1. If battery is not fully charged and you disconnect clamps from drained battery then add jump leads directly from car with health battery,l to drained battery terminals, will this charge the drained battery and if so how long would you need to leave it for?

  2. If you try to jump start a car but there is a problem with the no start cars main relays or starter motor, and you try to jump start car by putting jump lead cables on clamps of no start car, would the battery still charge? I imagine It will still not start charge since the relays are not working and some circuit is not complete or will the jump cables being on the clamps which are on the battery be enough even if the car does not starT?

  1. If the answer to 1 is it should charge up and it doesn’t, would this mean the battery is at fault?


  • You should never charge a dead battery using another donor vehicle, it can damage the alternator of the donor car, you should always use a battery charger on dead batteries. – Moab Sep 2 at 14:52
  • What do you mean by a ‘dead’ battery and how do you know it is dead? I have always jump started cars where the battery drained overnight but u don’t know if it was considered dead or not? – James Wilson Sep 2 at 14:54
  • 2
    I think what Moab is getting at, if you have a battery charger use it instead of a donor vehicle. Just better for all pieces/parts involved. – Pᴀᴜʟsᴛᴇʀ2 Sep 2 at 14:55
  • Having started cars hundreds of times this way in the dead of Winterpeg in my youth, old cars will start if your cables have very low contact resistance with high force and contact area. Never cheap clamps. I would even park where I could reach a donor car in case I got stuck without a charger. the alternator gives a couple extra volts but current is limited by your clamps. Use V+ and chassis in case V- is corroded or loose. Try to twist terminal in case it is loose slightly to test. Squeeze clamp aggressively might be necessary to cut thru oxide insulation. 1 mOhm ESR is hard to get with – Tony Stewart Sunnyskyguy EE75 Sep 3 at 16:21
  • The alternator diodes are always reverse biased doing this but you can generate Flyback voltage from cable inductance from intermittent contact after high current charging, so clamp quickly then twist , squeeze to increase contact area. Yet this is not much different than ACU flyback. we had 9Vdc under load starters on wheels (heavy) for -40V that would generate good spark and never failed to start even if you had to wait a minute. These dead battery symptoms are due to under charging from some diodes blown or a weak cell that you can test with a hydrometer. – Tony Stewart Sunnyskyguy EE75 Sep 3 at 16:23

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