dual batteries, Dual alternators. Space shuttle?
dwilson said:
Hi all,
ready to install my hall effect amp sensors for my GRT EIS 4000....following Bob N.'s dual alternator/dual battery setup. Bob has a shunt amp meter on the output of each alternator.....seems to me I want to monitor the battery lead more.
concern monitoring the battery lead (with)........ current flow may be a couple hundred amps for a short time during engine cranking. Will that high current flow damage the amp sensor? Thanks, Duane
Duane good question? I could tell you where using Bob's book, with a system like that.
Bob says put the amp meter on the alternator, and I agree, you want the alternator, since that is where the power is coming from.
My vote is put the hall effect on the main alternator out put only. If it dies you will know (zero amps). If the back-up alternator works, you know it, because the volts will be above 12.6v battery voltage. I suspect the second alterantor is for lower capacity and emergency use only? So if you had a gauge on it. it would read zero most of the time, which is kind of a waste.
WHY NOT THE BATTERY?
Look, think of it this way, the battery serves as a reservoir and to get the engnine started. After engine start and the alternator energized, the battery's job is done. All the power is from the alternator. After the battery charges back-up (less than a minute or two if in good condition), its waiting as back-up or for the next engine start. Kind of boring and no need to watch it.
The battery DOES serve a very important secondary function, a stabilizing function, handle surges, like if a high load item is switched on, e.g., landing lights or pitot heat. Once the alternator catches back-up (in seconds), the battery goes back to essentially zero current in (charge) or out (dischrage). Again boring.
THE POINT is you will get a lot more out of the AMP meter on the alternator. How much the alternator output is, 10 amps, 20 amps, 40 amps and so on, is way more important. The ampmeter on the battery reads near ZERO most of the time in flight.
What if the alternators goes dead? Voltage goes to about 12.6v and you LAND asap.
With two batteries you probably have enough juice to fly till the tanks are empty. A little electrical load shedding, turning off everything except what is absolutly needed, may be in order. Its a good idea to know what your EMERGNECY or MIN equip electical load (amps) is. You don't need a meter for that. Knowing the estimate of how long the battery will last, with the MIN equip, is a good idea.
VOLTS is a great indirect indication of battery state. Normally it should read about 12.6 volts with no alternator. It will stay there and drop off slowly. As it drains and voltage drops at a faster rate. Most radios and lightspeed electronic ignition can run down to 10 volts. Once down to 10 volts, the battery is pretty shot.
So you can live without an ampmeter on the battery.
IMPORTANT: Good battery maintenance is important. In cars we drive batteries until they don't work at all. In a plane you should CHANGE the battery OUT on a preventative schedule.
It's NOT a bad idea to have an amp meter on the battery, just given the choice of EITHER batter or alternator, the alternator is more importnat to monitor.
Some airplane manufactures and car makers, especially in the old days, had an ammeter on the battery. It usually was a CENTER needle gauge, sometime called a load meter, that deflected left or right for discharge or charge. If the needle was just right of center (no political comment) the battery was not being drained, which was its normal position. Right after start for a minute or so as the battery took some current to charge, the needle would be way right. If the needle went the other way, the battery was draining, like with the engine off and headlights on. When you where driving along and turnned the lights on, the needle would jump left for a second and than go back to just right of zero. On old cars you could get the battery to drain with the engine running because the alternator or generator could not keep up, especially at idle rpm, so the battery made the difference.
Since you are using a EIS4000 engine monitor I can tell you the HALL EFFECT sensor can handle the two way current flow in or out of the battery, but the display does not show negative numbers, but there's a workaround explained in the manual. With the hall effect on the alternator "B-lead", it works better, since all readings out of the alternator are positive or in one direction.
You mention a SHUNT can not handle the start current. You are correct. The hall effect will not be damaged but it will not read that high, unless sized for that.
To explain about shunts and the starter wire draw/current, you could make it work, but you would need a BIG 400 amp shunt or a hall effect sensor. Also you need a gauge scalled for say 400 amps. Can you image reading 10 amps on a 400 amp scaled gauge? That is why we dont do it and we don't care what the starter draws.
IF you want to measure the battery wiht a shunt, put it on the smaller battery wire to the aircraft buss. The starter wire is ignored.
With a HALL EFFECT, it does not matter. You can put it right on the POS BAT wire to the master relay (or contactor). When you start, the starter current will go thru the HALL EFFECT, but it does hurt it, but you will not get any useful reading on the EIS4000, for one, it usually is not ON during start. Unless the EIS4000 has its own power, the voltage drops to 8 or 9 volts during start and the EIS drops out and reboots. Some people don't turn the EIS on until after start.
With you dual dual system you could go all the way and have 4 shunts (hall effect meters) and 4 amp meters (or use more EIS4000 AUX inputs or a switch). I don't suggest you do that or need it, but you sure would have the total picture.
Can I asked what made you go to dual alternators and batteries? I mean there are 10's of thousands of single engine planes all over the world flying with one battery and one alternator. The space shuttle does not have that much stuff.
With an electrically dependent engine and/or panel, electical redundancy is needed and a great idea, but I think either, (1)-alternator and (2)-batteries -OR- (2)-alternators and a (1)-battery is plenty. Does Bob have any stats to justify his rocketship electial design? Just a question.