Battery charging

[George and Dragon]

I know this has been done to death but unfortunately I'm still not sure I have a good understanding...

 

So, I have some rather naive questions about battery charging. I may not be asking the right questions – if so I hope someone will help me out.

 

For the sake of simplicity I'd like to start with cheap lead acid leisure batteries.

 

I've read various people saying that to maintain the capacity they need to be fully charged which I accept. What I don't really understand is how different methods of charging get to fully charged.

 

I guess the simplest example is the boat that goes into a marina and is on shore power 90% of the time. Presumably the charger should be set up to automatically bring the battery up to full charge even if it takes a long time to do so?

 

Where I'm finding it harder is with boats that don't have shore power available most of the time. Does a PV system make a big difference? How big should the PV system be if (for example) the boat normally used 1300 Wh each day? Would a system that big fit on the roof of a 60ft NB?

 

Without PV how many hours would a 12V 150A alternator need to run to put back all 1300Wh?

 

And what happens if the batteries remain at say 90% SoC for a day before they're charged? What about 80%? 70%? Longer periods?

[WotEver]

Have a read of this and see if it helps any:

 

 

5 minutes ago, George and Dragon said:

Does a PV system make a big difference?

From March to October yes, an absolutely huge difference. From November to February not so much. 

7 minutes ago, George and Dragon said:

Without PV how many hours would a 12V 150A alternator need to run to put back all 1300Wh?

8 to 12 hours. 

[Tony Brooks]

This ^^^^^   especially the last bit.

 

Also remember that shore line chargers tends to drop into float voltage too early if you are aboard and using a lot of battery power every day. To maximise battery life you also need some form of battery monitoring so you get a handle on their state of charge throughout your time aboard. I don't know if @WotEver's piece linked above says anything about battery monitors that count Amp hours but if not the only accurate read outs are Amps, Volts, and Amp hours out. The rest will tell lies unless  jump through hoops and they will encourage you to destroy batteries.

 

 

[WotEver]

11 minutes ago, Tony Brooks said:

I don't know if @WotEver's piece linked above says anything about battery monitors that count Amp hours but if not the only accurate read outs are Amps, Volts, and Amp hours out. The rest will tell lies unless  jump through hoops and they will encourage you to destroy batteries.

No, it doesn’t say that specifically. It would be a good paragraph to add to that post but it’s way past editing time unfortunately. 

[TheBiscuits]

15 minutes ago, WotEver said:

No, it doesn’t say that specifically. It would be a good paragraph to add to that post but it’s way past editing time unfortunately. 

Request a mod edit by reporting your original post and including the text you want adding.  It's a good bit to add to the topic.

[WotEver]

2 minutes ago, TheBiscuits said:

Request a mod edit by reporting your original post and including the text you want adding.  It's a good bit to add to the topic.

Good idea. When I have a bit more time I’ll work out where it needs to go and word something to be dropped in.

[TheBiscuits]

9 minutes ago, WotEver said:

Good idea. When I have a bit more time I’ll work out where it needs to go and word something to be dropped in.

Just don't report yourself as a spammer

 

[WotEver]

18 minutes ago, TheBiscuits said:

Just don't report yourself as a spammer

 

Dun

[George and Dragon]

15 hours ago, WotEver said:

15 hours ago, George and Dragon said:

Without PV how many hours would a 12V 150A alternator need to run to put back all 1300Wh?

8 to 12 hours. 

Are you sure?

 

How about to put back 2600Wh? Which would be equivalent to discharging a 12V 440Ah system to half capacity. What if my estimated consumption is way out and I only need replace 650Wh?

 

Are other battery chemistries better?

[Alan de Enfield]

12 minutes ago, George and Dragon said:

Are you sure?

 

How about to put back 2600Wh? Which would be equivalent to discharging a 12V 440Ah system to half capacity. What if my estimated consumption is way out and I only need replace 650Wh?

 

Are other battery chemistries better?

To a certain extent the size of the alternator is almost irrelevant after the 1st 1/2 hour. Putting it simply, the batteries control the charge level , just as an example :

 

You need to replace 150Ah

 

For the fist 1/2 hour your alternator may be running at 100A

The the next half hour it'll run at  50A

For the next 2 hours it'll run at 20 amps

For the next 3 hours it'll run at 10 amps

For the next 5 hours it'll be ever reducing until you get down to about 2 amps

 

The battery can be considered to be 100% charged when it has been charging at 1% battery bank capacity for in excess of 1 hour with no change to the charging current.

 

Lets assume you have a battery bank of 3x 100Ah batteries.

 

1% is 3amps so if your alternator has been charging at 14.4v and a current of 3A with no change for an hour, you can consider the battery charged.

 

 

It will take pretty much the same time to replace 200Ah, that's lead-acid battery chemistry.

If you want faster charging batteries you can pay £5,000 and get Lithiums, but that opens up a whole new can of worms.

Edited September 4, 2020 by Alan de Enfield

[nicknorman]

14 minutes ago, George and Dragon said:

Are you sure?

 

How about to put back 2600Wh? Which would be equivalent to discharging a 12V 440Ah system to half capacity. What if my estimated consumption is way out and I only need replace 650Wh?

 

Are other battery chemistries better?

I think 8-12 hours is a rather pessimistic with a modern 150A alternator. The problem is that most of the charge can be put back in quite quickly, but that last 10% or so takes a long time due to the way lead acid batteries work - put simply, the chemicals that need to react are not very mobile and are in short supply as the battery approaches fully charged, so the reaction slows right down and the charge can only be absorbed very slowly.

 

With a decent alternator I’d say you can put most of the charge back in 3 hours, another 3 hours to really get it properly fully charged. Which is something you need to do every few days to ward of sulphation.

 

Solar can help a lot - the general idea being that you use the engine in the morning to get the first part of the charging done - when the batteries can absorb a lot of charge quickly. Then leave the solar to put the rest in slowly over the remainder of the day.

 

I have recently found that having an external “fancy” alternator regulator box really makes quite a difference to charging speed, which is contrary to perceived wisdom on here, and to my own opinion before I fitted it!

 

As to other chemistries well of course LiFePO4 batteries have many advantages, mostly charging speed and no need to fully charge, but also carry a lot of disadvantages including being easy to trash if charged or discharged incorrectly, and very expensive at the outset.

2 minutes ago, Alan de Enfield said:

To a certain extent the size of the alternator is almost irrelevant after the 1st 1/2 hour. Putting it simply, the batteries control the charge level , just as an example :

 

You need to replace 150Ah

 

For the fist 1/2 hour your alternator may be running at 100A

The the next half hour it'll run at  50A

For the next 2 hours it'll run at 20 amps

For the next 3 hours it'll run at 10 amps

For the next 5 hours it'll be ever reducing until you get down to about 2 amps

 

Popular CWDF myth number 1. The above is not true. Do I really need to explain it again?

Edited September 4, 2020 by nicknorman

[Alan de Enfield]

6 minutes ago, nicknorman said:

Popular CWDF myth number 1. The above is not true. Do I really need to explain it again?

Not quite sure what the problem is - I've given an example showing him getting 75% of the charge in 3 hours, and the balance over 5-6 hours, similar to what youhave said :

 

With a decent alternator I’d say you can put most of the charge back in 3 hours, another 3 hours to really get it properly fully charged. Which is something you need to do every few days to ward of sulphation.

 

The OP has said he doesn't understand batteries so just trying to make it simple.

 

Yes we could get all technical, but that doesn't seem to be what the OP wants.

Edited September 4, 2020 by Alan de Enfield

[nicknorman]

14 minutes ago, Alan de Enfield said:

Not quite sure what the problem is - I've given an example showing him getting 75% of the charge in 3 hours, and the balance over 5-6 hours, similar to what youhave said :

It is the point about the size of the alternator not really mattering after the first 30 mins. But it does. A lot. Due to the soft regulation curve of any normal alternator. Even without my new regulator, our alternator would be charging at over 70A for several hours. If you only had a 70A alternator, after a couple of hours it would likely be charging at only 35A or less.

 

Yes the very last bit is probably fairly independent of the alternator size but the initial and middle part of the charging, which lasts many hours, is very dependant on it.

 

You are saying 9 hours, I am saying 6. Although as we’ve said before, it all depends on how you define “fully charged”.

 

And you say that the battery controls the charging. Well not really, it is the battery in combination with the soft regulation curve of a standard alternator that determines the charging current. When the battery has reached a state of charge such that the voltage is say 14v, the alternator will already have shut down to say 1/2 output or less. But if the alternator was capable of putting out its full output nearly up to the regulated voltage of say 14.4v, as it would be with a modern digital regulator, then it would still be producing full output. Of course this would push the system voltage up a bit more, but overall the charge rate is massively increased as I have discovered.

Edited September 4, 2020 by nicknorman

[Alan de Enfield]

10 minutes ago, nicknorman said:

Even without my new regulator, our alternator would be charging at over 70A for several hours.

Surely that would depend on your battery bank size and SoC.

 

You are not going to get "70A for several hours" going into 3x100Ah batteries at 50% SoC

Should we assume that the OP has a "modern digital regulator" ?

 

The OP is struggling to comprehend charging times and I don't think 'blinding him with science' and the latest hi-tech gizmo's and big battery bank that you have is going to help him.

[nicknorman]

8 minutes ago, Alan de Enfield said:

Surely that would depend on your battery bank size and SoC.

 

You are not going to get "70A for several hours" going into 3x100Ah batteries at 50% SoC

Should we assume that the OP has a "modern digital regulator" ?

 

The OP is struggling to comprehend charging times and I don't think 'blinding him with science' and the latest hi-tech gizmo's and big battery bank that you have is going to help him.

Yes of course it would. Neither of which we really know, although he did talk about 450Ah of batteries.

 

Anyway my point is mainly that when people say “it doesn’t really matter whether you have a 70A alternator or a 175A alternator, since the latter only speeds charging for the first half hour or so” this isn’t true unless the battery is extremely small, and is thus misleading.

 

And as to the “modern digital regulators” the likes of an Adverc or Sterling A2B are generally dismissed on here as being fairly useless, but I can now see that that is not correct. If getting a rapid charge into LA batteries is a priority, it would be worth installing one of these “hi tech gizmos”.

Edited September 4, 2020 by nicknorman

[WotEver]

With or without high tech gizmos you’re not going to get it below 8-10 hours. I was being generous with my 8-12 hours as without them it’s more likely to be over 12 hours. 
 

I’ve just taken a look and I see that Battery University agrees with me. It’s the lead acid chemistry that you can’t avoid. 
 

https://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

[nicknorman]

8 minutes ago, WotEver said:

With or without high tech gizmos you’re not going to get it below 8-10 hours. I was being generous with my 8-12 hours as without them it’s more likely to be over 12 hours. 
 

I’ve just taken a look and I see that Battery University agrees with me. It’s the lead acid chemistry that you can’t avoid. 
 

https://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

I disagree, based on personal experience. Although as I’ve said many times, it depends on what you mean by fully charged. You can make the hours needed to fully charge virtually anything you want, according to what % of capacity you want the tail current to be. I think most people say 2% or 1%. 2% we can reach by late lunchtime (say 4 or 5 hrs). 1% takes several more hours but is reached by end of cruising day say 7 or 8 hours. Nobody charges their batteries for over 12 hours using an engine so I don’t see it as helpful to mention these sorts of figures as they are in practice unobtainable unless on shore power.

 

Battery University talks about 3 to 5% of capacity as being fully charged, and 8-10 hours, which is simply not true unless you are charging with a bicycle dynamo!

 

But in seriousness as we know it is all about compromise. To be kindest to you batteries, you charge em slowly, maybe around 10% of capacity, which takes hours and hours. But being kindest to your battery is not the only consideration, there is also the question of engine wear, fuel consumption and nuisance. So in practice people charge their batteries much faster than for optimum life, hence the sorts of times I’m talking about to get current to 2% or 1% of capacity.

[nicknorman]

Anyway here is some real-world data from our boat a few years ago with the standard 175A alternator and regulator, and 450 Ah of LA batteres. SoC in purple, LH axis. Current in red, RH axis. Under 3 hours to go from 57% to 93%. With a 70A alternator it would take a lot longer. Of course still a few more hours to get to 100% SoC, according to what one's definition of fully charged is!

 

 

[George and Dragon]

This is all very interesting

 

Thanks for the debate @nicknorman and @Alan de Enfield. You're right that I don't really understand batteries beyond the basic O level chemistry model, but it seems to me that there's an awful lot of trouble waiting to be found. 

 

How do you determine SoC? Are sophisticated instruments required? All I have is a voltmeter and an ammeter. It sounds like some kind of instrument that records both and allows a graph to be plotted would be helpful but the thing that really gets me is the necessity to run the engine almost interminably to get that last bit of charge into the battery. If I believe @WotEver's 8-12 hours then 1.3kWh which would cost me 20 pence at home is costing £10 of diesel.

 

A new set of batteries is £400. It really doesn't seem to make sense...

 

On 04/09/2020 at 15:52, nicknorman said:

With a decent alternator I’d say you can put most of the charge back in 3 hours, another 3 hours to really get it properly fully charged. Which is something you need to do every few days to ward of sulphation.

Does this mean that getting the battery back to 90% most days and then running the alternator 8am to 8 pm once a week would prevent sulphation? Or is that too simplistic? Battery university seems to be saying that sulphation won't occur if the cell voltage is above 2.05V.

Is discharging 4 times to 75% SoC equivalent to one discharge cycle? How many discharge cycles should I expect from cheap LAs anyway?

 

I'm beginning to understand why so many people expect to change their batteries every couple of years!

 

[Alan de Enfield]

8 minutes ago, George and Dragon said:

Does this mean that getting the battery back to 90% most days and then running the alternator 8am to 8 pm once a week would prevent sulphation? Or is that too simplistic?

 

You will get some sulphation if you do not immediately recharge to 100%, all you can do is plan to do a cost evaluation as to the most economic way to recharge looking at Engine wear & tear / fuel costs / replacement battery cost / life of battery if partly re-charged.

 

What you are proposing is what many will do, if you cruise for 4 or 5 hours a day (every day) then your batteries will be pretty much always re-charged each day, if you only move every 2 weeks you are going to need to put back as much as you reasonably can each day, and top up to 100% at 'weekends'.

 

This is where (for 6 or 7 months of the year) Solar comes into its own.Run the engine for a couple of hours in the morning to get the bulk charge back in, and then let the solar 'trickle charge' (say) 10 amps for the ret of the day (all for 'free').

 

14 minutes ago, George and Dragon said:

Is discharging 4 times to 75% SoC equivalent to one discharge cycle? How many discharge cycles should I expect from cheap LAs anyway?

Yes.

The battery sales blub will tell you how many cycles you can get - it can be as low as 100, with cheap batteries so in theory going down to 75% and back up to 100% (DAILY) means the (100 cycle) battery(s) will be OK for about 400 days.

[WotEver]

9 minutes ago, George and Dragon said:

How do you determine SoC?

Several ways...

 

1. Measure the relative density of the electrolyte. 
2.  Look for a current in the region of 1% of battery capacity whilst charging at around 14.4V.

3.  Remove the surface charge (say by turning on the tunnel light for 10 or 15 mins) and measure the resting voltage. 
4.  Use a Smartgauge (but only truly accurate during discharge). 

 

13 minutes ago, George and Dragon said:

If I believe @WotEver's 8-12 hours then 1.3kWh which would cost me 20 pence at home is costing £10 of diesel

That’s why it’s always a compromise between time/cost/engine wear/battery life. 

 

14 minutes ago, George and Dragon said:

Does this mean that getting the battery back to 90% most days and then running the alternator 8am to 8 pm once a week would prevent sulphation?

Nope, but it will limit it. 
 

16 minutes ago, George and Dragon said:

Is discharging 4 times to 75% SoC equivalent to one discharge cycle? How many discharge cycles should I expect from cheap LAs anyway?

Some manufacturers supply tables or graphs which show number of cycles vs depth of discharge. 
 

18 minutes ago, George and Dragon said:

I'm beginning to understand why so many people expect to change their batteries every couple of years!

Simply consider them as a consumable and get on with your boating. Or, fixate on them, treat them carefully and probably get a longer life out of them. It’s all compromise. Charge them fast and hard and they’ll die of grid corrosion before they sulphate, don’t charge them fully enough often enough and they’ll die from Sulphation. As previously said, it’s all a compromise. 
 

A decent Solar installation will result in a much longer life for your batteries. You can start the charge off with a bit of engine running then leave the solar to take care of the long, slow, boring bit. But it’s not a lot of use from November to March. 

[WotEver]

On 04/09/2020 at 16:27, nicknorman said:

my point is mainly that when people say “it doesn’t really matter whether you have a 70A alternator or a 175A alternator, since the latter only speeds charging for the first half hour or so” this isn’t true unless the battery is extremely small

Absolutely. The Amphour ‘law’ holds true. So put a 450Ah bank on charge and at 80% SoC it’ll still take 90A if the charge source can provide it. At 90% it’ll be looking for only 45A etc... but it’s that last 10-20% that takes the time. 

[Tony Brooks]

8 hours ago, George and Dragon said:

How do you determine SoC? Are sophisticated instruments required? All I have is a voltmeter and an ammeter. It sounds like some kind of instrument that records both and allows a graph to be plotted would be helpful but the thing that really gets me is the necessity to run the engine almost interminably to get that last bit of charge into the battery. If I believe @WotEver's 8-12 hours then 1.3kWh which would cost me 20 pence at home is costing £10 of diesel.

 

 

I tried to warn you about "sophisticated instruments" in my post above. The majority tell lies and a liability unless in the hands of a sophisticated user who has fully understood the manual and conducts themselves in a way that minimises those lies. 

 

@WotEver covered the basics above and I got on perfectly well with an accurate digital ammeter and voltmeter. If you feel you must have a % charged figure then the best you can do is get a Smartguage but remember they are only accurate on discharge, they may well be several % out during charging so its best to ignore them at those time.

 

Actually I very much doubt that you will use anything like £10 of fuel for 10 to 12 hours of battery charging.I suspect that you are making the common mistake of using the engine manufacturer's  fuel consumption data. Those are taken at various speeds at FULL LOAD, even a 200 amp alternator at full output will never put the engine under full load unless its a very small engine. Just for charging using an 80 amp alternator at maximum output it would be loading the engine to around 2 to 4HP and after a comparatively short while that would start to drop to eventually well under one HP. Although its best for the engine and run time to always run charge at a speed where the ammeter has just stopped rising with any increase in speed that speed will gradually drops you adjust the speed down to keep the charge at maximum with the lowest revs, eventually you will be as close to idle speed as lack of vibration allows. I can't say how much fuel you will use but I find a week's cruising with six hour days tends to use between £40 and £60 of fuel. Just charging will use less and in most cases it will also provide you with hot water.

[WotEver]

I wonder if OP has read this...

 

[Tony Brooks]

1 minute ago, WotEver said:

I wonder if OP has read this...

 

Well you pointed him to it almost as soon as he started the topic so hopefully he did.