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dual battery isolation diode

L

larryj

Well Known Member
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=CMKAr5_ql6kCFRs5gwodYiOOwA&N=4294941369+4294887038
but my electrical / electronics is weak.
Thanks for your help,
 
What failure mode exactly are you looking to protect against?

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

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
 
Diode

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
 
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.
 
Larry if I understand what you are wanting to do, its not going to work because the diodes will prevent the batteries from charging.
 
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!
 
Last edited:
rocketbob said:
Larry if I understand what you are wanting to do, its not going to work because the diodes will prevent the batteries from charging.
Click to expand...

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.
 
In my rocket electrical system design I have two electronic ignitions, two electric fuel pumps and two backup fuel pumps. These components are connected to the batteries before the master switches. There is no "essential" bus. I had one in my RV-6 and never used it and didn't like the voltage drop across the rectifier diode that was recommended by electric Bob at the time I put it together.

AGM batteries we use are highly unlikely to fail short, unlike conventional lead-acid batteries where sulfation can cause a buildup at the bottom of the battery thus causing a short.

What concerns me is an overvoltage condition, ie, runaway alternator. The ND alternator I used in the RV and in the rocket have OV protection built in, but I've never tested it myself. I had the crowbar OV circuit I got from Electric Bob in the RV but removed it several years ago due to many nuisance trips, which caused more in-flight problems than the circuit was to solve. Switching transients and brief OV conditions are common. I'm thinking that a better approach would be to use a zero-cross detection circuit to shut down the alternator if any AC voltage is present on the main buss. If any of the diodes fail, this is what can fry everything. I've diagnosed bad alternators in cars with my oscilloscope this way, and I believe this is how the alternator testers used at your auto parts store test for a bad alternator.
 
Wow, thanks for the replies so far !! You can easily tell electrics are my weakest subject. Given what I'm trying to do, I need to have both batteries available for starting; I'm using an Odyssey PC625 with the "backup" PC310 to start a high compression IO-540. This Odyssey combination optimizes starting power (CCA) with overall weight and provides redundancy. After spending time with AeroElectric stuff and concluding it is all over my head I decided to go at this from a UI perspective; what do I want as a pilot to be looking at and using. That's how I ended up with 2 toggle switches; one for each battery.
Sounds like the voltage drop across any diode makes them a non-starter for the start-circuit (pun intended). And given a battery internal short is extremely unlikely, sounds like a diode between the 2 busses is what I need along with OV protection and warning ?????
 
Hi Larry,

Voltage drop wouldn't do any good for sure but the high current to your starter (potentially upto 680 Amps & 325 Amps for your chosen batteries) means you'd need some pretty hefty diodes to handle that load.

Yep, you don't want the diodes in the starter circuit.

Steve
 
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