Battery protection/charging for 4S Li ion - Page 1
Forum section: electronics / projects

author: LukeW
author_rank: Frequent Contributor
timestamp: September 26, 2017, 11:15:49 am
content: So, I have a 4S pack of discrete 18650 cells.They do not have any integrated protection, and I want to protect them, in terms of safe charging, balancing, over-voltage protection as well as over-discharge protection, with very low quiescent current to stop the battery crapping itself if the customer leaves the device unattended without charging for a month. It should have easy plug-and-play user experience, without fire or explosion or failure of the battery.There's an interesting thread here which deals with similar issues.https://electronics.stackexchange.com/questions/317019/diy-18650-4s-battery-pack-how-to-charge-how-to-build$2 inexpensive "battery management" boards are readily available from all the usual Shenzhen e-commerce sources, like this example.https://www.aliexpress.com/item/4S-6A-16v-Li-ion-18650-BMS-PCM-battery-protection-board-bms-pcm-for-li-ion/32674520843.htmlAre they actually worthwhile? Do they do anything?Are there any similar devices which are more reputable?What should I add to the battery, for a reliable and safe charging experience?I want an external DC plugpack supply, say 12V or 18V or something, which the user will plug in to the device to charge - I want the actual battery charging electronics kept inside the unit with the battery, enclosed where the user does not access it except for plugging in the supply.So the battery charger circuit also has to remain connected to the battery, without discharging it.Any particular modules or silicon or products you would recommend?

author: LukeW
author_rank: Frequent Contributor
timestamp: September 30, 2017, 09:24:50 am
content: Okay, so have a look at this and tell me what you reckon please.The 4S Li-ion pack (with no other protection circuits is connected to a Turnigy Compact B4 balance charger, and the charger is left integrated inside the system.12v power is plugged into the input side of the charger to charge the system.2x DPDT relays are used to isolate the battery from the charger balancing port, so there is no quiescent back-discharge into the charger.The main output port of the charger is just connected in parallel with the balancer port (to the two outermost terminals of the balancer connector.)The bottom or "ground" end of the battery is left connected always, because there are only four relay poles available to isolate the four lines.The relay coils are in parallel with the 12VDC input side of the balance charger, so when the power connector is plugged in the relays pull in to connect the battery to the charger.From there, a low-voltage protective cutoff consists of a MCP6541, a MCP1703 and a FET. These were chosen for their extremely good power efficiency, meaning minimal battery discharge.After the FET, there is a power switch, then load circuits.Completely discharging the Li-ions to the point that they're wrecked and cannot be charged is the main issue I'm concerned with here.I want confidence that this won't happen, even if the system is left switched on and stored for months without the battery being charged, if possible.Will this be expected to do the job?

author: LukeW
author_rank: Frequent Contributor
timestamp: October 09, 2017, 01:13:06 am
content: Anyone? Thanks.