dual battery isolation diode

[larryj]

I'm rigging a dual battery set-up in my Rocket; 1 larger and 1 smaller battery. I want 2 toggle switches in my panel; 1 for each battery. This gives me a simple selection of either battery or both batteries. Back close to the batteries are 2 solenoids; 1 for each battery. My debate on architecture is over but I'm still trying to figure out the executiion.

I believe all that is needed to add safety to this would be 2 diodes; to protect each battery in the event the other battery fails with an internal short. First off, is this true and a good way to proceed and second, what is a good diode ?? I've found what I'm thinking may be the answer here http://www.alliedelec.com/search/searchresults.aspx?Ntk=Primary&Ntt=diode&gclid=CMK Ar5_ql6kCFRs5gwodYiOOwA&N=4294941369+4294887038
but my electrical / electronics is weak.
Thanks for your help,

[krw5927]

What failure mode exactly are you looking to protect against?

A schematic, even if rudimentary, would be most helpful.

[Daver]

Its gonna take a diode with a large enough current capacity to handle starting loads.

Also a silicon diode (Si) has a forward voltage drop of about 0.7 volt if this matters to you.

I think a germanium diode (Ge) has a forward voltage drop of about half that.

A better idea might be to use a couple of high current relays with a little logic.

jJust thinking out loud.....

Dave
-9A flying

[Beancounter]

Larry
I have seperate switches for my 2 batteries. One alternator, one solenoid, and two busses. To prevent backfeeding, I used a Schottky diode bought off fleabay for around $5. I placed the diode between the primary and endurance buses. It is rated way in excess of the amps my endurance buss will ever see. My endurance bus is not connected to the starter so no starting loads thru the diode. I do see a forward voltage drop of around .3-.5v. I was aware that this will reduce the amp hrs available from my 2nd battery so I bought a bigger one.

Matt

[deene]

Check out Bob Nuckols at his AeroElectric Connection site for good design info for dual battery (and other) systems.

For the isolation diode, you want to use a schottky diode. They have low forward drops and are available with good ratings. Eric Jones at Perihelion Designs sells a nice one that I have used on several projects:

http://www.periheliondesign.com/powe...y%20Manual.pdf

[terry.mortimore]

Hi Larry:

B&C is another source for the diodes.

http://www.bandc.biz/essential-bus-diodes.aspx


Cheers, Terry.

[shuttle]

The referenced B&C diode is not a schottky diode. It is a bridge rectifier and has a higher forward voltage. Schottky diode's are a better bet to minimuse voltage drop.

[rocketbob]

Larry if I understand what you are wanting to do, its not going to work because the diodes will prevent the batteries from charging.

[elippse]

I'm using a circuit which I designed that uses two P-channel Mosfets to connect a master battery with an auxiliary battery. It will bring the voltage on the aux battery to within 50-80 milivolts of the main battery. It only works when the main battery has a higher voltage than the aux battery. It uses current limiting to keep the charge current to the 4AH a maximum of 2A. I designed this after I ruined two 4AH batteries when connected via a Schottky diode to the main buss. Why did this happen, you may ask? Well, if you really want to know, the aux battery had a slight keep-alive discharge current load which completely discharged it. As soon as the Master was operated, the main buss dumped tons of current into this poor little 4AH batterey, warping its plates. So beware, Schottky users, or this could happen to you!

[shuttle]

Originally Posted by rocketbob

Larry if I understand what you are wanting to do, its not going to work because the diodes will prevent the batteries from charging.

How about this set of connections.....

Aim: Auxillary Battery to provide alternate power source for Avionics. Does not aid engine starting.

1) Main battery to Contactor to Main Buss.
2) Main Buss to schottky diode to on-off switch/relay to Avionics/Ebus. (Avionics Master Switch)
3) Auxillary Battery to schottky diode to switch/relay to Avionics/Ebus. (Avionics Aux Battery Switch)
4) Main Buss to schottky diode to 3 ohm resistor to Auxillary Battery.

Aux battery is charged via route 4) with resistor managing/limiting current inflow.
Routes 2) & 3) allow either or both batteries to feed the Avionics/Ebus.
Appropriate CBs/fuses applied in appropriate locations.
Alternator feed comes in at the usual place.

Does not quite matches Larry's original spec but is there or there abouts.