Parallel Charging Question

Mr Fox

Mr Fox

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Aug 31, 2017
204
Marshall 22 Portland, ME
Hello everyone

I am curious to know about charging batteries in parallel in my setup (when the switch is set to both)- I have heard various opinions, from "the alternator will see the charge of the higher one and therefore never charge the lower" and "the lower will draw down the higher so both batteries will never get charged" to "it's just fine, the alternator will put out enough and the battery that can take more will".

I have 2 group 27 100AM FLA batteries. These are wired through the 1/2/b/o switch, which is make before break. The alternator is connected to the switch. The wiring in my boat is correct and proper for this setup. The automatic bilge pump runs off battery #2, which is what I typically run my minimal house loads off of. I have a not smart at all stock Hitachi alternator.

I do know how to use the switch correctly, and it works just fine for my current needs. I will be upgrading the system once these perfectly good batteries eventually die.

For the moment I'm just looking to learn some more, so thanks!!!
 
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Sumner

Sumner

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Jan 31, 2009
5,254
Macgregor & Endeavour 26S and 37 Utah's Canyon Country
When the batteries go south I'd replace them with two 6 volt true deep cycle batteries in series. A better setup in my opinion. More amp hours, more efficient charging,

Sumner
 
Johnb

Johnb

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Jan 22, 2008
1,421
Hunter 37-cutter Richmond CA
That is an arrangement I used for years. As long as the batteries are similar the charge will go the one that needs it. About once per month I would attach a shore powered charger and do an equalization charge. Other than that make sure the levels are good
 
Mr Fox

Mr Fox

.
Aug 31, 2017
204
Marshall 22 Portland, ME
Johnb said:
That is an arrangement I used for years. As long as the batteries are similar the charge will go the one that needs it. About once per month I would attach a shore powered charger and do an equalization charge. Other than that make sure the levels are good
Click to expand
Thanks for the info. Sadly my marina has no power hook ups in my section- can I equalize them by motoring for an extended period every now and then?
 
D

dlochner

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Jan 11, 2014
11,398
Sabre 362 113 Fair Haven, NY
Mr Fox said:
Thanks for the info. Sadly my marina has no power hook ups in my section- can I equalize them by motoring for an extended period every now and then?
Click to expand
No. Equalization requires charging at a voltage higher than normal. The not so bright Hitachi alternator will not do that.
 
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B

Benny17441

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May 24, 2004
7,131
CC 30 South Florida
It sounds like you are doing OK. To avoid some of the issues you mention and to provide redundancy to the system I practice the following. I alternate the use of batteries with every outing, the purpose is to provide both batteries with a similar number of cycles for a balanced useful life for both. Keeping them isolated also prevents a failed battery from bringing down the other. The use of just one battery at a time facilitates the charging while the 2nd battery is maintained fully charged and ready to start the engine if needed. Only if the battery in use is weak to start the engine will I combine both batteries for a short period of time. On extended trips where I may need more battery capacity I will pack a third battery or an automotive type battery pack to provide emergency power to start the engine. On those instances I will combine both batteries for the duration of the trip. If both batteries are of the same age and have been submitted to similar use then they will act very much like a larger single battery. It is no secret that the combined larger battery is more efficient than using each in a single fashion. One other thing, it is very hard to charge batteries to 100% with just the alternator as the average charge will require from 5 to 7 hours. The largest enemy to battery useful life is to not charge them back to 100% after every use. To allow them to remain partially charge generates sulfation which will shorten their life. That could be the difference between batteries lasting 2-3 years vs. 5-7 years. Equalization helps to combat sulfation but cannot be done with the alternator. About upgrading make sure you have the need for it, a larger battery bank may require a larger alternator and inboard battery charger. Also under used batteries promotes sulfation. I also like wet cell batteries as if one should fail in a trip I just get off the baoat and it is easy to find a replacement almost anywhere. For coastal cruising it is convenient to purchase batteries from Walmart as warranties and replacements can be found everywhere. Hope this can give you some ideas. We also have added a Honda 2000 gas powered generator which allows us to recharge batteries on the hook and run refrigeration and 120V appliances, but that is a theme for another day.
 
S

sesmith

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Jul 1, 2010
962
Catalina 350 Lake Huron
When set to both, the batteries will go to the same voltage, so charging charges both of them. The downside with this setup is that if one of the batteries is weak or bad, it takes them both down.

Unless you need the capacity, it might make more sense to use the switch on one bank only with the other as a reserve. You could add a combiner, and wire the charging circuit to charge your house bank only, and never have to worry that the reserve is fully charged as the combiner would take care of that.
 
Maine Sail

Maine Sail

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Feb 6, 1998
11,667
Canadian Sailcraft 36T Casco Bay, ME
For those batteries using the BOTH position to charge, is a non-issue. The issue comes when you are no longer charging and you forget to take it off BOTH..

The article below discusses parallel-charging in a pretty in-depth level. While the article is about ACR's, automatic charging relays are nothing more than automated (based on voltage) parallel charging or electronic BOTH switch...

Making Sense of Automatic Charging Relays


.
 
shemandr

shemandr

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Jan 1, 2006
7,063
Slickcraft 26 Sailfish
I think MS has addressed this before but I wonder if the situation has changed: Why not carry one of those battery packs for emergency starts instead of trying to maintain a battery reserved for starting? I have one in my car and have used it and it was effective for that engine.
 
Whatfiero1

Whatfiero1

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Mar 29, 2017
576
Hunter 30t 9805 littlecreek
Buy a 3rd identical battery and switch out every month for one others and take home and put on charge. I run group 24's. Which are same that's in Toyota car so I keep swapping out in car over winter to keep exercised
 
DArcy

DArcy

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Feb 11, 2017
1,702
Islander Freeport 36 Ottawa
I use one at a time but switch to both before starting the engine. Once you get in the habit it's easy to remember to switch back to one or the other after shutting down the engine. This works for me but I also plug in a smart charger while at the dock. Before using shore power to charge between uses I did find it hard to keep up just running the engine and the batteries did not last as long. I was getting about 3 to 5 years out of them but with shore power charging I get about 6 years. If you can't plug in to shore power to charge I suggest getting solar or wind to top them up. Even a small 10ish watt solar panel will work surprisingly well for battery maintenance.
 
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Mr Fox

Mr Fox

.
Aug 31, 2017
204
Marshall 22 Portland, ME
Maine Sail said:
The issue comes when you are no longer charging and you forget to take it off BOTH..

The article below discusses parallel-charging in a pretty in-depth level. While the article is about ACR's, automatic charging relays are nothing more than automated (based on voltage) parallel charging or electronic BOTH switch...
.
Click to expand
Thank you- that actually answered the question and my desire to learn more about the process.

I am aware my current setup leaves room for operator error- when I upgrade things (hopefully I can hire you if you are available when the time comes) I will be removing this potential.
 
Maine Sail

Maine Sail

.
Feb 6, 1998
11,667
Canadian Sailcraft 36T Casco Bay, ME
Mr Fox said:
Thank you- that actually answered the question and my desire to learn more about the process.

I am aware my current setup leaves room for operator error- when I upgrade things (hopefully I can hire you if you are available when the time comes) I will be removing this potential.
Click to expand
What ever happened with the LFP battery?
 
Mr Fox

Mr Fox

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Aug 31, 2017
204
Marshall 22 Portland, ME
DArcy - Islay Mist said:
Even a small 10ish watt solar panel will work surprisingly well for battery maintenance.
Click to expand
I could definitely do a small panel- I can sew one onto the dodger and hook up a charge controller, but I will have to wait for the end of this season to do that when I haul out.

Unless doing a portable one is worth it- I could hook it up to the battery with the bulge pump attached to it...
 
Mr Fox

Mr Fox

.
Aug 31, 2017
204
Marshall 22 Portland, ME
Maine Sail said:
What ever happened with the LFP battery?
Click to expand
It is still sitting in my basement! I just can’t justify chucking the ok batteries yet, (also I installed a new head, plumbing, holding tank, resealed the port lights, repaired the PO cockpit leak and teak sole damage this year, that basically blew this years budget). Next year I hope it can go in as we had discussed.
 
Sumner

Sumner

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Jan 31, 2009
5,254
Macgregor & Endeavour 26S and 37 Utah's Canyon Country
shemandr said:
I think MS has addressed this before but I wonder if the situation has changed: Why not carry one of those battery packs for emergency starts instead of trying to maintain a battery reserved for starting? I have one in my car and have used it and it was effective for that engine.
Click to expand
Exactly, messing with inefficient manual switching batteries via a switch or other means is just too much work and not very good way to maintain your batteries or charge them for use. Make life simple and parallel the batteries and charge all at once and take a battery pack with you. If you do have a separate start battery get...

http://1fatgmc.com/boat/end-1/endeavour-elect/page-5.html

... a combiner and be done with it. The money spent doing this will be paid for in the extended life of just a battery or two,

Sumner
============================================================
1300 miles to The Bahamas and Back in the Mac...
Endeavour 37 Mods...
MacGregor 26-S Mods...http://purplesagetradingpost.com/sumner/endeavour-main/endeavour-index.html
Mac Trips to Utah, Idaho, Wyoming, Canada, Florida, Bahamas
 
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