I'v been away for quite some time, but recently I noticed the SB urge again so I decided to start with some engineering for the good cause.
My plan is to build a battery powered electromagnet lock combined with an Arduino (ESP8266) controlled relay. It's still early days and I'm trying to figure out how much battery I need. I'm looking at 18650 cells; preferably a well known / decent quality one - for instance the Panasonic NCR18650B which is rated at ~3350mAh. Ideally I would go with 2 cells given the space they need, but I'm trying to figure out if they can deliver enough and how to avoid damaging them. Since this is not my specialty, I hope to get some directions from you.
I plan to use the following components
- electromagnet draws ~250mA
- ESP module + relay draws ~120mAH
The module are designed to run at 12V input. Using two 18650 cells adds up to ~7.4V, which need therefore to be boosted in order to make 12V. It looks like this buck/boost module could work [1]. This module also draws some power and there is of course loss in converting, so my guesstimate would be that the entire thing draws around ~420mA.
An online battery calculator tells me these cells give enough power to last 5,5 hours at that demand - is that right? Are two of these cells powerful enough to run this setup for that long? I'm guessing these cell in reality deliver less current so it's probably a little shorter, but even so I would prefer them even to run much shorter, just as a safety feature should my Arduino fail. Perhaps a 2000mAh would even be better given these exist form a well know vendor.
Also how can I protect them from dropping below a cut-off voltage in order to avoid damage?
Lot's of question - hope I'm asking the right ones.
Looking forward to your thoughts.
[1] https://www.aliexpress.com/item/15952-F ... 30697.html