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Car jump-start batteries

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Old Croc
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    Posted: 28 August 2018 at 5:15pm
Seen a few of these around capable of doing up to 19V. Anyone tried using them as an easy way to do 12+V rigs? On the surface they seem like a pretty no nonsense way to get a fairly high power battery that'll do higher voltages than a standard USB bank without it weighing like it's made of lead...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lost eden Quote  Post ReplyReply Direct Link To This Post Posted: 30 August 2018 at 10:11pm
They're just a lithium ion battery inside, so why waste money on the casing/leads/crocodile clips when you could just buy a similar battery on its own?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote slaz Quote  Post ReplyReply Direct Link To This Post Posted: 31 August 2018 at 9:33am
Originally posted by lost eden lost eden wrote:

They're just a lithium ion battery inside, so why waste money on the casing/leads/crocodile clips when you could just buy a similar battery on its own?


There's quite a bit more than "just a battery" inside.

1) A charging circuit designed to accept a range of input voltages around 12V (not just a fixed voltage from the supplied AC adapter - so it can be charged from e.g car batteries).

2) A DC-DC converter board to power laptops (e.g. 19V) ... which if its reliable and supplies enough current like 4A or so, would do nicely for certain chip-amp modules).

3) If a half-decent design, they'll have a BMS circuit to balance the cells, monitor temperature, provide short protection etc.

4) You get stuff like multiple USB output ports (well OK could easily be replaced by a "power bank" thang).

5) The cables and casing could actually be quite useful for sound system use.


My concern with these (certainly the cheap ones) would be about the quality of the cells, and overall reliablity. The vendors will often claim silly optimistic ratings for the battery capacity.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote mobiele eenheid Quote  Post ReplyReply Direct Link To This Post Posted: 31 August 2018 at 3:03pm
My concern is mostly with the exaggerated claimed capacity.

- One good price performance brand claimed 12 - 19 V and 12 Ah but the pdf stated 41.1 Wh (so roughly 3300 mAh)

- A Chinese brand claimed 83 Ah but supposedly also consisted of 4 x 3300 mAh cells in series.

So buyer beware!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote valve head777 Quote  Post ReplyReply Direct Link To This Post Posted: 31 August 2018 at 4:08pm
Are these really designed for longish term output? To my mind, they basically just need to give a highish current output for a very short duration (less than a 30 seconds) in order to crank an engine. Maybe the laptop output is supposed to be used when plugged into a car power socket. I have seen ones with a small lead acid battery. Most of the time, these are used to boost a battery that can't quite start the engine, but is not completely dead.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Meat Quote  Post ReplyReply Direct Link To This Post Posted: 01 September 2018 at 8:10am
Thanks, there's some great food for thought there.

In terms of DIY projects for alternatives which provide all the features described above is there much out there?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote slaz Quote  Post ReplyReply Direct Link To This Post Posted: 01 September 2018 at 10:21am
Originally posted by valve head777 valve head777 wrote:

Are these really designed for longish term output? To my mind, they basically just need to give a highish current output for a very short duration (less than a 30 seconds) in order to crank an engine. Maybe the laptop output is supposed to be used when plugged into a car power socket. I have seen ones with a small lead acid battery. Most of the time, these are used to boost a battery that can't quite start the engine, but is not completely dead.


Well - the o/p thats meant for cranking an engine will almost certainly be wired directly to the battery terminals of a 3S pack (highly likely flat LiPo cells). Any intervening electronics would have to be rated for very large currents like 50-100A ..... so not likely.

So yeah - even my 3S 5Ah LiPo pack - a well-known brand name sold for use with RC models.drones -  could almost certainly deliver say, 50-60-70A for a few seconds at a time .... thats something like 1 horse-power, which I think should turn over a car engine. As to how many times it do that on a single charge - no idea really.

If buying one of those jump-starter things I'd wanna see inside it to check the cell capacity .... I would NOT trust what it says on the Chinese box. Then I'd want to run tests on it to see if the capacity marked on the cells is anywhere near the real-world capacity.

If buying thru ebay/amazon etc. you should get 30 days to evaluate - so could return it.

Now if using it for a sound system via the laptop o/p (19V) - should be possible to test that - using known load impedance, music with a guessable crest factor. and your ears to establish when the amp is on the edge of clipping .... and preferably monitor the cell-pack voltage (i.e. before the DC-DC converter). See how long it runs ....

Problem will be though - if the thing is put together as a sealed unit with no screws to access its gubbins, you likely won't be able to return it if you open it up with a dremel.

A decent clue to the capacity of the fitted cells will be the overall weight.

My good brand 3S 5A pack is about 500g I think. If you assume the cells are about 80% or so of the overall weight of the gadget you shouldn't be too far away.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote bitSmasher Quote  Post ReplyReply Direct Link To This Post Posted: 01 September 2018 at 2:11pm
Have seen and considered the idea of using these, but convinced myself they'd be cheap shit cells not even worth the time to experiment with
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