Too many variables. What is the amp hour rating of the battery? What is the internal resistance of the switch? The resistance of the cables? The resistance of the starter motor? The temperature? How clean are the battery clamps, and what is the resistance of the battery cables? The resistance of the starter solenoid?
Tiny in automotive terms.
Dunno
Mine has 16mm² cables. The resistance of the alligator clamps is more likely the limiting factor.
Non of which matters since the power pack makers claim a maximum output.
Really? My jump packs have 17 Ah batteries.
Product ID: BSL1117 Cold Cranking Amps: 240 Voltage: 12 Termination: NUT & BOLT Chemistry: SLA OR VRLA VALVE REGULATED S Weight: 13.3 Width: 2.99 Length: 7.13 Height: 6.57
My truck uses a size 27 battery with these specs:
Product ID: MTP-27 Amps: 1000 Cranking Amps: 1000 Cold Cranking Amps: 810 Voltage: 12 Termination: A Pro-rata Warranty: 85 Weight: 47.4 Width: 6.81 Length: 12.06 Height: 8.88
So, it has 24% of the rating of the truck battery which isn't 'tiny'. It is intended to start a vehicle with a run down battery, not one with mechanical problems. The portable pack is designed with a different type of battery, as well. The available current is determined by the plate area and thickness.
I've seen someone use 12V alarm batteries to jump start a service truck. They were rated at 7 Ah.
No, all of it is important. If the resistance of the clamps is as high as the leads, they will overheat since all the resistance is at the same spot rather than distributed along the length of the cables.
BS. They all matter, if you aren't an ignorant troll.
What do you think the short circuit current of a fully charged 12V car battery is?
I suppose it depends on your definition of tiny, but 17 amp.hr is a lot smaller than any battery in any car I've owned or seen. Both my current ones have 70 Ah types.
Err, you don't appear to have given the amp.hr capacity - the very thing you mentioned.
Batteries? Parallel them and they become like a larger one.
Are you trolling? The maker of the jump start pack claims a maximum current. Since they can't possibly know exactly what the starter motor etc draw is, just how is it relevant?
My point is (with experience of several jump start packs including expensive ones) is that they will not do lots of starts of a vehicle with a flat battery without permanent damage to the SLA. Somewhere round a dozen or so seems to be it.
Yeah, but how many minutes would your 70Ah crank for, and how many seconds does it need to crank for (assuming there's no other problem with the engine)?
Gee, that's damn close to the 24% figure isn't it? 17/.24 = 70.83
Very few car batteries are given an amp our rating in the US since they are not used without a charging circuit. If you look at the given spces you can get an idea though.
It is very relevant, if you stop and think about it.
You keep changing the subject.
Think you should re-read my posts. Perhaps you addressed your questions to the wrong person.
Great. So never used with the engine stopped? All car batteries in the UK have the amp.hour marked on them. As well as max current, etc.
I have an electronic battery tester - an expensive device. This gives a instantaneous readout of the capacity in amp.hours. If you don't know what it should be it is fairly useless.
On 12/6/2010 8:58 AM Michael A. Terrell spake thus:
OK, your impeccable documentation has won me over. I withdraw my objection; the current claims on those jump-packs (300 amps, 400 amps) must be believable after all. I'm a little surprised, though.
In the case of the Schumacher IP-55 model (12V 4Ah battery) the descriptive term that's not been mentioned so far, is 400A Peak, for this model.
Manufacturers know that Peak is a (multi)million dollar word, whether it's used with watts, HP, or amps.. because Peak distorts actual specifications. Consumers love big numbers, and manufacturers know it.
With a properly rated current shunt, and a peak-reading amp meter attached, one could determine the peak current available from such a small battery.
As Dave P points out, the number of times that a small battery can deliver the somewhat severe duty discharge rates, is going to be a very limited number of times.
I didn't purchase this Schumacher power pack as an emergency car battery jump pack, but instead, only as a portable power supply.
-- Cheers, WB .............
That was really the only point I was trying to make. The output from my cheap one was enough to start a car on a few occasions - then not. What the actual peak power is/was didn't much concern me.
Mine - despite being quite a few years old - is still fine for this, with the battery holding its charge well. It also has a built in compressor, so gets used for tyres. Slow, but does it.
It was remarkable value. Any cordless power tool of its age would have had a dead battery long since.
I used to work in a TV production facility where some of the prog was made in the 'studio' - some on location. So there was a fleet of small trucks used just for this location work. That location might only be a few minutes drive away. And the trucks left unused at other times. So by nature many suffered from flat batteries just when needed.
The availability of cheap jump start packs was very appealing. Easy to store and carry. But in practice, even with the low cost, weren't reliable enough. So units were made up using a sack trolley and car battery with a charger attached. So the same sort of idea only larger.
Sure, but vehicles that are used that way on a regular basis can have a dual battery system so the starter is on a separate battery. I first installed some on a pair of large paddle boats at an amusement park in the early '70s so they could run cart machines for the music while the
283 CID V8 engines were shut down.
What good does that do the average owner? Do they stand around bars playing 'My dick is bigger' over car batteries?
Instantaneous? Right. It gives an WAG. Battery testers have been around for generations. Just because they add a little electronics doesn't make them magic. Does your car battery tell you the discharge rate to achieve the marked Ah rating?
In the US you can find a combination battery charger & large lead acid battery built to jump start vehicles. Garages use them, and I've seen them at large malls. The charge can put out enough current to start a typical vehicle but if no AC is available they use the battery.
You don't half talk rubbish. Battery testers used to be large devices. This is the size of a DVM. And gives an accurate readout of the overall battery condition. Not just its voltage or how much current it can deliver. It's invaluable for quick testing of a lead acid battery.
It's also a standard in the UK. 20 hour rate.
Then explain how it can determine the Ah rating. it sounds like it tests the battery's ESR and guesses the Ah rating.
So you can leave your headlights on for 20 hours with no problems?
You cannot take an "instantaneous" reading of a cell's Ah capacity. Such a measurement requires actually draining the battery.
It doesn't 'guess' anything. It gives a pretty accurate reading. I'd suggest you find out for yourself how they work.
So you've no clue *either* about how battery capacity is measured?
Hint. Headlights on dip (and the other lights which will be on with them) amount to around a 12 amp load. That's with 55 watt tungsten headlights. At a 20 hour rate that would suggest a 240 amp.hour battery just for them still to be sort of working. But not without problems. Please try it on your car.
I dunno how it actually achieves this, but in practice it's what it does. And is pretty accurate if you check it against doing an actual discharge. Nor does it need the battery to be fully charged to test it.
I've used it on many batteries of different capacities - and on known good ones gives a readout within something like 5% of the stated capacity. Unless the charge is too low for it to work.
For me, it's invaluable.
While we're on batteries...
Yesterday whiel doing a perfunctory cleanup I came across the Canon 580EX II flash I'd packed in a carrying case (when visiting a friend) almost a year ago. The 2700 mAh NiMH PowerEx (MAHA) cells had not been charged since then. When I turned the flash on, it came to full charge in less than three seconds.
The cells measured 1.274V, 1.283V, 1.285V, and 1.286V, all higher than the nominal 1.25V of a NiMH cell. (NiMHs generally come out of a full charge at around 1.4V.) That's pretty good performance for a cell that's supposed to "drop dead" within a few weeks of its last charge.
So I ask... Where did this belief that NiMH cells rapidly self-discharge come from? I never believed it, and here's strong evidence it just isn't true.
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