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I have a question with regards to surge and continuous amperage ratings as they apply to battery disconnect switches.

I recently had a battery kill switch installed on my light duty truck as a theft deterrent. As far as I can tell it’s the Pico P/N 5575PT. (Shop supplied the kill switch and parts)

I started doing some research after the fact and found that the specs on this particular switch are 350 amps of continuous DC and 500 amps surge DC.

I’m a bit concerned now since I’m thinking that the surge current would be equal to the CCA of the battery (Duralast AGM) which is rated at 900 amps. Continuous amperage would be from the alternator which has a maximum output of 180 amps.

With all of this being said I’m now afraid that the disconnect switch itself is below where it needs to be for starting, assuming the starter pulls all 900 amps every time.

The switch itself is inline or the negative battery cable which (correct me if I’m wrong) -1.5v less than the positive terminal.

I’m hoping I’m just paranoid and overreacting to this.

What’s the worst thing that can happen assuming this switch is truly not sufficient?

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  • \$\begingroup\$ If the switch can't handle the current it may melt or catch fire. It may also weld itself closed. However, just because the battery can supply 900A doesn't mean the starter actually draws that much. \$\endgroup\$
    – Drew
    Commented Jun 5, 2023 at 17:49
  • \$\begingroup\$ It's pretty hard to push 500A through reasonably sized copper conductors. It would take only 17ft of 12awg copper wire to limit the current to less than that. And that's with no motor at all, just a dead short. The battery itself will also have some ESR. \$\endgroup\$
    – Drew
    Commented Jun 5, 2023 at 17:55
  • \$\begingroup\$ Drew - thanks for the response, with that being said do you think the 500 surge amps rating of the switch is safe in my case or should I start looking for something with a higher rating? \$\endgroup\$
    – PhOeNiXpIoLe
    Commented Jun 5, 2023 at 18:46
  • \$\begingroup\$ Big diesel engines (4, 5 or more litres) can easily take 1000A to start depending on the age of the truck. Starters have moved on over time and are more efficient so if it is under 10 years old you might find that to be some 500 to 600 A. Cold Cranking Amps are usually defined as the capability of the battery at -18 deg C - so very relevant where I live in the mountains in Switzerland, not so critical for my dad in the South of England... \$\endgroup\$
    – Solar Mike
    Commented Jun 6, 2023 at 6:16

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I’m thinking that the surge current would be equal to the CCA of the battery

The Cold Cranking Amps (CCA) is not an output current. It "kind-of" represents the current available from a battery. What “cold”means is not clear.

When the starter is engaged, the starter motor can draw the battery voltage down as low as 9V. But it is the current that is required to turn the engine over. The bigger the engine the more current is required.

The battery voltage internally is still 12V but the internal resistance of the battery drops about 3V while starting.

The draw by the starter motor varies on the engine size. Here is a clip from Alvin Power showing the draw for several start motors. You can see that for most engines the current draw is well within these specifications.

enter image description here

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  • \$\begingroup\$ Hmmm, I always read CCA as being when the engine was cold (and the oil more viscous) rather than the battery. Living in the mild Irish climate may have prejudiced my understanding. Anyway, in most cases the engine and battery temperature will be the same (but maybe they test the rating with cold engines and warm batteries to help meet the specs!). \$\endgroup\$
    – Transistor
    Commented Jun 5, 2023 at 18:33
  • \$\begingroup\$ Russell - thanks for the response, the truck has an 5.7L 8 cylinder engine so I would like to say 175-225 amps is well within the power draw range of the starter. Given this information, would you agree that the 500 amp rating of the switch is well within the required surge amperage of the battery/electrical system? \$\endgroup\$
    – PhOeNiXpIoLe
    Commented Jun 5, 2023 at 18:51
  • \$\begingroup\$ @PhOeNiXpIoLe: I am just a voice on the internet. What I presented I found elsewhere. If that informtion can be trusted, then I would say that that the switch will not break. To be confident you should find a data sheet fir the starter, or perhaps it is in the owners manual or a repair manual. \$\endgroup\$
    – user319836
    Commented Jun 6, 2023 at 0:20
  • \$\begingroup\$ @Transistor: Thank you \$\endgroup\$
    – user319836
    Commented Jun 6, 2023 at 5:46

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