Removable 18650 Battery Management System (BMS) - Page 1
Forum section: electronics / projects

author: skipper762
author_rank: Contributor
timestamp: October 09, 2019, 03:18:13 am
content: Hi all,I have a device that takes two 18650 batteries which are removable for recharge. The device needs to have battery protection most likely in some form of BMS as they have all the niceties for safety.However, all BMS's I have looked into require a reset after an under-voltage shutdown has occurred (AKA the device has run out of battery and has shut down).This means when charged batteries are put back into the device, the BMS won't reenable the output until a charge voltage is applied. The device doesn't and can't have a charger to provide this reset charge voltage. Is there any know BMS's out there that have this ability?Thanks.

author: viperidae
author_rank: Frequent Contributor
timestamp: October 09, 2019, 05:42:19 am
content: Can you just use protected 18650's?

author: amyk
author_rank: Super Contributor
timestamp: October 09, 2019, 11:29:55 am
content: Quote from: viperidae on October 09, 2019, 05:42:19 amOr the standard DW01 protection circuit (two of them, one for each cell).

author: kjr18
author_rank: Regular Contributor
timestamp: October 09, 2019, 11:41:50 am
content: Quote from: amyk on October 09, 2019, 11:29:55 amUnless those would be attached directly to cells, they still need to get this reset voltage. Using protected cells would be good enough.This device that OP mentions, needs them serial or in parallel?

author: skipper762
author_rank: Contributor
timestamp: October 09, 2019, 07:41:11 pm
content: We have thought about using protected cells, however, it relies on protected cells always being used.If the user was to replace the protected cell with an unprotected cell, the device would have no protection.Like kjr18 mentioned, the DWO1 BMS still needs the "resetting" charge voltage applied to release the Undervoltage lockout (UVLO).As well as using an off the shelf BMS we have tried rolling our own from a TPS2565 hot-swap controller as this has over current, short circuit and under-voltage protection (without the lockout).However, the Q current of the hot-swap chip is too high (250uA, about 100 times the sleep current of the rest of the device).

author: jhpadjustable
author_rank: Frequent Contributor
timestamp: October 09, 2019, 08:23:20 pm
content: Take a look atthe S-8250B datasheet, page 15. This is the behavior of the version with power-down function:QuoteFor the version without the power-down function:Quote

author: skipper762
author_rank: Contributor
timestamp: October 09, 2019, 09:58:10 pm
content: jhpadjustable, that looks promising.. have you ordered these chips before... if so where did you get them from?

author: jhpadjustable
author_rank: Frequent Contributor
timestamp: October 10, 2019, 12:30:41 am
content: I haven't, just ran across the datasheet on their site.I see Digi-Key stocks some very similar Diodes International parts, such as the AP9101CA (the A means no power-down mode) with a handful of setpoint/pin configurations. You don't need to worry about the overcharging protection limits since your application isn't charging them. Check 'em out, and if they don't suit, at least you have a name for the anti-feature that's giving you grief so you can find other protection ICs that don't have it.A fix that should work for many protection ICs is to add a momentary switch, maybe under the battery door somewhere, that shorts across the protection MOSFETs so that the protection IC wakes up. The user changes the batteries then presses the button. Solved!

author: skipper762
author_rank: Contributor
timestamp: October 10, 2019, 06:46:34 am
content: Now that more like itIf that chip works I owe you a beer or 6.We've ordered a few of the Diodes INC chip and spun a dev PCB... I'll post when I've had a chance to test it.The biggest problem with trying to find these bespoke chips is actually knowing what keywords to look for.We actually contemplated the button solution and even got as far as building it. However, the constraints of the enclosure mean a button is a bit hard to do and if the button ever failed it would completely bypass the BMS.We made a digital version of the above circuit, using an RC circuit to momentarily fire a load switch across the BMS when the battery was first installed.However, this felt extremely janky and was the last resort in case we couldn't find a BMS that did what we wanted.Thanks everyone for your help!