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Old 10-13-2017, 07:59 PM   #1
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Adding a second house battery bank

Hello fellow TFers.
In a recent post I mentioned that I plan to add a second house battery bank.
Context: I currently have 1 house bank, made of 2 group 31 batteries, 105Ah each, and 1 separate starting battery for my single engine. These are charged using 1 dual charger (25 amp) and 1 alternator.
Issue: While I have a small energy footprint, the house bank capacity is a bit too low. I spent most of the time 2 to 3 days at anchor. The main energy burn comes from the fridge, the remaining is only lights and radio.
My plan: I would like to add a second house bank made of 4 GC2 batteries. This second bank would be used solely for the fridge while the initial bank will be used for lights and bilge pumps, fresh water pump, head etc... This should allow me to have 48h capacity without running the genset.

Now some questions:
Should I install a second charger for this house bank as my current charger already connect to start and initial house bank?
Would a single alternator enough for 2 house bank and the start battery? ( I would like to avoid installing a second alternator).
Any advice or comment on what I plan to do?

Any comment welcome

L
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Old 10-13-2017, 08:15 PM   #2
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One large bank is a better design with less complication. One of the advantages is generally a lower draw down rate as there are more batteries to share the load.

Ted
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Old 10-13-2017, 08:35 PM   #3
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Originally Posted by O C Diver View Post
One large bank is a better design with less complication. One of the advantages is generally a lower draw down rate as there are more batteries to share the load.

Ted


My point of adding a second one is to keep advantage of the 2 old batteries for what is drawing the less. New bank will sustain 48h of fridge usage with 50% discharge, while old bank would have days of capacity for lightning and pumps. This would allow me to keep using my current batteries instead of tossing them to trash.

L
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Old 10-13-2017, 09:05 PM   #4
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Using the rule of thumb that the charger amp output ought be at least 10% of total battery amperage, you would need more charge for the extra bank.
Having 2 house banks would keep the existing older but still viable batts separate from new ones. You could have 2 banks until the older ones need replacing, and then combine them one bank(though by then you might be concerned about combing these new ones with the "newer new" ones). The extra charger could be sized to the intended enlarged house bank.
Without knowing the output of the existing alternator, it may or may not be able to serve all the proposed batts. If not, you could upgrade the existing, and perhaps add an external regulator to get the best from it.
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Old 10-13-2017, 09:13 PM   #5
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Quote:
Originally Posted by BruceK View Post
Using the rule of thumb that the charger amp output ought be at least 10% of total battery amperage, you would need more charge for the extra bank.
Having 2 house banks would keep the existing older but still viable batts separate from new ones. You could have 2 banks until the older ones need replacing, and then combine them one bank(though by then you might be concerned about combing these new ones with the "newer new" ones). The extra charger could be sized to the intended enlarged house bank.
Without knowing the output of the existing alternator, it may or may not be able to serve all the proposed batts. If not, you could upgrade the existing, and perhaps add an external regulator to get the best from it.


Thank you Bruce.
Indeed I was thinking that my current charger would not be enough for another bank so I guess I will have to add a dedicated charger for the new bank or a new more beefy charger for the whole.
Like you wrote having 2 banks would allow to cycle between new and old. When the current batteries will be toasted, the gc2 bank would become the new old and would be replaced by new gc2. Doing this would allow to use batteries till their bones as old ones would always be connected to less drawing device ( not sure I am clear lol )
I would really like to avoid changing the alternator as it is brand new. Not sure of its power output though.

L
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Old 10-13-2017, 09:13 PM   #6
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I'm with the other responses. You would benefit from a larger charger and if you purchased a 3 bank charger you would be able to use the setup you're talking about. You just need to make sure that all 3 banks are the same battery technology which it sound like they would be (all flooded lead acid).

I believe your engine alternator could be used to charge all 3 banks if you used some combination of battery paralleling switch and or an echo charger or automatic charging relay. It also depends on how big that alternator is and what your expectations for it might be. Keep in mind that a typical alternator can really only put out about 1/2 it's rated power for any length of time.

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Old 10-13-2017, 09:16 PM   #7
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Originally Posted by kchace View Post
I'm with the other responses. You would benefit from a larger charger and if you purchased a 3 bank charger you would be able to use the setup you're talking about. You just need to make sure that all 3 banks are the same battery technology which it sound like they would be (all flooded lead acid).

I believe your engine alternator could be used to charge all 3 banks if you used some combination of battery paralleling switch and or an echo charger or automatic charging relay. It also depends on how big that alternator is and what your expectations for it might be. Keep in mind that a typical alternator can really only put out about 1/2 it's rated power for any l night of time.

Ken


Thank you KChace.
I am wondering if it would be better to have 1 bigger charger or to add a new second charger just for the new bank and keep the current charger in its current setup.

L
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Old 10-13-2017, 11:06 PM   #8
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I have 3 48 volt banks. They're made up of 4-8dd Dyno lead acid batteries. Currently they're all the same age, One bank will run a double door reefer, chest freezer, usual lights and computer for about 48 hours. I use a diesel stove, but in warm weather use a couple induction plates. The inverter either charges (if on power) or draws off the bank that's on. If unattended and the batteries get low, the inverter starts a generator.
I've tried it both ways, single bank on line or all 3. I seem to go longer with three than the total of the 3 banks run as singles. With all 3 banks I can go a week at anchor (depending on who's on board).
I also can charge with a single alternator on one of the mains if they're running. And then I don't need a generator when steaming unless I'm heating water. I can use the water heater with the inverter but it runs the alternator at max if the reefer and freezers are running, too. I avoid hot weather, so usually no AC.
I don't use newer style batteries because I'm 69 and get 10+ years out of the Dynos.
There are chargers available that will charge multiple banks of different age batteries, each as they need.
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Old 10-14-2017, 02:06 AM   #9
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An advantage of combining batts into one house bank is reducing the chance of pulling one of 2 separated banks too low, if you spread the load over more batts you are less likely to overdraw.
Your existing charger output of 25A is <10% of the batts currently being charged, so a bigger capacity charger which can do all 3 banks could be better, but if what you have works and cost is a consideration adding a charger for the extra bank is reasonable too. But if you are going to have one house bank at some point, the bigger charger has merit.
Someone else may know this. If you fit a 3 bank charger as needed now, and later have only 2 banks, will the charger concentrate all its output into 2 banks, or does 1/3 of the output get lost? I`m thinking not, but others will know.
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Old 10-14-2017, 06:25 AM   #10
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Most Group 31 batts I have seen were truck starting batts,

not Deep Cycle as required for a house bank.

Find out if you have start or true deep cycle batts as first step..
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Old 10-14-2017, 07:42 AM   #11
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"And then I don't need a generator when steaming unless I'm heating water. I can use the water heater with the inverter but it runs the alternator at max if the reefer and freezers are running, too. I avoid hot weather, so usually no AC."
------------------

If you are steaming, why would you need an inverter to heat water?
Most engines have a small heat exchanger that will heat water for the shower etc.
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Old 10-14-2017, 09:41 AM   #12
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Quote:
Originally Posted by Lou_tribal View Post
Context: I currently have 1 house bank, made of 2 group 31 batteries, 105Ah each, and 1 separate starting battery for my single engine. These are charged using 1 dual charger (25 amp) and 1 alternator.

My plan: I would like to add a second house bank made of 4 GC2 batteries. This second bank would be used solely for the fridge while the initial bank will be used for lights and bilge pumps, fresh water pump, head etc... This should allow me to have 48h capacity without running the genset.

Think I'd try to avoid adding complexity, and just increase the existing house bank to 6x or even 8x GC2s and beef up your charger. Assume contiguous space, access, etc. Bag the old G31s, or save them (if possible) for replacing start batteries when/if necessary.

All that Peukert stuff applies... but mostly I'm thinking if space allows, it's a relatively simple matter to make up the jumpers.

That said, a slightly more complex approach would be to put your house bank on it's own dedicated (and much larger) charger. An advantage would be that all charging amps go where you want them to go, the house bank would therefore recharge faster, and you wouldn't have any issues with imbalances between bank size. Original charger could remain dedicated to start and genset batteries...

And even more complex than that, make the new one an inverter/charger, if having an inverter would be useful.

You could do some of that in stages. Max out the batteries now. Do something about the charger(s) later, etc.

-Chris
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Old 10-14-2017, 09:43 AM   #13
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Quote:
Originally Posted by BruceK View Post
Someone else may know this. If you fit a 3 bank charger as needed now, and later have only 2 banks, will the charger concentrate all its output into 2 banks, or does 1/3 of the output get lost? I`m thinking not, but others will know.

Our original 3-bank charger came with instructions about this kind of set-up. Essentially, unused terminals on the charger get jumped to a used terminal... so it becomes a 2-bank or a 1-bank charger...

-Chris
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Old 10-14-2017, 10:15 AM   #14
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Originally Posted by BruceK View Post
Someone else may know this. If you fit a 3 bank charger as needed now, and later have only 2 banks, will the charger concentrate all its output into 2 banks, or does 1/3 of the output get lost? I`m thinking not, but others will know.
All of the better multi bank chargers can send any portion of their total output to the different banks as required and have no problem running with an output not connected.

Ken
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Old 10-14-2017, 10:18 AM   #15
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Originally Posted by Lou_tribal View Post
Thank you KChace.
I am wondering if it would be better to have 1 bigger charger or to add a new second charger just for the new bank and keep the current charger in its current setup.

L
Well you could certainly do it either way, but in my opinion it would be a lot simpler to have one multibank charger than 2 singles. In addition, if you got a new multi bank charger you could take advantage of the recent advances in more sophisticated battery charging that the newer chargers offer.

Ken
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Old 10-14-2017, 12:17 PM   #16
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The way to accomplish the OPís request would be. Connect the charger and alternator to the largest bank of batteries. Then use two echo chargers. One between the two banks and one between the main bank and start battery.

If you use a combiner you will be over charging one of the battery banks. You can use a multi bank charger but now you canít use the alternator to charge all banks without combining.

Most the world still doesnít understand the issue of combining dissimilar batteries.
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Old 10-14-2017, 12:35 PM   #17
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Yes to above.

But honestly go with a bigger GC bank and toss the old 12V, really have little value, between Peukert, KISS, no additional infrastructure, clearly the way to go IMO.
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Old 10-14-2017, 12:50 PM   #18
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One house bank made up of similar batteries with an echo charger to the start battery and a 1/2/all/off switch for emergencies is the best solution in most cases.
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Old 10-14-2017, 01:38 PM   #19
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Thank you all for your input. This is good matter for thought.

L
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Old 10-14-2017, 02:47 PM   #20
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My vote is for a single larger house bank (6xGC) with a switchable combiner/ACR/echo charger to share the charge with the start battery when needed, i.e. on alternator.

Upsize your charger (60-100A) to a 2- or 3-bank charger.

If the budget allows, upgrade the alternator to a high output (100A) alternator with external regulation.

Add a Battery SOC meter to monitor the electron flow, voltages and battery SOC.

If you won't be running your genset regularly, ensure you have a way to keep its battery fully charged during long periods of inactivity.
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