Advice on charging batteries

[Riverboat]

Hello, new to the forum and am looking for some advice please.

 

We have recently acquired a boat without any wiring diagrams or instructions.

On the boat we have a bank of twenty-one x 38Ah 12v batteries connected to create a 24v string. 

The daily energy requirement onboard is approximately 3500-Watt hours.

The battery string is connected directly to a Victron Energy MultiPlus Compact 24 | 1600 | 40 inverter / charger.

There is also an ML8500w portable generator on deck which I guess is for charging the batteries via the MultiPlus Compact 24 | 1600 | 40.

All the appliances on the boat are served from the inverter by 230v, there are no DC circuits.

My questions are:-

1.    What is the maximum AC current that the inverter will accept from the generator when charging the batteries? 
2.    Is the generator big enough? 
3.    Is the charger big enough?
4.    Most importantly, I want to know approximately how many hours we need to run the generator per day? What is the formulae that I should use to calculate this based on the above information?
 

[Tony Brooks]

Is this a wind up?  21x12V batteries can not be connected in a string to produce 24V. You need 20 or 22. Perhaps it is 20 for domestic use and 1 for engine starting at 12V but then 38Ah is rather light for engine starting unless it's a car size petrol unit or a smaller outboard.

 

I also can't understand why anyone would use 21small batteries rather than fewer but larger.

 

You seem to have about 380Ah @24 volts and a 40 amp charger. That seems OK as long as you run it for long enough.

 

Your load looks like about 146 Ah per day, so if the batteries were new they are a tad undersized for optimum life. (don't discharge below 50% and fully recharge as often as possible, at least once a week). As time goes on they will sulphate, and have less or far less capacity.

 

Google suggests that is a 6000 Watt generator so it should power the inverter with a some output left over for use on board, but it's an open frame site generator so will be noisy. Best moor well away from other boats. It is also petrol powered with that entails safety wise in resect of CO fumes petrol fumes. Not a nice item for a boat.

 

You will have to run the generator for many, many hours to fully recharge, and even if we gave you a number it may not be enough to get the batteries fully charged. You need some form of battery monitoring to see when the batteries are more or less fully charged, so what instruments do you have? No instruments and assuming lead acid batteries 10 to 12 hours a day but maybe more. You may get away with 4 hours or so during the week, plus a very long charge at weekends, but this will be at the expense of battery life. Solar will help a lot in the summer.

 

Please read, learn and understand the Battery Charging Primer that is at the top of the maintenance forum, ask for clarification if you need to.

[PeterF]

You can download the inverter manual from

 

https://www.victronenergy.com/support-and-downloads/manuals#item=manuals-multiplus

 

The victron numbering of inverters is 24 = 24V, 1600 = 1600VA (not quite the same as Watts but close), 40 = 40A charging current into the batteries at say 30V max. So this is about 1200W so about 5 to 5.5 Amps st 230V.

[Tracy D'arth]

You cannot have 21 12v batteries in a 24v string. It must be an even number, 20 maybe?

Allowing for inverter losses that consumption is around 180 Ah out of a maximum battery capacity of 380 Ah @24v.  

The question then is not how big a generator but how much charge the batteries will accept and how long it will take to get the charging current down to  less than 2% of the capacity.

I think you will struggle to keep the batteries charged fully and that you will severely shorten their life quite quickly.

[nicknorman]

Not sure how you can have 21 12v batteries to create a 24v battery? Surely it must be an even number! I’ll assume there are 20. Maybe one is a 12v battery for the engine?
 

1/ The Victron can not only charge the batteries, but also pass through the generator power to the boat’s services. If it is just being used as a charger then it’s about 40A x 28.8v = 1.2kw, maybe a bit more due to charger inefficiency. That is about 5 or 6 amps at 230v.

 

2/ the generator is pretty big, 6kw I think. Way to big to just power than charger. Big, thirsty and very noisy. I hope you don’t have any neighbours! My advice would be to get a smaller suitcase-type generator that will be much quieter and more economical, and create much less nuisance for you and your neighbours.

 

That said, you power audit is quite heavy for a boat. Rather than thinking about it as charging the batteries then using the 3.5kwh of battery power, can you try to use the higher power devices (eg washing machine etc) whilst the generator is running, so the generator powers the heavy users directly. This is massively more efficient than getting the batteries to power everything.

 

3/  I think you have 380Ah at 24v? A 40A charger is a little on the small side. However a really important point about lead acid batteries is that they become more and more reluctant to accept current as the state of charge increases. So whilst with low batteries they will take the full 40A, and would take a lot more with a bigger charger, for the last several hours of charging they will be taking less than 40A. So a bigger charger will speed up the process initially, but it won’t make a huge impact on the overall time taken to fully charge the batteries.

 

4/ As I mentioned, lead acid batteries take a long time (say 8 hours or more) to properly fully charge, because trying to get the last 20% or so into them simply cannot be rushed. And if you don’t regularly fully charge the batteries they will sulphated and rapidly lose capacity, irreversibly. What some people do is carry out a “top up” charge for a few hours a day, then say once a week do an 8 or 10 hour full charge. You can see from that why you want a small, quiet and economical generator for this phase of the charging, when maybe as little as 5amps (at 24v) will be going into the batteries.

[Riverboat]

4 minutes ago, Tracy D'arth said:

You cannot have 21 12v batteries in a 24v string. It must be an even number, 20 maybe?

Allowing for inverter losses that consumption is around 180 Ah out of a maximum battery capacity of 380 Ah @24v.  

The question then is not how big a generator but how much charge the batteries will accept and how long it will take to get the charging current down to  less than 2% of the capacity.

I think you will struggle to keep the batteries charged fully and that you will severely shorten their life quite quickly.

Yes, you are correct, it is 20 batteries. Thank you for your comments.

[nicknorman]

Two other things to consider, one is to have some sort of battery monitoring so that at least you know the current going into the batteries from the charger. This allows you to know when you can stop charging - when the charging current has decreased to maybe 5A or so. Novices tend to think you just have to charge until a certain voltage is reached, but this is a big mistake.

 

Secondly have you thought about some solar? If you are going to live aboard, solar power is a massive boon and in summer you may not need to run the generator much at all. And it’s silent!

[Tracy D'arth]

What do you run to use 3500 Wh  of electricity a day?  Its a huge load for a canal boat.

[Riverboat]

14 minutes ago, Tony Brooks said:

Is this a wind up?  21x12V batteries can not be connected in a string to produce 24V. You need 20 or 22. Perhaps it is 20 for domestic use and 1 for engine starting at 12V but then 38Ah is rather light for engine starting unless it's a car size petrol unit or a smaller outboard.

 

I also can't understand why anyone would use 21small batteries rather than fewer but larger.

 

You seem to have about 380Ah @24 volts and a 40 amp charger. That seems OK as long as you run it for long enough.

 

Your load looks like about 146 Ah per day, so if the batteries were new they are a tad undersized for optimum life. (don't discharge below 50% and fully recharge as often as possible, at least once a week). As time goes on they will sulphate, and have less or far less capacity.

 

Google suggests that is a 6000 Watt generator so it should power the inverter with a some output left over for use on board, but it's an open frame site generator so will be noisy. Best moor well away from other boats. It is also petrol powered with that entails safety wise in resect of CO fumes petrol fumes. Not a nice item for a boat.

 

You will have to run the generator for many, many hours to fully recharge, and even if we gave you a number it may not be enough to get the batteries fully charged. You need some form of battery monitoring to see when the batteries are more or less fully charged, so what instruments do you have? No instruments and assuming lead acid batteries 10 to 12 hours a day but maybe more. You may get away with 4 hours or so during the week, plus a very long charge at weekends, but this will be at the expense of battery life. Solar will help a lot in the summer.

 

Please read, learn and understand the Battery Charging Primer that is at the top of the maintenance forum, ask for clarification if you need to.

Hello Tony, thanks for your reply, that is helpful. My mistake, it is 20 batteries. They are "Powersafe" SBS40 by Enersys. I don't know why they previously installed so many small batteries rather than a few larger ones. Point taken about the safety issues with petrol generators on board. There is only a digital voltmeter connected to the batteries, no other instruments. 

How do you arrive at the approx figure of 10 to 12 hours? Just wondering how I can calculate, even if it is very roughly.

 

Thanks.

 

 

[Riverboat]

33 minutes ago, nicknorman said:

Two other things to consider, one is to have some sort of battery monitoring so that at least you know the current going into the batteries from the charger. This allows you to know when you can stop charging - when the charging current has decreased to maybe 5A or so. Novices tend to think you just have to charge until a certain voltage is reached, but this is a big mistake.

 

Secondly have you thought about some solar? If you are going to live aboard, solar power is a massive boon and in summer you may not need to run the generator much at all. And it’s silent!

Thank you. I will investigate a battery monitoring device. Yes, I am going to get some solar panels but hope to live on board during the winter months.

[GUMPY]

9 minutes ago, Riverboat said:

Powersafe" SBS40 by Enersys

Very very very expensive when new £300+ each.

They are VRLA batteries,  so check the charging voltage it won't be as high as some recommend😯

Float voltage is 13.62v at  25degc I can't find a figure for max charge voltage.

They advise only 300 cycles as they are intended as a standby battery rather than boat use. I suspect someone got a job lot from somewhere 😎

 

[nicknorman]

23 minutes ago, Riverboat said:

Hello Tony, thanks for your reply, that is helpful. My mistake, it is 20 batteries. They are "Powersafe" SBS40 by Enersys. I don't know why they previously installed so many small batteries rather than a few larger ones. Point taken about the safety issues with petrol generators on board. There is only a digital voltmeter connected to the batteries, no other instruments. 

How do you arrive at the approx figure of 10 to 12 hours? Just wondering how I can calculate, even if it is very roughly.

 

Thanks.

 

It’s not really a calculation, it’s just how it is for any lead acid battery starting from a well discharged state. This is because the chemical reaction that converts the electricity into chemicals, has a limited rate. The higher the state of charge of the batteries the less easily the reaction can take place, so everything slows down. So a graph of the charging current vs state of charge would be a curve that gradually tends towards nearly zero.

 

The initial state of charge (how much energy you’ve used) will obviously have some effect on what happens /how long the early part of the charging takes, but the majority of charging time is that needed to get to a fully charged state from then on.

 

Its worth bearing in mind that there is no black and white answer to this, because “fully charged” is not really defined. As I said, charge current gradually tails off over many hours, but you never quite reach fully charged. If you stop charging when the current is say 7.6A (2% of capacity) that is reasonably fully charged. But if you continued charging for another 2 hours, the current might be 3.8A (1% of capacity). If you carried on charging for many more hours it might get down to less than 1A. But of course that means you’ve been charging for 15 hours perhaps.

 

So there has to be a compromise between endless generator running, vs not quite fully charging the batteries which will slightly reduce their life. This is where solar comes in, you can put the bulk of the charge in with the generator in the morning, then when the charge current has subsided, leave the solar to slowly and inexorably get the state of charge up to near 100%.

 

Lead acid batteries are really annoying in this respect, but they are what they are! This is why a number of us have switched to lithium batteries, which are much more like a “bucket of electricity”, taking full charge current right up until they are fully charged, and anyway not needing to be fully charged. But of course lithium batteries bring their own set of issues and problems than need careful management.

Edited October 8, 2021 by nicknorman

[MtB]

19 minutes ago, Riverboat said:

How do you arrive at the approx figure of 10 to 12 hours? Just wondering how I can calculate, even if it is very roughly.

 

It isn't a calculation, it is a function of the type of battery. The chemistry of any lead-acid battery dictates that from 50% SoC, they take around ten hours to fully charge.

 

 

[Tony Brooks]

25 minutes ago, Riverboat said:

Hello Tony, thanks for your reply, that is helpful. My mistake, it is 20 batteries. They are "Powersafe" SBS40 by Enersys. I don't know why they previously installed so many small batteries rather than a few larger ones. Point taken about the safety issues with petrol generators on board. There is only a digital voltmeter connected to the batteries, no other instruments. 

How do you arrive at the approx figure of 10 to 12 hours? Just wondering how I can calculate, even if it is very roughly.

 

Thanks.

 

 

 

Others have answered, but there in no calculation, it is based on empirical observation. You can't calculate it.

 

You need to keep charging until the charging currant has dropped to roughly 1to 2% of bank capacity at 14.2 volts or more. I expect you won't be able to do that with your charger because it will drop down to a float voltage of around 13.6V long before the batteries are full charged.

 

You need an accurate voltmeter (digital) and ammeter to monitor batteries or a so-called battery monitor, but at least on scale on most of those will encourage you to destroy your batteries unless you fully understand the manual and act on that understanding.

 

[Riverboat]

44 minutes ago, nicknorman said:

It’s not really a calculation, it’s just how it is for any lead acid battery starting from a well discharged state. This is because the chemical reaction that converts the electricity into chemicals, has a limited rate. The higher the state of charge of the batteries the less easily the reaction can take place, so everything slows down. So a graph of the charging current vs state of charge would be a curve that gradually tends towards nearly zero.

 

The initial state of charge (how much energy you’ve used) will obviously have some effect on what happens /how long the early part of the charging takes, but the majority of charging time is that needed to get to a fully charged state from then on.

 

Its worth bearing in mind that there is no black and white answer to this, because “fully charged” is not really defined. As I said, charge current gradually tails off over many hours, but you never quite reach fully charged. If you stop charging when the current is say 7.6A (2% of capacity) that is reasonably fully charged. But if you continued charging for another 2 hours, the current might be 3.8A (1% of capacity). If you carried on charging for many more hours it might get down to less than 1A. But of course that means you’ve been charging for 15 hours perhaps.

 

So there has to be a compromise between endless generator running, vs not quite fully charging the batteries which will slightly reduce their life. This is where solar comes in, you can put the bulk of the charge in with the generator in the morning, then when the charge current has subsided, leave the solar to slowly and inexorably get the state of charge up to near 100%.

 

Lead acid batteries are really annoying in this respect, but they are what they are! This is why a number of us have switched to lithium batteries, which are much more like a “bucket of electricity”, taking full charge current right up until they are fully charged, and anyway not needing to be fully charged. But of course lithium batteries bring their own set of issues and problems than need careful management.

 

Hello, that's very helpful. Perhaps in the longer term I should consider lithium batteries. I will need to check if the Victron Energy MultiPlus Compact 24 | 1600 | 40 inverter / charger will work with lithium batteries.

 

[David Mack]

There are two sides to the battery charging question, and you have asked only one of them: How much do I need to put back into the batteries? You haven't addressed the other question, which is how much you take out. How have you calculated your energy demand? Have you looked at what could perhaps be reduced, or run directly from the generator rather than via the batteries? How about turning the fridge off in winter (keeping perishables outside the main cabin or in an under-floor locker)? How often do you plan on moving the boat, and for how long at a time, so that the main engine can do most of the charging?

[Riverboat]

2 minutes ago, David Mack said:

There are two sides to the battery charging question, and you have asked only one of them: How much do I need to put back into the batteries? You haven't addressed the other question, which is how much you take out. How have you calculated your energy demand? Have you looked at what could perhaps be reduced, or run directly from the generator rather than via the batteries? How about turning the fridge off in winter (keeping perishables outside the main cabin or in an under-floor locker)? How often do you plan on moving the boat, and for how long at a time, so that the main engine can do most of the charging?

David, thanks for your comment. I am now beginning to realise I need to try to reduce the energy demand. Unfortunately, the boat doesn't have an engine, it's an old barge which is permanently moored up to the riverbank. I was thinking of investigating gas fridges. To calculate demand I have simply looked at the Watts that each device consumes and then multiplied by the number of hours per day. Then added it all up to get a total Watts per day figure.

 

I have just joined this forum and am amazed how helpful everyone is.

 

[Riverboat]

12 minutes ago, David Mack said:

There are two sides to the battery charging question, and you have asked only one of them: How much do I need to put back into the batteries? You haven't addressed the other question, which is how much you take out. How have you calculated your energy demand? Have you looked at what could perhaps be reduced, or run directly from the generator rather than via the batteries? How about turning the fridge off in winter (keeping perishables outside the main cabin or in an under-floor locker)? How often do you plan on moving the boat, and for how long at a time, so that the main engine can do most of the charging?

There is also an old wind turbine on the boat which is not connected, I guess it no longer works. Do many people use them with permanently moored river boats? Do you think it's worth investigation? Perhaps add solar for the summer months and a wind turbine would help during the winter months?

[IanD]

48 minutes ago, Riverboat said:

 

Hello, that's very helpful. Perhaps in the longer term I should consider lithium batteries. I will need to check if the Victron Energy MultiPlus Compact 24 | 1600 | 40 inverter / charger will work with lithium batteries.

 

Yes it is, but it needs reprogramming to suit them. You will also need either lithium batteries with a built-in BMS (Battery Management System), or to buy/fit a BMS (ready-made or DIY) if you use bare lithium cells without one. You can't just connect the Multiplus to a bank of lithium batteries without a BMS and hope that it will work...

 

Also lithium batteries are not cheap if you don't want to go down the DIY route. You have 24V 380Ah of lead-acid batteries, to get the same usable capacity you need something like 24V 200Ah of lithium batteries. A ready-made battery with built-in BMS will cost you about £4000, for example:

 

https://www.bimblesolar.com/batteries/lithium-batteries/24v-lithium/victron-lithium-24v-200ah

 

If you're happy to go DIY then you can do it for about half the price, still over £2000 though (see the 24V 200Ah set):

 

https://shop.gwl.eu/Winston-12V-sets/?cur=1

Edited October 8, 2021 by IanD

[Tony Brooks]

Just now, Riverboat said:

There is also an old wind turbine on the boat which is not connected, I guess it no longer works. Do many people use them with permanently moored river boats? Do you think it's worth investigation? Perhaps add solar for the summer months and a wind turbine would help during the winter months?

 

How much good a turbine will do is dependent upon the wind and as this is an inland forum the consensus is that they only produce a fraction of their rated output. Look at the data sheets and you will find their rated output is in near gale force winds. They also tend to be noisy, especially if their pole is mounted on the boat. So it depends where you are on the Medway. If it's in a creek on the estuary, you are likely to get stronger and less fluky winds than if you are above Maidstone.

 

Being a barge then, I suspect that as long as the cost is doable, a large solar array would provide more than enough charging (say) April to September and make a significant contribution during a lot of the winter, especially if you turn the fridge off during the colder months.

 

We may be able to give some advice if you post your power required calculations.

2 minutes ago, IanD said:

Yes it is, but it needs reprogramming to suit them. You will also need either lithium batteries with a built-in BMS (Battery Management System), or to buy/fit a BMS (ready-made or DIY) if you use bare lithium cells without one. You can't just connect the Multiplus to a bank of lithium batteries without a BMS and hope that it will work...

 

Well, I suppose it would "work" but probably for a very short time before cells started to fail.

[Ex Brummie]

The wind turbine might still work, but they tend to be noisy. If permanently moored, you may be able to mount it on the bank to minimise any noise transference.

Can't type as fast as Tony.

Edited October 8, 2021 by Ex Brummie
to explain duplicate post.

[IanD]

48 minutes ago, Riverboat said:

There is also an old wind turbine on the boat which is not connected, I guess it no longer works. Do many people use them with permanently moored river boats? Do you think it's worth investigation? Perhaps add solar for the summer months and a wind turbine would help during the winter months?

Wind turbines are noisy and have pathetic output in real life. A big solar array will fit on your boat and give you a fair bit of power in the summer, but much less in the winter, you'll still need to use the generator.

 

You need to decide how much power you need. Don't use the peak power figures for solar panels, to get kWh/day in summer multiply the panel rating by about 3.5, so a 1.8m x 1.1m 350W panel (costing about £100) will give you about 1.2kWh/day on average -- more on sunny days, less on cloudy/rainy ones. In winter you might average 0.2kWh/day...

 

https://www.bimblesolar.com/345w-canadian-solar-panel

https://www.bimblesolar.com/offgrid/mppt/victron-mppt/30a-victron-mppt-100-30

 

You also need a suitable MPPT controller to charge the batteries which together with all the other bits needed will probably add about 50% to the panel cost. Looks like you need 4 panels (2 series, 2 parallel) which will cost you something like £600 total. Should mean you don't have to use the generator at all from probably April to September, maybe a bit longer -- more panels extends this period but you'll need to spend a fortune to get enough power all year round.

 

Edited October 8, 2021 by IanD

[Col_T]

52 minutes ago, IanD said:

Also lithium batteries are not cheap if you don't want to go down the DIY route. You have 24V 380Ah of lead-acid batteries, to get the same usable capacity you need something like 24V 200Ah of lithium batteries. A ready-made battery with built-in BMS will cost you about £4000, for example:

 

Sterling Power website reckon their 200Ah lithium, with built in BMS is £999 - https://sterling-power.com/products/lithium-batteries?variant=40493851115709. Are these lithiums any good?

[Tony Brooks]

21 minutes ago, Col_T said:

 

Sterling Power website reckon their 200Ah lithium, with built in BMS is £999 - https://sterling-power.com/products/lithium-batteries?variant=40493851115709. Are these lithiums any good?

 

Probably, but note that the link seem to imply that you need a B2B to charge them from an ordinary alternator (via the start battery).

[IanD]

1 hour ago, Col_T said:

 

Sterling Power website reckon their 200Ah lithium, with built in BMS is £999 - https://sterling-power.com/products/lithium-batteries?variant=40493851115709. Are these lithiums any good?

 

You'll need two of those. Low-cost lithium batteries with inbuilt BMS are a bit of a minefield, especially if you connect them in series, because the BMS can't talk to each other for cell balancing, and there's the issue of what happens when one of them decides to disconnect because of over/undercharging. Some are also distinctly dodgy about current ratings, especially cheap batteries from China which I suspect the Sterling ones are -- there's more information here than you're ever going to want to read...

 

https://marinehowto.com/lifepo4-batteries-on-boats/

 

But they're sold in the UK by Bimble who have a good reputation, claim 150A/300A maximum continuous charge/discharge rates, and they *are* half the price of the Victron batteries...

 

https://www.bimblesolar.com/batteries/lithium-batteries/12v-lithium/Lithium-12v-200ah-5year

 

And Sterling came out quite well when tested here:

 

https://www.pbo.co.uk/gear/lithium-batteries-for-boats-reviewed-12-of-the-best-lithium-boat-batteries-tested-62244

Edited October 8, 2021 by IanD