Battery charging circuitry - help wanted
Posted: 14 Jul 2017, 19:47
As you may know, I have a long-running side project where I am reviving a classic 1980s 'luggable' Zenith laptop with modern-ish internals, to create a vintage yet useful laptop with an Alps (integrated dome) keyboard.
For this, I need to construct a Li-ion protection+charging circuit. Strangely though, this is not readily available, so I will need to construct it myself... without setting the house on fire. And since we have at least a handful of electrical engineers on DT, I thought I'd reach out and see if anyone would lend their advice.
My goal
is to build an internal battery pack with 4 li-ion cells in series (providing 12V-16.8V) using unprotected LiNiMnCoO2 cells (good balance between safety and specific energy), add a BMS for protection and a proper lithium-ion charger; and panel mount a barrel power jack so I can charge/power the laptop using a regular laptop power adapter.
This should not be *that* hard to find; after all, every laptop out there must include a charging circuit that does all of what I need.
I have watched every YouTube video about li-ion charging and DIY battery packs and frankly, 95% of the DIYers are building insanely unsafe battery packs consisting mainly of shrink tubing and hope. I would like my project to be a bit safer without having to remove my battery from the laptop every time I need to charge it.
So if you are an electrical engineer or if you have experience with this sort of thing, I would love to hear from you -- at this point I could use all the help I can get.
For this, I need to construct a Li-ion protection+charging circuit. Strangely though, this is not readily available, so I will need to construct it myself... without setting the house on fire. And since we have at least a handful of electrical engineers on DT, I thought I'd reach out and see if anyone would lend their advice.
My goal
is to build an internal battery pack with 4 li-ion cells in series (providing 12V-16.8V) using unprotected LiNiMnCoO2 cells (good balance between safety and specific energy), add a BMS for protection and a proper lithium-ion charger; and panel mount a barrel power jack so I can charge/power the laptop using a regular laptop power adapter.
This should not be *that* hard to find; after all, every laptop out there must include a charging circuit that does all of what I need.
| Challenge | Possible solution |
| Provide 12V 1A (continuous more like 400mA) to the LCD (anywhere between 9-18V will do) | Direct connection to the BMS output pads |
| Provide a stable 5V 3A (continuous more like 300mA) to the single board computer | DROK Buck converter from the 12V rail to 5V 5A |
| Protect the Li-ion cells from over-charge, over-discharge, over-current and short circuits | BMS circuit board |
| Charge the cells correctly (ideally w. balance charging, but CC/CV will do) | 4s li-ion charging board, or a BMS with built in charging logic |
| When adapter is plugged in, power the 12V+5V rails from the adapter only while charging the batteries (because powering them from the BMS would interfere with the charging logic and potentially provide insufficient voltage during the CC phase) | Normally-closed Solid State Relay(s) cutting off battery power to the load |
| When adapter is plugged in or out, and power switches between adapter and battery pack, single board computer must not lose power for even a moment, as it could reboot the system | Two 2.7v 10F supercapacitors in series at the 5V load should be able to handle a brief outage or ripple on the order of a few hundred milliseconds |
So if you are an electrical engineer or if you have experience with this sort of thing, I would love to hear from you -- at this point I could use all the help I can get.