Battery Protection

[Garry Wright]

I have been looking for information regarding protection of the cable and the master relay to the main buss/starter relay. This zone appears to be routinely ignored for faults by aviation designers. There are a few comments on the Van's site that dismiss faults in this zone as extremely unlikely but as a former electric utility guy, that is not very satisfying. A fault in this zone without protection to clear the fault could be disastrous - in homebuilts and in certified aircraft. In homebuilts, it seems even more unwise than in certified aircraft because we do not have a legacy of thousands of identical examples to demonstrate this unlikely event is truly unlikely.

Other comments I have found rely on the master relay being able to interrupt this heavy fault current (up to 500 amps into a dead short limited only by the internal resistance of the battery). This strategy also requires that the pilot immediately shut off the master and not try putting it back on. It requires that the master relay be in good condition and that it be able to interrupt this extraordinary current flow. Check out the ratings. This is an unlikely sequence to say the least.

Others have used a large slow blow fuse rated to carry probably double the inrush current of the starter, assuming the feeder cable and return circuits are relatively low impedances. This could be a satisfactory approach as long as the maintenance routines took into account the number of start cycles the fuse has endured and replaced the fuse occasionally.

Are there any other thoughts on how a fault in this zone should be protected?

[FlyingRon]

Fortunately lead acid (as opposed to NiCads or the more exotic batteries) have a fairly reasonable internal resistance. It's hardly much different than which for years sold without any main over current protection, you might find fusible links on the main cable. What makes you thinks that there's a lot of failure in the main battery cable and master relay that this is an overwhelming safety concern to the point of introducing another point of failure .

[Garry Wright]

Fortunately lead acid (as opposed to NiCads or the more exotic batteries) have a fairly reasonable internal resistance. It's hardly much different than which for years sold without any main over current protection, you might find fusible links on the main cable. What makes you thinks that there's a lot of failure in the main battery cable and master relay that this is an overwhelming safety concern to the point of introducing another point of failure .

It is true that conventional lead acid batteries have a high current limiting resistance and that the likelihood of a fault is very low. It is also true that it is an experimental and thus riskier proposition than with certified aircraft. One thing that is changing is that batteries like the Odyssey series, have much lower internal resistances and people are not taking this into account but rather treating the situation just like no design change has occurred. The other element is the consequence of a permanent fault occurring in an airborne machine. What would one do with a hot electrical fire at an unpredictable fault location. Many owner built and maintained aircraft have long cables, to help with weight and balance. These are often difficult to inspect as well as aggravating the risk due to length.

I have a line on marine fuses for this purpose. I now need to settle on sizing wisely.