Best technique to keep a second car battery charged onboard ?

[Kestrel]

I have a second battery in my vehicle (powering an inverter), and am interested in an inexpensive on-board charging system to top it off every 4-5 days.

It is an AGM battery:
https://www.dropbox.com/s/btu90gaxaejqjfh/20190720_171757.jpg?dl=0

This battery is fully independent of the cars' electrical system & not connected in any way - long story, but I am not interested in doing so.
The battery is installed so that it already incorporates a charging jack, with which I can utilize my 110v trickle charger at home.

Near the battery is an extra cigarette lighter adaptor; I am thinking of two options to utilize this for a charging setup (during vehicle operation):

1:
Amazon/eBay direct cable (cig lighter adaptor -> charging jack), ~$10.
Very basic, but includes an inline fuse. I would handle the charging process manually every 5 days or so, with a separate voltmeter to verify battery voltage.

Or, 2:
Amazon/eBay cig lighter adaptor charger, ~$20.
/Supposedly/ will not overcharge, but /many/ reviews indicating that this feature has been unreliable, cooking the battery. Also, reports of having the opposite polarity than what is indicated by the wiring color coding.

For simplicity sake, I am inclined towards the simpler approach; as I would be simply using 'line voltage' from the car's electrical system, this would be safer for the $$$ battery yes ? The inconvenience of handling this manually, and perhaps not obtaining a full charge is not an issue.

Thoughts or suggestions ?
Again, for various reasons I am not interested in integrating this AGM battery with the cars' electrical / charging system.

Thanks in advance,

 

[tatasal]

Perhaps an inverter (that can power a Battery Tender with trickle charging) or even a hobby charger, set to a very low charging current but powered by your main battery will do the trick.

 

[FRITZHID]

I'd suggest a proper battery charger/isolator between your cars stock battery and the aux battery. This will allow your cars charging system to charge the aux battery while keeping it isolated from your cars main system (other than the charging).

Sent from my SAMSUNG-SM-G900A using Candlepowerforums mobile app

 

[fivemega]

Again, for various reasons I am not interested in integrating this AGM battery with the cars' electrical / charging system.

A simple voltage detector circuit can connect second battery to car charging system and immediately disconnect when not charging. This means extra electric power will go to AGM to charge but will never go reverse.
(Edit): Similar advice suggested on post #3

You can also use solar charger for second battery.

 

[lightfooted]

I'd say keep it simple and just make sure that the port you plug it into is only on when your vehicle is turned on so that it isn't a drain on the primary battery.

 

[SilverFox]

Here is another option but it seems to be a little more expensive.

https://www.sterling-power-usa.com/sterling-power-usa12vto12vbatterymaintainer.aspx

Tom

 

[Lynx_Arc]

This looks interesting
https://www.amazon.com/dp/B011J76CFE/?tag=cpf0b6-20

 

[IonicBond]

I have that Tecmate / Optimate TM500 DC-DC charger and recommend it for a maintenance purpose like this with a caveat.

HOWEVER, the output is only 2A, which of course would mean a LONG time to drive around between a deep discharge from the main battery with the inverter. But I suppose if one is driving a few days between top-off's, then maybe.

The secondary issue is that using a small amperage maintainer to repetetively charge a large battery is very inefficient, and actually harms the battery in the long run. Although kestrel's battery is agm with low internal resistance, that helps, but still not recommended as a long-term repetetive cycling solution for a battery that large.

The low amperage recharge on a deep discharge large battery can also be misleading as there is more of a shallow-depth "surface charge", rather than the proper deep-surface charge with a much larger amperage. That means while the charger itself is excellent, the bigger the battery is, the more this unit should be rightly called a "maintainer" for close-to-fully charged batteries, than a deep-discharge "charger". There's a tipping point.

As long as you keep this in mind, and PROMISE to get it on the regular 110v heavy current charger regularly, then maybe you could get away with it.

Another popular option might be to look into the CTEK D250S charger / controller. RV guys like it because it makes it simple to have a secondary battery like this take a charge from the alternator, OR a solar panel without having to buy a dedicated solar charge controller - it's already in there - and will automatically switch to whichever is more efficient at the time. $$$ though but perhaps worth itl

 

[Kestrel]

Many good & helpful replies, thank you. Not sure where I'm going at the moment, but will certainly pursue something from the above. :thumbsup:

 

[Wonder]

I'd suggest a proper battery charger/isolator between your cars stock battery and the aux battery. This will allow your cars charging system to charge the aux battery while keeping it isolated from your cars main system (other than the charging).

Sent from my SAMSUNG-SM-G900A using Candlepowerforums mobile app

Makes total sense to me. Any other option is just chasing your tail.

 

[BigusLightus]

Many years ago I read an article in one of the mechanics magazines (don't remember which one) and they were advising the simplest way was their favorite way of dealing with two batteries in a car. A simple switch made the most sense. For example:
https://www.perko.com/catalog/battery_switches/148/medium_duty_battery_selector_switch/

 

[Kestrel]

Many years ago I read an article in one of the mechanics magazines (don't remember which one) and they were advising the simplest way was their favorite way of dealing with two batteries in a car. A simple switch made the most sense. For example:
https://www.perko.com/catalog/battery_switches/148/medium_duty_battery_selector_switch/

Well huh ! :huh: I'm liking the looks of that ...

 

[KITROBASKIN]

Hey Kestrel, wondering if you acquired something to achieve your goal here..
Did want to say that our (now) secondary photovoltaic system Absorbed Glass Mat battery pair, got lousy after a good many years (11? of very light duty because we were living in civilization with grid power) but turns out it was only one battery that was horrible, so I kept the better one, connected it to our back-up 600W pure sine wave inverter for simple jobs where portability is needed (using a hand truck to move the golf cart sized battery and inverter). Anyways, when I use it, afterwards I simply connect it to the home system in the morning of a sunny day (through alligator clips, wire, and the kind of twist plug we use in our 12V home to differentiate from AC plugs) and disconnect when the sun is not shining on the panels and before the home system takes on a significant load. Being AGM, it never needs an equalization charge. The charger/controller keeps things on keel. Our new (larger) AGM batteries use extremely close low voltage, bulk, float, and high voltage values as the old ones. Of course I will need to be occasionally checking the resting voltage of the old AGM (someday it will kick the bucket) and not allow it to drag down our good home batteries by leaving them connected when the sun doesn't shine. Checking is easily accomplished by turning on the inverter and viewing the voltage available. Long story short, the cheap way for an able person is to manually switch to charge the multipurpose battery, seems like; that is if your charging capability is sufficient for your electrical use. One opinion.

 

[Kestrel]

Hi, thx for the bump; wow nice setup & it sounds like you know your stuff.

Yes, the suggestion from BigusLightus was key (& I can't thank him enough for the tip).
I went with that Perko 8501 switch; made for boats & built like a TANK (mfg in the US).

The big capability that made me go in that direction was the four-way 2-All-1-Off setting option, and doing 'make-then-break' electrical connections.
Wont' get into the mundane details, but it covers all the bases I wanted in a perfect fashion.