Is a large battery bank necessary?

[smac999]

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?

Most boats are tied to a dock 355 days a year. and are used for a couple weekends a year. Being able to leave the dock fri and have 3 days of battery and then plug back in Sunday night is a perfect solution for Many boaters.

Having a few days of battery power is also a big safty thing.

Hitting 14v while charging is still a long way from full

A 40’ boat with an 800ah bank and a 200a charger is pretty normal around here.

[CruiseN]

Originally Posted by rslifkin

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%.

That's how I saw it too.

Agreeing with many above. Batteries go through stages of charging. Flood charging when flat that reduces to bulk charging that then can look full but really just almost there as final charge is just tiny amounts to get level high.

Like a tide. Standing at low Mark, tide turns, low Mark floods. Then bulk of tide keeps filling. Finally although high the gentle last rise.

Without gentle last rise batteries don't last as long regarding leadacid, AGM,gel cells.

Lithium like a man made sink. Up, down, wherever. Needs to be above 10% ish for pick-up to be in liquid but can withstand sitting half full.

Sizes vary to suit vessels.

[moseriw]

Of course not! You can use three AAA monocells to enpower a 20 kWh e-motor

[Clivevon]

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?

The OP's Broom 42 is a motor boat I believe, mostly intended for river or estuary use rather than longer offshore cruises, so his battery needs are likely to be very different to many sail cruisers. Anyway, hopefully now he has a better understanding of why we like big battery banks. They don't necessarily cost more - the upfront cost is higher but they last longer so probably actually cost less overall.

[Moontide]

Andrew-C


Pretty sure you are not a long term cruiser. If true, I think you're missing the whole point of having enough battery power to run for 2-3 days with no minimal solar (or wind) output. Never having to run a generator is very nice, and can save a good bit of money, and not interfere with the music.

Just because it can supply 2-3 days of power does not mean it takes 2-3 times as long to charge. One only needs to replace the daily usage, not the entire bank so unless you've gone 2-3 days with minimal charging you don't need to charge 2-3 days worth of energy.



Also, your charging method probably does not even get close to getting the batteries fully charged - probably only 80% or so. Good chargers have 3 stages. Bulk, absorption and float. Bulk gives max charge up to a voltage around mid 14 volts (depending on battery type). This is constant current increasing voltage mode.



Once they hit the absorption voltage smart chargers switch to a constant voltage decreasing current mode. The current drops as they get closer to full. Even with a 100 amp charger it will start reducing the current to much less than that. This is why it takes so long to fully charge lead acid batteries (Lithium whole different story).


Once the batteries are full the chargers go into float mode which is a constant voltage very low current rate. Voltage at this stage is usually in high 13's. It is common for the absorption stage take longer than the bulk stage charging.



The charging profile of lead acid is what makes solar (and even wind if in an area with over 10-15 knots) such a good charging source. I will put out current for many hours at whatever current the batteries need at that time, without the cost, maintenance or noise of a generator.


As with most things, it is different strokes for different folks. For a longer term outing your method is not the best (IMHO) but for weekends or slightly longer it may work fine. All depends on what you want.

[Dockhead]

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?

I never knew anyone with a battery bank that lasted 2 or 3 days.

Mine (440 amp h * 24v) lasts a bit more than half a day in a long passage under sail.

A larger battery bank not only gives more reserve time and power, but works less hard. A given load is less C so less Peukert (in case of lead batts) and a given amount of consumption is a lesser depth of discharge. Also acceptance rate is proportionately higher so you can store more power in the same amount of time, if you have power available.

This is less important with lithium, but quite important with lead. If you want to charge no more than once a day, then if feasible your battery bank should cover your daily needs with 50% of nominal capacity. A bit bigger than that is better, because then you can go a day and discharge to a healthier depth of discharge, and you have some reserve.

Another factor is how many days you spend off grid, and whether you are sail or power, and whether you have a generator, and whether you have solar. If like someone above said you only anchor out 3 days a year, then I'm not sure what point there is investing in a large battery bank. If you anchor out 365 days a year like some of our people here, then you will wear out a lead bank quickly if it's not big enough to avoid discharging deep. Most motorboaters I know just run their generators all the time. Sailors on long passages need big reserves because they might not be able to run either generator or main engine on a heel. Etc. etc. etc.



There is no one size fits all. It depends on your use pattern, how often you are willing to buy new batteries, and other things.

[MagicBeans55]

I think the battery situation is like children decades ago on a farm// you have as many as you need depending on how big your farm is.. Train them well and they help you and eventually help themselves.. Abuse them or don’t train (charge) them properly and you don’t get much out of them only problems..Itall depends on what you grow and the size of your farm equipment.. cheers

[FlyingScot]

Larger house bank makes solar more efficient. Same solar 1440wp we once had 450ah @12v. Now with 900ah we harvest more solar. We use less energy underway than at anchor. Yes even with all instruments and radar and autopilot on 24/7.

[Chotu]

There are a LOT of different dynamics here and everyone uses their boat differently, so the answers will be all over the place.

The OP has a good point and I realized the same thing during my design process.

I chose to maximize my usable electricity per day, while minimizing my weight and not spending a fortune on lithium.

The sweet spot was 440 AH. The entire boat runs perfectly off this. Deep chest freezer, refrigerator, watermaker, all nav stuff, etc.

How can this possibly be true??

Look closely at WHEN you use your power. I realized we are asleep at night, when the batteries are needed. So, the batteries are only running the refrigerator and freezer at night and occasionally the nav instruments. 440 AH does that just fine and is topped off to 100% charge by noon or so.

My rather unique design uses 1500 watts solar to achieve this.

The only time we need to run generators is for air conditioning or baking in the (also very lightweight) electric oven.

Other loads like dishwasher, watermaker, slow cooker, clothes washer, etc etc can run off 1500 watts of solar without touching the battery bank DURING THE DAY, which is the only time they are used.

My big breakthrough to eliminating hundreds of pounds of batteries was realizing the time of day you use the load has everything to do with how much battery capacity you need. If solar is your main power source

[Andrew-Calliope]

Many replies simply miss the point (while others appear to be towing a tennis court full of solar panels).

What's important is (a) the capacity of the battery bank (without discharging it too much) (b) the daily consumption and (c) the charging capacity (d) how long you are prepared to run additional charging (beyond solar and wind) for.

For most people using a fridge and a freezer (and those without vast solar panels) external charging will be necessary and this should be done within an acceptable time. Whatever it is, having a battery bank larger than around twice this capacity is probably wasted. What's important is matching the battery bank to the daily consumption and recharging capacity. If you want to go 2-3 days without recharging, then you have to run the charger 2-3 times as long so there is no gain.

In my catamaran I allowed to run the generator for up to hours a day which, with a large battery charger would re-charge the batteries and make enough water for 24 hours. One of my points is that, as standard, most boats are equipped only with small chargers designed for use in marinas. With a generator you should upgrade the charger. (Just a note that my Sunware solar panels failed completely as did their replacements, so I question solar panel reliability.)

Also apologies for saying the cut off was over 14 volts. The cut-off is when the charge going into the batteries (as read by the shunt) reduces to around 15 amps. After that, there's not much point in trying to get any more in.

[Pete7]

Originally Posted by nigel1

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

Ah, they have updated their website and a lot of really good stuff has gone sadly. Charles was a fan of flooded lead acid and fast charging, even if that meant topping up with water more often. Flooded lead acid are now becoming expensive and scarce. Seems the demand is for SLA instead or is it good business practise selling SLA knowing they can't be topped up? Therefore they will have a shorter life, leading people buying more of your batteries

https://sterling-power.com/pages/product-information-1

[MAJICDAN]

I have tried to keep my batteries above 70 percent.
With cloudy days I figured that I needed enough batteries for three to four days without much solar.
That is how I came up with two banks of ten golf cart batteries. They seem to last me about five years. I buy them at Sams when they are on sale for about $70 dollars apiece. I do fine when I am not sailing. My auto pilot uses a lot of power. I usually have to run the generator to charge about every 6-8 days. The five solar panels, are theoretically 1450 watts but one half to two thirds of them are usually covered by the sails. I have three 120 amp, and one 80 amp chargers for the house batterie bank

[svfinlandia]

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?

I guess it depends. If you have an electric range, electric oven and a household size electric refrigerator you would certainly need a much larger battery bank then I have.

My last house bank was two 100 amp NAPA deep cycle/starting batteries that lasted for five years and only cost $89 each.
They recently finally died due to a shorted cell in one of them and I upgraded them to two 220 amp hour golf cart batteries. Hope you get at least seven years out of them.

Al, S/V Finlandia

[MAJICDAN]

I have always relied on solar and my wind generators Rather than running my generator every day..
I have a large enough house battery bank that the only time I have to use my generator is when I am sailing. My autopilot takes more power than my five solar panels can provide

[noelex 77]

Originally Posted by Chotu

There are a LOT of different dynamics here and everyone uses their boat differently, so the answers will be all over the place.

The OP has a good point and I realized the same thing during my design process.

I chose to maximize my usable electricity per day, while minimizing my weight and not spending a fortune on lithium.

The sweet spot was 440 AH. The entire boat runs perfectly off this. Deep chest freezer, refrigerator, watermaker, all nav stuff, etc.

How can this possibly be true??

Look closely at WHEN you use your power. I realized we are asleep at night, when the batteries are needed. So, the batteries are only running the refrigerator and freezer at night and occasionally the nav instruments. 440 AH does that just fine and is topped off to 100% charge by noon or so.

My rather unique design uses 1500 watts solar to achieve this.

Lots of solar and a small battery bank is a useful option to consider. The durability and long term reliability of rigid solar panels makes it a hassle free and very inexpensive option. It works best in areas/seasons of consistent solar insolation such as the Med in summer. It is helpful if there are discretionary loads such as a watermaker.

With lead acid batteries battery life will be a shorter, but replacement is not expensive. It can work well in the right situation. I have never used this option with lithium, but it has potential. The batteries will have a hard life, but by the time their cycle life is reached there are likely to be better and cheaper options available.

Many boats are much better suited to a large battery bank, but don’t dismiss a small battery bank, especially if you have room for a large solar array.