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I'm trying to size a bulb to limit DC booster pack current (harbor freight viking lithium) from overloading the 12V aux port (cigarette lighter) wiring which is fused at 15A. I'm ideally looking to achieve 6-10A range; either slightly below 6A so that the small charge port can handle it, or as high as 10A and use the booster clamp port rated for ~400A (hence needing a limiter).

I'm currently using a 12.8VDC 100W bulb in series via the positive lead from the booster pack to the 12V aux port. If I connect directly (no bulb in series) using the 6A charge port, its overload alarm activates when turning the vehicle ignition to run (glow plugs active, diesel during winter scenario). Adding 100W bulb does not trigger the alarm, so it's obviously pulling less than 6A. I am connecting two DC sources in parallel.

When it's cold, the car battery measures ~12.25V and that drops to 11.4V or less when glow plugs are active. When cranking the starter, it sags to 8-9V, even though it's a relatively fresh/new battery (~80AH). The boost pack is typically 13.5V no load, bulb is slightly lit when glow plugs active due to increased voltage potential from booster pack to car battery. The potential obviously increases when starter is engaged (i.e. brighter bulb response).

Until I snag a decent clamp meter or accurate DMM, there are only estimates as to the boost current allowed with the bulb in circuit. I noticed the other Q&A showing a nice chart with 10 Ohm resistor vs bulb cold/hot resistance.

Is there a formula to help size the optimal bulb, or perhaps electrically violating an AC bulb for better performance? Let the Coulomb crunching commence...

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    \$\begingroup\$ That's a very confusing explanation. Can you just simply say if you are trying to charge the lithium battery starter pack or are you trying to help out your car's starter by running DC into your cigarette lighter port? If it is trying to start, why don't you just clamp it onto your battery directly? \$\endgroup\$
    – Vince Patron
    Commented Jan 8, 2018 at 22:37
  • \$\begingroup\$ Or power the glow plugs from either battery via dual diodes. (higher voltage supplies the glow.) \$\endgroup\$
    – D.A.S.
    Commented Jan 8, 2018 at 22:50
  • \$\begingroup\$ Are you trying to charge the booster pack from the car or trying to power the car from the booster pack? \$\endgroup\$
    – Andrew Morton
    Commented Jan 8, 2018 at 22:57
  • \$\begingroup\$ using booster pack to supplement healthy battery or at least counteract voltage sag that lead-acid cells are renowned for when in low temp environments. key issue with dc sag/brownout is less voltage for starter yielding less rpm to start motor. \$\endgroup\$
    – beyond electric
    Commented Jan 8, 2018 at 23:21
  • \$\begingroup\$ Seems like supplementing 100A to 200A starting current with 10 to 15A from cigarette lighter really won't add much. It would be better to put a separate battery/power system that is to the glow plug and ignition circuits so that those can run at full voltage. This secondary system can be charged by the vehicle when it is running. \$\endgroup\$
    – Vince Patron
    Commented Jan 9, 2018 at 22:21

1 Answer 1

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100W 12.8Vdc bulb
R(hot)= V²/W [Ω]
R(hot) = 12.8²/100W = 1.64 Ω , 0.16 cold, 0.5 ohm dim? ( verify?) Cramp Amp rating is done at 7.5V , you are getting 8 or 9V.

Thus for Glow plugs load during hard starting I would suggest your setup with 100W in series is a reasonable simple solution.

If you want more voltage into the glow plugs, say so...

Impressions:

I think the boost pack is intended to fast charge the dead battery in minutes but not directly run the starter motor in the cold at 500 CCA. Adding the 100W lamp only supplies a small percentage of power to the glow plugs and starter motor during the actual start, while no current limit triggers the overcurrent warning. You may correct me if my impression is wrong.

I think if the LiPo boost pack charges a cold battery with 100Amps that if this raises the electrolyte temperature it would be useful to restore the cold battery ESR and efficiency.

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  • \$\begingroup\$ Many of those lithium-iron boosters can start the car within seconds of attachment. I see ratings of 200 Amps or more on them. The battery capacity is only in the 100 Wh range but the ESR of those batteries are so low that they so can handle very high starting currents. \$\endgroup\$
    – Vince Patron
    Commented Jan 9, 2018 at 0:32
  • \$\begingroup\$ it depends on the battery condition greatly. If the battery is weak and undercharged or start current is low from warm weather, then I agree that is possible. If the battery is drained but has a low ESR, or is in a start mode when cold then it will be as the OP has stated with overload alarm activated. It all depends on actual cranking amps and V drop/ESR ratios \$\endgroup\$
    – D.A.S.
    Commented Jan 9, 2018 at 0:49
  • \$\begingroup\$ Yeah, I saw a consumer reports test where they "froze" (as in ambient to 0 deg-C, not frozen electrolyte) both car battery and booster battery pack and it did not work well. Of course, booster battery pack at room temp did much better. They didn't "freeze" the engine though. That would definitely be a much bigger load for the starter. \$\endgroup\$
    – Vince Patron
    Commented Jan 9, 2018 at 22:25
  • \$\begingroup\$ on youtube jerry bskorner has some torture test vids on power pack performance as well as verifying some of the claimed output abilities. froze a few of them, left outside during canadian winter, then tested their output ability. \$\endgroup\$
    – beyond electric
    Commented Jan 10, 2018 at 10:27

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