Battery jump starters

[Lance]

I'm an electrical engineer. What's your qualification?

If you're an electrical engineer, why did you say

The problem with modern cars is the parasitic drain when the vehicle is not being used. I measured this on my Audi and it was about 100mA, which is 2.4A/day or 17A/week

?

100mA is 100mA and if it's all day it's still 100mA. You're confusing current draw with energy.
100mA draining for an hour is an energy drain of 100mAh.
So in a week you have drawn 16.8Ah.

/pedant but you did say you were an electrical engineer and this is pretty basic stuff.

[DealorNoDeal]

If you're an electrical engineer, why did you say ?

100mA is 100mA and if it's all day it's still 100mA. You're confusing current draw with energy.
100mA draining for an hour is an energy drain of 100mAh.
So in a week you have drawn 16.8Ah.

/pedant but you did say you were an electrical engineer and this is pretty basic stuff.

Yep, I have a habit of missing out the "h".

[ns1]

/pedant but you did say you were an electrical engineer and this is pretty basic stuff.

I do love it when a know-it-all gets shown up.

Oh, and BTW, you can pick up a jump starter for a lot less than a new car battery, and besides who in their right mind would want to lug a spare car battery around. Idiot!

[d000hg]

Willies waved, another query. We were advised that jump-starting should be avoided whenever possible on modern cars as it risks damage to all the computer gubbins - and a battery started is preferable.

Sounds like that could be a bit dodgy to me - it was from a mechanic but it's no guarantee they'd be an expert on the electrical side. Anyone heard the same?

[d000hg]

Oh, and BTW, you can pick up a jump starter for a lot less than a new car battery, and besides who in their right mind would want to lug a spare car battery around. Idiot!

The jumper posted above costs about the same as a regular battery (though not a fancy stop-start one). Maybe he has RWD and it helps in the snow

[Lance]

Willies waved, another query. We were advised that jump-starting should be avoided whenever possible on modern cars as it risks damage to all the computer gubbins - and a battery started is preferable.

Sounds like that could be a bit dodgy to me - it was from a mechanic but it's no guarantee they'd be an expert on the electrical side. Anyone heard the same?

I'd heard the same. However very mechanic I know will still jump start cars.
If you attach a fully charged battery to a totally dead battery whilst still in situ there will be a surge of current but the volts aren't going to jump above what the charged battery can provide so I don't see an issue.
I've also heard that you shouldn't remove a battery from a modern car to charge it as that messes up the computers.

They are either mutually exclusive or the manufacturers just want to cover their arses when things go wrong.

[WTFH]

Willies waved, another query. We were advised that jump-starting should be avoided whenever possible on modern cars as it risks damage to all the computer gubbins - and a battery started is preferable.

Sounds like that could be a bit dodgy to me - it was from a mechanic but it's no guarantee they'd be an expert on the electrical side. Anyone heard the same?

I'm only a mech eng, so I'll share my opinion.
Push starting a modern car is frowned upon, but using a second car, charging device, or whatever, is OK if you do it carefully and follow the instructions. It's always useful to carry leads, etc, as part of the equipment in your car, in case you get into problems, or you choose to help someone else.

Push starting a modern automatic car is a no-no.

[DealorNoDeal]

If you attach a fully charged battery to a totally dead battery whilst still in situ there will be a surge of current but the volts aren't going to jump above what the charged battery can provide so I don't see an issue.

Agreed. And "jump starters" should be fine too assuming they supply 12.6V or thereabouts.

I've also heard that you shouldn't remove a battery from a modern car to charge it as that messes up the computers.

I hadn't heard that but I was told it could lock the radio (and I don't have the keycode for mine). It's a bit of a rigmarole but, just to be on the safe side, whenever I change a battery, I do the following:

1) connect the new battery to the charging points (under the bonnet on my car)
2) disconnect and remove the old battery
3) connect the old battery to the charging points (so both batteries are connected at the same time)
4) disconnect the new battery from the charging points
5) install the new battery (in the boot on my car)
6) disconnect the old battery from the charging points

I've never measured it but I wonder what the voltage drops to when you crank the engine? The starter motor draws a lot of amps, so the voltage must drop a bit. If you've only got a partially charged battery, the voltage will drop even further. (If the battery is flat, then the voltage may not drop at all because there probably won't be enough juice to activate the alternator solenoid.)

Anyway, clearly the electronics (computers etc) must be able to cope with a drop in the voltage to some extent but maybe just not to zero?

[WTFH]

I've also heard that you shouldn't remove a battery from a modern car to charge it as that messes up the computers.

The first few production models of the DB9 had a great issue - the OLED instrument cluster not only displayed what gear you were in, but also controlled it. Any gear change went via the display to the gearbox.
If the display had an issue, then the gearbox got stuck.
One of the connectors on the display had a tendency to have issues. Guess what went through that connector.

[Lance]

I've never measured it but I wonder what the voltage drops to when you crank the engine? The starter motor draws a lot of amps, so the voltage must drop a bit.

If you know the internal impedance of the battery, and the current that the starter draws you can calculate the voltage drop.
But measuring the voltage drop is easier (and safer) than the current draw.

An older battery will drop more than a new battery. A screwed battery could well read 12.6V but drop to under 10V when trying to crank, and likely won't drive the starter (depending on engine size, temperature, etc.)