Pete1122 Posted November 30, 2013 Share Posted November 30, 2013 I am just finishing up my wiring project and wanted to run some numbers by the group. Currently my Alternator is putting out 14.25 volts with 13.75 volts reaching the battery. Does that sound right? How does the voltage drop sound? Quote Link to comment Share on other sites More sharing options...
Gulfstream Greg Posted November 30, 2013 Share Posted November 30, 2013 Which battery? If its to the coach battery measure the voltage on both sides of your isolator. The mechanical ford types are prone to voltage drops because of carbon build up on the contacts. Bad grounds can also cause problems. You will see some drop in voltage at the coach battery because of the distance and if the wire is too small of a gauge. If its the starting battery check all your grounds. Voltage drops have 3 possibility's Too small gauge and too far a wire run. Bad isolator contacts. Bad connections. Quote Link to comment Share on other sites More sharing options...
Pete1122 Posted November 30, 2013 Author Share Posted November 30, 2013 13.75 volts to the coach battery located near the rear of the motorhome. I'm wondering if that's normal voltage at idle. At the isolator it's 14.25. Quote Link to comment Share on other sites More sharing options...
Gulfstream Greg Posted November 30, 2013 Share Posted November 30, 2013 That is low but if its all the way at the back that is a long ways. Can you tell what the gauge of the wire is? And is the ground side using a wire or the chassis for the complete circuit. For that distance I would think 10 gauge at least but 8 or 6 would be better. And what is the voltage at both sides of the isolator? Quote Link to comment Share on other sites More sharing options...
Pete1122 Posted November 30, 2013 Author Share Posted November 30, 2013 That is low but if its all the way at the back that is a long ways. Can you tell what the gauge of the wire is? And is the ground side using a wire or the chassis for the complete circuit. For that distance I would think 10 gauge at least but 8 or 6 would be better. And what is the voltage at both sides of the isolator?10 gauge wire from the battery to the isolator and then 10 gauge wire to my old battery compartment which is where my auto 12 volt auto resetting breakers are. From there I used 4 gauge wire to extend to the back where I relocated the batteries. I also upgraded my ground wire to 0 gauge. The isolator is using the chassis for the ground, if I recall the voltage on both sides of the isolator where the same, or close. I measured the voltage from the isolator to the breakers in the old battery compartment and they were above 14 volts, I seem to be dropping to 13.75 from the breakers to the new battery compartment. That is surprising to me considering I used 4 gauge wire to extend that length. I suppose I should also test while the truck is running at a higher RPM. Other thoughts? Quote Link to comment Share on other sites More sharing options...
WME Posted November 30, 2013 Share Posted November 30, 2013 Splices??? Bad connectors????? The max v drop for 12v is .36v, so your V at the battery is a bit low. BTW 16 ft of 4ga wire has a max load handling of 50 amps with the .36v drop figured in. So how much current are you pushing? Quote Link to comment Share on other sites More sharing options...
Gulfstream Greg Posted December 1, 2013 Share Posted December 1, 2013 The isolator is using the chassis for the ground, if I recall the voltage on both sides of the isolator where the same, or close. Other thoughts? What are the new batteries at the new location using for ground, the isolator does not care? Also disconnect the batteries and see what the voltage is. You might have a bad battery. And are the batteries the same? By pass the auto reset breaker in the old compartment. I would also add a breaker to just right after the isolator and one at the new battery location for safety. Your wire runs are a considerable distance from the isolator and converter. Chances of a short on those runs becomes higher. Just takes a bit of road shake making a wire rub against something. Big gauge wires set things on fire! I think those breakers are available up to 75 amps. Quote Link to comment Share on other sites More sharing options...
Pete1122 Posted December 1, 2013 Author Share Posted December 1, 2013 The new batteries are using the same ground as before, I just extended the ground. I replaced the wire with thick 0 Gauge. The batteries are brand new 6 volt golf cart batteries. I did install a fuse right at the new battery location. I am starting to think your idea of by passing the auto reset breaker in the old compartment might be a good idea. That is where my #10 wire transitions to the new #4 wire and the voltage drops. Maybe I should run a single length of #4 from the isolator straight to the battery? Or keep the breaker and go #4 from the isolator to the breaker? Or am I just overthinking this? Is 13.75 volts from the alt to the batteries at idle good enough? Quote Link to comment Share on other sites More sharing options...
WME Posted December 1, 2013 Share Posted December 1, 2013 Every .1v you lose in the wiring is a .1 less in your battery. You have a limited amount of power to use, so you can't afford to lose any. Quote Link to comment Share on other sites More sharing options...
Vanman Posted December 1, 2013 Share Posted December 1, 2013 Yes I would go after that. Sounds high to me, unless the current draw is really high that seems a bit much. I have replaced/repaired almost every connection made by the coachbuilder. What part is wired with a #10 wire? that's too small to be feeding your coach battery. What starts almost all electrical fires is arcing and in low voltage high current ckts connections are crucial. Go point to point with a digital meter and see if it's mostly one or two items or a slight drop at many places, don't forget to check the ground side too. john Quote Link to comment Share on other sites More sharing options...
Pete1122 Posted December 1, 2013 Author Share Posted December 1, 2013 The only part that is a #10 wire is under the hood at the isolator to my old battery compartment. From the isolator to that point using #10 my voltage is 14.25. It used to extend a #10 wire about 6 inches to the battery from a self resetting fuse. I replaced that 6 inches of #10 wire with 8 feet of #4 to the new battery compartment. From that self resetting fuse to the battery is where I am getting that drop. Quote Link to comment Share on other sites More sharing options...
Pete1122 Posted December 1, 2013 Author Share Posted December 1, 2013 Thank you all for your help, I went through every connection and made sure everything was tight. It looks like I had a bad connection somewhere. I am now getting 14.25 volts at the battery and the isolator is putting out 14.75. Quote Link to comment Share on other sites More sharing options...
WME Posted December 1, 2013 Share Posted December 1, 2013 Close but no cigar, still too much of a drop. .5v vs .36v max. Check/clean the grounds. Go to a bigger wire as last choice. Quote Link to comment Share on other sites More sharing options...
Maineah Posted December 2, 2013 Share Posted December 2, 2013 Get rid of any thing 10 gauge. Mind you you're dealing with trying to charge 450 amps of batteries (if they are T105's). I would say your voltage is in the ball park those big batteries have a pretty fair resistance if they are discharged at all. Quote Link to comment Share on other sites More sharing options...
Bob C Posted December 3, 2013 Share Posted December 3, 2013 You currently have 10 ga wire at the start of your wire run. It does not matter what you have after that because you are limited by the power that you can get downstream. You may have less power loss through the larger wire but you are still limiting power from the isolator to the old box. Quote Link to comment Share on other sites More sharing options...
Pete1122 Posted December 3, 2013 Author Share Posted December 3, 2013 I went through all my connections again, cleaned grounds and tightened all the connections. I am now at a .26 voltage drop, I may quit while I am ahead. With that being said I may go ahead and upgrade the remander wire to 4 awg. Quote Link to comment Share on other sites More sharing options...
WME Posted December 3, 2013 Share Posted December 3, 2013 Before you think this is all BS and a waste of your time, you have gained about 10% of useful charge Quote Link to comment Share on other sites More sharing options...
Gulfstream Greg Posted December 4, 2013 Share Posted December 4, 2013 The new batteries are using the same ground as before, I just extended the ground. I replaced the wire with thick 0 Gauge. The batteries are brand new 6 volt golf cart batteries. I did install a fuse right at the new battery location. I am starting to think your idea of by passing the auto reset breaker in the old compartment might be a good idea. That is where my #10 wire transitions to the new #4 wire and the voltage drops. Maybe I should run a single length of #4 from the isolator straight to the battery? Or keep the breaker and go #4 from the isolator to the breaker? Or am I just overthinking this? Is 13.75 volts from the alt to the batteries at idle good enough? Calculating voltage drop is a tricky calculation. Current load on the end of the run (battery needing a charge and current flowing into the battery) is part of the calc. Someone correct me if I am wrong. But if your battery is fully charged IE both are at the same voltage say 6.8 volts each then the voltage will be high but if they are being charged (because they are at a low voltage) and current is flowing into the batteries then the voltage will show low. In other words high current low voltage, low current high voltage. Maybe I have that backwards but my testings show high current low voltage. I think #4 all the way to the isolator and bypass the breaker at the old location. But I would have a fuse or breaker at both ends (batteries and isolator) reason being that line is always hot. If there is a short two breakers protect the isolator alternator side and the battery side. As a test separate both batteries and do a voltage test. Both should be exactly the same or real close. A 12 volt battery at 12.7 volts is considered 100% charged. A battery at 12.5 volts is considered 90% charged and at 12.06 volts 50% charged. Consider that when looking at voltages. It might be that they just are not fully charged. If you disconnect (or engine off) the batteries at the isolator what voltages do you see at the batteries and at the end of the wire at the isolator? That will tell you your true voltage drop. And you say the batteries are using the same ground as before, what is that exactly? Quote Link to comment Share on other sites More sharing options...
Maineah Posted December 4, 2013 Share Posted December 4, 2013 Two things the little circuit breakers will have to go they will not flow enough current. What I did on mine was use a two way radio fuse assembly rated at 50 amps. Second if you have a solid state isolator no matter what you do you are going to have a .7 volt drop that is the break over voltage of the diodes that is why they have cooling fins because the loss translates into heat there is no way around that other than replacing it with a relay type. Quote Link to comment Share on other sites More sharing options...
Gulfstream Greg Posted December 8, 2013 Share Posted December 8, 2013 A ground from the battery to the closest chassis point would be good also. DC current acts much like water through a pipe. The smaller the pipe the less flow. Also DC flows from negative to positive direction, not pos to neg as many think. I agree with Maineah about the voltage drop from a solid state isolator. Remember though that both the starting and coach circuit would see that drop so is it relative. The question here is the drop at the coach batteries. I'm still not clear what isolator he is using. And what the voltages are on all legs of the isolator. And what voltage of each battery is sitting at disconnected. And yes those auto circuit breakers have small terminals so even though they can be rated at high amperage they really have no flow rate, again the water pipe comparison. Anyhow most likely it's connector and wire size causing it unless one of the batteries is bad. Quote Link to comment Share on other sites More sharing options...
Vanman Posted December 8, 2013 Share Posted December 8, 2013 This post got me checking mine, .14 VDC difference across truck / coach batts. Not ideal, but short of up-grading the wiring size from truck battery to isolator to coach battery I will live with that. john Quote Link to comment Share on other sites More sharing options...
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