Dissimilar battery banks grounding

Ask a question...get an answer (or two).
Post Reply
Benorado
Bottom Sanding Grunt
Posts: 7
Joined: Tue Apr 06, 2021 10:16 am
Boat Name: The Flying Spur
Boat Type: Cape Dory 30C
Location: USA
Contact:

Dissimilar battery banks grounding

Post by Benorado »

Situation: I have two battery banks aboard my sailing vessel (Cape Dory 30C) that are switched via a break-before-make 1-2-Off rotating selector switch.

One (new) bank is Lithium Ion, the (legacy) other is AGM. I’m keeping both, and I’m not interested in charging them both via the alternator. The alternator will charge the AGM bank (reserved for engine duties), the photovoltaics charge the Lithium Ion.

Where I’m at: I need to provide a common ground to the Lion bank. The AGM has a grounding cable to the engine block. The location of this ground is the motor mount bolt. I wish to avoid pulling the bolt, adding another ground cable separately to the Lion bank, and then re-connect the mount bolt and potentially have to adjust the rest of the engine mounts as a result (and align the prop shaft).

Question: Is it unthinkable to run a cable from the negative post on the Lithium Ion bank to the negative post of the AMG bank (the AGM negative post that hosts the engine block grounding cable), thus accomplishing my objective?

When I searched for an answer to my question, the search string I used was “dual battery bank grounding”. If my question(s) has already been addressed accept my apology and kindly post the link to the existing article.

Regards,
-Benjamin
S/V The Flying Spur
-Benjamin
Aboard The Flying Spur
User avatar
atomvoyager
Moderator | Revitalizer of Classics
Posts: 421
Joined: Thu Jul 12, 2018 7:48 am
Boat Name: Atom
Boat Type: Pearson Triton
Contact:

Re: Dissimilar battery banks grounding

Post by atomvoyager »

Benjamin,
Welcome to the group. I don't have experience setting up lithium batteries yet but have done some research on it and expect to do so soon. I hope to avoid the dissimilar banks issues by using all lithium but my next customer may want to save money by keeping an AGM for the engine. If so, I need to research all the issues further. As you know, running two battery types is problematic because of the fire danger or at least damaging the batteries. As I understand it, the lithium's try to equalize their voltage difference to AGM at too high a rate causing an overheating imbalance. There are ways around it but I wonder if the advantages of your lithium house bank are outweighed by the disadvantages of running an AGM engine bank. In your case with modest sized banks and electrical needs it seems overly complicated to not have all lithium but maybe there's something other than cost that makes it reasonable to do it that way. I guess you bought a break-before-make 1-2-Off switch to avoid the danger of the typical switch that also has a Both or All position. That said, to answer your question, you can connect the two bank's negatives any way you want since with the positives not connecting, unbalanced current will never flow. Besides going bank to bank you could also T into the original engine ground cable at any point with a battery terminal stud.

Here's a thread on the subject. This guy seems to have figured out how to charge both banks with solar and alternator. That seems much better than not being able to use the engine alt to replenish the house bank, which is a common need on a cruising boat:

https://www.cruisersforum.com/forums/f1 ... 27623.html

From your YT channel it seems you're setting up for cruising to the Caribbean soon so it seems that a single battery (100 AH ?) is not going to be enough unless you have unusually low electrical needs. I would think 200AH is the minimum that will get you by and 300 would be more common, even with the greater efficiency of lithium over AGM. The larger bank will carry you through overcast days or when you want to run more battery loads than normal. This is even more important if you don't have ability to charge with the engine. And even if you did, who wants to run the engine frequently just to charge batteries?
User avatar
atomvoyager
Moderator | Revitalizer of Classics
Posts: 421
Joined: Thu Jul 12, 2018 7:48 am
Boat Name: Atom
Boat Type: Pearson Triton
Contact:

Re: Dissimilar battery banks grounding

Post by atomvoyager »

This video may have answered my question as to why it might be better to have an AGM battery bank with Lithium house bank:

https://www.youtube.com/watch?v=sqT27KbxRec
Benorado
Bottom Sanding Grunt
Posts: 7
Joined: Tue Apr 06, 2021 10:16 am
Boat Name: The Flying Spur
Boat Type: Cape Dory 30C
Location: USA
Contact:

Re: Dissimilar battery banks grounding

Post by Benorado »

Thanks for your thoughtful response James. I found a location where I can place a bus bar for the negative term that the existing cable attached to the engine block will reach, so I’m going to head over to WM and grab one today.

Good point on the bank size. I used one of the several available sizing calculators, and if I run the refrigerator above freezing, I’ll have nearly 3 days of spare power running the Lithium Ion bank down to 40%. Real world though might make things a bit clearer, and if I can’t get by then I will have to get another battery. Once my new selector is in place, I’ll have better data. Your observations comes from far more experience than mine, so I might just need to bite the bullet!

Clark can come off as a kooky guy at first, but he know his stuff. I think he’s working on some sort of logic board to balance charge controlling between dissimilar banks (I think), he’s a software engineer.
-Benjamin
Aboard The Flying Spur
Post Reply