This is a bit of a long one, and I am still not done figuring it out. But, it was one of those issues that lead me down a rabbit hole, and when I came out the other side I knew a ton more about how the boat works. 

  • Thing #1 – When we were at anchor and the house banks were at 53%, the solar panels didn’t seem to be putting out enough charge. 
  • Thing #2 – After plugging into shore power at 77% SOC, the Magnum shore power charger just went straight to float (instead of bulk, as expected). The next day, the house banks were still only at 80%, but the charger said “battery full”. 

This lead me down a winding path of poorly-translated charger manuals, battery theory, and system integration philosophy, which I will mostly leave out here. But, here’s what I learned in the nutshell. 

When the batteries were low and I saw this on the charger:

I’m like, “Why aren’t the panels putting out as much as possible? Shouldn’t they be in ‘Bulk’?” The answer was, of course, that they were. “Bulk” charging just means that the charger is putting as much as it can into the batteries. In this case, the panels are generating 88.56 watts, which the MPPT charger is converting into something that the DC system can use, in this case 7.7 amps at 12.5 volts (though, how it’s converting it up to 96.25 watts is a question for another day). The voltage is determined by the resistance of the battery, not by some setting on the charger. So the charger cranks as much current (amps) into the battery as possible and the voltage builds as the battery resists the charge. In this case, the sun was still relatively low in the sky.

Lesson Learned #1 – The solar MPPT charger will push as much current into the batteries as it can up until the voltage hits the absorb setting (14.4v in this case). If the sun is low or obscured, it will be less than if the sun is directly overhead. I thought that the voltage of the system was a set value, but that’s only true when the batteries are charged enough that they create some resistance to the charging sources. Once that resistance passes the absorb threshold, the charger will hold the voltage stable and continue to lower the charge to keep it there.

Lesson Learned #2 – The reported voltage of the DC system is a combination of all of the load, all of the charge, and passive resistance of the various components.

In practice this means that the voltage I see on different components will be different. So, in this case, the panels saw 12.5v. But, the battery monitor saw 12.31v. So we’re seeing a .3v difference between the charge output and all of the resistance in the system (the batteries and any devices using power).

Since the battery monitor (BMK) is a shunt that is the last link between all of the negative bus items and the battery, it is the most accurate measure of the voltage on the negative battery terminal.

Note that the solar charger is bridged directly to the charging input on the positive side of the batteries.

So any current from the solar panels is combined with current from the charger.

Lesson Learned #3 – the reported current in the system will be different depending on where it’s being measured. This one really confused me. I was trying to do simple math – if the DC + AC loads are X (a negative number) and the solar panels are outputting Y, the the reported load on the battery monitor should be X + Y. Nope. While that’s generally true, there are so many devices on the system (all fluctuating) the numbers will not add up exactly. Ultimately, the battery monitor will tell you what is going into or coming out of the house bank. But, trying to add up the loads across the system is imprecise. However, it is directionally true. So, at one point, the sun got bright enough and the batteries were charged enough (we ran the generator) that the resistance (voltage) increased and we saw a net surplus on the BMK. This means the sun was charging the batteries enough to produce a small surplus (over the house loads).

Also, the components all measure with different levels of precision. From the manual: “The DC Amps meter displays the amount of current going in or out of the battery. A negative number shows the amount of current being removed from the battery. A positive number shows the amount of current delivered to the batteries. This meter converts AC amps to display DC amps, so the accuracy below one amp AC (~10 amps DC @ 12 VDC) is not detected. When the current detected is greater than one amp AC, the accuracy of this meter is ±20%.” I’ll trust the BMK more than the regular DC volts.

Lesson Learned #4 – There are race conditions between the three phase solar panel charger and the three phase shore power charger. Each are using different voltage levels and thresholds for absorb and float settings. Also, since voltage is a trigger, the charging volts from one system can cause the other to go into an incorrect state. In most cases, this just means that the chargers take turns doing what they can to charge the batteries, (backing off when the other one is working harder). In some cases, it can prevent charging.  This has been a problem for a while.

In our real-world example, we returned from a cruise at 77% SOC with tons of sun. When we plugged into shore power, the solar panels were producing enough current (20.9A) that the battery was resisting at 13.7v, so the shore power charger went right into float (13.3 volts at the shunt). This is by design! From the manual: “If the battery is >13.0 VDC… then the battery was already charged and the charger automatically goes to Float charging to keep from overcharging the batteries.”

The next day, the batteries were still at 80%, though the charger said “full” Once again, this was explained in the manual. “After four hours of float charging, the charger turns off and “Full Charge” displays (charger is now in Battery SaverTM mode). If the battery voltage drops to ≥12.6v… the charger automatically initiates another four hours of float charging.” But, what happened is, the solar panels + shore power float got the batteries to 80% (~12.7v), so the second float cycle was never triggered.

The best way to avoid this is to force a Bulk / Absorb / Float cycle when you first plug into shore power. There’s a function on the Magnum remote under CTRL to do just this. When I tried it, the charger jumped into bulk and very quickly hit absorb. Right now, the absorb cycle is set to run for two hours, which is about right to get from an 80% charge to float.

There will be other race conditions. This is exacerbated by the chargers having slightly different set points for their various modes and triggers.

Start Bulk IfStart AbsorbAbsorb timeFloatBattery fullSkip to float
Magnum<= 12.8v>= 14.5v2 hrs13.5v for 4 hoursafter 4 hours float*>=12.9v
Tracer<= 12.6 (?)>= 14.4v2 hrs13.8vNA – stays in float if possibleNA

*When in “battery full” mode. The Magnum will enter absorb if the voltage drops to 12.6 and bulk if it drops to 12.1. 

Lesson Learned #5 – System integration is hard. The Tracer MPPT Solar charger uses different defaults than the Magnum. The magnum was set to the “wrong” type of AGM (Lifeline) (though since these are OEM batteries, there are no specs immediately available). There are also a number of settings like maximum charge rate, Absorb cycle time, etc. that probably need to be optimized. Don’t rely on the default settings and don’t assume the builder/commissioner set it up right. Even solid boats with experienced owners get this wrong.

Also, for posterity, here’s the cheatsheet for charge level per voltage for AGMs.

  • 100% – 12.8v+
  • 75% – 12.6v
  • 50% – 12.3v
  • ‘25% – 12.00v