Using my solar to keep starting battery topped off

[zoti]

I have a solar setup with 2 100W AGM batteries and a 100W panel on top.

My van sits and after a few weeks of not running the starter battery will get too weak. I charge it back up with a battery charger and it will work fine as long as I drive the car but will again goo too low after sitting.

I'd like to use the solar to also trickle charge the main battery.

I already have a setup that enables me to charge the AGM batteries while the engine is running off the alternator so I am wondering what I need to do the opposite and charge the starter battery off the solar.

[fxr]

What voltage do your AGM batteries get to when charged? If they routinely go to more than ~13.8V, then a simple diode (with a switch) will do the deed for you. Doesn't have to be a big one, a 6A one will suffice. However, it must be switched so that the AGM batteries don't try to help the starter when starting, as that will blow the diode. Assuming you have some sort of relay that does this already to disconnect the alternator from the AGM batteries when starting, then you just need another relay with its coil in parallel with the first one to switch the diode.

https://www.amazon.com/gp/product/B007Z7OQ5G

Less than $15 all told.

The much more expensive way is to invest in a Renogy or Victron DC-DC charger which will automatically take care of the starter battery as well as the AGM ones. $200 upwards. However, this also makes you Lithium battery capable should you wish to do that in the future.
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Jim Crowther
1984 1.9l EJ22 Westy Wolfsburg Edition
Vespa GTS 300

[jimf909]

OP, it sounds like you have a solar controller to charge the aux. AGM batteries. It also sounds like you have some sort of arrangement to send alternator current to both the starter battery and AGM aux. batteries.

It sounds like you want to send current from the solar panels to the starter battery when the van is idle for weeks. A few ideas from rudimentary to fancier:

- A simple jumper from the post of an aux battery to the starter (assuming they share a ground).
- FXR's suggestion
- a battery combiner such as a Yandina that will automatically combine batteries when voltage exceeds 13v on one side (alternator to aux. batteries when driving, solar to starting battery when on solar). This won't be an optimal charge pattern but it's waaay better than allowing a starting battery to repeatedly drop to 50% SOC.
- and so on...

The single most important thing is to not allow the starter battery to repeatedly drop to 12.4, 12.3. 12.2 volts or lower. That'll eliminate recharge cycles from the battery's life.

Is your van w/in reach of an AC outlet? If so, consider keeping the starter and aux systems separate and just set up a battery maintainer plugged into AC to keep the starter battery charged.
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- Jim

[dobryan]

If you are outside and the front faces something like South get a small 10W solar panel that sits on the dash and plugs into the stock cig lighter socket on the dash. I do this for the euroBus and it works great to keep the starting battery topped up while sitting.
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Dave O
'87 Westy w/ 2002 Subaru EJ25 and Peloquin TBD

"To travel hopefully is a better thing than to arrive." Robert Louis Stevenson

MD>Canada>AK>WA>OR>CA>AZ>UT>WY>SD
https://www.thesamba.com/vw/forum/viewtopic.php?t=620646

Building a bus for travel in Europe (euroBus)
https://www.thesamba.com/vw/forum/viewtopic.php?t=695371

The Western Syncro build
https://www.thesamba.com/vw/forum/viewtopic.php?t=746794

[Gnarlodious]

For this I installed a cheap programmable voltage controller that switches on the relay at about 13.6 volts. Search eBay for a device called a “12v voltage control switch overvoltage protection module”. I actually wrote a whole post about this solution:
How to safely run a flooded and AGM battery together
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Vanagon ’83 diesel AAZ w/Giles injection, 5spd 4.57R&P+TBD and a '78 diesel Rabbit

[4Gears4Tires]

[dobryan]

[fxr]

[Andymon]

I've been using this for a few years with no issues.https://www.amazon.com/gp/product/B07W97ZBXT/ref=p...psc=1[url]
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"Blanch" '85 Westy
"Too many freaks and not enough circuses".

[4Gears4Tires]

Oh cool. Very handy for my rallycross car that has a slow electrical leak. I think it's the keyless entry which I don't want to remove. This is easy enough to throw on the back seat shelf.

Thanks!
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'87 Syncro
Ferric Oxyhydroxide Superleggera Edition

[Abscate]

In 4 weeks a good battery will lose 15-20% charge max.

If it won’t start your car, it’s dead, Jim.

Check for parasitic drains
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.ssS!