Killed two 500A TE Kilovac contactors with only 250A. How is this happening?


New Member
Aug 8, 2021
Hi everyone.

Strange issue I'm having, I have a 1200AH 24v LiFePo4 battery bank connected to four Victron Miltiplus 24/3000/70 charger/inverters. I'm using a REC BMS and a TE Kilovac 500a contactor.

After charging the bank at around 250a for 3.5hrs I discovered the contactor would not close after opening the circuit. I could hear the actuator inside but no current passed across the terminals. I called my vendor and they RMAed me another. This time, after a couple hours the opposite happened, the contactor was now stuck closed even without power.

I discovered on both contactors that if I 'worked' them on the bench rapidly with +24vdc they would free up and work again. It looks like both got stuck from what I can only assume was overheating.

I am CERTAIN I did not exceed 250A-280A and these are rated for 500A.

Any idea what's going on here? Are there any phenomenons where multiple chargers being combined can create an electrical situation that results in heat generation or some sort of harmonics? These terms may not be applied correctly, I'm just at a loss as to why this has happened twice in a row. Doesn't seem possible to be two faulty units, it must be me, but what?


Happy Breffast!
Oct 26, 2021
Sounds like one corroded and couldn't make contact.

Sounds like the other lightly welded itself closed.

Should be no issue charging with multiple chargers.

They are marketed as, "lowest cost sealed contactor in the industry"

There might be a reason why they're the "lowest cost".


Solar Addict
Sep 27, 2020
I have a Kilovac contactor on my system EV200AAANA- no problems with it working - it’s been on for 6 months. I know if it tries to connect or disconnect under a large load that can cause problems. These are rated for 9-36v - so should be no problem on the voltage.

Do you have a pre-charge circuit. The spark from one inverter starting up can be tremendous - four - four times as much! If you do have a pre-charger - maybe adjust the time for longer.

Good luck figuring it out.


May 20, 2020
You are energizing your system with the contactor.. that is a big no-no.. Before engaging the contactor, short an old fashioned standard 60 watt incandescent light bulb across the contactor's terminals. The light bulb will glow, then the glow will fade after a few seconds.. hold for an extra 3 to 5 seconds, then engage contactor and remove bulb.

What is happening is that the contactor is closing and the capacitor bank inside your inverter/charger is sucking up 1000 amps or more for like 10 milliseconds.. but this is enough to cause severe arcing inside the contactor that exceeds its ratings. Each time the contacts arc, they blow away material, or can weld themselves together.

Its a really good way to destroy contactors if that's your goal...

For convenience, you can wire the contactor through the bulb with a momentary (push and hold) switch.... this way, you just push the button and hold for 10 or 20 seconds before engaging the system.

Some folks visit ebay and spend money on fancy power resistors in the 40 to 100 ohm range.. but a light bulb actually kind of works better because its cheaper and provides some visual feedback of the process working.


Offgrid Cabineer, N.E. Ontario, Canada
Oct 29, 2019
Rural NE Ontario Canada
REC BMS has a Pre-Charge Module, this is required for ALL Lithium Based battery systems. THEY KNOW THIS why they did not tell you... you have to ask them.


Pre-charge unit is a device that charges the input capacitors of the system components before the main contactor switches on. Pre-charge device eliminates high inrush currents at the switch-on of the contactor and prolongs the contactor lifespan dramatically

  • low power consumption
  • integrated 66 Ω power resistor
  • 2 A contactor coil drive
  • 120 V DC max system voltage
  • time set of 2/4/8/12 s delay
  • reverse polarity protection
  • over-voltage protection
  • suitable for 12, 24 and 48 V systems (11-68 V DC)


New Member
Aug 8, 2021
Hi All,

Thank you all for the great feedback. Originally I did NOT opt for the pre-charge module. The datasheet for these contactors even at high inrush current showed very good life expectancy, even more so at lower voltages like 28vdc. I assumed it was an opportunity to up-sell an unnecessary accessory. After the first failure, I figured I'd add one for the $60 they cost.

The new contactor has iterally been used ONCE and that was WITH the pre-charge module in place.

If I can get it to work reliably again this morning I'll measure current through the pre-charge wire and see if 4-seconds is enough to satisfy demand. If not, I don't see anything in the documentation to increase this?? Maybe the correct setting is 4 x 4 seconds in my case?


Renewable Energy Hobbyist
May 3, 2020
Kenwood, California
Any idea what's going on here?
I welded the contacts on a Kilovac contactor once by having an erratic trigger signal that caused the precharge circuit to be bypassed and the surge welded the contacts. Is your trigger voltage 24 volts or 12 volts?


Solar Enthusiast
Dec 22, 2020
I haven't heard the best things about Kilovac contactors, nothing concrete but a lot of hearsay. Tesla was using them in the early model S units but ended up switching to gigavac. Gigavac is what I'll be using in my system, and they are pretty affordable too. $100 for my 48v coil unit, with optimizer built in, and aux contacts.