Side effect of solar + alternator charging

A few months I noticed something odd about my isolator charging system.    Then started to see a pattern.  Finally figured it out and wanted to share since this issue could affect others.

We all know what a solar install does, but for issue I will restate the obvious:  the solar panels generate power whenever there is sufficient light.  I have an overpaneled system (570w) so I get some power generation even in relatively low insolation.

[The battery isolator (a Battery Doctor 75/100A, a single voltage sending relay (sVSR)) connects the banks when the starter battery side sees 13.4v.  It disconnects a couple of minutes after the starter side voltage drops below that setpoint.

observed oddness

The oddness: sometimes the isolator didn’t disconnect after a drive.  I thought it was a glitch so I unhooked the ground from the isolator to reset it.

Didn’t happen, didn’t happen, then happened again.  After a while I realized it was disconnecting as expected only at night.  Whaaaat?

Oh, I get it now.

1.  driving the vehicle raised the starter battery side >=13.4v, which connected the batteries

2. turning off the vehicle should allow starter side to fall <13.4v, thereby allowing the disconnect…

3. but in daytime the panels are putting >-13.4v to the house side, which is connected to the starter side which still sees that same voltage

4. so the isolator doesn’t disconnect until the sun goes down

the downside

“Great!”, one might say, “my starter battery is getting fully charged!”

Yes, but depending on battery type and specs, the absorption voltage may be too high for it.  Or maybe it’s a “maintenance free” battery where you can’t add more water.  Or maybe you don’t want 14.8v (or 15v+ equalization) backfeeding into your vehicle electronics.

Also consider:  house batteries are deep cycled and the starter battery is not.  In general, deeply cycled batteries require higher Vabs and Vfloat so we can see that even if they are both flooded lead acid their charging profiles should be different.

my solution


I figured a high voltage disconnect (HVD) should do the trick, so i ordered this one off Amazon.  I played with it and couldn’t figure out what it was doing (Chinese electronics documentation is notoriously poor).

I read some articles where people had figured out some of the functions.  I had been mis-understanding that the relay could be configured for high- or low-voltage cutoff by toggling NO vs. NC.

Instead, it has two setpoints and the toggle.  So there are three voltage conditions it can experience:

• higher setpoint

• at or between the setpoints

• < the lower setpoint

Toggling controls what happens in between the two.

The relay is now sits between chassis ground and the isolator’s tiny ground wire that is there to allow it to power itself (it already has a connection to starter  battery positive).

higher setpoint

I settled on 13.8v quicly because

1. I didn’t want to feed any more than that back to the van or to the starter battery.

2. 13.8v is the voltage at which barefoot LiFePO4 battery banks are fully charged.

I don’t have lithium while I am tied to one spot, but will when I can stay out of freezing temps.  I want to play with the 13.8v setpoint to see how it affects the system.

Am I missing out on some high current charging by limiting the voltage?  Yes, but it’s already spiking to 40A when the house bank is fully cycled to 50% DoD.  And 220Ah/40A is quite close to the C/5 charging limit for flooded lead acid batteries.

lower setpoint

I don’t have to worry about dropping voltage pulling down the starter battery, as the isolator unilaterally disconnects at <13.4v.

That left rising voltage.  Setting the value higher than 13.4v would thwart the function of the isolator, but would be a way of adjusting that setpoint if desired.   Setting it equal to or a little less than the setpoint could (and did!) result in rapid ping-ponging between states.

I configged this lower setpoint to 13.0v and left it there.  I think it could be arbitrarily set anywhere from 12.7-13.2v with no practical difference.

the effect

The batteries are connected when the starter side is >= 13.4v, and then disconnected when the voltage of either bank is >13.8v.