Assemble a very long autonomy UPS


My network and home automation installation have become essential to life in our home. Without smarthome controller, the automatisms disappear. Without a server, no more mail, music, TV, internet access :-(.  A power breakdown  is therefore an increasingly feared event, but more than that the deterioration of some device due to power hazardswould be disastrous.

After an unexpected shutdown of the server probably due to an overloading on mains, I decided to set up my long autonomy uninterruptible power supply whose elements were waiting patiently.



  • UPS unit being able to backup power supply during several hours, ideally 4 hours (maximum time for a minor issue repairing  at ERDF )
  • Maximum power sufficient to power eedomus, server and network rack, so around 75 Watts of power.
  • Limited cost
  • No risks on the setup with mains

Requested parts

We can find some regularly sold when internal battery is dead.
I’ve bought personnaly 2 of those for 20€.





A car battery
I’ve selected a 60Ah battery myself.
Around 80€ in a supermarket




Battery terminals
Around 6€ in a supermarket




Multi-strand cable 1,5mm2 black and red.
2 meters of each color
Around 3€ in DIY store





A battery tray for camping cars
The tray is dedicated to the storage of batteries, and resistant to possible leakage of liquid
Around 25 € in specialized camping  shops



Conventional computer UPS use lead-acid batteries. In small models, those are 13.8V batteries, so the same voltage as a car battery.
The APC 500 is one of those old models. It should also be noted that this is an offline inverter, thus providing an average level of protection: the load is directly connected to the mains in normal conditions, and the power supply is switched to the generator from the battery only in specific case (Power failure, overvoltage). This switching takes a few milliseconds which makes the difference with professional UPS. However, an offline inverter provides a sufficient level of protection for a domestic installation.

It is therefore possible to install an external car battery outside of this UPS housing. There are however essential elements to be taken into account:

  • A car battery is not waterproof, it is degassed. It is therefore imperative to store it in an airy room
  • The inverter is capable of delivering 500W which represents 40 amps on 12V. It is imperative that the cables connecting the UPS to the battery make are at least 6mm2. I put 4 stranded cables of 1,5mm2 in parallel for each poles, ie 4×1,5mm2 = 6mm2. If you have doubts, add cables !
  • The voltage of a battery is low, but the peak currents that can be supplied are enormous (> 400 amps). It is imperative to carry out the modifications in complete safety by ensuring at each stage that the insulation between the two poles is good at all points of the system.
  • A battery may have a defect in time, such as sinking. The acid can cause a fire. It is necessary to store the battery in a tray designed for this purpose and resistant to acids.
  • The battery charging circuit is not designed for such a battery size. The full charge of the battery therefore takes nearly a week. However, it doesn’t create any particular problem in normal use.


The changes to be made are as follows:

  • Remove the old battery from the UPS.
  • Unscrew the old battery cables from the electronic board
  • Fix 2x 4 cables of 1.5mm2, length about 35cm, to the battery terminals
  • Welding new cables instead of old battery cables
  • Make vents on the sides of the inverter in the metal housing. The UPS is not equipped with a fan because it works normally just during 15 minutes.
    To do this, make holes on each side of the case located there :
  • Put the battery in the battery tray
  • Cut the contour of the tray cover so that it can fit over the battery. This will be the isolation between the battery and the UPS
  • Put the UPS on the top of cover on the battery
  • Secure the terminals on the battery. It is preferable to add a little grease inside the terminals before mounting to ensure a better electrical contact.


For a price of about 130 € I now have an UPS allowing me to provide 75 watts during 3 hours, tested and approved.
My network and server bay (including server, eedomus home box, ethernet switch, adsl router, wifi access point) and radio alarm clocks in the bedroom are now protected from over-voltage and breakdowns.

There remains a problem of location. This set is bulky and heavy (~ 20kg), it must be placed on the ground.
In my case his location was planned in a clearance under the stairs, not far from the electrical panel and the server bay. A specific circuit for the clock radio was also reserved.

One day of plaster and electricity and the whole installation was functional.







Vincent Recipon

Propriétaire de ce blog. Owner of this blog.

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4 Responses

  1. Bagu says:

    Bonjour, quels sont les caractéristiques à rechercher pour le choix de l’onduleur ?
    Car il est compliqué de trouver un apc 500 dans les prix énoncés dans votre article.
    Or je dispose de plusieurs autres onduleur de cette marque, cela serait impec si je pouvais m’en servir.

    • Bonjour,

      Il faut que l’onduleur que vous achetez soit équipé d’un batterie au plomb de 12V.
      De ce que j’ai pu voir, les onduleurs utilisent toujours des batteries au plomb (moins chères). Par contre leur tension varie en fonction des modèles. Si cette information n’est pas précisée directement sur les spécifications de l’onduleur, allez voir du côté des batteries de rechange pour le modèle que vous voulez achetez et vous aurez plus de détails.


  2. enr37 says:

    très intéressant mais du coup, un peu plus de détails photos auraient été nickel sur la partie “bricolage” : soudure, perçage, toussa…

    • Bonjour,
      Bien vu tu as l’oeil. Pour la petite histoire cela fait un bon moment que l’onduleur était prêt. Les photos de son montage ont disparues avec la mort de mon précédent smartphone 🙁
      Il ne reste que le récit et les éléments d’achat, pas du tout envie de tout ressortir pour réaliser de nouveau des photos…

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