I've been thinking about this, and I'm confused. How come it takes 16 hours or whatever to fully charge the battery, but you can get enough juice in to start the car in 15 minutes (or half an hour, which the quickest I've ever tried)?
How many amps does a trickle charger deliver?
Well, a trickle charger will often be an amp or less, so won't start a car very quickly. I was thinking about your standard mains powered battery charger. About 5-10 amps output on average.
Battery is typically 70 or 80 amp-hours capacity so a 5 amp battery charger will take at least 16 hours to fully charge a flat one (probably double that, as the current will tail off as the battery gets full).
However, if the battery is otherwise in good nick, it doesn't have to be
fully charged to start a car.
To start a car you will probably need to deliver a very high initial peak of about 500 amps to get it cranking followed by perhaps 150-200 amps for a couple of seconds before it fires.
So. let's say 500 amps for a second plus 200 amps for 2 seconds. That's 900 amp-seconds. 3600 seconds in an hour so that's 900/3600 = 0.25 amp hours. A 5 amp charger will supply that in a few minutes.
Now, there will be huge inefficiencies in the battery that I haven't taken into account but you can see that it doesn't need a lot of charge to start a car. Just a lot of current for a short period of time.
You need to start looking at one rated at 600 CCA minimum, and avoid the cheap chinese crap ones that the likes of maplin sell (look around the £100 mark for a booster that actually works).
Agreed. I got caught with a flat battery in the Kit Car last year. There was a Honda dealership round the corner just shutting up shop, so I asked if they could help. They gave me a (physically) very small booster pack and said they had no idea when it was last charged but it was the best they could do.
Not expecting a lot I connected it up and it fired up straight away.
The difference was that this was a professional quality pack.
Kevin