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Old 23-06-2018, 10:53   #16
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Re: Two house battery banks - necessary?

I could see another use of splitting the house in two battery banks. In my case I have 900Ah and 500W solar. Even on a sunny day the batteries will never reach 100 SOC as the consumption eats most of the solar.

With two banks I could run the generator early morning taking the bank up to 80 SOC. Then isolate one bank and let the solar charge the bank that is isolated to 100 SOC. With all the solar I think I can reach 100% SOC in one or in worst case two days.

With two banks I could probably get both banks up to 100 SOC once week. The price is that batteries will be cycled deeper as Iím using only 50% at the time. But I would think PSOC is more damaging than cycling?

One question is how damaging it is to combine both banks when one bank is at 80% and the other 80% ? Will that create balancing problems in the long run.
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Old 23-06-2018, 10:55   #17
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Re: Two house battery banks - necessary?

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I could see another use of splitting the house in two battery banks. In my case I have 900Ah and 500W solar. Even on a sunny day the batteries will never reach 100% SOC as the consumption eats most of the solar.

With two banks I could run the generator early morning taking the bank up to 80 % SOC. Then isolate one bank and let the solar charge the bank that is isolated to 100% SOC. With all the solar I think I can reach 100% SOC in one or in worst case two days.

With two banks I could probably get both banks up to 100% SOC once week. The price is that batteries will be cycled deeper as Iím using only 50% at the time. But I would think PSOC is more damaging than cycling?
One question is how damaging it is to combine both banks when one bank is at 80% and the other 100%? Will that create balancing problems in the long run.
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Old 23-06-2018, 11:05   #18
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Re: Two house battery banks - necessary?

No to that last.

If the banks are composed of different batts, that is the only justification, it is poor design for many reasons.

Parallel charge both via genny early AM until the solar can finish the job.

If you fail some days, so be it, but just 1-2 times a week is not enough.

Best to sell the banks and buy one big one, but not so big you can't recharge daily.

Keep consumption lower than input is required to keep it healthy.
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Old 23-06-2018, 11:18   #19
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Re: Two house battery banks - necessary?

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No to that last.


Keep consumption lower than input is required to keep it healthy.
Good advice for most things, except food
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Old 23-06-2018, 11:33   #20
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Re: Two house battery banks - necessary?

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No to that last.

If the banks are composed of different batts, that is the only justification, it is poor design for many reasons.

Parallel charge both via genny early AM until the solar can finish the job.

If you fail some days, so be it, but just 1-2 times a week is not enough.

Best to sell the banks and buy one big one, but not so big you can't recharge daily.

Keep consumption lower than input is required to keep it healthy.




Have 900 Ah today. 450 W solar do not charge the bank to 100 SOC with consumption as normal. More solar I can not install whiteout cluttering the boat.

Off course 1-2 times a week is not perfect. But it is better than what most cruisers manages.

Please explain why 2 banks is not good? I want to learn.
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Old 23-06-2018, 14:14   #21
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Re: Two house battery banks - necessary?

Google Peukert effect.

Split bank means much lower total capacity.

Deeper discharging reduces lifespan.

Greater voltage sag on high current draws.

Longer charging because higher SoC reached faster.
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Old 23-06-2018, 15:03   #22
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Re: Two house battery banks - necessary?

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Google Peukert effect.

Split bank means much lower total capacity.

Deeper discharging reduces lifespan.

Greater voltage sag on high current draws.

Longer charging because higher SoC reached faster.


Thanks for your answer.

The idea was to only use split bank lets say 2 days/week getting the batteries up to 100% SOC. The other 5 days it would be used as on big house bank.

I would only use split banks when on the hook. When doing passages I would use the one bank because of reasons you mentioned.

I do agree that deeper discharges is not positive. But PSOC is even worse. It is not cycles that kill batteries in real life. It is sulfatation caused by PSOC.

Greater voltage sag will not be a problem as I will not have any high current draw.

Longer charging ? I understand what you mean. But I do not think it is a significant difference going 50-80 SOC bank on a 450 Ah bank vs 65-80 SOC on a 900 Ah bank.

The problem with the 900Ah bank is that the available solar do not reach 100% SOC during a sunny day. With a resting 450 Ah bank it would work i think. That is the disadvantage with a bigger house bank.

Running the generator or engine for 5-8 hours to get to 100% SOC is not an alternative.

My concern however is that I create some balancing problem when going from 2 banks to 1 bank as they will never be in the same SOC when i flip the switch?

If anyone have any insight on this please fill in.
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Old 23-06-2018, 15:16   #23
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Re: Two house battery banks - necessary?

As I said no need for two bank components to match SoC when joined.

They will equalize over time, which wastes energy, but no harm done.

Assuming robust wiring.

You can measure the amps flow at the time, you'll see NBD.

Are all the batts the same model and age?
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Old 23-06-2018, 15:50   #24
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Re: Two house battery banks - necessary?

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As I said no need for two bank components to match SoC when joined.

They will equalize over time, which wastes energy, but no harm done.

Assuming robust wiring.

You can measure the amps flow at the time, you'll see NBD.

Are all the batts the same model and age?


Thanks for taking the time answering.

I think I will give it a try as we spend many weeks on the hook in the Caribbean. LFP or Firefly would solve the PSOC problem. Unfortunately not easy to source at location.

It will be a new bank with 8 x T-105 at 12V.

A 1-2-all-off switch is not a big deal.
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Old 23-06-2018, 16:05   #25
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Re: Two house battery banks - necessary?

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I have about 960AH capacity on my boat, split between 2 house banks. They each have separate switches so they *can* be isolated, but is there really a need for this? The only reason I can think of is in case one bank has a battery that shorts, I could isolate it and continue on. But is that common enough to merit the trouble?

I also have a separate starting battery, separated from the house bank with an automatic charge relay.
Similar set-up here, except 1 house bank comprised of 2 batts. that are combined via a 1-2-ALL-OFF switch (not paralled at the batts). Switch always kept on ALL unless I'm equalizing, or in the event of a failure or emergency (has never happened). Then I have separate ON-OFF switches for separate eng & gen start batts.

The eng start batt. is an 8D (same as the 2 house batts) which seems like overkill for my 80hp propulsion engine. I have my fuel polisher wired to it so it gets a bit more use, but often thought of at least having the option of combining it with the house bank for add'l capacity on long passages under sail (and longevity). Not sure I'd want to do it permanently, as having the option of preserving unused batt. capacity for engine starting seems prudent. I also have two alternators which charge the eng start & house banks separately, and it would be expensive (but maybe worthwhile) to replace those with a single large capacity one.

What might be the best (i.e. easiest) way of pulling this off?
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Old 23-06-2018, 23:03   #26
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Re: Two house battery banks - necessary?

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Similar set-up here, except 1 house bank comprised of 2 batts. that are combined via a 1-2-ALL-OFF switch (not paralled at the batts). Switch always kept on ALL unless I'm equalizing, or in the event of a failure or emergency (has never happened). Then I have separate ON-OFF switches for separate eng & gen start batts.



The eng start batt. is an 8D (same as the 2 house batts) which seems like overkill for my 80hp propulsion engine. I have my fuel polisher wired to it so it gets a bit more use, but often thought of at least having the option of combining it with the house bank for add'l capacity on long passages under sail (and longevity). Not sure I'd want to do it permanently, as having the option of preserving unused batt. capacity for engine starting seems prudent. I also have two alternators which charge the eng start & house banks separately, and it would be expensive (but maybe worthwhile) to replace those with a single large capacity one.



What might be the best (i.e. easiest) way of pulling this off?


Two alternators is redundancy. Instead of changing to one alternator I would make sure both are charging the house bank. There is very little consumption from a starting the motor. Charging the start battery from a DC/DC like Balmar DUO charger is more than enough.

Fuel polisher do belong on the house bank.
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Old 23-06-2018, 23:32   #27
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Re: Two house battery banks - necessary?

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Two alternators is redundancy. Instead of changing to one alternator I would make sure both are charging the house bank. There is very little consumption from a starting the motor. Charging the start battery from a DC/DC like Balmar DUO charger is more than enough.

Fuel polisher do belong on the house bank.
My (admittedly limited) understanding is that having two alternators charging the same bank won't help since the higher output one will always cancel out the other. In my case, the eng alt is an oem 50a and the house alt is an 80a (I think). The eng alt is wired through the starter, and the house alt is wired through the 1-2-ALL switch that controls only the 2-batt house bank. I have a VSR relay which charges the genset batt from the eng batt.

Probably less complicated than I'm trying to describe it, but certainly a bit outside the normal setup, or so I've been told. Do you agree that an 8D (255ah) is overkill for starting an 80hp diesel?
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Old 24-06-2018, 00:16   #28
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Re: Two house battery banks - necessary?

Two separate banks that can be charged and discharged separately OR in parallel is a very good setup. Most users leave the switch in the "Both" position, and this has distinct advantages if you use your batteries heavily, especially with flooded cell lead acid batteries due to Peukert effect. However, if you practically never discharge below 90%, it is possibly better to keep them isolated and switch from one to the other every day. Shorted cells don't happen all that often, but they do happen. And it will pull down any battery that is in parallel with it. However, if you discharge more deeply or if you occasionally run heavy loads even for a short time, then it is probably better to keep them in parallel. The important thing is that in the event of a casualty, you can isolate if the situation calls for it, or run both packs together if needed.



One exception... if the banks are of different capacity, different brand, or significantly different age, it is best to not parallel them unless you have a reason to do so. And BTW, a two bank system enables you to replace half at a time, so you have one bank of relatively new and dependable batts.



An isolated engine start battery is a good thing. Even an important thing. TWO isolated start batteries is even better. A handy means of connecting house batteries to the starter circuit is important, if you only have one start battery and you cannot start the engine by hand crank. JMHO and YMMV.
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Old 24-06-2018, 00:26   #29
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Re: Two house battery banks - necessary?

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My (admittedly limited) understanding is that having two alternators charging the same bank won't help since the higher output one will always cancel out the other. In my case, the eng alt is an oem 50a and the house alt is an 80a (I think). The eng alt is wired through the starter, and the house alt is wired through the 1-2-ALL switch that controls only the 2-batt house bank. I have a VSR relay which charges the genset batt from the eng batt.



Probably less complicated than I'm trying to describe it, but certainly a bit outside the normal setup, or so I've been told. Do you agree that an 8D (255ah) is overkill for starting an 80hp diesel?


Do not worry about one alternator shutting off. Your engine alternator will probably never shut off as I suspect it to have internal regulation.

When one alternator shuts off voltage will be high and batteries will not absorb more than what one alternator will put out. When the voltage is low both alternators will charge in parallel.

Another alternative is to control both alternators with one regulator. In that case you need to prepare your engine alternator for external regulation.

Above is what I done on my boat. But I did that as I wanted a smarter regulation not to dry out AGM batteries with constant high voltage.

Will revert to flooded batteries this year which mean I would had been O.K. with the dumb regulator on my std. engine alternator. Actually it can be an advantage as it never goes in float.

An 8 D, 255 Ah start battery is an overkill. I have a 55 Ah start battery with high CCA. Has been working great for many years. Engine is 75 HP 4-cylinder. It is CCA (cold cranking amps) that is important.

If you would have an engine that is hard to start and you are sailing in cold areas a bigger battery like your 8D is probably a better choice than my 55Ah. But I always have the option to parallel my starter battery with the house bank. Knock on wood have never happened in 20 years of cruising ;-).
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Old 24-06-2018, 09:29   #30
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Re: Two house battery banks - necessary?

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Do not worry about one alternator shutting off. Your engine alternator will probably never shut off as I suspect it to have internal regulation.

When one alternator shuts off voltage will be high and batteries will not absorb more than what one alternator will put out. When the voltage is low both alternators will charge in parallel.

Another alternative is to control both alternators with one regulator. In that case you need to prepare your engine alternator for external regulation.

Above is what I done on my boat. But I did that as I wanted a smarter regulation not to dry out AGM batteries with constant high voltage.

Will revert to flooded batteries this year which mean I would had been O.K. with the dumb regulator on my std. engine alternator. Actually it can be an advantage as it never goes in float.

An 8 D, 255 Ah start battery is an overkill. I have a 55 Ah start battery with high CCA. Has been working great for many years. Engine is 75 HP 4-cylinder. It is CCA (cold cranking amps) that is important.

If you would have an engine that is hard to start and you are sailing in cold areas a bigger battery like your 8D is probably a better choice than my 55Ah. But I always have the option to parallel my starter battery with the house bank. Knock on wood have never happened in 20 years of cruising ;-).
Very helpful info Oceansailor -- thank you.

Yes, both my alternators are internally regulated, and they are both probably undersized for my banks of 8D's. But the system has been working for many years now, and I probably don't use the engine as much as most, so I have thus far declined to incur the expense & time to convert to external regulation. It's interesting that you mention the potential problem of overcharging AGM's with such a setup since there's no float mode. In another recent thread, I was advised that the problem is with undercharging, since internal regulation typically limits voltage to ~14v and Lifeline specs closer to 14.3-14.4 (for bulk/absorption). My two alt voltage meters (analog) typically show closer to 14.4v but I really need to confirm that with my digital meter.

The other problem I've read about with only one alt doing all the work is that the other could overheat, esp. with a large AGM bank. But your comments suggest that the lower capacity alt never actually stops working with internal regulation, so maybe this is also not a problem.

Yes, I always suspected an 8D was overkill for engine starting. This was the way the boat was originally configured AFAIK. Why would that be the case? Old school thinking, i.e. make sure the engine will start no matter what? Like you, I also have crossover relays that bring in the house bank if the eng batt is low, and I also have a portable lithium pack for backup.

In fact, according to the engine manual, my small G24 generator start batt. has enough CCA (at 72ļF) to start my engine as well, along with the same crossover relay. So I think I have engine starting covered, and perhaps should proceed with combining my eng start 8D with the house bank as it would increase overall capacity by one-third.
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