32 replies to this topic

[Mthomas]

I am also interested in increasing the rate of charge to my house batteries from the alternator when driving/running. My diesel has an over 200 amp alternator, and should be able to supply some decent current.  I am a bit of an electrical newby and had a couple questions. This is not meant as a criticism, just trying to understand:

 

1. why such high amperage breakers? I do not think anything in a FWC draws over 30 amps. I believe the current MPPT supplied by FWC only draws 15 amps and most of the smaller units I have seen people using for these solar systems draw 30 or less, so that would be it for charging from the alternator, wouldn't it? Wouldn't it make sense to have a breaker trip well before the max draw of the charger?

 

2. I also do not understand the rational behind a breaker on both ends of the supply wire. 

 

3. What is the purpose of a breaker in the ground return to the batteries?

[ClimberRob]

I am also interested in increasing the rate of charge to my house batteries from the alternator when driving/running. My diesel has an over 200 amp alternator, and should be able to supply some decent current.  I am a bit of an electrical newby and had a couple questions. This is not meant as a criticism, just trying to understand:

 

1. why such high amperage breakers? I do not think anything in a FWC draws over 30 amps. I believe the current MPPT supplied by FWC only draws 15 amps and most of the smaller units I have seen people using for these solar systems draw 30 or less, so that would be it for charging from the alternator, wouldn't it? Wouldn't it make sense to have a breaker trip well before the max draw of the charger?

 

2. I also do not understand the rational behind a breaker on both ends of the supply wire. 

 

3. What is the purpose of a breaker in the ground return to the batteries?

 

Pick a breaker that fits your individual system and needs. If you are charging straight off of your truck without a DC-DC charger and have a large enough wire, you will likely be pushing more amps than is healthy for your battery. 25 amps is a maximum safe charge to push into a single 100 AH AGM battery, for example.

 

A breaker on both ends of the wire will limit damage to the truck battery, as well as camper battery. If you only have breakers on one end, one of the batteries would still be connected to that wire once the single breaker trips.

 

An issue with the negative wire can lead to arcing, increased resistance, and possible fire. Isolating that wire with a breaker is simply additional insurance.

[Vic Harder]

My diesel has an over 200 amp alternator, and should be able to supply some decent current.  

 

1. why such high amperage breakers? I do not think anything in a FWC draws over 30 amps. I believe the current MPPT supplied by FWC only draws 15 amps and most of the smaller units I have seen people using for these solar systems draw 30 or less, so that would be it for charging from the alternator, wouldn't it? Wouldn't it make sense to have a breaker trip well before the max draw of the charger?

 

2. I also do not understand the rational behind a breaker on both ends of the supply wire. 

 

3. What is the purpose of a breaker in the ground return to the batteries?

1) Yes, you alternator will supply all the current that will go into any potential load connected to it.

 

The biggest load -- and the reason for the bigger wires -- is  your camper battery.  Assume for a minute that the camper had been drawing 10A all night, and over a 10 hour period your camper battery was nearly completely discharged.  So, it is 100AH short of "Full".  Your alternator will try to push all 200A into that empty battery as fast as it can.  With my 130A alternator and a 250AH camper battery bank, I saw 90A when the batteries were at 55%.  That's a lot of juice!

 

2) Likewise, both ends of the +ve wire are connected to batteries.  The breakers protect that wire and your life by being able to disconnect either/both ends of the wire from any battery.  

 

2) Not sure I get the ground wire breaker thing myself.  I haven't seen/done that myself.

[Mthomas]

interesting ideas in this article, follow the link back to his blog. 

https://cookfb.files...ring-scheme.pdf

[Vic Harder]

Sounds very similar to what others here are saying. keep in mind that this guy has a huge battery bank. 2/0 AWG is not necessary for the typical FWC without an induction cooktop. he is recovering 150AH of overnight use. most of us are using 10-45 AH.

[ClimberRob]

 

 

2) Not sure I get the ground wire breaker thing myself.  I haven't seen/done that myself.

Vic,

 

A fuse or breaker in the negative wire protects the conductor/wire. There are various scenarios where the negative wire can become a serious hazard. A fuse on the positive side will sufficiently protect the equipment. Fuses/breakers on the negative side will isolate the wire and protect the operator. You could also place a diode on the negative side, if you find that easier.

Edited by ClimberRob, 10 September 2019 - 03:55 AM.

[rando]

I can't really think of a scenario where a fuse/breaker on the negative wire would be required or help.    If there were to be a short on the negative wire (which would have to be a short to a positive wire) then the the current would also have to flow through the positive wire, and the fuse/breaker on that wire would trip. 

 

The only place that I can think of where fuses are commonly used on ground wires is for equipment (usually ham radios) that have a multiple grounds (negative wire and antennas) that are wired directly to the battery with skinny wire.   This is done so on the off chance that the ground strap on the battery breaks and you try and start the car, it could pull the starter current through the radio.   This is not an issue in the camper scenario as there is only one ground (the cable we are talking about) and it hefty enough to handle the current anyway.

 

A diode is also not going to work - the ~0.7V drop across the diode would kill the ability to charge the camper battery (not to mention getting super hot at 30+ Amps).

 

Is there a scenario I am not considering here?

[ClimberRob]

There are quite a few ways you could end up with current flowing through the negative wire. Reverse polarity hookup, shorted battery (internally), etc. People hook up equipment improperly quite often, which can result in a short.

 

Many 12v electronics have a 16v diode across the negative connection. It works quite well. Obviously a fuse or breaker is the correct choice for the battery connection.

[rando]

In all these scenarios, wouldn't the same current be flowing through the positive wire to the camper battery, and therefore be protected by the circuit breakers on that wire?  The only reason you would need a separate breaker on the negative would be in the case where you could have an (excessive) current flowing on the negative wire which is not also flowing on the positive wire.  I can't think of any scenarios where this would be the case.

 

Not trying to be argumentative here, but trying to make sure we are giving good advice.    A breaker on the negative for a dual battery is not something I have ever run across before, so I want to make sure it actually serves a purpose and that I am not missing something. 

[ntsqd]

Rando beat me to it. Ham radios usually have a fuse in the ground wire. It is there mostly to protect the radio from reversed polarity. Ask me how I know this.

 

I used 285 series 80A breakers, an Anderson SB120 connector, and 6 ga. Ancor paired cables. I chose the paired cables because they have an extra layer of insulation to protect them from road debris and chafing - AND I detest split loom. I arrived at 6 ga. from measuring the total circuit length and assuming 80A max charge, then consulting Ancor's technical page on sizing DC conductors. Run cable for the full circuit length, don't try to ground to the truck's frame or something.

 

A lead- acid battery's internal resistance will self limit the max actual charge rate. They're not super-capacitors. No knowledge if you're using one of the more advanced chemistry batteries.