Upgrading Jetech LXM-500 UPS
After finding this old UPS I’ve decided to upgrade it with a 12V 55A car battery (the original battery was a 12V 5A).

Schematic of the comunication cable with PC
The communication cable used by this UPS is a straight pin-to-pin DB9 (Serial, RS232, etc).
The software is not available on Internet (trust me, I’ve been searching for hours before I found it in an old dusty box).

All files are the same (for search engine optimization only):

Replacing Original Battery with Car Battery
As the UPS can handle 500VA/250W at least I must take care of the ampere capacity of the cables.
The original cable was an 12 AWG cable which it can handle 25 ~ 30 Amp (source). As I like to take precautions I use an 10 AWG cable which can handle 30 ~ 40 Amp.

The easy part is to connect the 10 AWG cable with the battery using standard connectors. The hard part is to connect in a cheap and less complicated way both cables without soldering. For this purpose I’ve used two Feed Through Terminal per cable (they are rated 380V and 10A, so using two I can pass 20A).

LXM-500 UPS Circuit

LXM-500 UPS Circuit


 
Connector

Connector


 
Battery with Connectors

Battery with Connectors

Removing the annoying beep sound
When this UPS is running from the battery supply it emit an annoying beep to alert the user. To ‘correct’ this you can simply remove the dammed buzzer.

Removed Buzzer

Removed Buzzer

Safety
As you may know, if you short-circuit a car battery (lead-acid) it can generate Hydrogen and Oxygen gas. With a little spark you can have a big fire or even an explosion. To avoid this try to put the battery it a ventilated container, secure all connections and avoid the access of other people (children, stupid guys, etc). As a container I’ve used a toolbox with some baking soda at the bottom (the battery can leak acid so you can neutralize it with this).

As some finals words I don’t take any responsibility on this home-made super UPS and try to do things in the right way.