connecting the house batteries

[climbing coastie]

I'm getting close to buying some AGM batteries for my van conversion (if I can get the mechanical issues fixed).
My question is can I hook up my wires to the battery or should I go from the alternator?

I will have about a 15 foot run from the vehicle battery (under the drivers floorboard) to my house battery and am planning on using 4 or 6 gauge. I’m not sure if it matters, but I will not be using the chassis as a ground and running positive and negative wires for everything.

Since my starting battery is an AGM I’m guessing that I don’t have to worry about if the alternator will do the job or destroy the house batteries. I plan on putting solar on at some point, but need to get house batteries in before winter to run my heater.

 

[Optimistic Paranoid]

While it‘s always preferable to tap directly off the alternator, it's not always possible.  Depends on where the alternator is located, and how thick the wire is.  Thicker is always better.  Tapping off the engine battery ain't the end of the world.  Don't forget to fuse this hot wire as close to the alternator or battery as possible.

Grounding the house battery(s) to the frame is not the best practice.  It's acceptable on a vehicle with a real frame, not at all a good idea on unibody vehicles.  On those, the return runs through the spot welds holding the body together, and these can crack on older vehicles.  You are better off running a separate ground wire to the engine battery

Regards
John

 

[B and C]

Don't forget that you are now making your chassis (engine) battery part of your house battery system this way.  You will be draining from all three, not just the two house batteries.  You may think about a continous duty solenoid to connect the two backs together.  This would allow you to "separate" the two battery banks.

You might want another fuse close to your house batteries too as that long cable is hot on both ends and always connect the negative battery cable last.

Thicker wire is always better.

Brian

 

[SoulRaven]

Going from the engine battery to the house batteries via a solenoid/isolator and a fuse on each end is fine, but don't plan on running an electric heater.

 

[SternWake]

Agreed with all posted above.

If there is not a means to actually separate the house batteries with the engine off, you do not have a house battery but a very large engine battery in 3 different parts.

There are a dozen different ways to isolate the house batteries, or perhaps it is better to say isolate engine battery from house loads.

Taking power from the engine battery for house batteries is OK, but the weak link is the original alternator charging circuit which must now also carry the current for the house bank.  It was never designed for this purpose and recharging of house battery suffers for it.

Taking power for house batteries right from alternator (+) bypasses the original alternator circuit, and is usually a shorter circuit path as well.  This method can also trick the voltage regulator into allowing a higher voltage be held for longer,  quickening the recharge process, and perhaps by a substantial amount as a discharged battery that requires 45 amps to be held at 14.4v will only require ~12 amps to be held at 13.7v, and perhaps not even that.

You can certainly run the house battery to feed off the engine battery, and run a ground cable from house battery to engine battery too.

Ideal would be house battery (+) to alternator (+) and the house battery (-) cable would go not to the engine battery, but an Alternator mounting bolt, or the alternator  (-) stud, if one exists.

Another possibility is to just add another cable from Alternator (+) to engine battery (+) and beef up the grounds too, but this is basically more cable than just taking power from the alternator(+) in the first place.

Keep in mind some lower Dollar AGMS say to not exceed 30 amps of charge current per 100AH of capacity, where as the higher $$ AGMs will say there is no limit on charging current, and in fact they say not to allow less than 20 amps per 100AH or in the case of Odyssey, not to allow less than 40% when cycled to 50% State of charge.  If you have the lesser$$ AGMS and an alternator that can provide more than this, over cabling able to pass this much easily, then such AGMS can be fed at too high a rate.  They could exceed the ability of the battery to recombine the gasses and the vents Might open.

In such cases then one should not really be seeking the thickest shortest charging path possible from the alternator.  One can limit it by choosing 6 or 8 awg cabling rather than going for 4 or 2awg cable.

There are lots of variables, platform specific variables.  If the vehicle allows high voltages in the high 14v range, then that pressure differential between depleted battery and alternator is going to allow huge recharging amps to l flow, if the cabling is thick enough to allow it to flow.

 

[ccbreder]

I read of alternator problems as the house bank puts heavy load on the alternator.

 

[SternWake]

I read of alternator problems as the house bank puts heavy load on the alternator.

Anything that has to work harder is going to wear out quicker.

One should factor in the inconvenience and cost of batteries that are not fully recharged, cannot deliver for the user, and have a much shortened lifespan, vs the cost of having to replace the alternator sooner.

It is the Heat generated which really takes a toll on alternators when they are putting out anywhere near their maximum.

In my case, , I can swap out the alternator in 15 minutes, and in 2004 bought a lifetime warranty alternator.  While i got a bunch of badly remanufactured ones in the beginning I sent to an early grave as as i did not have solar then, the latest one lasted since 2006, and O'reilleys honored Kragen's lifetime warranty and I walked out of the store with another remanufactured unit, for free.

So it is worth it to me to have the alternator feed the battery as much as it can, voltage limited of course, but someone whose alternator runs 300$ and requires 4 hours of shop labor, might just want to use thinner cabling to limit what the alternator can feed the house bank of batteries, and extend its life.