
author_rank: Super Contributor
timestamp: March 31, 2023, 09:44:02 pm
content: Do your battery packs have over/under voltage BMS protection per cell? You need to implement that if not.QuoteEfficiency of 80% is not terrible for such a cheap item. But for self discharge you could set up some circuitry so that disconnecting the input will also disconnect the output of the DC/DC.What current are you charging at?

author: dobsonr741
author_rank: Frequent Contributor
timestamp: March 31, 2023, 11:44:51 pm
content: Take a look at LTC4162 or in general the LTC4000.

author: Oxmstr
author_rank: Contributor
timestamp: August 07, 2023, 11:31:32 am
content: Thank you for the replies.Just came back again to search for right part, these green "300 W" step downs are quite bad. I'd love to pay more than twice for it to not blow up after few months when using them with 60 W output with 19 V input...Yes the batteries have 3S BMS, that's not a problem.I can't see LTC4000 part in stock, although I saw a module use ltc3780https://www.analog.com/en/products/ltc3780.html5$ for 30 pieces.

author: Oxmstr
author_rank: Contributor
timestamp: August 24, 2023, 11:53:19 am
content: As I have no clue how to search for appropriate part,I just went to lcsc dc-dc controllers, in stock and sort by availabilityhttps://www.lcsc.com/products/DC-DC-Controllers_11270.htmlI'm looking at prices of around 0.5 to 1.5 usd per 30+ pieces.There islm3485, but it uses pmos fet, so worse efficiency than nmos (as far as my knowledge leads me to believe)lm25085another pmos buck controller, also quite expensive at 1.2$xr75100synchronous step down, on expensive side of thingsI'm unable to find chips that include battery charging in their typical applications on the datasheets, maybe I should be looking at cheaper ICs, but I'm willing to spend more for better reliability and efficiency.I'm not sure how to know if there won't be any voltage on the input side, while there is a battery connected on the output.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: August 27, 2023, 12:24:20 am
content: Take a look at MAX17703. Analog.com will send you 5 samples for free. You can break the tube for single pricing (use the checkbox)Here is the parametric search on analog.com:https://www.analog.com/en/parametricsearch/11471#/p5574=30|max&p5555=10|max&qsfv=chem|Li-Polymer_vmax|30_icharge|10&p5380=Li-Polymer

author: Oxmstr
author_rank: Contributor
timestamp: August 31, 2023, 07:02:43 am
content: I will check later mouser for battery management IC.I've checked dc-dc converters instead of controllers on lcsc and foundSIC471, but it seems too good to be true, for 3$ you get so much. I don't know what the catch is there, all you need is a coil and some capacitors, then bam, super capable step down converter.Although I didn't see in the evaluation boards any kind of potentiometer to set the voltage and current, only fixed examples, and don't know if there will be voltage on the input if battery is connected on the output. The current limit equation seems quite complex.I'm not sure you can find more capable chip for this price.