Sizing a battery bank

[jetzi]

Another battery thread... but I have read a number of the existing threads and something I am still not totally clear on is just how many Ah of usable battery bank I need. I'm working my way through @Tony Brooks's site and I'm reading the Victron Energy Unlimited book. My install date is 2020-04-01 which gives me six solid months to do my homework and shop around for deals.

 

Totally understand that it is premature for me to be asking for advice, so please feel free to ignore me if it is just more reading that is required.

I have done the power audit and I would like capacity to generate 3.5kWh on my heaviest day. The inverter will only be used for a washing machine and a hairdryer, so I'll hopefully only need the 2kWh every other day at worst.

 

 

I'm thinking of getting 4x 315W bifacial panels which - on my white boat roof - I understand are well suited to overcast diffuse light days. These panels total an absolute maximum of 1600W.

 

• The longest day of the year is 16h 40min. If I approximate the output to be an average of 50% over the day, that's 16h 40min * 800W = 13.3kW.
• The shortest day of the year is 7h 50min. If I approximate the output to be an average of 10% over the day, that's 7h 50min * 160W = 1.25kW.
• October is around 10h of daylight. If I approximate the output to be an average of 25% over the day, that's 10h * 400W = 4kW.

If I'm being pessimistic enough with these figures, that should mean I'm running entirely from solar for at least 8 months of the year. The rest of the time, I'll be running my 70A alternator long enough to generate up to 2kW, which if I take it to output 50A, means running my engine for up to 4 hours on a washing day in the dead of winter.

 

My guess is that you want a battery bank to last you at least a skipped day of no charging whatsoever on your heavy load (300Ah/day). And perhaps 3 skipped days if you're rationing yourself (100Ah/day). So that suggests around 300Ah of capacity, for my needs.

 

I haven't decided on LA's vs Li's (leaning towards LA's) but either way I'll need the same usable capacity, right?

Am I right in saying, then, that I would need around 6 of my current 110Ah LA which at 90 GBP each would set me back 540 GBP. I feel if I was going to go the LA route I may as well get as cheap as I can.

 

Alternatively, a cursory search turns up a 100Ah LiFePO4 drop-in battery for 662 GBP, so 300Ah of Li's would be a maximum of around 2kGBP. I'm sure that this cost could be brought down if I knew what I was looking for in a more complex system. So is it fair to say that Li is around 3x the price of LA? Note that my main motivation for Li's would be to cut down on charging time in the winter, I shouldn't have thought that the speediness of charging would make much difference in the summer since I'd be relying on the slow charging of the solar. Or is that wrong?

 

Sorry, I don't want to turn this into another Li vs LA debate. My real question is - do you have any thoughts on my figure of 300Ah usable capacity for my battery bank?

Edited September 24, 2019 by ivan&alice

[WotEver]

10 minutes ago, ivan&alice said:

My real question is - do you have any thoughts on my figure of 300Ah for my battery bank?

Yes. You need to at least double it if you’re using LAs because you never want to take them far below 50%. 

[jetzi]

2 minutes ago, WotEver said:

Yes. You need to at least double it if you’re using LAs because you never want to take them far below 50%. 

Yes, the LA bank I'm comparing the 300Ah Li to is 6x 110Ah batteries which is 660Ah, or 330Ah usable. I edited the final sentence to make it clearer.

Edited September 24, 2019 by ivan&alice

[Alan de Enfield]

11 minutes ago, ivan&alice said:

My guess is that you want a battery bank to last you at least a skipped day of no charging whatsoever on your heavy load (300Ah/day). And perhaps 3 skipped days if you're rationing yourself (100Ah/day). So that suggests around 300Ah of capacity, for my needs.

 

I haven't decided on LA's vs Li's (leaning towards LA's) but either way I'll need the same usable capacity, right?

Am I right in saying, then, that I would need around 6 of my current 110Ah LA which at 90 GBP each would set me back 540 GBP. I feel if I was going to go the LA route I may as well get as cheap as I can.

 

Many would say that you want a usable capacity of 3x daily usage.

If you are going to use 100Ah for 2 days, and 300Ah on the third day you are looking at a usable capacity of 500Ah.

 

Bearing in mind that your usable capacity should be no more than 50% of total capacity you should be looking for a 1000Ah battery bank.

[Tony Brooks]

But there is a big difference between generating 3.5 kWh and actually getting 3.5 kWh back into the batteries. To do the latter you will need to generate probably at least 20% (for LA) more electricity and maybe far more.

 

Providing the battery capacity is not normally the problem, properly recharging is

Edited September 24, 2019 by Tony Brooks

[WotEver]

13 minutes ago, ivan&alice said:

The shortest day of the year is 7h 50min. If I approximate the output to be an average of 10% over the day, that's 7h 50min * 160W = 1.25kW.

No chance. Probably more like 1 or 2%. Remember that the sun is very low in the sky and passing through loads of atmosphere before it reaches your panels. 

[Alan de Enfield]

Just now, WotEver said:

No chance. Probably more like 1 or 2%. Remember that the sun is very low in the sky and passing through loads of atmosphere before it reaches your panels. 

I suggested 1% in Ivan's previous thread, MtB suggested 0.1% in deepest darkest February.

[WotEver]

Just now, Alan de Enfield said:

I suggested 1% in Ivan's previous thread, MtB suggested 0.1% in deepest darkest February.

Well yeah, in that region for sure. 
 

And it takes up to 12 hours to fully recharge a LA battery from 50% so you don’t ever really want to go that low in the winter. 

[jetzi]

1 minute ago, Alan de Enfield said:

Many would say that you want a usable capacity of 3x daily usage.

If you are going to use 100Ah for 2 days, and 300Ah on the third day you are looking at a usable capacity of 500Ah.

 

Bearing in mind that your usable capacity should be no more than 50% of total capacity you should be looking for a 1000Ah battery bank.

That seems excessive. If for some reason I was unable to charge my batteries I would not be using 300Ah.

 

That said, 1000Ah of LA's would be about 9x 110Ah which is 810 GBP. Just seeing how cheaply you can put together a really, really big bank of LA's makes it rather appealing.

 

Would it be better to have a huge LA bank for charging from solar or a more adequately sized Li bank? I'm wondering if having LAs charged primarily from solar might be problematic if they are so huge that they don't get right to 100%?

[WotEver]

2 minutes ago, ivan&alice said:

I'm wondering if having LAs charged primarily from solar might be problematic if they are so huge that they don't get right to 100%?

A lot of folk get this wrong. The size of the bank does not in itself impact the recharge time. It’s how much you’ve taken out of it that matters. 
 

Take 200Ah from a 600Ah bank and it will take more or less exactly the same length of time to recharge as it would if you took 200Ah from a 1000Ah bank. 

[Alan de Enfield]

Consider type 625 batteries. They about 200-230Ah (depending on make) weigh about 50-53kgs each and cost £160 - £170 each.

 

5 of those would be sufficient (I have 6 in my bank)

 

 

2 off @ total £310

 

https://www.ebay.co.uk/itm/PAIR-X-2-Exide-EG2153-PROFESSIONAL-210ah-1200CCA-12v-Type-625-Commercial-Battery/312550343978?epid=1654183694&hash=item48c573c52a:g:q~MAAOSw9bFcneB5

[WotEver]

.

Edited September 24, 2019 by WotEver

[jetzi]

21 minutes ago, Tony Brooks said:

But there is a big difference between generating 3.5 kWh and actually getting 3.5 kWh back into the batteries. To do the latter you will need to generate probably at least 20% (for LA) more electricity and maybe far more.

Thanks, this is an obvious point that I just missed. I was pessimistic with my calculation, but not enough - I estimate needing 3.3kWh on a bad day, so +20% is about 4kWh. I'll use that figure for my calculation. Why would it be "far more" than 20%, would I be able to do something to improve this? (For example, using high quality components, a MPPT charge controller for the solar, etc).

 

20 minutes ago, WotEver said:

No chance. Probably more like 1 or 2%. Remember that the sun is very low in the sky and passing through loads of atmosphere before it reaches your panels. 

 

18 minutes ago, Alan de Enfield said:

I suggested 1% in Ivan's previous thread, MtB suggested 0.1% in deepest darkest February.

I'm still not convinced that it would be that low. The salespeople cry 10%. Is the 1% you're expecting an average over the 8 hours of daylight, or is it 1% peak? In any event, I'm not expecting to rely solely on solar in winter, as I said I'm expecting to run the engine for 4 hours on a bad day in winter. If I have to do another hour or two of running to get the last 1.25kW because the solar gave me nothing at all and I'm on my last pair of drawers, then so be it. I have a hospital silencer now so the neighbours should be OK!

 

The more interesting figure that I'd like critiqued is 10h of daylight in October, at 25% average output from the panels = 4kWh per day. this is plenty even given @Tony Brooks's correction to add 20% to my generation.

8 minutes ago, WotEver said:

A lot of folk get this wrong. The size of the bank does not in itself impact the recharge time. It’s how much you’ve taken out of it that matters. 
 

Take 200Ah from a 600Ah bank and it will take more or less exactly the same length of time to recharge as it would if you took 200Ah from a 1000Ah bank. 

Hm, that's an interesting point. So realistically the more batteries you have, the longer they will actually last because you'll never discharge them much? In your example, if you never use more than 200Ah, your 600Ah bank would be 66% discharged but the 1000Ah one only 80% discharged. And LAs like to be as charged as possible as long as possible right?

Edited September 24, 2019 by ivan&alice

[jetzi]

6 minutes ago, Alan de Enfield said:

Consider type 625 batteries. They about 200-230Ah (depending on make) weigh about 50-53kgs each and cost £160 - £170 each.

 

5 of those would be sufficient (I have 6 in my bank)

 

 

2 off @ total £310

 

https://www.ebay.co.uk/itm/PAIR-X-2-Exide-EG2153-PROFESSIONAL-210ah-1200CCA-12v-Type-625-Commercial-Battery/312550343978?epid=1654183694&hash=item48c573c52a:g:q~MAAOSw9bFcneB5

Thanks for the tip! Are these LA batteries?

 

What's the advantage of these?

[Alan de Enfield]

14 minutes ago, ivan&alice said:

The salespeople cry 10%. Is the 1% you're expecting an average over the 8 hours of daylight, or is it 1% peak?

It is an actual Wh achieved compared to Wh 'in theory' over daylight hours .

 

I'm not on the boat at the moment but I have the Wh generated by month over the last couple of years.

 

From memory the actual between about 8:00 and 15:00 in Winter is around 1%, the actual from 03:00 to 22:00 in the Summer is around 70% the theoretical.

 

A passing cloud even for a few seconds drops the average down considerably.

 

18th Feb, for a few minutes. I was achieving 3.3 amps, in a clear blue sky. Cold but Sunny.

It didn't last long and I was back to fractions of an amp.

The theoretical output should have been 14 amps

 

Remember that solar panels are rated 'at the equator' with the overhead sun at midday.

 

At 53* North we have the sunlight travelling much further thru a 'mucky' atmosphere we are lucky to achieve 75% of the rated output for any considerable time.

I have said before, I actually work on an achieved 50% of theoretical output.

 

My panels are some 'special' low light panels that Bimble had - apparently they are designed to work better in higher latitudes with lower angles of incidence of sunlight.

 

 

 

 

9 minutes ago, ivan&alice said:

Thanks for the tip! Are these LA batteries?

 

What's the advantage of these?

LA with screw-tops so re-fillable. (A must really).

Edited September 24, 2019 by Alan de Enfield

[jetzi]

7 minutes ago, Alan de Enfield said:

LA with screw-tops so re-fillable. (A must really).

Does it work out cheaper, or are these better quality? I would have thought that having more, smaller batteries would be better because then you could swap out a dud cell?

[Alan de Enfield]

1 minute ago, ivan&alice said:

Does it work out cheaper, or are these better quality? I would have thought that having more, smaller batteries would be better because then you could swap out a dud cell?

I don't know - it was the way the boat was set up and I have just replaced like-with-like.

 

They work for me, they fit in the battery boxes, they are Bl**dy heavy to move into place, and are re-fillable.

 

I reckon you'd get 5 off then for £700 (battery sellers are always open to deals on multiples)

 

Many 110Ah are rated as 100Ah with different labels on them so 9 or 10 of them will be £900 ish.

 

£200 pays for another couple of solar panels !!!

[jetzi]

4 minutes ago, Alan de Enfield said:

I don't know - it was the way the boat was set up and I have just replaced like-with-like.

 

They work for me, they fit in the battery boxes, they are Bl**dy heavy to move into place, and are re-fillable.

 

I reckon you'd get 5 off then for £700 (battery sellers are always open to deals on multiples)

 

Many 110Ah are rated as 100Ah with different labels on them so 9 or 10 of them will be £900 ish.

 

£200 pays for another couple of solar panels !!!

Yeah, you're right on that. I think Li's are just far too expensive to make real sense. Better to get a humungous LA bank. The only time the Li's will really help me is when I'm charging from the alternator in winter.

 

I'll keep in mind the suggestion and try to haggle to get multiples.

21 minutes ago, Alan de Enfield said:

From memory the actual between about 8:00 and 15:00 in Winter is around 1%, the actual from 03:00 to 22:00 in the Summer is around 70% the theoretical.

This is good to know. 70% of the theoretical (i.e. 1600W) over 19 hours is a whopping 21.3kWh.

 

I'll treat whatever I get from November to February (inclusive) as a bonus, and rely on the alternator and perhaps a laundromat to keep the leccy usage down.

[Richard10002]

Merely anecdotal but it seems to be generally suggested that a high use of power on a daily basis is around 130Ah, average use around 100Ah, and low use around 70Ah.

 

Your figures seem reasonable until we reach the inverter.... What are you using the inverter for that will consume 200AH?

 

I'm not saying you cant use 300Ah per day. I think we use about 14Kwh per day in our house, (and I'm now beginning to wonder what we use it all on  ).

 

I ask because, whilst I am the only one on board most of the time, (apart from poppy the dog  ), I have almost every electrical home comfort and seem to use around 110Ah per day. I don't have a washing machine or a dishwasher.

 

As a matter of interest, 500w of solar supplies all/most of my power from around mid March until mid September. The rest of the year, it provides pretty much nothing.

 

Personally, I dont think it is worth attempting to provide power from solar over the winter months. Shore power, daily cruising, or generator, seem more sensible, (only IMHO obviously).

 

If you do use 300Ah per day, I like to have a bank which will provide 2 days of power without falling below 50% of nominal capacity, thus 1200 Ah, give or take.

 

Having said that, if you do go 2 days without charging, and get down to 50%, you will need to put 600Ah+ back in over the course of a day - a challenge in itself..

[Alan de Enfield]

6 minutes ago, Richard10002 said:

What are you using the inverter for that will consume 200AH?

 

We had this discussion in an earlier thread :

 

1) Washing machine

2) Alice's hair dryer. (1Kw = 100Ah)

Edited September 24, 2019 by Alan de Enfield

[Richard10002]

2 minutes ago, Alan de Enfield said:

 

We had this discussion in an earlier thread :

 

1) Washing machine

2) Alice's hair dryer. (1Kw = 100Ah)

OK! Wow!  

[MtB]

6 minutes ago, Alan de Enfield said:

(1Kw = 100Ah)

 

Ahem. Dimensional analysis, anyone? 

 

[Bobbybass]

We had a Travelpower on our last boat...for washing machine..hair dryer etc...plus running the battery charger from it. 

We still gave up on the washing machine on most occasions...and used a laundry.

 

[WotEver]

10 hours ago, Mike the Boilerman said:

 

Ahem. Dimensional analysis, anyone? 

 

Pedantic nitpicker

[Jen-in-Wellies]

Would it be worth leaving space for a larger battery bank and installing only a small lead acid one first? Say three or four flooded lead acid batteries. Then you can learn what it takes to keep them charged properly over the first winter or two and if you get it wrong and wreck them, then it isn't too expensive to replace. I destroyed the capacity of my first set of batteries very soon doing pretty much everything wrong. Second set lasted longer doing only some things wrong.

 

Jen