Electronic – Confusion regarding jump starting a car

The use of 4 x AA Alkaline would usually be safe BUT does exceed the USB spec and damage may occur in some cases. I have seen IC's in this role with max operating voltages of 5.5V (which is ludicrous) - you'd hope designers had more sense, but it can't be guaranteed.

While some devices may use converters between charge input and battery, many don't (probably most). A LiIon battery has max charging voltage of 4.2V so a 5V nominal USB input will usually meet this need with enough headroom for a linear regulator.

An Alkaline cell can be nearly 1.6V when fully charged - about 1.55V is common or 6.2V for 4, and up to 6.4V may be seen. There is not much energy in this initial high voltage "tail" and voltage falls to 1.5V or below very quickly.

So, you should be safe, but YMMV, alas.

A solution would be to use an LDO (low dropout voltage) regulator OR a clamp regulator which takes the peak energy out of the battery or a series diode to drop 0.4 to 0.8V (Schotky / Silicon).

• LDO is best solution but you want as little drop as possible.

• Clamp to drain peak battery voltage is unusual but viable. A zener could be used but is too inexact. An eg TL431 clamp regulator in a TO92 or other largish package (to get OK dissipation capability) ould do. A TL431 plus a transistor would be safer.

• Series diode is cheap and easy but prevents full battery use. Say minimum usable battery voltage is 4.6V (may be higher). At 1.15V/cell there is still some battery capacity left. Adding a Schottky diode increases minimum battery voltage to 4.6 + 0.4 = 5V or 1.25 V/cell. Some capacity wasted. At the top end a 0.4V drop diode results in Vbattmax of say (1.55V x 4 - 0.4) = 5.8V or 1.45V/cell."Almost certainly safe".

Using NimH works but is more marginal at bottom end and safer at top end. At 4.6V, V per cell is 1.15V where NimH still has modest energy left. At top end Vmax = say 1.35V, maybe 1.4V for short periods at start. 4 x 1.4V = 5.6V. Very probably safe.

"Ground" is just a code word which, in this case, refers to the "current return common" circuit node. There is a complete circuit because everything electrical in the car, such as the starter motor, also connects to ground in order to return current to the minus terminal of the battery through the ground. The car's chassis is used for this return network, and so the entire chassis is an extension of the minus terminal of the battery.

During jump-starting, we connect the boosting battery to ground rather than to the dead battery's - terminal for the simple reason that this provides a more direct return path to the good battery which is powering the dead car: the return current does not have to travel through the dead battery's minus terminal hookup cable and then to the jumper cable, but can go directly from the chassis ground to the jumper cable.

A more direct return path allows for better current flow and less voltage drop, like plugging a big appliance directly into an outlet, rather than via an extension cord.

In case you're also wondering why the plus jumper connections are made first, then the minuses. This is because there is no harm done if you leave the minus jumper dangling in the chassis of the car. Anything it accidentally touches is likely to be ground. If you connect both alligator clips on one end before connecting the other end, the other end is now live and you can accidentally touch the clips together to create a short circuit. If you connect the minuses/grounds first and then go to connect one of the pluses, you can create a short circuit, because the opposite side plus is probably dangling and touching something that is grounded.