Is a large battery bank necessary?

[Andrew-Calliope]

I've never understood why people put in such large battery banks. Surely the only important thing is what the daily battery consumption is and how long it takes to recharge them? Having a battery bank that can last two or three days is not much point if it takes two or three times as long to recharge them.

I've always relied on a generator and 240v battery charger, but I've always had to upgrade the charger since as standard they are usually quite low-powered. Installing a large charger - 100amps - means that I only have to run the generator for one or two hours a day to get all the power we need. Also, I never bother to charge to 100%. Once the input is reading over 14 volts then it's as good as fully charged.

What does anyone else think?

[steve77]

I'll stick with lead-acid batteries here since I'm assuming that's the people you are seeing putting in large battery banks. And I don't think you want a treatise here, so I'll be very basic.

A battery's life is related to the depth of discharge. A lead-acid battery that is discharged to 80% and then recharged will last much longer than a battery that is discharged to 50% and then recharged.

So, one reason to have a large battery bank is to lengthen the life of the bank, and lengthen the time between spending money on replacing batteries.

Cheers!

Steve

[chrisr]

although it's true that a voltage of 14v or so can mean the battery is fully charged, i think there is more to it eg at what amps ?

our multi-stage charger reads 14.6 v long before the battery bank is fully charged (the first step is at about 97% i think). it is charging at over 55 amps at this point

it then progressively scales down the charge rate (amps) and at each step the voltage is back up to 14.6v, but can take another half an hour to get to fully charged.

on the last step the charge rate is only about 10 amps

i believe that it's important to get the battery bank to a really fully charged state frequently eg we do this every single day.

cheers,

[valhalla360]

Originally Posted by Andrew-Calliope

I've never understood why people put in such large battery banks. Surely the only important thing is what the daily battery consumption is and how long it takes to recharge them? Having a battery bank that can last two or three days is not much point if it takes two or three times as long to recharge them.

I've always relied on a generator and 240v battery charger, but I've always had to upgrade the charger since as standard they are usually quite low-powered. Installing a large charger - 100amps - means that I only have to run the generator for one or two hours a day to get all the power we need. Also, I never bother to charge to 100%. Once the input is reading over 14 volts then it's as good as fully charged.

What does anyone else think?

If you are willing to run the generator/engine occasionally to charge the batteries, you can reduce the size of the battery bank.
- If you motor frequently or you only weekend, that can be a good option as running the motor purely to charge the batteries will be rare.
- If you are a purist sailor or you anchor in place for longer periods of time, being able to handle overcast periods without starting the motor is handy.
- For the same power consumption, it wont' take 2-3 times as long to recharge them with a large battery bank.

A small battery bank brings other issues.
- Can it even absorb 100amps? There are limits even for batteries at 50%. It gets worse as you get near full charge.
- If you don't charge to 100%, it's hard on lead acid batteries. Lead acid are happiest at 100%.
- Voltage is not a good estimate of charge state when trying to fast charge. You are likely seeing the chargers voltage not the batteries. You need to stop charging and let the battery sit for a while in order to get a reliable reading from voltage.

If you have minimal consumption of electricity and your engine charging regime never lets it below 50%, you can get away with it. If you get it wrong, it's a few hundred dollars for a new battery bank. As you get into larger systems to feed larger consumption patterns, the cost of getting it wrong and destroying a battery bank is a lot more expensive.

[Pete7]

Originally Posted by Andrew-Calliope

Once the input is reading over 14 volts then it's as good as fully charged.

No it is not.

What batteries have you got and what does the manufacturer say?

If FLA then get them on a nice 14.8v charge and follow Charles Sterlings advice for fast charging all be it at the expensive of a bit of water. They will thank you for it, if its not too late.

Pete

[Stu Jackson]

While this discusses one or two house banks, it touches on depth of discharge and battery life.


Also, getting to 14V is NOT getting to a full bank.


__________________________________________________ __________


IS IT BETTER TO HAVE ONE OR TWO BATTERY BANKS FOR HOUSE USE?
(By Nigel Calder - I DIDN’T write this!!!)

This appears in Calder’s excellent Boatowners Manual. I bought the 2nd edition in 1998.

The [formerly – pre-1990s] popular arrangement of having two house banks alternated in use needs scrutiny before I go any further.

LIFE CYCLES: As we have seen, the life expectancy of a battery in cycling service is directly related to the depth to which it is discharged at each cycle - the greater the depth of discharge, the shorter the battery’s life.

This relationship between depth of discharge and battery life is NOT linear. As the depth of discharge increases, a battery’s life expectancy is disproportionately shortened. A given battery may cycle through 10% of its capacity 2,000 times, 50% of its capacity 300 times and 100% of its capacity around 100 times.

Let’s say, for arguments sake, that a boat has two 200-ah battery banks, alternated from day to day, with a daily load of 80 Ah. Each bank will be discharged by 40% (80 Ah of one of the two 200 Ah banks) of its capacity before being recharged. The batteries will fail after 380 cycles, which is 760 days (since each is used every other day). If the two banks had been wired in parallel, to make a single 400 Ah battery bank, this bank would have been discharged by 20% of capacity every day, with a life expectancy of 800 days, a 5% increase in life expectancy using exactly the same batteries!

But now let’s double the capacity of the batteries, so that the boat has either two 400 Ah banks, or a single 800 Ah bank, but with the same 80 Ah daily load. The two separate banks will be cycling through 20% of capacity every other day, resulting in a total life expectancy of 1,600 days. Doubling the size of the battery banks in relation to the load has produced a 210% increase in life expectancy. The single 800 Ah bank will be cycling through 10% of capacity every day, resulting in a life expectancy of 2,000 days - a 25% increase in life expectancy over the two (400 Ah) banks, and a 250% increase in life expectancy over the single 400 Ah battery bank!

There are two immediate conclusions to be drawn from these figures:

1. For a given total battery capacity, wiring the (house) batteries into a single high capacity bank, rather than having them divided into two alternating banks, will result in a longer overall life expectancy for the batteries.

2. All other things being equal, any increase in the overall capacity of a battery bank will produce a disproportionate increase in its life expectancy (through reducing the depth of discharge at each cycle).

FOR BATTERY LONGEVITY, A SINGLE LARGE (HOUSE) BANK, THE LARGER THE BETTER, IS PREFERABLE TO DIVIDED (HOUSE) BANKS.

[jt11791]

Originally Posted by steve77

A battery's life is related to the depth of discharge. A lead-acid battery that is discharged to 80% and then recharged will last much longer than a battery that is discharged to 50% and then recharged.

Isn't that exactly backwards? Did you mean to write:

A battery's life is related to the depth of discharge. A lead-acid battery that is discharged to 50% and then recharged will last much longer than a battery that is discharged to 80% and then recharged.

[valhalla360]

Originally Posted by jt11791

Isn't that exactly backwards? Did you mean to write:

A battery's life is related to the depth of discharge. A lead-acid battery that is discharged to 50% and then recharged will last much longer than a battery that is discharged to 80% and then recharged.

Depends what you mean by 80%.
- Taken from 100% to 80% of full charge...this should result in a long battery life
- Taken from 100% to 20% (80% used)...this will wear out a battery bank quickly.

This presumes Lead Acid batteries.

Lithium doesn't really have a problem going down to 20% of full charge. They also can take a fast charge pretty darn close to 100%. Of course, that comes at significant cost for the same watt-hr.

[C420sailor]

No, not with flooded lead acid. You’re thinking of types that develop a ‘memory’.

The further you venture into discharge land, the more lead sulfate you get on the plates. Charging, in theory, puts the sulfate back into solution...but it isn’t perfectly efficient. Any sulfate left behind crystallizes and coats the plates, making them less and less effective. And if you discharge it and leave it for a while? Crystal city.

FLA batteries should be maintained as close to 100% as possible, and discharged as little as possible.

[rslifkin]

Originally Posted by jt11791

Isn't that exactly backwards? Did you mean to write:

A battery's life is related to the depth of discharge. A lead-acid battery that is discharged to 50% and then recharged will last much longer than a battery that is discharged to 80% and then recharged.

I think he was referring to 50% and 80% SoC, not depth of discharge. So discharging to 80% SoC (80% full) is easier on the battery than 50%.

[CatNewBee]

Originally Posted by Andrew-Calliope

I've never understood why people put in such large battery banks. Surely the only important thing is what the daily battery consumption is and how long it takes to recharge them? Having a battery bank that can last two or three days is not much point if it takes two or three times as long to recharge them.

I've always relied on a generator and 240v battery charger, but I've always had to upgrade the charger since as standard they are usually quite low-powered. Installing a large charger - 100amps - means that I only have to run the generator for one or two hours a day to get all the power we need. Also, I never bother to charge to 100%. Once the input is reading over 14 volts then it's as good as fully charged.

What does anyone else think?

Depends on your loads. Sometimes a large bank is simply necessary to produce the current wanted.

Chemistry and inner resistance is another thing when it comes to charging.

A GEL battery will take much longer to charge than a AGM battery, simply by the higher resistance of the gel-bound acid, that slows down the ion flow. The charge current depends on the battery and not only on the charger. A LFP battery can be charged at 1C and quick charged at 3C, a GEL may accept 0.1C.

Also other people may need more power than you, because they use more electric devices. So a big bank is simply necessary. Some may even be reluctant to run the generator daily for charging, they rely on wind and solar, so they need a longer resilience storage, as weather varies.

3 days energy independence powered off the battery is a good starting point. With a bigger bank comes a bigger charge sources demand. We can charge 250A off the generator and up to 100A from solar. If you do not want to run your noisy generator all night long for the A/C, a large bank may be a good idea too.

Thete are many good reason for large battery banks.

[donradcliffe]

The size of the optimum battery bank depends on

1. Your consumption rate (I've been on boats that burned from 2 to 20 amps).
2. Your charging sources (solar, genset, dockpower, main motor).
3. Your preferences on noise, or lack of it.
4. The length of your typical sail.

[nigel1]

Originally Posted by Pete7

No it is not.

What batteries have you got and what does the manufacturer say?

If FLA then get them on a nice 14.8v charge and follow Charles Sterlings advice for fast charging all be it at the expensive of a bit of water. They will thank you for it, if its not too late.

Pete

Pete, do you have the full link, the link provided is to the main Sterling website.

[hpeer]

Originally Posted by Andrew-Calliope

I've never understood why people put in such large battery banks. Surely the only important thing is what the daily battery consumption is and how long it takes to recharge them? Having a battery bank that can last two or three days is not much point if it takes two or three times as long to recharge them.

I've always relied on a generator and 240v battery charger, but I've always had to upgrade the charger since as standard they are usually quite low-powered. Installing a large charger - 100amps - means that I only have to run the generator for one or two hours a day to get all the power we need. Also, I never bother to charge to 100%. Once the input is reading over 14 volts then it's as good as fully charged.

What does anyone else think?

Well you poked a hornets nest. People think all kinds of things informed by their boat, how they use it, where they use it, their interests, and how they value money.

I am lazy and cheap and am set up for the Caribbean. I have 4 GC lead acid batteries in series parallel, 12v and roughly 420AH. I have a good wind generator and solar panels. Ours is a very simple boat with few things many folks consider necessities. The biggest draw by far is the ice box. It has an Aries for long distance and a CPt for motoring.

It is seldom I need to run the Honda eu2000 to charge the batteries. They are relatively inexpensive and common, if they die I buy a new set.

Others will have very different opinions, which fit their needs. My way works for me.

Now my small boat l, 33’, doesn’t have a fridge, its biggest draw is the AP or the radar, or diesel heater fan. It has a 13hp diesel with a larger alternator, not much bigger than many gen sets. That boat is used in Northern waters where exotic batteries are uncommon. It has 2 GC batteries in series.

The use case for that boat is that in fog I am motoring anyway. Other times I run the engine as needed to keep the bank up. In light winds, using the AP, I am likely motoring; with good wind I run the wind vane. Otherwise I run the engine as a gen set to charge the batteries. I would like a bigger bank and may do it someday, but what ai have works adequately if not perfectly.

[LoudMusic]

It very much depends on the operator's intended use. We have a large lithium bank and a lot of solar. We do not have a generator and I prefer to not charge from my main engines.

My battery bank can charge and discharge at 400A safely. My solar can output 150A. We use about 740Ah per day, using our excessive electronics, electric galley, water heater, and average of one small load of laundry (with dryer) per day. Most days we produce about 810Ah. But some days we don't! And having a much bigger bank than we use allows us to go a couple cloudy days without having to worry about power.

Having a bigger battery bank allows us to get through those cloudy days without having a generator on board.

We also watch the weather and if it looks like we're going to have several days with minimal sun we will seriously back off our consumption. At that point we can probably go five days without trouble, which we've come close to several times in the past two years.