RV Battery Combiner Comparator Circuit

[For The Popcorn]

I'm working on resolving some potential issues on our 1976 GMC Motorhome. One issue is battery charging. The engine alternator charges the engine battery and the house battery via a battery isolator – both batteries charge, but the loads are isolated.

The house batteries are also charged by a solar panel or a 120VAC (North America) – 12v charger which can be operated from 'shore power' or the generator.

There's a continous-duty boost solenoid that ties house and chassis batteries together. If the chassis battery is dead, this may allow the engine to start by paralleling the batteries.

The downside of this system is that the chassis battery can't charge from the solar panel or on-board battery charger. My thought to fix this issue was to replace the battery isolator with a battery combiner – in simple terms a relay to tie the batteries together if either side is charging.

Then I realized my boost solenoid is already most of a battery combiner. If the voltage on the house battery is greater than around 13 volts, closing the boost solenoid will charge the chassis battery.

And so my question: Got any unique way to monitor battery voltage for some level over 13 volts and provide an output to activate a relay? I'll probably use an "Arduino relay module" that includes a transistor driver as I believe the boost solenoid draws a fair bit of current. Ideally, the circuit should be very low power so as not to drain the house battery during days when no solar or shore power charging is available. Hard to believe that days or weeks could go by with no appreciable solar charging in Seattle

 

[danadak]

Simple Arduino Nano or ATTINY85 would suffice. Could put them to
periodic sleep to conserve power, Iavg < 1 mA I would think.

Here is code using mBlock. I did not put hysteresis in the test for 13V,
you can easily add that, you get the idea. Mblock takes block design
and generates code for Arduino. In this case you would take code
generated and add the sleep functionality. Plenty examples on
web doing that. Although the ATTINY85 running at 1 Mhz and 2V
only draws .5 mA, so maybe use that approach.

You can use either a Nano board or an Attiny85 8 pin minidip, using a
Nano board to program it.

Regards, Dana.

 

[For The Popcorn]

A micro seems like overkill (says the guy who would prefer a micro solution)....

I don't think hysteresis is a big issue – I believe there's a pretty distinct difference between battery voltage at rest vs. charge voltage.

Just noodling around, my thought at the moment is a low-power, low quiescent-current linear regulator, say with an 8 volt output, and a comparator fed via a voltage divider.

 

[rjenkinsgb]

The solar power system presumably has some form of control to isolate the solar panels & prevent them discharging the battery.

If you could pick up power to run the monitoring circuit from the panel side of that, via a small buck reg if appropriate, it means zero power from the batteries.

Add a small 120 > 12V PSU & combine the two sources via diodes and you automatically have control power any time charging power may be available.
eg.

IRM-02-12 | Mean Well Encapsulated, Switching Power Supply, 12V dc, 167mA, 2W | RS

uk.rs-online.com

Then just an LM311 type comparator with a pot & divider from the aux battery with a zener for reference (and a bit of filtering & hysteresis) and you can control the charge link relay / contactor.

 

[danadak]

A micro seems like overkill (says the guy who would prefer a micro solution)....

I don't think hysteresis is a big issue – I believe there's a pretty distinct difference between battery voltage at rest vs. charge voltage.

Just noodling around, my thought at the moment is a low-power, low quiescent-current linear regulator, say with an 8 volt output, and a comparator fed via a voltage divider.

These days I think of Micros as simply more capable OpAmps and Comparators,
with some decent enhancements to make life easy. Insofar as the hysteresis always
cognizant of load generated noise and charger ripple (especially in cheap chargers).

I do see folks using CMOS logic parts and RC to generate timing, but ugh thats a recipe
for real crappy timing accuracy and drift. Todays low end processors with production
trimmed onchip timing make that problem go away for many designs. Even onchip
trimmed Vrefs terrific.

I did a lot of rats nest OpAmp and Comparator and Transistors type design, no
longer anxious to go back there. I love the mixed signal routable parts for design these
days. Especially the GUI tools that go with them.

Its all good.


Regards, Dana.