MacGyver's UPS



  • Could any of you electrical engineering types tell me whether it's dangerous to supply (I think force-feed is more appropriate in this case) a regular PSU using car batteries? And yes I know about PSU's for carputers, but I'm really talking about normal, day-to-day desktop PSU's here.

    I'm asking because I was talking to some people about this job I had back in 2005, and the manager invented a very... interesting... kind of UPS. It was quite function during the time I spent there, but I'm also wondering about long term hazards.


  • Discourse touched me in a no-no place

     Well, you'd have to run it through an inverter first since car batteries supply 12vDC and regular power supplies don't take it as an input. However, it is quite alright. This is exactly what regular UPSes do - they're 12vDC=>120vAC inverters packaged with a small 12v lead acid battery (though they typically use sealed batteries so you can handle them roughly, and most car batteries are not sealed).

    A typical car battery wouldn't work well after more than a few complete charge/discharge cycles, though. You'd want a deep cycle marine/racing battery for that. 



  •  There is several DIY approaches for this:

    1. As weng says: Battery charger -> Battery bank -> Inverter -> ‘puter

    Some inverters take care of power source switch over from mains to battery.  This is basically system building a standard UPS..... for which commercial products have long since been developed and improved (e.g status communications, power lows and highs etc).

    2.  High voltage battery bank -> ‘puter.  

    It is possible to feed a large battery bank DC power directly into PC switch mode PSUs, for most PSUs at around 260Vdc however autoswitching PSU are less fussy on input voltage.  This approach eliminates the inefficiency of the inverter but it has its own issues: different PCs will have different PSUs leading to different drop out voltages, very cheap PSUs may have rectifiers that will not last so long, computer accessories and peripherals may not all work on DC (older wall wart linear transformers), switch over from mains power will be ..... interesting.

     3. Replace ‘puter PSU for DC type.

    Just run PCs from 12 – 48v DC from battery bank and continually charge battery/power pcs.  This is similar to telecommunications equipment.  Not cheap.

    4. Add batteries to an existing UPS.

    I have seen some place just connect more SLA batteries to the existing UPS, either on the ‘external battery’ connector or connecting directly to the internal battery bank.  Work well but not in design or accordance with UPS design – therefore no warranty.

    As weng says, car/truck batteries are NOT the right type. Should be sealed/gel lead acid.



  • @Renan said:

    Could any of you electrical engineering types tell me whether it's dangerous to supply (I think force-feed is more appropriate in this case) a regular PSU using car batteries? And yes I know about PSU's for carputers, but I'm really talking about normal, day-to-day desktop PSU's here.

    I'm asking because I was talking to some people about this job I had back in 2005, and the manager invented a very... interesting... kind of UPS. It was quite function during the time I spent there, but I'm also wondering about long term hazards.

    Google published their server designs a few years back.  They have a sealed 12V battery in every server instead of (or perhaps in addition to) centralized UPSes.  Of course, they probably have a custom motherboard design that doesn't have a 5V or -5V rail rather than using an inverter to run the power supply directly.


  • @Jaime said:

    @Renan said:

    Could any of you electrical engineering types tell me whether it's dangerous to supply (I think force-feed is more appropriate in this case) a regular PSU using car batteries? And yes I know about PSU's for carputers, but I'm really talking about normal, day-to-day desktop PSU's here.

    I'm asking because I was talking to some people about this job I had back in 2005, and the manager invented a very... interesting... kind of UPS. It was quite function during the time I spent there, but I'm also wondering about long term hazards.

    Google published their server designs a few years back.  They have a sealed 12V battery in every server instead of (or perhaps in addition to) centralized UPSes.  Of course, they probably have a custom motherboard design that doesn't have a 5V or -5V rail rather than using an inverter to run the power supply directly.

    I think they use a specific PSU for that. I had one in my car, which drew power from the battery to feed a mini-itx board in the carputer.


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