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Old 27-07-2020, 13:34   #61
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Re: Best batteries for $2k budget

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BS. It take much longer to put in the final 20% charge and the larger bank will have more Ah above 80% SOC to fill. If you like to abuse your batteries like most people and operate in the 50 to 80% SOC range you are correct but otherwise you are dead wrong. It's a common misunderstanding leading to gourmet charging systems and batteries. Marketing at it's best. Absorption phase won't get your batteries anywhere near 100% SOC. It takes a long time to fill a battery and it is limited to acceptance rate of the battery.

Sorry, you are wrong. If we are talking constant amp-hours, the big bank takes them faster. If you are talking about percentage charge, then big and small banks are the same. But we were talking about a given number of amp-hours.
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Old 27-07-2020, 14:30   #62
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Re: Best batteries for $2k budget

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Sorry, you are wrong. If we are talking constant amp-hours, the big bank takes them faster. If you are talking about percentage charge, then big and small banks are the same. But we were talking about a given number of amp-hours.
Charge efficiency drops significantly as SOC increases. The larger bank, 800 Ah, drops to 75% SOC after consuming 200 Ah. The 600 Ah bank drops to 66% SOC. The smaller bank will charge at a higher average efficiency. That's why a lot of people chose to cycle between 50 and 80% SOC. Not only the acceptance rate drops way off above 80% SOC, the efficiency also drops.

The misconception that discharging to no less than 50% SOC is etched in stone despite the manufacturer published data showing it more economical to discharge deeper exemplifies the more is better philosophy.

What people prioritize in selecting a bank should ultimately dictate what they chose. Weight, size, charge efficiency, maintenance, cycle life, etc must all be considered in a logical and realistic manner. That is rarely done. I know a guy that bought a HUGE battery bank because he needed a laptop for "work". He justified the 800 Ah bank because the laptop was for "work", as if it mattered what the laptop was for.

The need to justify ones purchases often defies logic. There is no best mousetrap.
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Old 28-07-2020, 01:16   #63
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Re: Best batteries for $2k budget

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Originally Posted by kmacdonald View Post
Charge efficiency drops significantly as SOC increases. The larger bank, 800 Ah, drops to 75% SOC after consuming 200 Ah. The 600 Ah bank drops to 66% SOC. The smaller bank will charge at a higher average efficiency. That's why a lot of people chose to cycle between 50 and 80% SOC. Not only the acceptance rate drops way off above 80% SOC, the efficiency also drops. . . .

This is not logical. Why do you assume that the smaller bank is being run below 50% while the larger bank is being run in absorption? That's just weird. If you want to run your bank below 50% SOC, they're your batteries, but you can do that just as well with 800AH of batts as you can with 600AH of batts.


Smaller banks do NOT have greater charge efficiency. An 800AH bank has about ("about" because 800 is nominal only) 240AH of capacity between 50% and 80%. A 600AH bank has 180AH of capcity between 50% and 80%. If you choose to run them down to 30%, then the 800AH bank has 400AH in the range of 30% to 80% and the 600AH bank has 300AH. You can run more power at higher efficiency through a larger bank, than through a smaller one.


A larger battery bank has almost only advantages compared to a smaller one, but for initial acquisition cost and weight. But a larger bank, although it is more costly to acquire, is not necessarily more costly over the lifetime of the bank, because it will last longer. All other things being equal, a battery's life is determined by the amount of power cycled through it. If you double the number of batteries you halve the amount of power each battery handles. And all things are not equal -- a smaller bank will more often get run down lower because it has less capacity, which shortens life further. In the long run, a larger bank may be actually cheaper.


You might NEED to run the batts down to 30% on a regular basis, with a small bank, whereas the same power needs can be met without going below 50% with a larger one. The life of the smaller bank will be disproportionately shortened. We had a thread about this a few years ago where a lot of real experts brought a lot of real facts -- I was surprised myself at the result, but it is so.
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Old 28-07-2020, 03:23   #64
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Re: Best batteries for $2k budget

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This is not logical. Why do you assume that the smaller bank is being run below 50% while the larger bank is being run in absorption? That's just weird. If you want to run your bank below 50% SOC, they're your batteries, but you can do that just as well with 800AH of batts as you can with 600AH of batts.


Smaller banks do NOT have greater charge efficiency. An 800AH bank has about ("about" because 800 is nominal only) 240AH of capacity between 50% and 80%. A 600AH bank has 180AH of capcity between 50% and 80%. If you choose to run them down to 30%, then the 800AH bank has 400AH in the range of 30% to 80% and the 600AH bank has 300AH. You can run more power at higher efficiency through a larger bank, than through a smaller one.


A larger battery bank has almost only advantages compared to a smaller one, but for initial acquisition cost and weight. But a larger bank, although it is more costly to acquire, is not necessarily more costly over the lifetime of the bank, because it will last longer. All other things being equal, a battery's life is determined by the amount of power cycled through it. If you double the number of batteries you halve the amount of power each battery handles. And all things are not equal -- a smaller bank will more often get run down lower because it has less capacity, which shortens life further. In the long run, a larger bank may be actually cheaper.


You might NEED to run the batts down to 30% on a regular basis, with a small bank, whereas the same power needs can be met without going below 50% with a larger one. The life of the smaller bank will be disproportionately shortened. We had a thread about this a few years ago where a lot of real experts brought a lot of real facts -- I was surprised myself at the result, but it is so.
Charge efficiency = power stored divided by power in. Above 75 or 80% SOC the efficiency drops. Below that it might be around 95% and above 95% SOC it may be less than 50%. If you cycle the battery between X and 80% SOC you have a charge efficiency of about 95%. If you cycle between 80 and 100% SOC you might only have a 70% charge efficiency. Above 95% SOC charge efficiency may drop below 50%. So not only does the acceptance rate drop off with increasing SOC the efficiency also drops off.

Before solar cruisers almost never cycled between X and 100% SOC. That would have required long daily engine run times. With solar it's possible to cycle between X and 100% SOC. Batteries will last much longer if not cycled to PSOC. This allows you to take advantage of deeper discharges while still maintaining acceptable battery cycle life. Battery usage by cruisers has fundamentally changed with inexpensive solar. It's not like driving your daddy's Buick.

Another advantage of solar is that battery banks can be sized for energy required during the night and not a 24 hour period. With adequate solar you only need to store enough energy for the night. During the day solar provides the power AND recharges the batteries. It's another case of traditional wisdom outliving it's relevance and usefulness.
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Old 28-07-2020, 05:34   #65
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Re: Best batteries for $2k budget

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Originally Posted by kmacdonald View Post
Charge efficiency = power stored divided by power in. Above 75 or 80% SOC the efficiency drops. Below that it might be around 95% and above 95% SOC it may be less than 50%. If you cycle the battery between X and 80% SOC you have a charge efficiency of about 95%. If you cycle between 80 and 100% SOC you might only have a 70% charge efficiency. Above 95% SOC charge efficiency may drop below 50%. So not only does the acceptance rate drop off with increasing SOC the efficiency also drops off. . .

As everyone knows.



Not relevant to the question of how efficient are small banks versus large ones, since both large and small banks have the same bulk and absorption phases.
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Old 28-07-2020, 06:01   #66
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Re: Best batteries for $2k budget

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As everyone knows.



Not relevant to the question of how efficient are small banks versus large ones, since both large and small banks have the same bulk and absorption phases.
Sure it is. The smaller bank will discharge to a lower SOC where charging is more efficient. Deep cycling a battery will also provide more energy over it's lifetime. The published cycle life vs DOD graphs show exactly that. Why don't people believe the published facts? Is it that damage done by PSOC cycling is blamed on deep cycling or maybe PSOC cycling is offset by shallow discharging?
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Old 28-07-2020, 06:08   #67
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Re: Best batteries for $2k budget

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Originally Posted by kmacdonald View Post
Sure it is. The smaller bank will discharge to a lower SOC where charging is more efficient. Deep cycling a battery will also provide more energy over it's lifetime. The published cycle life vs DOD graphs show exactly that. Why don't people believe the published facts? Is it that damage done by PSOC cycling is blamed on deep cycling or maybe PSOC cycling is offset by shallow discharging?
Why couldn't you discharge a larger battery to a lower SoC as well. Sure you CAN keep a larger battery at a very high state of charge and yes that would be less efficient. Not a single person has disagreed with that.

However you don't have to. In fact with a larger battery you can get by with NEVER as in not a single second of a single day ever charging beyond 80%.
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Old 28-07-2020, 06:18   #68
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Re: Best batteries for $2k budget

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Why couldn't you discharge a larger battery to a lower SoC as well. Sure you CAN keep a larger battery at a very high state of charge and yes that would be less efficient. Not a single person has disagreed with that.

However you don't have to. In fact with a larger battery you can get by with NEVER as in not a single second of a single day ever charging beyond 80%.
You can. You can oversize your battery bank all you want and carry the extra weight. Cycling between X and 80% SOC used to be standard procedure for most sailors before solar. Solar has or should have changed that practice. It's a different ballgame now. Solar works all day at no additional expense. Engine running time is expensive.
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Old 28-07-2020, 06:23   #69
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Re: Best batteries for $2k budget

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Why couldn't you discharge a larger battery to a lower SoC as well. Sure you CAN keep a larger battery at a very high state of charge and yes that would be less efficient. Not a single person has disagreed with that.

However you don't have to. In fact with a larger battery you can get by with NEVER as in not a single second of a single day ever charging beyond 80%.
If you PSOC cycle you will reduce battery life significantly. If you don't want solar that's something you have to live with, or run the engine five hours a day.
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Old 28-07-2020, 08:26   #70
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Re: Best batteries for $2k budget

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My understanding is that Bigbattery.com is making their batteries from established EV and hybrid vehicle battery modules. They are used but fully tested to comply with the specification for the pack. These are much higher quality battery packs than true "no name brand" stuff. I will never waste my money on a lead acid battery again....



I take good care with my lead acid Trojan battery's, I have had two replacements' in the last 20 years. first set T105 lasted 10 years, I am now on the second set: T145, 4 years now, great performance both sets were, are, (6) six volt golf cart batteries.
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Old 29-07-2020, 16:22   #71
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Re: Best batteries for $2k budget

I stumbled across this from Victron:

Efficient
In several applications (especially off-grid solar), energy efficiency can be of crucial importance.
The round-trip energy efficiency (discharge from 100% to 0% and back to 100% charged) of the average lead-acid battery is 80%.
The round-trip energy efficiency of a Li-ion battery is 92%.
The charge process of lead-acid batteries becomes particularly inefficient when the 80% state of charge has been reached, resulting in
efficiencies of 50% or even less in solar systems where several days of reserve energy are required (battery operating in 70% to 100% charged
state).
In contrast, a Li-ion battery will still achieve 90% efficiency even under shallow discharge conditions.
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Old 29-07-2020, 18:06   #72
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Re: Best batteries for $2k budget

That is true and you can find lots about how charge inefficient lead acid batteries are when charging 80-100% state of charge.

But in a cruising boat using mostly solar, who cares about charge efficiency really? I don’t care about charge efficiency, all I care about is that I can make up amp hours out with amp hours in.
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Old 29-07-2020, 18:23   #73
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Re: Best batteries for $2k budget

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That is true and you can find lots about how charge inefficient lead acid batteries are when charging 80-100% state of charge.

But in a cruising boat using mostly solar, who cares about charge efficiency really? I don’t care about charge efficiency, all I care about is that I can make up amp hours out with amp hours in.
The application of that knowledge is what's important. Battery sizing, solar sizing, and what range of SOC you use are affected by it. You may not have that charge capacity to make up amps out when cycled between 60 and 100% SOC but might have the charge capacity to make up amps cycling between 40 and 80% SOC. Boats are limited in the amount of solar they can have so cycling range may be dictated by that. You may not be able fully charge large batteries to 100% SOC with available solar but could deeper cycle smaller batteries to 100% SOC.
If that knowledge hasn't been considered in an energy system it hasn't been optimally designed.
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Old 30-07-2020, 07:01   #74
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Re: Best batteries for $2k budget

Couple of years ago in the Bahamas I would ask people about their batteries (the cruisers). Some would know a lot and some would know a little. Near as I could tell was that overall each group got about the same life out their batteries.

That shows just how little difference in real life off the grid cruising these small nitpick things are.

I thought I was pretty knowledgeable and followed pretty much the charging/discharge "rules" for my FLA. One of them still didn't make it to the 4 year mark.

BTW - I have 640W solar. If I doubled it I bet it would at best save 30 minutes/day charging and maybe get to 100% at 2pm instead of 2:30pm. Either way by 12-1 on a good sunny day the battery acceptance is already clamping down on the current.
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Old 30-07-2020, 07:37   #75
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Re: Best batteries for $2k budget

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It takes longer to put the last 200 Ah back in a 800 Ah bank than a 600 Ah bank. Don't oversize the bank for longevity sake. Large banks will run at PSOC more than smaller banks defeating the gains of lower DOD cycling.

That might be where you and Dockhead crossed wires.
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