change over from charged to discharged battery using for starting of 100 kva DG set

[ghaudesh]

I have 2 no. 24V battery( Exide, PULSE) along with separate charger for each of them for starting a 100kVA Diesel generator (Cummins, Jackson) set.I have to manually change the battery when one of them gets discharged. I want to built a system to automatically changing from discharged to charged battery and these changeover circuit need to run with 220v dc supply i have as backup. I have searched a lot but got nothing. Please help.

 

[FvM]

A changeover controller must cooperate with the motor start controller to avoid changeover during start. I don't believe there's a standard product covering the requirements. It would be usually build with a small PLC. For the power switching, regular starter contactors are an option.

 

[chuckey]

Its a fiddly job but in principle it can be done. A couple of things, one is that the fuel cutoff relay must also be driven and most importently are you sure that if the battery chargers are switched off that the diesel starts? - must be checked!
So for long life, you must only SLIGHTLY discharge the batteries, don't run them flat. You will need a second solenoid and a pair of 24V driving relays with 50A contacts, this is so you can interlock the two solenoid drives so only one solenoid can be actuated at any given time. From these two relays there will be two wires, one is the normal "start" wire, the other the select A/B wire. For the fuel cutoff valve i would put in another relay driven from the Start signal, this relay takes the output of BOTH batteries via diodes and puts in on the valve when the start signal is present. Depending on its design, a current of 10+amps will be used.
Now for the monitoring system, use a supply from each battery, diode OR'ed and stabilised down to +12V. Use a OPamp to differentially compare the two battery voltages, with a very low gain, so a voltage difference of .5V will drive the output from a low to a high. There should be a large capacitor from the -ve input to the output*. this is to make the amplifier very slow to react to change in voltages from the batteries, else the drop in battery terminal voltage during cranking the diesel would cause a battery change over. Use a transistor to drive a relay to power the select A/B line.
Frank

* make this capacitor switchable, else it will drive you crazy during testing/setting up