Battery jump starters

[DealorNoDeal]

Why does putting load on a nominal 12V battery cause the voltage to be a bit lower and charging it cause it to be higher, and when the battery is flat/dying why does the voltage drop much more rather than just the ampage?

Imagine an extreme load; a short circuit made of really heavy gauge copper wire. The voltage across the terminals will drop to nearly 0V. Other loads (starter motor, headlights) will just reduce the voltage by less. The reason this happens is, as Lance mentioned previously, the battery has internal impedance. Think of it as though there is a resistor inside the battery.

I don't know why charging leaves the voltage (initially) higher but it's probably a chemistry thing. I imagine other rechargeable batteries do this, not just lead acid.

The voltage drops more for a flat/dying battery because the internal impedance has gone up.

ps. Lance beat me to it.

[DealorNoDeal]

Not the best diagram but hopefully you get the gist.

[DoctorStrangelove]

Stone me, that's reminded me of "Sideways Sid"*, an Esteemed Customer with "A First Class Degree in Physics" who, however, wouldn't agree with said diagram & proceeded to argue the hind legs off a donkey that he was right & we were wrong.

Oh, Esteemed Customers, how we loved 'em.

I forget exactly what his argument was but his maths was wrong & our maths gave the Rite Anser.

*So named since he was wall eyed & you could never be sure who he was looking it. <- see wot I did there?

He never got further than the first year IIRC.

[d000hg]

Inneresting stuff. Made sense to me.

[Lance]

Inneresting stuff. Made sense to me.

most if it is GCSE Resistors in series - Calculating resistance - CCEA - GCSE Physics (Single Science) Revision - CCEA - BBC Bitesize

I did it at A level as there was no electronics O' level in my school. It's great simple practical knowledge.

[Paralytic]

most if it is GCSE Resistors in series - Calculating resistance - CCEA - GCSE Physics (Single Science) Revision - CCEA - BBC Bitesize

I did it at A level as there was no electronics O' level in my school. It's great simple practical knowledge.

I do love a bit of humblebragging

[DoctorStrangelove]

most if it is GCSE Resistors in series - Calculating resistance - CCEA - GCSE Physics (Single Science) Revision - CCEA - BBC Bitesize

I did it at A level as there was no electronics O' level in my school. It's great simple practical knowledge.

Look on the bright side, you did your A level in SI units.

Back in the dawn of time, mine included Imperial, CGS, and MKS units.

Who'd have thunk that the CGS unit of capacitance was the centimetre.

Not something that has ever proved of any use for me to know.

[WTFH]

Look on the bright side, you did your A level in SI units.

Back in the dawn of time, mine included Imperial, CGS, and MKS units.

Who'd have thunk that the CGS unit of capacitance was the centimetre.

Not something that has ever proved of any use for me to know.

You think that's bad?
Do you know how many Foals of power are required to move a Firkin along a Furlong in a Fortnight?

[DealorNoDeal]

I remember doing a practical at Uni, where you had to design an op-amp circuit. Once you'd calculated the required component values, you went and collected them from the stores technician. One guy couldn't understand the bemused look on the technician's face when he asked for a several Farad capacitor.

[d000hg]

most if it is GCSE Resistors in series - Calculating resistance - CCEA - GCSE Physics (Single Science) Revision - CCEA - BBC Bitesize

I did it at A level as there was no electronics O' level in my school. It's great simple practical knowledge.

Knowing the formulae and understanding the principles is trivial GCSE stuff. Deducing that the reason a battery drops voltage under load is due to internal resistance very definitely isn't GCSE level... obvious once you know of course.