Seamless Power Source Switching for EFI: Lithium Battery to Generator, No Back-Charge

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How can I automatically switch an EFI system from a lithium battery to an engine generator without a drop-out, while preventing the generator from back-charging the battery?

Use diode OR-ing (steering diodes) if the generator voltage will always be at least about 1 V higher than the battery, and use Schottky diodes to keep the forward drop and power loss as low as possible [#21670890][#21670893] At your stated 2.5 A load, the example Schottky part discussed has about a 0.3 V forward drop at 125 °C and around 0.75 W dissipation, so a bench test is a good idea to confirm the EFI will tolerate the reduced supply voltage [#21670893] The tradeoff is reverse leakage; the cited diode was about 1 mA at high under-hood temperature, which may be acceptable if the battery is only connected during use [#21670893][#21670894] If the diode drop is not acceptable, the next step is an ideal-diode or hot-swap controller such as the Linear Technology LTC4359 [#21670895] For vehicle-style use, choose parts rated for automotive service, including spikes up to 60 V, 24 V jump-start conditions, and reversed battery connection [#21670897]

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Automatic source switching without battery back-charge
#1 21670889 24 Jan 2014 13:35

Casey Fordyce Casey Fordyce

Anonymous

Automatic source switching without battery back-charge
Post #1 21670889 24 Jan 2014 13:35

Hello,

I'm looking for direction in how I would go about switching between two power sources automatically without a drop off. My application is a small engine EFI system that I would like to perform the power on start up from a lithium battery pack and then switch power draw over to an on engine generator after the engine has started and the generator is making power. I would like the battery to always be available to provide power if the generator fails, but not to be charged by the generator. I would consider the battery portion a total loss system where the only time I draw power from it is during the start up and I would externally charge the battery after several uses.

I have discovered ORing controllers and hot swap controllers, but I'm not enough of an electronics guy to decipher whether I need something that fancy or not.

Thanks,

Casey
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Steering diode works if generator stays 1 V higher
#2 21670890 25 Jan 2014 16:48

DAVID CUTHBERT DAVID CUTHBERT

Anonymous

Steering diode works if generator stays 1 V higher
Post #2 21670890 25 Jan 2014 16:48

You're on the right track with diode ORing (steering diode) and hot swap controllers.

If the engine generator when operating will always output at least 1 volt more than the maximum terminal potential of the battery a simple steering diode circuit will work. Additionally, the circuit will have to be ok with the 1 volt diode drop. If the diode voltage drop and power loss are unacceptable then a Hot Swap type of circuit is the solution.

If you'd like more information post the voltage and current involved.
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Request for hot swap circuit sources
#3 21670891 26 Jan 2014 02:09

Casey Fordyce Casey Fordyce

Anonymous

Request for hot swap circuit sources
Post #3 21670891 26 Jan 2014 02:09

David,

Thank you for your response. I'm not sure on the voltage difference yet. It is a three phase generator. For now I'm going to use an ATV type rectifier/regulator so that voltage will be ~14.4. I'm looking at a max current draw of 2.5A. The battery voltage with a full charge could either be 12.6V for a fully charged 3 cell lipo or 14.8V for a fully charged 4 cell. I wouldn't think either of those voltages would be at that level for very long.

Do you have any sources I could research for the Hot Swap circuit?

Thanks,

Casey
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#4 21670892 26 Jan 2014 02:14

Casey Fordyce Casey Fordyce

Anonymous

Post #4
21670892 26 Jan 2014 02:14

Something like this?
http://www.hobbyprojects.com/the_diode/steering_diode.html
Schottky diode choice and leakage tradeoff
#5 21670893 26 Jan 2014 08:35

DAVID CUTHBERT DAVID CUTHBERT

Anonymous

Schottky diode choice and leakage tradeoff
Post #5 21670893 26 Jan 2014 08:35

If the diode forward drop is not a problem the circuit in your link will work. I would use Schottky diodes for minimum voltage drop and minimum power dissipation. The disadvantage of a Schottky diode is the reverse leakage.

A suitable diode is the VISHAY V30100S-E3/4W available at Digikey for $1.73. This diode has a reverse breakdown of 100 volts and that is more than high enough to withstand automobile voltage spikes.

http://www.vishay.com/docs/88941/v30100s.pdf

Looking at Fig. 3 in the datasheet we see that the forward drop at 125 deg C is 0.3 volts. The power dissipated is then 0.75 watts which will cause the diode (with no heatsink) to be 50 deg C above the ambient temperature.

And looking at Fig. 4 we see that the reverse leakage current is roughly 1 mA at the highest temperature to be expected under the hood. Will your battery be ok leaking off a few mA-hours per day?
Suggesting active ideal-diode alternatives to diodes
#6 21670894 28 Jan 2014 10:40

Casey Fordyce Casey Fordyce

Anonymous

Suggesting active ideal-diode alternatives to diodes
Post #6 21670894 28 Jan 2014 10:40

David,

Let me do a little more research on my end on the voltage requirements for the system, but I think the forward drop won't be an issue. I'll rig up a bench test with the VISHAY parts you recommended and see how it works.

The battery will only be connected during use, so the leakage is not significant.

If I wanted to take this to the next level and use something fancier than diodes, where should I start looking?

Thanks,

Casey
#7 21670895 28 Jan 2014 11:52

DAVID CUTHBERT DAVID CUTHBERT

Anonymous

Post #7
21670895 28 Jan 2014 11:52

The Linear Technology LTC4359 Ideal Diode controller is something to look at for your application.

http://cds.linear.com/docs/en/datasheet/4359fa.pdf
#8 21670896 28 Jan 2014 17:27

Casey Fordyce Casey Fordyce

Anonymous

Post #8
21670896 28 Jan 2014 17:27

How about this guy to be able to beard board my test circuit?

http://www.intersil.com/content/dam/Intersil/documents/fn31/fn3183.pdf
#9 21670897 28 Jan 2014 17:37

DAVID CUTHBERT DAVID CUTHBERT

Anonymous

Post #9
21670897 28 Jan 2014 17:37

That part is a low current part and it is good to only 15 volts. For a vehicle I think a part rated for automobile service should be used. To meet that requirement it must handle spikes to 60 volts, 1 minute at 24 volts (for a double battery jump start), and a reversed battery.
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#10 21670898 28 Jan 2014 17:39

Casey Fordyce Casey Fordyce

Anonymous

Post #10
21670898 28 Jan 2014 17:39

Ok. I did find some MSOP-8 prototyping boards that convert it to .1" header spacing. I'll give those a shot.
#11 21670899 30 Jan 2014 02:15

Saty Barole Saty Barole

Anonymous

Post #11
21670899 30 Jan 2014 02:15

Hi, can anybody, Elaborate this topic because I am thinking to give a seminar on it in my college. so any one suggest me more about it.
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Topic summary

The discussion addresses the design of an automatic, seamless power source switching system for a small engine EFI application, transitioning from a lithium battery pack used at startup to an on-engine generator once running. The goal is to prevent back-charging the battery from the generator and maintain battery availability if the generator fails. A simple diode ORing (steering diode) circuit can work if the generator voltage exceeds the battery voltage by at least 1V, but this introduces a diode forward voltage drop (~0.3V with Schottky diodes) and power dissipation. Schottky diodes such as the VISHAY V30100S-E3/4W are recommended for minimal voltage drop and power loss, with considerations for reverse leakage current. For more advanced solutions with lower losses, hot swap or ideal diode controllers are suggested. The Linear Technology LTC4359 Ideal Diode controller is cited as a suitable example for such applications. However, automotive-grade components must handle voltage spikes up to 60V, reversed battery conditions, and transient loads. Prototyping with MSOP-8 breakout boards is mentioned for testing. The system voltage is approximately 14.4V from a three-phase generator rectified by an ATV-type regulator, with battery voltages ranging from 12.6V (3-cell LiPo) to 14.8V (4-cell LiPo) and a maximum current draw of 2.5A.
AI summary based on the discussion. May contain errors.

Seamless Power Source Switching for EFI: Lithium Battery to Generator, No Back-Charge

How can I automatically switch an EFI system from a lithium battery to an engine generator without a drop-out, while preventing the generator from back-charging the battery?

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