Battery Bank Size?

[Polishicebreaker]

I'm being lazy here, but could anyone advise what size battery bank I'd need to run a 3kw inverter?

 

 

many thanks    Ed

[rusty69]

I would think a single battery of low nominal capacity would run a 3KW inverter.

 

If, however you intend to put a load on that inverter, and combine it with other DC loads, I suspect you will need to give a few more details before you will get any meaningful answers.

[Tony Brooks]

I have not worked it out but typically large inverter boats tend to use between abut 440 and 660 Ah worth of batteries but if you only intend to draw a maximum of 1Kw for a few minutes then maybe a bit less, and the larger the load and for longer the more you need. Also will you be running your engine when running high loads.

[rusty69]

If a 3000 KW (3000VA?) inverter can operate continuously at that rating at a given temperature, a draw of 300A could be put on a battery bank. For short periods of time,however, the continuous rating can be exceeded (peak power), so it could be more than that. Not forgetting mr Peukert (on lead acid batteries,not lithium), and the desire not to go too far below the 50% capacity 'rule', and assuming no net input into the bank whilst discharging, could be that a bank size of 600 Ah plus would be sufficient (assuming a 12V bank).

 

It really comes down to what load you will be putting on the inverter, and for how long. Combined with any DC loads, and your charging regime,it all comes back to that all important power audit.

Edited September 24, 2022 by rusty69

[Thames Bhaji]

Victron (and probably others) give a recommended minimum battery capacity for each of the their inverters. You can find this in the installation guides which are all on their website. Along with sufficiently sized cables, this is in part to minimise DC ripple when higher loads are applied, which I believe inverters don’t like very much. 

[nb Innisfree]

1 hour ago, Polishicebreaker said:

I'm being lazy here, but could anyone advise what size battery bank I'd need to run a 3kw inverter?

 

 

many thanks    Ed

I managed to fit in 8 x 120ah AGMs for Victron 3kva Multiplus, worked a treat.

One advantage of a larger bank is less voltage drop under load towards end of batt life, therefore delaying replacement. 

[BEngo]

3 hours ago, Polishicebreaker said:

I'm being lazy here, but could anyone advise what size battery bank I'd need to run a 3kw inverter?

 

 

many thanks    Ed

Once you have a suitable battery size  do you have a suitable charging set up that is able, in a reasonable time,  to put back all the electricity which has gone out?

 

N

[Tony Brooks]

20 minutes ago, BEngo said:

Once you have a suitable battery size  do you have a suitable charging set up that is able, in a reasonable time,  to put back all the electricity which has gone out?

 

N

 And that is probably the most vital thing unless you want to destroy batteries.

[ditchcrawler]

21 minutes ago, BEngo said:

Once you have a suitable battery size  do you have a suitable charging set up that is able, in a reasonable time,  to put back all the electricity which has gone out?

 

N

And that is the important and often difficult bit, Its easy to buy wire, batteries and inverter.

[Polishicebreaker]

thanks for your responses. 

 

would probably be drawing around 1.5 - 2kw for shortish periods. 

 

Currently on shoreline but trying to spec for potential off-grid scenario/cruising.  ( in case have to leave london in a hurry!!:)

Engine has 120A alternator, currently only 1 solar panel but plenty roof space so I'd probably stick more on. 

 

Would also consider getting small genny although that begs the question why not just run the genny for heavy usage instead of invert? 

I suppose it's more hassle, unless I go water cooled but prob too expensive.

 

 

many thanks   Ed

[MtB]

36 minutes ago, Polishicebreaker said:

thanks for your responses. 

 

would probably be drawing around 1.5 - 2kw for shortish periods. 

 

 

In which case, a smallish battery bank will probably suffice. 

 

 

[Polishicebreaker]

Thanks very much for all your advice. 

 

Now hunting for Inverters.  I can't afford a Victron, but I don't want the cheapest either. 

 

Just wondering if anyone has experience of these:   https://www.ebay.co.uk/itm/195325903451

 

cheers     Ed

[Tracy D'arth]

5 minutes ago, Polishicebreaker said:

Thanks very much for all your advice. 

 

Now hunting for Inverters.  I can't afford a Victron, but I don't want the cheapest either. 

 

Just wondering if anyone has experience of these:   https://www.ebay.co.uk/itm/195325903451

 

cheers     Ed

I have fitted a couple of these, they are still working AFAIK.  Their quoted quiescent current is optimistic, I found that it was over one and a quarter amps.

[Richard10002]

24 minutes ago, Tracy D'arth said:

I have fitted a couple of these, they are still working AFAIK.  Their quoted quiescent current is optimistic, I found that it was over one and a quarter amps.

I wonder if the quoted 0.6A is a type of standby current?

 

Having said that, I cant see mention of a standby facility.

[Chewbacka]

9 hours ago, Richard10002 said:

I wonder if the quoted 0.6A is a type of standby current?

 

Having said that, I cant see mention of a standby facility.

The 0.6A is the current taken by the control electronics when the inverter is on, but there is no load.  So the inverter will take 0.6A all the time it is on.  Plus as it is (claimed) 90% efficient, it will take from the supply additionally 110% of the ‘load’.

[MtB]

8 hours ago, Chewbacka said:

The 0.6A is the current taken by the control electronics when the inverter is on, but there is no load.  So the inverter will take 0.6A all the time it is on.  Plus as it is (claimed) 90% efficient, it will take from the supply additionally 110% of the ‘load’.

 

I'm no electronics designer but figures like this always surprise me. Both how high the quiescent current always is (I'd have guessed at milliamps rather than almost whole Amps as a layman), and at how high the claimed efficiency is. I'd have thought 90% was far too high a value given the amount of cooling built into these hefty and weighty pieces of electronic engineering, and the sheer amount of work they must be doing to be constructing the shape of a 'pure' sine wave for the output. 

 

I also have my suspicions about just how 'pure' the sine wave shape actually is, too! 

[Chewbacka]

1 hour ago, MtB said:

 

I'm no electronics designer but figures like this always surprise me. Both how high the quiescent current always is (I'd have guessed at milliamps rather than almost whole Amps as a layman), and at how high the claimed efficiency is. I'd have thought 90% was far too high a value given the amount of cooling built into these hefty and weighty pieces of electronic engineering, and the sheer amount of work they must be doing to be constructing the shape of a 'pure' sine wave for the output. 

 

I also have my suspicions about just how 'pure' the sine wave shape actually is, too! 

The quiescent current is high because even when there is no load the machine is still doing “the amount of work they must be doing to be constructing the shape of a 'pure' sine wave for the output”.  As to efficiency the figure given is the best efficiency over the entire output range, probably near the top end, what you never see is the efficiency at say 5~20% load which is typical for an inverter on 24/7 running a fridge etc, that would be interesting when making a buying decision.

 

Back to the original question, the data sheet for my inverter recommends (assuming you may on occasion actually run it on batteries at full power for a while) 5 times the supply current as amp hours, so for a 12v 3000w inverter that would be 1250Ah.  That will not be cheap!!!!

Edited October 5, 2022 by Chewbacka

[Polishicebreaker]

here's another one in the lower/mid price range 

 

https://www.ebay.co.uk/itm/195361947519

[Chewbacka]

There are a few YouTube videos of inverter testing and with cheap no real brand the inverters often are incapable of the rated output, and some go bang under test.  That said some are fine.  How to tell is the question!!!

 

When fitting out my boat some years ago I had a 1kW(2kW peak) unbranded inverter that I used with a 600w drill.  If pulling the drill speed trigger slowly it was fine, pull it quickly and the inverter would shut down, until one day it went pop and clouds of smoke were released.

 

Personally I would avoid unknown/unbranded inverters and I would never leave one ‘On’ if away from the boat, just too risky.

 

[Polishicebreaker]

6 minutes ago, Chewbacka said:

There are a few YouTube videos of inverter testing and with cheap no real brand the inverters often are incapable of the rated output, and some go bang under test.  That said some are fine.  How to tell is the question!!!

 

When fitting out my boat some years ago I had a 1kW(2kW peak) unbranded inverter that I used with a 600w drill.  If pulling the drill speed trigger slowly it was fine, pull it quickly and the inverter would shut down, until one day it went pop and clouds of smoke were released.

 

Personally I would avoid unknown/unbranded inverters and I would never leave one ‘On’ if away from the boat, just too risky.

 

Thanks, yes does seem risky.   Could you recommend anything that's cheaper than Victron. I don't mind paying a bit more but can't afford 800 quid, or is this my only option. 

 

Cheers     Ed

maybe something like this 

 

https://uk.renogy.com/3000w-12v-to-230v-240v-pure-sine-wave-inverter-with-english-standard-socket-with-ups-function/

 

or still  too cheap?

[Polishicebreaker]

 

crikey there's hundreds!!          terrible music though 

 

 

[Puffling]

I'd avoid any high power inverter where the 12v cables connect to terminals secured by push lugs. Quite are few are made like that.

 

With 200A flowing through that termination you need clamps that can be screwed or bolted down.

[IanD]

18 hours ago, Chewbacka said:

The quiescent current is high because even when there is no load the machine is still doing “the amount of work they must be doing to be constructing the shape of a 'pure' sine wave for the output”.  As to efficiency the figure given is the best efficiency over the entire output range, probably near the top end, what you never see is the efficiency at say 5~20% load which is typical for an inverter on 24/7 running a fridge etc, that would be interesting when making a buying decision.

 

Back to the original question, the data sheet for my inverter recommends (assuming you may on occasion actually run it on batteries at full power for a while) 5 times the supply current as amp hours, so for a 12v 3000w inverter that would be 1250Ah.  That will not be cheap!!!!

Peak efficiency -- which can be as high as 96% for the bigger Victron models -- is usually at something like 30% of maximum power, according to (well-hidden!) data.

[Chewbacka]

18 hours ago, Polishicebreaker said:

Thanks, yes does seem risky.   Could you recommend anything that's cheaper than Victron. I don't mind paying a bit more but can't afford 800 quid, or is this my only option. 

 

Cheers     Ed

maybe something like this 

 

https://uk.renogy.com/3000w-12v-to-230v-240v-pure-sine-wave-inverter-with-english-standard-socket-with-ups-function/

 

or still  too cheap?

No experience of Renogy.  If buying from the lower cost range I would look for a brand that has reviews over a number of years, that at least indicates they are good enough to survive as a brand, and if there are no horror stories then probably not dangerous.  I would not expect them to perform to spec ie running at rated output for long periods .  For some high frequency low cost inverters their peak power output may only be sustainable for about a second, so if using with a fridge or washer (motors have a high startup power) I would look for one with a couple of kW output.  Remember the quoted output in kW assumes a power factor of 1.  So a device with a power factor less than one (motors etc) will require more inverter power than the appliance rating plate indicates.  So again an ‘oversized’ inverter is a wise choice.

[Alan de Enfield]

24 minutes ago, Chewbacka said:

 Remember the quoted output in kW assumes a power factor of 1.  So a device with a power factor less than one (motors etc) will require more inverter power than the appliance rating plate indicates.  So again an ‘oversized’ inverter is a wise choice.

 

I found when I was investigating battery chargers that their pf varied tremendously from 1 to as low as 0.6.

The information is either well hidden in the paperwork or was only available via a direct request to the manufacturer.

Edited October 6, 2022 by Alan de Enfield