Options for using my solar to keep starter battery charged over winter

[a very small camper van]

Thanks for starting up this forum! I was going to post a question on your youtube channel but this is much better!

I have a nice solar set up in my minivan camper which I am quite proud of and which works well.
Now that winter is around the corner here in Canada, the van will basically be parked on the street for the next 6 months.
My starter battery is getting old and has trouble holding a full charge but it is sitting in a van with 100 watts of solar on the roof which is doing nothing other than float charging the house battery.
I am wondering if there is a simple and inexpensive way to use that solar to keep the starter battery topped up.
I know I could buy a DC to DC charger or set up a seperate charge controller but its a bit over my budget (as is a new battery right now).

The two option I am considering are: option 1: to run a cable from the 12v socket coming from the solar panel/house battery to the always on 12 v socket on the dash.
When the panel is charging a volt meter in the 12v socket reads 14 v, which should be enough to charge the starter battery. This would effectively connect the starter battery and the house battery to the same charge controller. Is this problematic? They are both AGMs but different sizes and ages.

Option 2: I have a small (150 watt continuous) inverter which I can plug into the 12v socket coming from the house battery and I also have a 3.5A genius charger which I use as a "shore power" set up. I could plug it into the inverter and feed the power to the 12v always on socket on the dash.
This has the disadvantage of stepping the 12v up to 110v then back down to 14v to charge the battery but has the advantage that it keeps the two batteries separate.
As an extra layer of protection I happen to have a "battery saver" cable which I bought for my koolatron thermoelectric cooler, which will cut off if the house battery voltage drops too low.

There is also a 3rd option, which is that the 150 amp Battery Doctor isolater that I have has a button that I can push to use both batteries to start the car... so in a pinch...

Which of these option seems best? If these ideas are no-no's what exactly would be the problem? I am trying to understand all my van electrics as well as I can. Does anyone have other ideas?
I have included 3 diagrams showing the basic set up and the options.

Any help is appreciated!

 

[SolarRat]

There are many ways to do it, and you covered all but the easiest .

My truck has a bad control module that always draws down the battery when sitting. I got tired of disconnecting the batter after every use so I slapped a small solar panel on the dash and plugged it into the cig lighter. I do similar on my tractor that has a bad battery that doesn't hold charge.

I used the 30w version in the following link. It's nowhere near a true 30w but it keeps the batteries topped off when not in use just fine.

US $12.32 |30 W/50 W/60 W/70 W Zonnepaneel Dual Usb Output Solar Cellen Poly Solar panel 10/20/30A Controller Voor Auto Jacht Batterij Boot Charger-in Zonnecellen van Consumentenelektronica op AliExpress

Smarter Shopping, Better Living! Aliexpress.com

nl.aliexpress.com

Although since you already have a big panel on top of the van you might as well tap into it. Your basic setup should work just fine.

 

[a very small camper van]

Ah yes, there is that option too! I actually have a tiny solar trickle charger I bought from Canadian tire.
I never believe it does anything, but maybe something a bit bigger.
But as you say, I do have the 100 watt panel just hanging out on the roof.
Can someone tell me if my other options could work and if not why? I mostly want to understand what goes on in my system.

Thanks

 

[SolarRat]

Basic setup-looks good with start battery isolated
Option 1- wires from the fusebox to battery bypasses the isolator...why do that? Also with bad start batt you'll blow that fuse every time you start.
Option 2- stay away from inverters if not needed...KISS

All you are really trying to do is keep the start battery at float voltage right?

And you really should just replace that battery as it may not even last through the winter no matter what you do.

 

[a very small camper van]

Not sure I understand (or maybe my quickie diagram isn't clear)

in option 1, my understanding is that the battery isolator provides a form of one way "valve", letting the start battery/alternator charge the house battery but unless I push the manual overide it keeps current from flowing back to the start battery.

Why would it blow the fuse (which fuse? The one in the 12v socket on the dash or the 12v socket coming off the house battery?) when I start, anymore than it would blow the fuse of an accessory plugged into the 12v socket on the dash?

And I know I should replace the battery, but I have 270.00 reasons to put it off as long a possible

 

[SolarRat]

The isolator is fine. But in option 1 you (I'm assuming) also have a direct connection between start and house batteries (through the fuse block) which is going to flow both ways. When you crank the engine the bad starter battery will go low, the starter is still wanting more current and will pull it from wherever it can...in this case through that fused wire and from the other battery.

Maybe your diagram (inside the fuse block) isn't clear...are the two batteries in any way connected in that fuse block? Are they both feeding the block or??? Why are both batteries going to the same fuse block?

 

[a very small camper van]

Okay, I think I follow.

But if it was set up only while parked - which it will be for months at a time over winter - is there any reason that this set up would harm anything?
It will be a cigarette lighter style 12v plug into the socket on the dash. very easy to unplug if I needed to use the van during the winter.
(I know I could forget, but I think the risk is low)

The fuse block is where the 12v socket is connected on the house battery side, that is where the power should come from.
I forgot in my original post to add that I was looking for advice about setting up a DIODE to make sure the power only flowed one way - doesn't help with the possible overload if the starter motor tries to pull power from the house battery, but will make sure the starter battery does not get drained to feed the house battery.

Any advice on what kind / size diode would work if I do try this option?

Thanks

 

[brassmonkey001]

Swap your isolator for a VSR.

12V Voltage Sensitive Relay - 140A | 072733 Durite - Leaders in Vehicle Safety & Auto Electrical Parts | Durite.co.uk

Product Description and Specification for 12V Voltage Sensitive Relay - 140A | 072733

www.durite.co.uk

 

[SolarRat]

Any advice on what kind / size diode would work if I do try this option?

The isolator is basically a diode. If you hook it up one way (the normal way) your alternator will charge the house bank but the starter can't draw from the HB. If you hook it up the other way, your HB/solar will keep the starter battery charged, but then the starter can draw from the HB through the isolator (possibly exceeding it's limits).

If you aren't trying to start the van while all this is hooked up none of this matters. Just disconnect if starting.

I'm sure you can get a new battery from Walmart for under $100....or craigslist for $50.

 

[a very small camper van]

Swap your isolator for a VSR.

12V Voltage Sensitive Relay - 140A | 072733 Durite - Leaders in Vehicle Safety & Auto Electrical Parts | Durite.co.uk

Product Description and Specification for 12V Voltage Sensitive Relay - 140A | 072733

www.durite.co.uk

The one I have is the Battery Doctor 150 amps, which I believe is a voltage sensitive one.

 

[a very small camper van]

The isolator is basically a diode. If you hook it up one way (the normal way) your alternator will charge the house bank but the starter can't draw from the HB. If you hook it up the other way, your HB/solar will keep the starter battery charged, but then the starter can draw from the HB through the isolator (possibly exceeding it's limits).

If you aren't trying to start the van while all this is hooked up none of this matters. Just disconnect if starting.

I'm sure you can get a new battery from Walmart for under $100....or craigslist for $50.

If I did put a diode on the cable running from the house battery to the starter via the 12v sockets, what kind of diode would I need? Amp rating? type?

Prices I am finding for the size and type of battery I have run about 250 -275$ where I am in Canada. Not a huge sum, but one I would like to put off a bit longer. And the battery is not that bad, just not in top shape for sitting outside in -20C without any long drives to charge it up but needing to be started to move it for snow clearing, as it is parked on the street.

 

[brassmonkey001]

The one I have is the Battery Doctor 150 amps, which I believe is a voltage sensitive one.

Some VSRs are bi-directional and some are not. I'm pretty sure the one I linked to is bi-directional, so as long as it detects a charging source above a certain voltage, whichever side of the relay it is, then both batteries will be charged. Once the charging source is removed then the voltage drops and shuts off the flow of current, so draining one side will not drain the other.

 

[a very small camper van]

Some VSRs are bi-directional and some are not.

Veeeery cool, I did not know that. I'll look into that. Thanks