- The 30 amp breaker you refer to seems like a lot for the 12 volt on your relay, or is the '12 SLA batteries' you mention actually 12 batteries of the SLA type?
I'm looking at a single, external 12 volt 7Ah SLA battery connected to the controller with 10 feet of 18AWG zip wire. The run out to the pad is a 100-foot 16/2 medium-duty extension cord. Could I get away with something along the lines of a 10 or 15 amp breaker? I'm thinking of just putting it inline on the positive lead coming from the battery, before the controller, using these:
Quality rubber fuse holder fits ATC/ATO size fuses. Holds and encloses fuse securely. Hinged cover protects fuse from impact, water and dust. 12-inch wire loop is made of 18 gauge flexible stranded red wire. Fuse not included.
ATC style resettable circuit breakers are compatible with most ATC/ATO type fuse blocks. Resettable, Type III. Available in 10, 15, 20, 25 or 30 amps.
- Your thoughts?
One 12V SLA battery, 12aH on the relay box, 7aH on the controller box.
At the time I built the relay system I was relying on things I was reading when I was doing the design. The design I stole mine from (it was 10 years old and all of the parts were Radio Shack, so I had to research equivalent parts) used a 30A breaker, so I cloned it.
The thing that has not been mentioned to this point is that my relay system can fire 4 pads at the same time, so it could generate a fair amount of current. The launch controller on that system only has a 10A breaker, but the relay box has a 30A. I used automotive relays that are also fused at 30A. Since a fused relay could be disastrous in a relay system, I overbuilt it on purpose. So, in theory, no relay in that system should ever become fused due to overcurrent, nor should the entire relay box melt down if the combined draw of the relays exceeds 30A.
In retrospect I may have foregone the individually fused relays and used low voltage relays instead. When I did my testing, only one of the 4 relays would fire due to the voltage drop over the 100-foot distance (again, I didn't calculate voltage drop ahead of time, lesson learned). The choice was to either go to 24v or use a bigger wire gauge on the cable. Since the box was built, switching to 24v wasn't really an option, plus the fact that 24v batteries aren't easily had. If I had known more at the time, I could have switched to using a 6S LiPo, but I was LiPo illiterate at the time. Consequently, since I was using DB-9 connectors on the cable and there were no standard DB-9 cables that were not small gauge wire, I bought 100-foot rolls of 18-gauge wire and built my own.
My docs for the relay system are attached below.
P.S. I did not put continuity checks on the relay pad at the time because I couldn't figure it out in my head but have designed that in now. I haven't made the mod yet but intend to. The docs include the design with and without and the BOM includes the necessary add-on components. The .dsn file is a TinyCad file.