How does the diode pack on lipo jumper packs stop the alternator chaging it up? - Page 1
Forum section: electronics / beginners

author: yada
author_rank: Frequent Contributor
timestamp: May 26, 2017, 11:31:03 pm
content: I understand that the diode only lets voltage flow in one direction, but when the alternator is running the polarity doesn't change: I can see how the diode would stop you from reversing the leads as plus would be connected to the band end of the diode, but how does it stop current (not voltage) from going from the high Amps alternator to the lower amps lipo pack?

author: sleemanj
author_rank: Super Contributor
timestamp: May 26, 2017, 11:57:11 pm
content: Quote from: yada on May 26, 2017, 11:31:03 pmThe alternator outputs a higher voltage than the LiPo pack is, the diodes are thus reverse biased and block current flowing in that reverse direction.

author: yada
author_rank: Frequent Contributor
timestamp: May 27, 2017, 01:04:48 am
content: Quote from: sleemanj on May 26, 2017, 11:57:11 pmSo the +12.0v of the battery when connected to the +13.8v of the alternator the battery becomes 0v and alternator becomes +1.8 volts and then the diode blocks it?

author: retrolefty
author_rank: Super Contributor
timestamp: May 27, 2017, 01:06:43 am
content: Quote from: yada on May 26, 2017, 11:31:03 pm

author: sleemanj
author_rank: Super Contributor
timestamp: May 27, 2017, 01:57:27 am
content: Maybe this crude diagram in the hydraulic analogy style will help you understand.Voltage is thought of like water pressure, and a diode is like a one-way flap-valve.High pressure (voltage) on the left side and low on the right will push the valve (diode) open and allow current to flow.High pressure (voltage) on the right side, and low on the left side, will push the valve (diode) closed and no current will flow.

author: yada
author_rank: Frequent Contributor
timestamp: May 27, 2017, 02:32:50 am
content: I get it now, the diode is seeing that the alternator side when running is ~2 volts higher on that one side, so as far as the diode is concerned the + and - have switched. Now for more technical explanation you would use four diodes in parallel because they can only carry so much corrent? or because just having one diode would fail and let the reverse current flow through it? If each diode has a drop of 0.6v then putting four in parallel would drop .15v?

author: sleemanj
author_rank: Super Contributor
timestamp: May 27, 2017, 03:02:53 am
content: 4 diodes in parallel is for current, the current will, one hopes, divide between them more or less evenly.No it doesn't reduce the diode drop to use more diodes in parallel (ignoring other conditions which affect the diode drop of diodes).If the diodes are not well matched, then you may get into a situation where one or more of the diodes have a low enough forward voltage drop that they cause the other diodes to not be sufficiently forward biased and therefore not conduct at all.You can observe such an effect for example with two LEDs, take a red LED and a blue LED, the red one will have forward drop much much lower than the blue one, if you put them both in parallel with a single resistor shared by both, the red one will light and the blue one won't because the blue one will no longer be sufficiently forward biased to pass current.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 27, 2017, 02:11:51 pm
content: I'm not sure what this drawing is supposed to mean:- Only one diode is conducting?- One battery is reversed- There is no 1K resistor of course.

author: yada
author_rank: Frequent Contributor
timestamp: May 28, 2017, 11:50:49 am
content: Quote from: PA0PBZ on May 27, 2017, 02:11:51 pmI was trying different orientations to see which way the current would flow. The simulator wont work without the resistor. Also only one diode has current flow according to the simulator. I forget what my point was with the reversed batteries.

author: Audioguru
author_rank: Super Contributor
timestamp: May 28, 2017, 03:47:24 pm
content: What do you have that uses a Li-Po battery and an alternator??A Li-Po will explode when connected directly to an alternator or to the rectifiers in an alternator because a Li-Po needs a charger circuit designed for it.Nobody makes a "12V" Li-Po battery. One cell is 4.20V when fully charged then two cells make 8.4V and three cells make 12.6V. If they are charged to a higher voltage then they explode or catch on fire.Li-Fe battery cells have a little less voltage.Maybe your battery is a lead-acid battery?

author: yada
author_rank: Frequent Contributor
timestamp: May 28, 2017, 03:51:39 pm
content: Quote from: Audioguru on May 28, 2017, 03:47:24 pmI don't have either. I just wanted to know how the diode pack works.

author: Terrius
author_rank: Contributor
timestamp: May 28, 2017, 07:11:07 pm
content: Quote from: Audioguru on May 28, 2017, 03:47:24 pmI imagine yada is referring to the increasingly common LiPo jump starters. They are made from 3 LiPo cells in series, and designed to jump start a dead car battery. The jumper connector to the car battery contain a Diode pack to prevent back feeding of the batteries from the Alternator once the car starts.Google LiPo Jump Starter and you will find thousands of them. Bigclive recently even did a video on one, where he shorted the pack out to see what would happen.

author: Codebird
author_rank: Regular Contributor
timestamp: May 29, 2017, 10:08:59 pm
content: QuoteActually it means you are using the simulator wrong. I recognise that little java-sim - it's a handy tool for simulating simpler circuits without needing to wrestle with ltspice. It also doesn't consider a component connected to a wire that merely passes through the end point - it looks connected, but it isn't. You just need to delete the two long wires and connect it in steps from one compoent to the next, no shortcuts.

author: mikerj
author_rank: Super Contributor
timestamp: May 30, 2017, 10:59:41 am
content: Quote from: Audioguru on May 28, 2017, 03:47:24 pmOP is referring to the compact jump starter packs that are now widely available and use lithium polymer cells. They include a diode pack to prevent the cells being charged from the alternator. The cheap ones have no other protection and typical Li-Po failure modes will occur if shorted etc.

author: yada
author_rank: Frequent Contributor
timestamp: May 31, 2017, 12:05:05 am
content: Quote from: Codebird on May 29, 2017, 10:08:59 pmWhat do you mean by this? You mean always make the wire goes to the circle at the end of the components lead? I use that a lot because its much more visual and simple then ltspices number table.