I am considering making a 12V UPS for an ATX computer. I would like to know if my concept is sound, (at least halfway so :) )
It's insane that commercial UPS rectify+step-down mains to charge a 12V/24V battery, then invert/step-up this battery voltage back to mains voltage, only for it to be again recified/stepped-down to 12V DC inside the computer PSU. I want something more efficient.
- Use existing ATX power supplies (only the 12V rail)
- Use existing salvaged Li-Ion cells (not LiFePO)
- Super minimalist approach to component count and circuit complexity
- Maintain high efficiency by avoiding DC-DC conversion losses
To avoid losses, no DC-DC conversion is to take place on 12V. The ATX spec allows ±5% on the 12V rail. Let's be very naughty and stretch that to -10% on the low end, giving us 10.8 to 12.6V. Let's put 3 Li-Ion cells in series (including a suitable BMS).
For longevity and ease of finding a suitable power supply, we shall charge the cells to only 12.0V ≈ 4.0V/cell. On power loss, the cells discharge and under-voltage protection needs to kick in at 10.8V ≈ 3.6V/cell.
The 3.6V-4V range gives us slightly less than half of the cell's energy capacity, but that is sufficient. In turn, the longevity of the cells will increase fourfold. (Source: Battery University's Discharge characteristics and Prolonging articles)
For simplicity's sake, I'd like to avoid having to switch to the battery on power loss (i.e. offline UPS). Instead, the battery shall be permanently paralleled to the power supply (online UPS). Diodes to avoid having the battery back-drive the PSU shouldn't be necessary, as SMPS already have diodes on their outputs.
Charge current limiting
The battery can't exactly be parallel to the PSU, otherwise there would be no way to limit the charge current. Something like this circuit will be necessary:
ATX power is more than just 12V. Areas of concern:
3.3V and 5V rails
These are also required. They can either be supplied by a mini DC-DC converter behind the actual PSU/UPS, such as as these Inter-Tech or Mini-Box products (since those rails need very little power on modern systems, the switching losses are negligible)
Or by using a modern ATX12VO or pseudo-12VO mainboard, such as an Asrock Rack one which only uses the 12V rail.
The UPS would need some minimal logic to handle the ATX
PS_ON pin correctly. It should monitor
PS_ON and switch on/off accordingly, aswell as passing this signal through to the actual PSU.
The PSU signals to the mainboard when voltage rails have stabilized. Obviously this signal will drop when the PSU switches off during power loss, resetting the PC. The quick&dirty solution would be just tying this signal to 5V directly.
Will this work? Apart from being naughty and over-exploiting tolerances, are there any glaring mistakes or omissions?
Thanks a lot!