A question arose on the CruisersForum recently which I responded to:

My solar panel has a controller. I just talked to my electrical guy with 40 years of experience. He said the device with the highest voltage will provide DC to the batteries. The others will shut down (their controllers will).

Well, my thought is that your electrical guy with decades of experience is either over-simplifying it for you (and other customers) or he learned something 40y ago and never reflected on it, as how wrong he has been all those years…

The real world is more like this: if you have healthy batteries that are somewhat discharged to a level of, say, below 80% (that’s called 80% SOC, state of charge), their electrical resistance is very small, hence they can accept or absorb a lot of energy, which is the current that’s being fed into them.

Now, your producers all “see” (as in:measure) the voltage on the power bus, say 12.6V for FLA or 13.3V for LFPs. If your solar CC now produces 10A, it’s not going to raise the bus voltage until the batteries are charged higher, to say 85% or more. The CC will try to put out maybe 14.4V, but the batteries with their low resistance are pulling that down to wherever they are.

Next you shore power supply also sees the 12.6 or 13.3V and also puts out whatever current it can, maybe 15A. Again the bus voltage doesn’t rise at first, but there are 25A going into the batts.

If you then turn on the iron genny, the alternator tries to produce another 40A, if the batts can absorb all that current.

If they do, the voltage is still at the same level of 12.6 or 13.3V but it will now rise a lot faster, since there are now 40+25=60 Amps going in. Without the other two sources, the net current would now be 40A and the voltage rise would be slower.

Finally, as the bus voltage slowly increases because the batteries are getting full, each of those devices will throttle back, depending on its individual setting.
Only at that point one might say: they shut down. Only they don’t, they just stop producing energy until the voltage drops back to their lower set point. (Or they go from this “bulk phase” into an absorption phase, depending on their profile.)

You can check all that yourself if you buy, borrow or steal a DC-capable clamp ammeter, turn on the chargers and check each positive lead coming from them, then compare it to the positive lead going into your batteries. It should sum up, minus the cable losses and other consumers that are running at the time.
Note: If you get a negative current on the ammeter display, it just means the current goes the other way. If you clamp the meter on the other way (rotated 180 degrees), you would see the opposite sign yet the same value).

So in short, you can have many sources of current all feeding into one battery bank, albeit every source might behave a little bit different towards the end of this charge cycle. The standard alternator, for example, is the simplest of those devices and just turns off at around 14.4V and back on at its lower threshold/setpoint. It also puts out a pretty bad, pulsed DC which can, in fact, confuse other devices measuring the bus voltage.

Must be the weekend that I have time to write all this… hope it helps you and someone else down the track. I might just copy it into my blog as well, coming to think of it..

3 thoughts on “Can you charge your batteries from multiple sources?

  1. Reference your comments:

    What pleasure to you derive from your condescending and demeaning attitude in your posting? Is it not as much fun when you can’t be a dick about it? Not worth it, is it? If you can’t show just how much more clever you are than the other guy, why bother. Imagine all the great teachers of the world if they spoke to other people like you do? Just sayin ‘ ……

    1. There is a lot of misinformation out there on the net, confusing some, misguiding others. The post wasn’t meant as demeaning but expressed my attitude towards such things.

    2. @Derk: your post is completely out of line and doesn’t belong here! The accusations are also not accurat at all (after having read the CF link). If I were huey I’d block and delete you. Not worth having such content/people on a blog.

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