Solar Charger to Keep Chassis Battery Charged?

[B and C]

I am at the sticks and bricks right now.  I would much rather be almost anywhere but here.  Circumstances prohibit me from leaving at least for a few months.  Anyway...

My van is behind the house and I have not started the engine in about 3 months (I do exercise the generator monthly though).  Guess what, the chassis battery was down to 12.3v!  I put a charger on it and got it charged.  This got me to thinking (dangerous sometimes).  I have seen threads where suggestions were to use a DC/DC charger and they seem pricey for what they do.  Then there are the ACR's (automatic charging relay) and seem cheaper but not as cheap as a PWM Solar Controller.

I am just starting to look.  A little 10A SC should be able to keep the chassis battery up to snuff.  This is the first one I found that seems to fit the bill: https://www.amazon.com/dp/B07B49VQYK/ref=twister_B074XGS16W?_encoding=UTF8&psc=1
(10A PWM SC)

I would not be using solar directly to run it though.  I have 250Ah's of AGM battery with a MidNite Kid SC with the Whiz Bang Jr. and 200 (permanant) or 400 watts (with 200 watts of portable addon) of solar to keep the batteries happy.

Here is where I think I may get myself in trouble.  Instead of connecting a solar panel to the input of the SCC (solar charge controller), I would connect the solar input to my house bank instead and use that as a sudo solar panel.  I don't have room or a good way to run additional wires for a small solar panel and this would eliminate the need to buy, mount and run cables for it.  

Anywho, here is the plan as it is starting to take shape in my pea brain.  10 Ga. wire from + and - of house battery (15A fuse on + near battery) to SC input.  10 Ga. wire to chassis battery + and - with 15A fuse near chassis battery.  With an adjustable float voltage on the SC, I could adjust out any voltage drop to the chassis battery.  Oh, forgot a switch in the + for the SC input wire to make a chassis battery change easier or to turn the SC off for any reason someone might come up with here.


There are some great minds here and I am hoping for some feedback on whether this will or will not work.  So fire away and tell me I'm crazy (at least I have a good time ).

 

[Trebor English]

The inexpensive (cheap) PWM solar charge controllers are designed to work from a source that limits the current.  Solar panels do that.  250 amp hour AGM batteries don't do that.  When the little transistor switch turns on connecting the big battery to the starter battery the little transistor switch is likely to go poof.  

The controller you linked is different from mine.  Mine is blue.  The way it works is the pulse width modulating switching is done by switching the minus side.  The plus side is always on.  With a solar panel input either side could be switched.  If the source is your house battery with the minus tied to the chassis that minus side switching will fail.

 

[Trebor English]

It isn't obvious from the Midnight web page but there is a load circuit and a dump circuit.  You could take a signal that is on when the house batteries are charging or full and use that to turn on the coil of an automotive 1 inch cube relay.  The relay contacts can tie the house battery to the starter battery.  If you have 20 feet of #16 wire the current will never be high but it will keep the starter battery charged.  

Those relays are cheap.

 

[B and C]

"The inexpensive (cheap) PWM solar charge controllers are designed to work from a source that limits the current.  Solar panels do that.  250 amp hour AGM batteries don't do that.  When the little transistor switch turns on connecting the big battery to the starter battery the little transistor switch is likely to go poof."  

Maybe running a 20 or 24 Ga wire from the house battery to input to help limit current?  This will be used as a battery maintainer and not to bring a dead battery up to par.

"The way it works is the pulse width modulating switching is done by switching the minus side.  The plus side is always on.  With a solar panel input either side could be switched.  If the source is your house battery with the minus tied to the chassis that minus side switching will fail."

Minus side switching is a positive ground controller then?  So if I can find a negative ground one that would work?  I was really planning on using the chassis for the common - bus on this tiny system.  My batteries are at least 20 cable feet round trip apart.  I was planning to put the controller close to the house battery with the long run to chassis battery and only run the + wire with short chassis ground connections.

 

[Trebor English]

It is not positive ground.  The positive plus 12 doesn't connect to the chassis.  On mine the panel plus connects to the battery plus all the time.  The panel minus is connected to chassis ground, the battety minus, sometimes, on and off.  The positive is not grounded.  

If you really want to use one of these charge controllers connect the middle two wires, the battery wires, to your AGM house batteries, plus to plus, minus to minus.  Program the load output to 24 hour, not porch light, and set the voltage to 14 volts.  Connect the load outputs to a relay coil.  Connect the relay contacts to the two batteries.  When your house battery is above the 13.8 float level your starter battery will get charged.  

The old timey flashlights with two D cells have a switch on the side.  It switches the connection from the D cell minus to the bulb.  That is a minus side switch.  I don't think either end of the D cells is called ground.

 

[Trebor English]

The Midnight Kid web page says it has a dump output, excess power to feed to a water heater.  If that is no more than the battery voltage it could be run to the starter battery or used to control a relay to connect the two.

 

[Trebor English]

After thinking about it overnight I have another suggestion.  Use a diode, a regular silicon diode.  The 0.7 volt drop would be a good thing.  When the house batteries are being floated at 13.8 the starter battery would be at 13.1.  That's enough to maintain but not have any water loss to electrolysis.  When the house battery is absorbing at 14.4 the starter would be floating at 13.7.  Again not an electrolysis problem.  Overnight when the solar is not charging the vehicle battery can drop to 12.7 without any discharging of the house battery.  

The circuit needs enough resistance to protect the diode.  If the house battery is at 14.4 volts and during engine cranking the starter battery drops to 10 volts that's 4.4 volts.  The standard for cheap diodes, 1n4001, can handle 1 amp.  A 10 ohm resistor will keep the current below a half amp.  Maybe a dashboard lamp would have an appropriate resistance.  

This approach is consistent with kiss, keep it stupid simple.  One diode and one resistor is stupid simple.  It takes full advantage of what is already there and adds as little as possible.  The only failure mode I can think of is a short to ground in the wire.  At the house battery the diode will blow and the starter battery with the 10 ohm load will slowly die.  With charge controllers and relays the possible failures are too long to list.  

One more thought, make the connection between the systems at points that are fused.

 

[Trebor English]

Since it took 3 months for the starter battery to go down to 12.3 volts, do you have a battery disconnect switch?

 

[highdesertranger]

you should also be starting your van at least once a month. driving it a bit wouldn't hurt either. a battry is cheap compared to an engine or transmission. highdesertranger

 

[tx2sturgis]

Since you are parked at a house, why not buy a simple plug-in battery maintainer, like a Battery Tender, and hook that up?

Keep that thing on life support.

 

[B and C]

Thanks for everyone's input. I do appreciate it. It seems the easiest and seems to be the best way to keep the chassis battery charged is do as HDR suggested. When I exercise the genny, I'll fire the van up and move it around some to keep all the gears and bearings coated in oil. Don't know why I didn't even consider this. Doh!