Today I investigated whether a li-ion jump starter could work for my car. It's a hybrid vehicle and the battery is in the trunk (which can't be opened if the battery is flat). Under the hood, there's only a single positive terminal. The idea is that:
The car shouldn't be used to jump start other cars. The 12V lead-acid battery is not a starter battery but rather a different type of battery that can't be used to start engines (low peak current). Probably the wiring to the positive terminal under the hood is also not enough for jump starting other cars. The engine is started by the high-voltage hybrid battery.
When the car needs external power to start due to flat 12V battery, the external power is only used to boot up the computers and enable all 12V accessories. When the computers have power, they connect the high-voltage hybrid battery via a relay, and a DC-DC converter starts charging the 12V battery. The engine is started by the high-voltage hybrid battery, to charge the HV battery.
The worry I had was whether there is a good ground near the positive terminal (that is inside the fuse box), because there's really only positive terminal there and no negative terminal. With jumper cables, as the cables are separate you can connect the ground very far away from the positive terminal, for example to some bolt on the engine. However, with li-ion jump starters, the cables are short (to keep weight down) and the cable span is usually at most half a meter or less. So the good ground has to be way less than half meter away from the positive terminal.
So, I used my multimeter to test various metallic objects near the positive terminal for good ground. I connected one end of the multimeter to some part of the engine, and the other end of the multimeter to some metallic object near the positive terminal.
To my surprise, it seemed that finding good ground was very hard. Usually either of the multimeter probes had poor contact to the metal, probably due to oxidation. Very often, the multimeter read "above the range" (I used the 200 Ohm range), getting 20 Ohm was possible but getting below 1 Ohm took some real work. Finally I managed to locate a metallic object where getting good ground (below 1 Ohm) was reasonably easy if the probe was pressed hard enough, and the ground was close enough to the positive terminal to be usable with li-ion jump starters.
I consider this surprising because I have used an older car I used to have to jump start two other vehicles, and another older car I used to have was jump started from another vehicle. All three jump starts with traditional jumper cables worked just fine, I got good ground on the first attempt.
So my question is: when jump starting a car, and connecting the negative cable to a metallic grounded object, how common is it to have poor contact due to oxidation and needing to try again on some other metallic grounded object?
Should I carry a fine grit sandpaper with the li-ion jump starter to ensure I can always have good ground by removing oxidation layer from the grounded object?