Commenting on the engineering of battery-based jumpstarters for cars - Page 1
Forum section: electronics / general

author: poof
author_rank: Newbie
timestamp: October 26, 2017, 04:29:26 pm
content: Hi,I was browsing online shops for portable jumpstarters and I have noticed substantial differences between each model .For example I would like to propose 2 of the most reputable brands/models , an Anker 400A and a Bestek 600A .Anker 400ABestek 600ACan you give some advices based on what you see about the engineering of this 2 products or this class of products in general ?I'm trying to pin down on a list of good signs of good engineering .For example both have short cables, which, as far as I know, is something good for high-current applications ; but 1 has the "smart circuitry" on just 1 cable and the other is wiring both positive and negative into the same electronics : which one you think is better and why ? Can you suggest other things to look out for when buying this kind of devices ?

author: Mjolinor
author_rank: Frequent Contributor
timestamp: October 26, 2017, 04:33:41 pm
content: The "smart circuitry" is just some series diode arrangement to save it when you connect it the wrong way round. I would put money on the second one just being straight through on the -ve cable.

author: poof
author_rank: Newbie
timestamp: October 26, 2017, 04:45:53 pm
content: you mean the Anker one ?

author: tszaboo
author_rank: Super Contributor
timestamp: October 26, 2017, 05:04:18 pm
content: It has 3 Li-Ion battery inside, in series. When you jump start, itmassivelyoverloads the batteries, but it kinda works. If you connect it for too long, or any other "woopsie" happens, they blow up, like no tomorrow. The diodes are actually there to prevent reverse charge from inductive kick-back or the generator.That being said, I have one at home. I have a hybrid, which normally charges the small 12V from the HV battery. But only if the car is running. If I left a light on, it discharges the LV battery and I cannot start the car (happened once). With this, I only need to charge it to some 5V and the car "boots up" and I can drive away. So it is kinda useful.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: October 26, 2017, 05:38:22 pm
content: Yup serious case of not switching brain on before replying there. They are to stop you charging the battery once the engine starts.I meant that the protection is probably in only one lead even if both leads go through a little box.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: October 26, 2017, 05:40:16 pm
content: I have carried one of these for a couple of years and they really do work well. It will start my CDI Smart car when the battery is totally flat. I too have used it on my Prius, that is until my dad wrote it off. :bawling:

author: wraper
author_rank: Unknown
timestamp: October 26, 2017, 05:40:58 pm
content: Quote from: Mjolinor on October 26, 2017, 04:33:41 pmPlease explain how diode would add any protection? If you attach it in reverse polarity, high current still will flow. Also it would be some extremely powerful diode and power dissipation on it would be simply huge.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: October 26, 2017, 05:46:02 pm
content: Quote from: wraper on October 26, 2017, 05:40:58 pmYup. Me not thinking.It is only to stop you exploding the Liion by uncontrolled charging.

author: wraper
author_rank: Unknown
timestamp: October 26, 2017, 05:47:51 pm
content: Quote from: NANDBlog on October 26, 2017, 05:04:18 pmAgain, think about ballpack figure of how powerful diode(s) would need to be, and how high losses would be. It's simply not viable and provide almost no protection from anything.

author: wraper
author_rank: Unknown
timestamp: October 26, 2017, 05:49:12 pm
content: Quote from: Mjolinor on October 26, 2017, 05:46:02 pmwhy would it charge by current that can actually do damage? If there is any protection, it should be MOSFETs or relay.

author: tszaboo
author_rank: Super Contributor
timestamp: October 26, 2017, 05:54:27 pm
content: Quote from: wraper on October 26, 2017, 05:47:51 pmI saw a teardown*, and there were several TO263 diodes inside in parallel, each of them rated to about 10A. Surge current will be even higher.And yes, the protection works. Current flows only from the battery pack to the car, and not the other way around. Maybe the one with both leads have FETs in it.*BigClive

author: ovnr
author_rank: Frequent Contributor
timestamp: October 26, 2017, 05:58:06 pm
content: Quote from: wraper on October 26, 2017, 05:49:12 pmCharging a max 12.6V lithium pack to 14.4V will yield fireworks. And yes, it's just a pile of diodes. MOSFETs are more expensive, you know. And good luck finding a 500A surge-rated relay that'll fit and won't cost an arm and a leg.

author: poof
author_rank: Newbie
timestamp: October 26, 2017, 07:11:45 pm
content: I'm reading all the replies and now I have 2 more questions:- there is a better chemistry for an high-current rapid discharge battery cell than Li-Ion ? ( for a reasonably portable device )- are they just using 18650 ( or 26650 ) in series ?

author: ovnr
author_rank: Frequent Contributor
timestamp: October 26, 2017, 09:54:06 pm
content: They're generally lithium polymer cells, because they tend to have better power density. Think RC battery packs.

author: Chalcogenide
author_rank: Regular Contributor
timestamp: October 27, 2017, 04:47:17 pm
content: I own two jump-starters.One has 4 LiFePO4 cells in series, and the cable with a bunch of beefy schottky diodes to protect from reverse polarity and overcharging.The other one (much cheaper) has 3 4000 mAh LiPo cells (RC style, probably 20 or 30C rated) and a slightly more complex circuitry:- an automotive 70A relay to connect the car battery to the jump starter- an optoisolator on the car battery side to sense if the polarity is correct, and only then energize the relay coil- a recessed button to energize the relay manually (needed if the car battery is so flat that it won't turn on the optoisolator)- some other circuitry that I didn't analyze well, but that probably will disconnect the car battery in case of overcharging.