Got a handy new toy. A clamp on amp meter. Now easy to measure current flow. For those who SMB is based on the Chevrolet Express Van.

As can be seen when the van is first started the current flow to the house batteries is higher then it is after a few minutes of running. Since I have used this system for 15 years (first w one HB and then w two HB's and now with golf car batteries) I figure this is a non issue. Have always had good battery life. We do though when we have access to 110v while camping keep the batteries topped off so the alternator does not have to work as hard.

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Len & Joanne

The Green TARDIS

I have a Ford, but I'm curious about something. If I'm reading this right, over 100 amps is potentially available to your house batteries during highway cruising. Is that after the engine/Chevy systems take whatever power they need? Do you tend to see your house bank actually getting this much?

I have a friend with my same V-10 and alternator (120 amp, IIRC). He has a Victron "amp counter' battery monitor on the house bank, so that can show amps in. I was curious, so we started his rig the other day and I watched the gauge. We did not go down the highway, although he "revved" and I didn't see any change.

He says he has seen 90 amps, but that day it showed around 40 amps. Now, this may be because his house bank is typically up at 80% or higher SOC (due to solar panels). So I don't think the batteries would take any more amps even if they were available. He has #2 wire and about a 15' one-way run to the battery bank. This wire size was chosen as the minimum that would allow him to fuse such that he could "jump start" his start battery from the house bank and still be within the ampacity rating of the wire (but it's close) (in his case it would have been hard to fit a larger cable). Of course if only using for charging, then there is only voltage drop to worry about, and no need to fuse such that engine starting currents won't blow a fuse.

I've never known how much the engine/systems "take" from that 120 amps, nor how much really can come to the house bank (presuming it was at a low enough state of SOC to take it). So I'm interested in that part of the equation.

100 amps? No, I very much doubt it. Some amps go to the van for its needs.

As noted when the house battery is at 12.55v the alternator starts out supplying 38 amps but quickly over a very few minutes drops to 20 amps. And eventually as the battery gets up to 14.7v drop down to 0.65A-0.80A.

My in line fuse (self resetting) is rated for 50A. So first off the current will never get above that. Also remember that 18 feet of 6 ga. wire carrying the current has voltage drop and the higher the amps flowing the higher the line voltage drop. That is the alternator sees a voltage equal to the sum of the battery voltage plus the line loss. I used 6 ga. so it would "cushion/baby" the alternator when charging the house batteries.

Really will not know the value until we boondock, discharge the house batteries a good 50% and REMEMBER to measure the current flow when I start the van.

Doing some calculations on the back of a dirty napkin: My guess is that if the HB was REALLY down to 50% of charge (12.14v) that the alternator would be seeing (adding line voltage loss) 13.18v and generating 65A. If correct, my resettable fuse would have to cycle off/on a few times before the battery voltage got up high enough to limit the alternator current 50A or less. Really don't know if the house batteries have ever truly been that low. And if they had would I know the fuse was tripping as we drove down the road??

As you and your friend noted. I also do not see any increase in amp flow when I take the engine off idle to a higher rpm.

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Len & Joanne

The Green TARDIS

Thanks for the additional info. I think I must have slightly misunderstood your chart (how I got to 100 amps). Oops!