2S vs 2P 18650 for low power electronics device - Page 1
Forum section: electronics / projects

author: skipper762
author_rank: Contributor
timestamp: May 09, 2019, 12:04:33 am
content: Hi Everyone,I'm developing a low power electronics device that I'm hoping to last a significant amount of time (1 year +).I'm looking to use two 18650 cells as the are pretty ubiquitous, low cost and power dense.The batteries will be removable from the unit for charging/replacement - also justifying the use of 18650s as chargers are common.The radio in the unit can be powered directly from a LiPO cell (3.0 - 4.2 V), the rest of the circuitry is 3V3 and requires a regulated supply.During TX, the radio can draw bursts of up to 800mA.I am unsure whether to run the batteries in a 2S or 2P configuration.Running them in a 2P gives me a voltage that is correct for the radio and switching regs that produce 3V3 from a 1S voltage are common.However, as the batteries are charged out of the unit, I'm worried about cross charging (as they will be different voltages) when they are installed back into the unit and unequal draw from the batteries.Cross charging can be solved with a Schottky diode in series with each cell, however, this will drop the cell voltage 0.3~V reducing the operational life of the batteries and wasting power through the diode.Running them in 2S means I'll need a much grunter regulator to supply a regulated voltage to the radio and remaining circuity.I'll also have to integrate a more complex BMS chipset.These devices will need to get certification to work in Europe, so I'm trying to do this the "right" way.If you made it through all that - well done, I have the following questions for the hive mind:Am I missing anything?What would you do?Any other resources, datasheets, idioms or random thoughts that might help me?I am personally leaning towards the 2P approach as it makes the power regulation stage simpler and lower loss.Thanks in advance.

author: mariush
author_rank: Super Contributor
timestamp: May 09, 2019, 12:48:40 am
content: put them in series and use a single 3.3v switching regulator.Or, maybe 3.6v and then use small ldo for 3.3v if you want minimal ripple/noise.You could add a DC In jack to use a 9v or 12v wall wart adapter and add a battery charging circuit insidefor ex MCP73844 is relatively cheap, 1$ each if you buy 100... 1.3$ if you buy 1:https://www.digikey.com/product-detail/en/microchip-technology/MCP73844T-840I-MS/MCP73844T-840I-MSCT-ND/8637222See page 15 in datasheet, besides chip you only need mosfet and bunch of passives... cheap:http://ww1.microchip.com/downloads/en/DeviceDoc/21823D.pdfMCP73842 would also work. Or MCP73213(bit more expensive but no mosfet needed, no dealing with thermal sensor:http://ww1.microchip.com/downloads/en/DeviceDoc/20002190D.pdfSkyworks AAT3663 is also cheap, but may be hard to source it:https://www.digikey.com/product-detail/en/skyworks-solutions-inc/AAT3663IWO-8.4-2-T1/863-1544-1-ND/4753078Could use two low drop diodes so that your switching regulator auto picks the dc in power when adapter is plugged in. Since DC In would always be higher than battery output (8.4v) it's easy...A 12v 1A adapter could easily top up the batteries at let's say 500mA, while still having 3-4w available for your circuit.Probably even 9v 1a adapter would work.Now you just have to make sure that switching regulator can do 6v..12v +/- 20% -> 3.3v..4v at 800+mA - better make it at least 14v input voltage.The microchip battery charger ic can tolerate max 13.5v so up to a 12v adapter would be safe.

author: sleemanj
author_rank: Super Contributor
timestamp: May 09, 2019, 01:08:35 am
content: Since the cells will be user replaceable individually I would go parallel but incorporate protection (eg DW01+8205A for the typical Chinese solution) into your device for each cell rather than the diode drop, this will serve two purposes, firstly protect the cells from over discharge which you need to do anyway, and second it will limit any serious cross-charge with unequal cells as in such a situation the supplying cell will trip-out.And of course ultimately it also protects the cells against some failure in your device from shorting the cells, which would be bad too.

author: skipper762
author_rank: Contributor
timestamp: May 09, 2019, 01:35:21 am
content: mariush:Given the location and environment that these devices will be deployed in, adding charging ports/ circuity would be no use as there is no mains power nearby.sleemanj:The BMSs would manage the stabilisation, would I need to worry about unequal discharge after the batteries had equalised?

author: sleemanj
author_rank: Super Contributor
timestamp: May 09, 2019, 01:49:40 am
content: > would I need to worry about unequal discharge after the batteries had equaliseOnce they have reached an equal level of charge, then by definition of being parallel they will remain equal.Your concern is controlling how they reach that equilibrium.If each cell is protected, and the cells are equal, then they should by rights discharge equally. If they are protected and are unequal then for reasonable imbalances one will charge the other until it is equal. If they are protected and are unequal for unreasonable imbalances then in theory the charging one should trip-out (disconnect) when it attempts to supply too much current to the uncharged cell.

author: skipper762
author_rank: Contributor
timestamp: May 09, 2019, 02:58:28 am
content: sleemanj:Reading my sentence back, I realise I was being a bit of a nugget.I'll look into some BMS chipsets and see how I get on