Sizing a battery bank

[MtB]

12 minutes ago, Col_T said:

@WotEver, many thanks for this, and your patience! Another piece of the puzzle drops slowly into place!!

 

Mind you, at the same time a different piece of the puzzle will have probably fallen on the floor at the same time, without you noticing...

 

Happens like this for me anyway! 

[jetzi]

 

On 25/09/2019 at 21:46, Dr Bob said:

Firstly 300Ahrs use per day is rather a lot for winter use. If I read it right, this is mostly coming from the washing machine. I agree with Richard above. That is an awful lot. Heating the water in the WM by battery power is not a good idea. Far better to fill for the wash cycle with hot water. Rig up some valves to switch to hot water to fill at the start then switch to cold for the rinse etc. We start our washing machine when we run the engine for 1-2 hrs per day in the winter to get the most out of the alternator. That extra 100Ahr per day (heating the water) will take a few hours more engine each day. I pitty your neighbours. A bit antisocial. Aim for 200Ahrs per day.

 

I've said it a couple of times now, but you may have missed it. We will never use the washing machine's heater. It has a true cold wash button and I have sited it right next to the calorifier for hot water fill. It is not possible to know how much power the inverter would take to power the machine for a cold wash cycle. I know that with the heater the machine is 2300W and I know that a 1300W inverter runs the machine on cold wash. So, I have assumed 1000W.

 

Secondly, this is not every day but a worst case day. Please do tell me if that is not a good way to look at it? Worst case, I will be doing two hours of washing and 5 minutes of hairdrying. I estimated this to be 2kW of power for the inverter on top of the 1kW I use on a light day. If my battery bank is big enough I will be able to replace the heavy day over 2 or even 3 days.

 

Perhaps a better way to estimate would be on a weekly (or even monthly/annually!) average usage and average generation?

 

I realise that charging the batteries with the engine is antisocial and I'm trying to minimise this, including installing a hospital silencer, soundproofing the engine bay and mooring with the noisy end away from neighbours. I still have more I can do and right now if you are more than half a boat length away from us you wouldn't know the engine was running. I also want to get enough solar to run entirely off solar for 8 months of the year. The rest of the time we'll either ration our power strictly, use laundromats and/or go on nice long holidays to somewhere sunny! With those measures in place, I'd guess our winter usage would be closer to 150Ah per day on average.
 

On 25/09/2019 at 21:46, Dr Bob said:

On the choice of battery type, 2nd hand Li's from an EV should only cost around £1000 for 480Ahrs ....which should be ok for 200Ahrs use per day. Ok, you need a battery management system but that can be done for £200-£500 dependent on how complex you want it. You are looking at cheap LA's but if you dont get them up to 100% regularly and are thrashing the batteries by 200Ahrs of inverter discharge per day then you'll be replacing them every 2 years or less.  Having now had Li's for nearly 6 months (ok not in winter yet), I would defo go the Li route if I was in your position as they are so easy to manage with not having to look to see if they are full and always seeing the full power going in from the solar or alternator rather than the endless 10A to 20A LA tail current charging. If you are using 200Ahr/300Ahr per day then I can see you running your engine many many hours a few times a week just to get back to the magic 100%. Try and make it 200Ahrs!

My impression is that the big advantage of Li's is that you need less engine to charge them because they will suck up the power so much faster. However, this seems to be a lot less relevant if you are aiming for 100% solar generation for 66% of the year. Solar is naturally slow to charge batteries so it seems to me that you may as well use the cheaper LAs if you are generating mainly from solar, since you are slow charging them over the whole day anyway. Not so?

The reason I would get Li's is to minimise the amount of charging I would need to do in the deepest darkest bit of winter. This is less than a quarter of the time for us, so I am just really not convinced that this is going to be worth it, provided I get enough solar.

[jetzi]

On 25/09/2019 at 11:45, ivan&alice said:

If I was to upgrade this after a year, presuming my first set survives living with me for that long, what are the implications of adding fresh batteries to an existing bank? Or would I have to replace the whole bank whether they are showing signs of age or not?

@Jen-in-Wellies pardon but did you see this question? You suggested getting a smaller bank and then upgrading - would it be possible/advisable to (for example) get a 600Ah system now, and then in a year add another 600Ah? Or would I have to toss the first 600Ah and buy 1200Ah?

 

On 25/09/2019 at 22:02, WotEver said:

A battery will demand a charging current approximately equal to the amount by which it has been discharged. So a battery which has been discharged by 20Ah will accept a charge current of 20A. A battery which has been discharged by 15Ah will accept a charge of 15A, etc. This is known as the ‘AmpHour Law’ (even though it’s not a law at all, simply an observable phenomenon). 
 

So, a battery (whatever its size) that’s been discharged by 200Ah will demand a charging current of 200A. If the charger is unable to deliver that (how many 200A chargers do folk have?) then the charger will simply output its maximum current until the battery demands less (Bulk Charge). So in the case of a 50A charger it will output a constant 50A with a slowly increasing voltage until it’s put 150Ah into the battery. At that point the battery is only 50Ah discharged and will, as it gets fuller and fuller, demand less and less from the charge source (Absorption). 

 

All of the above is relevant only to Lead Acid batteries of course. 

Thanks @WotEver this is a superbly simple explanation. The takeaway is that it is the deficit of power in the LA battery bank (and the size of the charger) that determines how fast the battery charges, NOT the size of the battery overall.

So whether or you have a 10 kAh battery or a 1000Ah battery, if it is discharged by 200Ah and you have a 50Ah charger, they charge at the same speed.

So this implies, quite simply, that the bigger the better when it comes to LA battery banks. And the minimum is to ensure you never go under 50% SoC on your worst day of consumption. For me, that means a 600Ah LA bank minimum, but the bigger the better.

[Sea Dog]

8 minutes ago, ivan&alice said:

Worst case, I will be doing two hours of washing and 5 minutes of hairdrying.

I wouldn't do either of those things off my 440 Ah of LA batteries frankly. You're looking at around a 100 amp draw there and, whilst some folk may be content to do that, I only allow that sort of load with the engine running so the alternator can contribute. For me, it's no issue to combine that with cruising, so the engine is running anyway and I'm bothering no-one. That said, I wouldn't be keen to work a 1300 watt inverter that hard either.  

[jetzi]

3 minutes ago, Sea Dog said:

I wouldn't do either of those things off my 440 Ah of LA batteries frankly. You're looking at around a 100 amp draw there and, whilst some folk may be content to do that, I only allow that sort of load with the engine running so the alternator can contribute. For me, it's no issue to combine that with cruising, so the engine is running anyway and I'm bothering no-one. That said, I wouldn't be keen to work a 1300 watt inverter that hard either.  

Agreed, but 440Ah of batteries is much less than we have been discussing. @Alan de Enfield was recommending around 1000Ah and I have mentioned 600Ah as my bare minimum.

Also I have no inverter and I can buy a larger one than 1300W, but surely a 1300W inverter should be able to handle (a pessimistic) 1000W of load for 2 hours or 1200W for 5 minutes? Otherwise the power rating of the inverter seems to be rather irrelevant.

And yes wherever possible this would be combined with cruising. But I'm sizing the theoretical system at the moment, and my requirement is that I want to be able to handle my worst case day of 300Ah with no cruising (and indeed no charging whatsoever) and my best case day for around 3-4 days without any charging. There are any number of reasons why this might come up - mechanical breakdown, illness, iced in, good old fashioned laziness...

I'm sizing the system now so I am looking at the worst case and aiming the system to handle that. Doesn't that seem logical to anyone else?

[Murflynn]

11 minutes ago, Sea Dog said:

I wouldn't do either of those things off my 440 Ah of LA batteries frankly. You're looking at around a 100 amp draw there and, whilst some folk may be content to do that, I only allow that sort of load with the engine running so the alternator can contribute. For me, it's no issue to combine that with cruising, so the engine is running anyway and I'm bothering no-one. That said, I wouldn't be keen to work a 1300 watt inverter that hard either.  

err  --   what appliances do you have that draw 100A ?

[WotEver]

11 minutes ago, ivan&alice said:

The takeaway is that it is the deficit of power in the LA battery bank (and the size of the charger) that determines how fast the battery charges, NOT the size of the battery overall.

Yes. However it’s worth bearing in mind that the last 20% of charging (80% to 100% SoC) absorption phase takes much, much longer than the bulk phase. So you’d not notice a whole load of difference in charge time between a 100A charger and a 50A charger. Maybe 30 minutes quicker, which isn’t much, percentage wise, of an 8 hour charge cycle. 

Just now, Murflynn said:

err  --   what appliances do you have that draw 100A ?

Anything that draws a kilowatt, surely. 

[jetzi]

4 minutes ago, Murflynn said:

err  --   what appliances do you have that draw 100A ?

A 1300W inverter powering a washing machine using 1000W on cold wash.

[jetzi]

On 25/09/2019 at 12:55, Col_T said:

Our domestic PV setup is 5 off 250W panels on an east facing roof and 5 off 250W panels on a south facing roof. Some output numbers for you:

 

January 1st, 2018: the panels generated from 10:20=>16:50, outputting 0.931kWh

January 2018: the panels output a total of 57.3kWh, best day giving 5.0, worst day 0.1, average of 1.8kWh

July 2018: the panels output a total of 345.8kWh, best day giving 14.5, worst day 3.0, average of 11.2kWh

July 31st, 2018: the panels generated from 06:20=>20:40, outputting 11.931kWh

 

You seemed interested in October performance:

October 15th, 2018: the panels generated from 09:30=>17:20, outputting 0.53kWh

October 1st, 2018: the panels generated from 07:40=>18:40, outputting 9.96kWh

October 2018: the panels output a total of 153.3kWh, best day giving 9.97, worst day 0.33, average of 4.94kWh

 

Thanks for giving me some hard numbers to work with! My proposed system is around 60% of your 2500W system, however you mention that your solar is facing east and south, which suggests that I will get a little more on my boat as I will tilt the panels perpendicular to the sun. So if I give myself 75% of your figures, I get averages of 1.35kW in January, 9kW in July and 3.7kW in October (the reason I'm interested in October is because it is the last month of the year where I want to generate 100% of my power from solar).

 

And as I'm looking at around 3.2kW usage per day, this seems to validate my calculations (including the 20% extra for charging losses).

Of course a couple of days of no sun and I'll have to run the engine, but on average it works out.

[Alan de Enfield]

29 minutes ago, Murflynn said:

err  --   what appliances do you have that draw 100A ?

1200 watt hairdryer will draw ~ 120A from the batteries.

[rusty69]

Just now, Alan de Enfield said:

1200 watt hairdryer will draw ~ 120A from the batteries.

When I met you, you had no hair

[Alan de Enfield]

Just now, rusty69 said:

When I met you, you had no hair

I've been implanted since then !!

[Richard10002]

20 minutes ago, ivan&alice said:

A 1300W inverter powering a washing machine using 1000W on cold wash.

Given that the washing machine is the critical machine in this calculation, and your 1000W is an assumption, I would see if Hotpoint have an actual figure for the cold wash cycle of your machine. It might be 250W, or 500W, or 1000W. The actual figure could make a big difference to your worst case scenario.

 

Just a thought??? - It might even be possible to work it out for yourself if you were to run one cycle of the machine, with a DC clamp ammeter at an appropriate point. Watch the ammeter for the full length of the cycle, note the voltage during the cycle and you will get an idea of power used. W=VA, (more or less). If you only have a small battery bank now, run the engine while running the cycle, and the alternator will minimise the reduction in SOC of your existing bank.

[jetzi]

3 minutes ago, Richard10002 said:

Given that the washing machine is the critical machine in this calculation, and your 1000W is an assumption, I would see if Hotpoint have an actual figure for the cold wash cycle of your machine. It might be 250W, or 500W, or 1000W. The actual figure could make a big difference to your worst case scenario.

 

Just a thought??? - It might even be possible to work it out for yourself if you were to run one cycle of the machine, with a DC clamp ammeter at an appropriate point. Watch the ammeter for the full length of the cycle, note the voltage during the cycle and you will get an idea of power used. W=VA, (more or less). If you only have a small battery bank now, run the engine while running the cycle, and the alternator will minimise the reduction in SOC of your existing bank.

I've searched hard but I have not seen any figures for the cold wash cycle.

 

I don't have shore power and I don't have an inverter, so it's impossible for me to run the machine at the moment. I guess I could check into a marina and run a cycle to calculate how much power it uses. It would have to be one where you pay for the electricity by the meter and I would have to rely on its accuracy. You're right though, this would probably be worth it - I'll look into this idea.

[Richard10002]

1 minute ago, ivan&alice said:

I've searched hard but I have not seen any figures for the cold wash cycle.

 

I don't have shore power and I don't have an inverter, so it's impossible for me to run the machine at the moment. I guess I could check into a marina and run a cycle to calculate how much power it uses. It would have to be one where you pay for the electricity by the meter and I would have to rely on its accuracy. You're right though, this would probably be worth it - I'll look into this idea.

An email and phone call to hotpoint might, (only might), elicit an answer. You can bet your bottom dollar that they have the figures somewhere. I once called Raymarine about a technical issue that I could find nowhere.... I got put through to someone who pulled up a technical drawing, and sent it to me on the basis that I wouldnt tell anyone , so you never know. (The drawing is easily located in a variety of places on the web these days, but it was over a decade ago when I made the call).

 

I also think you said you know someone with the same machine who runs it via a 1300W inverter. I wonder if he might have the patience to run the test over one cold wash cycle.

 

I was wondering how often your worst case scenario might occur? If it was very occasionally, it is probably OK to let the batteries drop below 50% now and then - it isn't a hard and fast rule.

 

However, thinking a bit more about it, you are probably not far from your worst case scenario every day you use the washing machine. The hairdryer uses 10Ah so not a huge amount in your scheme of things. 

[jetzi]

48 minutes ago, Richard10002 said:

I also think you said you know someone with the same machine who runs it via a 1300W inverter. I wonder if he might have the patience to run the test over one cold wash cycle.

New information - he says he idly monitored the inverter which read 400W peak during a spin and 250W during the final spin. So I've recalculated everything based on a pessimistic 400W average during the whole cycle. I've also balanced it against @Col_T's solar figures and 30% charging losses (a little worse than @Tony Brooks's suggested 20%). I also adjusted my consumption figures to reflect more Eberspaecher in January and more fridge in July, and I'm only allowing 1 hour per day of washing machine in winter (if that's not enough, we can go to the laundry).

I worked out now that I will need to run the engine for 1.8h per day on average during January. Can anyone check my work here? The one limitation I can see is that taking an average of 40A from my alternator is not really accurate since this won't get them up to 100%, but if I get 1000Ah of batteries then this will get them to 96% in bulk and then I'll just have to charge to 100% weekly on a long cruise. In summer this will barely matter because the batteries will only be under 100% is when the sun isn't shining.

 

Edited September 29, 2019 by ivan&alice

[cuthound]

1 hour ago, Richard10002 said:

Given that the washing machine is the critical machine in this calculation, and your 1000W is an assumption, I would see if Hotpoint have an actual figure for the cold wash cycle of your machine. It might be 250W, or 500W, or 1000W. The actual figure could make a big difference to your worst case scenario.

 

Just a thought??? - It might even be possible to work it out for yourself if you were to run one cycle of the machine, with a DC clamp ammeter at an appropriate point. Watch the ammeter for the full length of the cycle, note the voltage during the cycle and you will get an idea of power used. W=VA, (more or less). If you only have a small battery bank now, run the engine while running the cycle, and the alternator will minimise the reduction in SOC of your existing bank.

 

Even easier if you use one of these between the inverter and the washing machine and add 10% to allow for the inefficiency of the inverter.

 

https://www.amazon.co.uk/Nevsetpo-Monitor-Consumption-Electricity-Calculator/dp/B07H1ZFVFM/ref=sr_1_6?adgrpid=55935645049&gclid=Cj0KCQjwrMHsBRCIARIsAFgSeI2xoymmovFOp5MQspira34x8GhoUvqIFcfr7belyE-JTASgONID368aAiU-EALw_wcB&hvadid=259075203254&hvdev=t&hvlocphy=1007147&hvnetw=g&hvpos=1t1&hvqmt=e&hvrand=10371853486310461302&hvtargid=kwd-298018762935&hydadcr=5087_1827855&keywords=plug+in+electricity+meter&qid=1569760763&sr=8-6

[WotEver]

Just now, cuthound said:

 

Even easier if you use one of these between the inverter and the washing machine and add 10% to allow for the inefficiency of the inverter.

 

https://www.amazon.co.uk/Nevsetpo-Monitor-Consumption-Electricity-Calculator/dp/B07H1ZFVFM/ref=sr_1_6

Good suggestion

oh, and I shortened your link. 

[cuthound]

1 minute ago, WotEver said:

Good suggestion

oh, and I shortened your link. 

 

Thanks, I didn't realise you could shorten them. I just cut and pasted it. ?

[jetzi]

6 minutes ago, WotEver said:

Good suggestion

oh, and I shortened your link. 

amazon.co.uk/dp/B07H1ZFVFM

You can actually shorten it even more... the only bit that identifies the product is dp/ plus the code. The rest is tracking stuff for amazon to spy on you.

Thanks for this, I think I will definitely get one of these when I have an inverter, but right now I cannot run the washing machine so it would be pointless.

 

Like I said, I'm working on the observation of 400W peak usage and pessimistically calling it 400W average.

[Sea Dog]

2 hours ago, Murflynn said:

err  --   what appliances do you have that draw 100A ?

Don't forget that something drawing 5a at 240v will cause the inverter to draw 100a at 12v.

[Alan de Enfield]

56 minutes ago, ivan&alice said:

New information - he says he idly monitored the inverter which read 400W peak during a spin and 250W during the final spin. So I've recalculated everything based on a pessimistic 400W average during the whole cycle. I've also balanced it against @Col_T's solar figures and 30% charging losses (a little worse than @Tony Brooks's suggested 20%). I also adjusted my consumption figures to reflect more Eberspaecher in January and more fridge in July, and I'm only allowing 1 hour per day of washing machine in winter (if that's not enough, we can go to the laundry).

I worked out now that I will need to run the engine for 1.8h per day on average during January. Can anyone check my work here? The one limitation I can see is that taking an average of 40A from my alternator is not really accurate since this won't get them up to 100%, but if I get 1000Ah of batteries then this will get them to 96% in bulk and then I'll just have to charge to 100% weekly on a long cruise. In summer this will barely matter because the batteries will only be under 100% is when the sun isn't shining.

 

 

That looks pretty fair to me BUT there is an old military saying :

 

"No battle plan ever survives contact with the enemy"

 

I am a BIG spreadsheet 'generator', & plan everything - some may accuse me of being OCD, but I deny it - (I may be CDO as that is far superior because all the letters are in the correct order).

 

I have designed exhibition stands, and once there found that there is not enough room to sit at the table, and so on and so on.

 

I have historically found that the military saying can easily be adapted to :

 

"No matter what your plan shows - real life will make it totally useless"

 

 

 

I think when real life actually 'hits' you will find that your estimates are considerably "out" and will inevitably be on the 'wrong side' - if you estimate you need a 600AH battery bank, get a 1000Ah battery bank, If you think you need a 1000w Inverter, get a 2,000 watt inverter (one day you will want to run that electric drill !!!!) etc etc.

 

You only get one chance to do it right first time, so 'over engineer' it from day 1.

Edited September 29, 2019 by Alan de Enfield

[Dr Bob]

4 hours ago, ivan&alice said:

A 1300W inverter powering a washing machine using 1000W on cold wash.

I dont think a cold wash machine takes anything like that.

Ours is a  2 year old 2.3Kw machine and we only use ours with the engine running. Our alternator puts out a max of 60A and when the washing machine is on, the charging amps drop to 20A when  the machine is spinning. I doubt if it takes more than 30Ahr total in a wash cycle....a 1hr wash cycle. So 2 washes plus 5 mins of hair dryer will not be more than 100Ahrs.

If I think on this week I will try look at the actual power usage.

 

 

[jetzi]

Just now, Dr Bob said:

I dont think a cold wash machine takes anything like that.

Ours is a  2 year old 2.3Kw machine and we only use ours with the engine running. Our alternator puts out a max of 60A and when the washing machine is on, the charging amps drop to 20A when  the machine is spinning. I doubt if it takes more than 30Ahr total in a wash cycle....a 1hr wash cycle. So 2 washes plus 5 mins of hair dryer will not be more than 100Ahrs.

 

Sorry @Dr Bob you're a bit behind ?

3 hours ago, ivan&alice said:

New information - he says he idly monitored the inverter which read 400W peak during a spin and 250W during the final spin. So I've recalculated everything based on a pessimistic 400W average during the whole cycle. I've also balanced it against @Col_T's solar figures and 30% charging losses (a little worse than @Tony Brooks's suggested 20%). I also adjusted my consumption figures to reflect more Eberspaecher in January and more fridge in July, and I'm only allowing 1 hour per day of washing machine in winter (if that's not enough, we can go to the laundry).

 

[Dr Bob]

22 minutes ago, ivan&alice said:

 

Sorry @Dr Bob you're a bit behind ?

 

I'm not keeping up