Two Lead Acid Batteries - Parallel or Switched?

crosstrnr

I still have the original 100Ah Interstate battery in my 2019 T@B Boondock Edge. Last year I replaced the battery in my TV with an 85Ah deep cycle lead acid battery. My alternator failed, and the repair shop also replaced that battery with a new one. Which leaves me with the working 85Ah battery.

My question:

1. What are the advantages/disadvantages in connecting both batteries in parallel? I am aware of the tongue weight issues -- curious about impact on the batteries and electrical system.
2. Alternatively, I have a three-way battery cut off switch which allows both batteries together or each battery separately. Same question -- pros and cons of this arrangement.

Lithium is on the long-term list, but the extra battery feels like an opportunity in the short-term.

Thanks in advance,

David

PS - I have external solar panels in a solar suitcase I use to charge an Ecoflow 2. I have disconnected the main battery and backfed the Ecoflow through the 12v port when boondocking. No solar controller separate from the one inside the Ecoflow.

Slackers

I'm not an expert on this topic but do know that an ideal configuration for parallel batteries is for them to match not only in volts, but also AH, age, and MFG. So switched.

pthomas745

Slackers is correct on the ideal configuration. 

There have been several threads on using "spare" batteries in the same way you mention: either connected in a non-parallel configuration so that each battery does its job, and then is switched out for the "spare" battery to take over.  (Like you do with your Ecoflow).  This has been done with various connectors (such as a 7 pin plug adapter) and other methods.  There are a couple of owners who have installed two "different" batteries with a battery switch that lets you choose what you want to use.

I have a 7 pin plug adapter and have also used a "spare" battery connected through a 12V port.

This is the thread with the various options.

https://tab-rv.vanillacommunity.com/discussion/7008/anyone-carry-a-spare-battery/p1