battery sizing

[MarkJ]

I hope I've got this right...

I've been sizing the battery bank for my hopeful build and reluctantly gone away from all-electric (winter heating...) so it's a more conventional load calculation.

Before I started I thought it was mostly about straight Ah capacity, but I now realise it can also be about peak current draw if you're planning any big currents.

One of the things I need to support is a 2kw hair dryer. This seems to driving my inverter size, but I'm thinking the impact of its current draw on the battery bank needs to be checked. I need about 240Ah energy capacity for my target of 2 days off-grid so I could meet that with 2 x 120Ah batteries, but will 2 batteries manage the current draw of the hair dryer? I think 'yes' because:

The hair dryer is actually rated at 1800W and I'm not sure that my wife uses it full power all the time, but on the other hand, I don't know the power factor either. So to be conservative, let's stick with 2000W and convert directly to12V - about 166A. Add an overhead for the inverter of 15% and we get around 190A. If my batteries are perfectly matched that would be 95A per battery.

I found a spec for some Leoch 120Ah Leisure batteries and the CCD table shows that 95A would give me around 20-30 minutes discharge time for a final voltage of around 1.7/cell. Since the hair dryer is on for 5 minutes typically, we would only be 5/20th=25% worst case discharged at the end of it. So all happy?

Am I on the right lines with this? One query is how to interpret the CCD table - it gives a choice of FV figures from 1.85 to 1.6. Not sure what that means.

Thanks!

 

[SquirrellCook]

Don't forget the 50% rule. Also with that kind of load I expect the batteries would gas quite badly.

 

[wildebus]

I hope I've got this right...

I've been sizing the battery bank for my hopeful build and reluctantly gone away from all-electric (winter heating...) so it's a more conventional load calculation.

Before I started I thought it was mostly about straight Ah capacity, but I now realise it can also be about peak current draw if you're planning any big currents.

One of the things I need to support is a 2kw hair dryer. This seems to driving my inverter size, but I'm thinking the impact of its current draw on the battery bank needs to be checked. I need about 240Ah energy capacity for my target of 2 days off-grid so I could meet that with 2 x 120Ah batteries, but will 2 batteries manage the current draw of the hair dryer? I think 'yes' because:

The hair dryer is actually rated at 1800W and I'm not sure that my wife uses it full power all the time, but on the other hand, I don't know the power factor either. So to be conservative, let's stick with 2000W and convert directly to12V - about 166A. Add an overhead for the inverter of 15% and we get around 190A. If my batteries are perfectly matched that would be 95A per battery.

I found a spec for some Leoch 120Ah Leisure batteries and the CCD table shows that 95A would give me around 20-30 minutes discharge time for a final voltage of around 1.7/cell. Since the hair dryer is on for 5 minutes typically, we would only be 5/20th=25% worst case discharged at the end of it. So all happy?

Am I on the right lines with this? One query is how to interpret the CCD table - it gives a choice of FV figures from 1.85 to 1.6. Not sure what that means.

Thanks!

Good you got away from the idea of Electric Heating (though doable if you towed a trailer full of batteries ).
You say "I need about 240Ah energy capacity for my target of 2 days off-grid". Do you actually mean 240Ah Capacity? as if you did, a pair of 120Ah Lead Acid will only give you 120Ah of [usable] capacity. Just mentioning that, but I am guessing you did factor that into the equation

Something to bear in mind with using an inverter is the voltage drop incurred whilst in use, not just the total power used. The smaller the bank, the greater the voltage drop for a given current draw - so you could have power in the battery bank but the voltage will drop too far to drive the inverter at the higher power.

Having said all that, I would say 2 x 120Ah batteries will do you for a long weekend including judicious use of a Hair Dryer.
On my T5 Camper, I had a pair of 110AH Leoch AGMs driving a 12V Fridge, assortment of lights and radio and occaisonal use of a 500W Induction Hob. WIth the 200W of Solar I would often leave site on a Sunday Afternoon with the same battery level I arrived with on the Friday Morning.

 

[MarkJ]

Thanks both - yes, I've factored in the 50%, plus, because the target is for summer off grid use, some solar. On that basis calculated capacity is about 120-130Ah, doubled up to about 240Ah.

What's the FV parameter quoted in the Leoch stuff? Is the the final voltage of a cell immediately after it's been knackered by a large current draw? Presumably it recovers after a while.

 

[wildebus]

Thanks both - yes, I've factored in the 50%, plus, because the target is for summer off grid use, some solar. On that basis calculated capacity is about 120-130Ah, doubled up to about 240Ah.

What's the FV parameter quoted in the Leoch stuff? Is the the final voltage of a cell immediately after it's been knackered by a large current draw? Presumably it recovers after a while.

Which 120Ah Leoch are you looking at? on Alpha Batteries there are three listed. (PS. the 120Ah is at the C100 Rating. At the 'recommended' C20 rating for comparison, it would be a 105Ah Battery).

This is the table for one of those 120Ah Batteries - the LAGM-120

1.80V is the per cell Voltage (=10.8V for Battery) when fully discharged at the battery rating. The way I look at those tables is go lower than 10.8V (1.80V/Cell) and you are permanently damaging the battery, not just hurting the cycle count.
So halve the quoted time for a given current pull and you would take the battery down to 50%. As an example, pull 126.1A for 7.5 Minutes instead of 15 minutes.
If you had a pair of these, you would look for half the drawn current in the table (so if drawing 180A, that would be 90A per battery. That is between 20 and 30 minutes for a F.V. of 1.80/Cell. Let's call in 25 Minutes for simplicity. So Battery dead in 25 Minutes.
If you pulled that load for 5 minutes that is 1/5th of the capacity. So after 5 minutes you would have 4/5th left = 80% SOC.

This is the way I roll with these tables anyway

 

[MarkJ]

Yep, that was the one. Not knowing that vital piece of info about 10.8V being the absolute minimum, I'd looked at a middle row.

So, yes, with my two batteries supporting 95A each for 5 minutes, and sticking to the 1.8V/cell row, we're in the 20 minute territory and therefore 25% discharged.

Thanks, that's very helpful. Think I'm happy now. Bloody big cables though. And thanks for the reminder about C20; I'd forgotten that.

You know, I do wonder how many people aren't told/don't find out about this stuff. So they plug in an inverter and pull huge amounts out of it and are probably disappointed that the electric fire goes out in 15 minutes. And when I collected both my vans and had the handover run through, I'm sure no-one said 'oh, don't discharge more than 50%.... ' So I guess some places probably make a handy living out of replacing batteries that have been unknowingly mistreated.

 

[wildebus]

The right C-Rating - C100/C20/C-whatever rating - really does depend on how you use your batteries. Personally, I think for most folk, the C20 is too low. I worked my actual usage for a typical off-grid weekend using the Amps drawn each minute against the various C-ratings in the table for my battery and IIRC, my personal C-Rating was around C115 including the use of a big inverter (but I do have a big bank that pushes the 'real' C-Rating up).
You can go below 50% for sure, but you are shortening the Service Life notably. Some Lead Acid batteries will go below 50% (I think mine are 20%. There is a range of Victron AGMs that genuine will go to zero SOC hundreds of times), but most don't like it at all.
And go for the lower Final Voltage row in the table and you will hurt the actual batteries ability to recharge and deliver (this is what I think anyway)
Oh, the other factor to consider .... All these capacity ratings assume a Battery Temp of 25C. For the UK, that is pretty warm in fact. As a battery is cooler, the lower the capacity it can give up and it is actually fairly significant, especially at Winter Temperatures. (on the plus side, a colder battery has a much better service life then one that lives in a hot environment).

Obviously more outlay, but consider if feasible a 3rd battery. Just like 2 batteries are worth more than 2 x single ones, 3 batteries are worth more than 1.5x 2 batteries (does that make sense the way I have put it?). You will get more than an extra 110Ah worth of energy by getting a third 110Ah battery and your battery lifecycle will be improved as you will not be discharging each as much.
I had 4 x 110Ah Leochs and when I checked them after taking them out a year later they were honestly just as good as when I put them in as the average load was so small. (only removed them to fit Phils old batteries to get even more capacity!)

Cables? good point. FWIW, I have 50mm2 between Batteries and I have 70mm2 from Battery Bank +ve and -ve to the Inverter/Charger on a cable length of under 1Metre. I changed from 50mm2 to 70mm2 as I could feel some heat in the cable (and heat = waste of course).

 

[wildebus]

Mark, after you posted this question, it made me think just what effect multiple batteries have on delivering high power (I mentioned intially about the voltage drop and how well would a bank drive an inverter).
So I did a test on my setup and put on a constant current similar to the one you are talking about on different battery setups to compare.
So load was a 2kW Hob running for 5 minutes (pulling 185A) and I tested this with a single 210Ah(@C10) Battery, a pair of 210Ahs and a triple set of the 210Ahs (and always starting with a fully charged bank each time).
So each time the load was shared further, with the idea of how that would effect the Voltage Drop.

The top graph shows the Current - as can be seen, same current each time. The next graph has the voltage shown as well and the difference is fairly dramatic (and the benefit of more capacity when pulling a high current)

 

[MarkJ]

That’s really interesting. Thanks for doing it,

As you say, a big difference. And although there is obviously a cost to buying a third battery, it needn’t be 50% more to get a lot of the benefit: 3 x 90Ah would be significantly better than 2 x 120ah for example and not as much as 50% extra.

 

[wildebus]

that is true. I guess a matter of getting the discharge tables for the different capacity batteries and appying some maths
(definately an advantage of having batteries from manufacturers that are good enough to provide this kind of data )

 

[trevskoda]

12v hair driers can be bought,or a towel is best as driers distroy hair.

 

[MarkJ]

Not according to the person who wields the tools....

 

[wildebus]

I used to carry a very handy portable hairdrier in my toolkit (used it on site as a engineer when repairing cicuit boards to find components with thermal failure )

 

[wildebus]

That’s really interesting. Thanks for doing it,

As you say, a big difference. And although there is obviously a cost to buying a third battery, it needn’t be 50% more to get a lot of the benefit: 3 x 90Ah would be significantly better than 2 x 120ah for example and not as much as 50% extra.

Just for info, when I did that 5 minute 2kW Hob test, it typically took 22Ah out the battery (bit more as a single battery, bit less as multiples)

 

[MarkJ]

Umm, isn’t 22Ah at 12V equal to 264W used in 5minutes, whereas shouldn’t a 2kw hob, running for 5 minutes consume 166W? Am I missing something?

 

[wildebus]

Umm, isn’t 22Ah at 12V equal to 264W used in 5minutes, whereas shouldn’t a 2kw hob, running for 5 minutes consume 166W? Am I missing something?

Don't forget any inverter overhead. Plus some other minimal loads. and maybe on for 5.5 minutes?
I'll download the info and have a look to see what was drawn ....

 

[wildebus]

Umm, isn’t 22Ah at 12V equal to 264W used in 5minutes, whereas shouldn’t a 2kw hob, running for 5 minutes consume 166W? Am I missing something?

Can't download the CSV for some reason from the VRM, but couple of screenshots showing the first (single battery) and second (dual battery).
Top Graph is Battery Volts and Current; Middle Graph is the AC Power out the Inverter (so actully under 2kw); Bottom Graph is the Total Power out the battery (I don't show it but the DC Load at this time is under 40W at all times so just noise really).
Difference between Middle Power Graph and Bottom Power Graph would be the Inverter overhead I would say and more than you would imagine.
FYI, the data is sampled only every minute, so for a very short time example, the lines can be quite 'sharp' (and note each data point is sampled every minute, but not ON the minute and not at the exact same second for each data input, so again, for very short time runs, the info may appear to be at odds on occasion. Useful to bear in mind when analysing the data from the VRM)

Single Battery

Dual Battery

 

[MarkJ]

Thanks - the two graphs, middle and bottom, do stack up: the Victron overhead is around 15% according to a table I've seen somewhere, so 1,724W + 15% = 2,025W - more or less matching the 2,141W shown. So it's your estimate of 22AH which seems anomalous, doesn't it? Never mind, I think we've converged on an answer! Thanks for your efforts to put my enquiring mind to rest....

 

[wildebus]

Thanks - the two graphs, middle and bottom, do stack up: the Victron overhead is around 15% according to a table I've seen somewhere, so 1,724W + 15% = 2,025W - more or less matching the 2,141W shown. So it's your estimate of 22AH which seems anomalous, doesn't it? Never mind, I think we've converged on an answer! Thanks for your efforts to put my enquiring mind to rest....

I took that 22Ah from the "last discharge number" and looking at what was happening at that time I related it to the test run. must admit I didn't actually do the math cross-check