Been dealing with a parasitic drain for a while now and haven't been able to use my maintainer as complex turns power off during the summer so the batteries have been cycling too much I believe. Anyways when I top up/charge the batteries separately then let them sit a few hours to settle the pass side battery still tests as 100%/SOH with 100/SOC, but the drivers side battery is much lower now at 78%/SOH with 100%/SOC so I believe it has an internal fault of some kind and is draining the system from 12.6v to 12v within a week and I think the battery was damaged somehow from the maintainer as it was on that side and used all last winter, but did not operate at -30c temps and I believe it would drain from that battery when not functioning properly.

Long story short, the pass side battery is 3 seasons old now, but still tests just fine so could I get away with just replacing the drivers side battery or would I have to replace both?

I'm also replacing the 90w solar on my truck camper and upgrading to 300w with a newer 40a MPPT and rather than using a plug in maintainer I'm going to get another smaller MPPT and run a secondary line off the 300w solar so I can keep my batteries topped up without damaging them.

Thanks for any insight here fellas

 

If money allows replace in pairs, if they were purchased together.

One question I had about your original maintainer, was it made to charge that specific type of battery (lead-acid, AGM, Lithium)?

I am no expert, but read about stuff.

Things I have learned recently about batteries is the new chargers (especially NOCO) are now introducing multi battery chargers, apparently using the wrong style charger on certain battery types is not good.

The new maintainer I will put in my truck is for all 3 types and a 2 bank. This will replace my old NOCO single bank that is for LEAD acid only and am going to buy AGM to replace my 5yo lead acid.

 

The alternator charges multiple batteries; a maintainer should not be an issue. Connections or an internal fault would be an issue.

Checking the CCA value and internal resistance would be a better way to determine if the 'good' battery can still be used with a new replacement. Let the battery sit for 24 hours disconnected before doing that.

 

weak battery steals from the strong one

Always buy then in pairs with same date codes or expect them to drain

Some maintainers will cook batteries if your terminals are not clean. I wouild use two solar chargers and a knife switch to separate them in winter.

Always scrape the teminal posts and the inside of the terminals.
Clean your battery grounds aka the other end not on the battery. This made a 50 rpm differenece in cranking speed on mine as it was a bit rusty after 20+ years

 

The "buy in pairs" is to have relatively equal CCA and resistance. I've got several years-old batteries that are still in the range of when they were new. It would be a waste to replace those.

 

that doesnt matter so much
what matters more is the voltage staying the same with age
There are some fords with two different battery sizes and thier voltages need to match or they drain each other down