Battery life using inverter

Jul 18, 2017
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Not sure of the calculation but if you have a 95ah AGM battery and it is fully charged, approximately how long will it last only powering a 140w 55cm television? We do have smaller TVs that are about 45watts which I am sure would last a lot longer. Contemplating getting a 300w inverter to connect to our old AGM battery due to number of power cuts we have experienced over the last month. Router has been sorted so thanks Sam
 
Nov 6, 2005
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95ah battery will give a nominal 1140 watt-hours (96ah x 12v) so a 140w tv would last a nominal 7 hours(1140 wh / 140w) assuming inverter efficiency of about 85% - BUT - an old battery will have reduced capacity - AND - it's life and capacity would be further compromised by discharging it right down - so probably good for 3-4 hours.
 

Sam Vimes

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Just doing the same sums but Roger beat me to it πŸ˜‰

You have to be careful with choice of inverter. Some cheap ones don't produce a clean 50Hz sine wave output and they can damage whatevers plugged in on the 240v side.
 
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Just doing the same sums but Roger beat me to it πŸ˜‰

You have to be careful with choice of inverter. Some cheap ones don't produce a clean 50Hz sine wave output and they can damage whatevers plugged in on the 240v side.
I am looking at the 'pure since wave' inverter. We have a normal 150w inverter in the caravan, but it must be about 20 years old so probably inefficient in comparison to ones built today.

Any suggestions on a reasonably priced inverter as a bit wary of what amazon offer. Gets boring sitting at home during a long power cut. LOL!
 
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Indeed so, but I was just relating my experience. Has any forum member actually wrecked any electronic item by powering it with a non sine wave inverter?
Just curious
 

Sam Vimes

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Yes, which is why I drew attention to the fact.

I have an inverter, the output of which is not a pure sine wave, so has some DC content.

It will run things that have a switch mode psu for example but not those whose first stage is just a transformer.

So as an example I can run my laptop but when I tried to charge my electric toothbrush, it died.

Could be costly to find out it's not going to work for you.
 
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Indeed so, but I was just relating my experience. Has any forum member actually wrecked any electronic item by powering it with a non sine wave inverter?
Just curious
I guess that using a modified wave inverter there should be no issue with a TV as TVs are fairly robust and the 240v even on mains gets converted down to 12- 15v. however with a laptop I would be a bit wary.
 
Mar 14, 2005
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In life so many things are compromises, The trade off for inverters is the efficiency against the purity of the sine wave output.

Quasi Sine Wave inverters (QSWIs) might achieve 90% input to output conversion under ideal conditions, those conditions are not across their entire working range, so Roger L's suggestion of 85% is more realistic though it can be lower depending on circumstances.

That efficiency is only due to the fact the output control devices (most likely Transistors or FET's) are being asked to switch quickly between discrete voltage levels to produce a stepped pyramid effect output voltage rather than progressively adjustusting their conduction between two levels as would be required to produce a pure smooth sine wave output.

Pure sine wave inverters (PSWIs) need their output devices not to switch but to smoothly transition between their limits. This in means they cannot be more than about 65% efficient at best, becasue of the time the output devices are actually transitioning between their on and off states means they are partially resisting current flow and thus converting that restied energy in to heat.

QAWI's can be bad for many types of mains equipment, especially those with inductive components like transformers and motors. When a QSWIs output steps between two voltage levels it does so very quickly which creates a high speed voltage change that is the same thing as a high frequency, which 50Hz transformers and motors trap in their coils, and it comes out as heat. In addition the high frequencies coupled with high power can saturate the inductors magnetic field preventing it from passing the normal power it should. This can result in some appliances running hotter than they should or simply getting hot and refusing to work.

Unfortunately you cannot assume that products that use a Switched Mode Power Supply (SMPS) will be immune to QSWIs output, some might work some might not. Many may work but are likely to run the SMPS warmer than on a 50/60Hz sine wave supply.

There is no simple way to predict which appliances will or won't work satisfactorily on a QSWI, suck it and see is answer, and it anything behaves unusually or get warmer than usual, stop using it.

Or take the hit on the efficiency and ensure you get a Pure Sine Wave inverter - Watch your power ratings and make sure your aware of continuously or just peak rated.
 
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I am looking at the Victron pure sine wave 300w at present, but it is bit more than I want to spend as could be days, weeks or even months before the next power outage so maybe an overkill? I do like the 5 year warranty
 
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Victron is good kit. Just make sure it's 300W and not 300VA they are not the same πŸ€”
Strictly the difference is not dependant on the type of supply but more the power factor of the loads you run on them, and here again the older transformer type products are usually fine on Pure Sinewave inverters, its the more modern items like poorly designed SMPWS, and some LED lamps that can react badly.
 
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The basic issue with the fiirst calculation is that an AGM or any other lead acid based battery shouldn't be discharged below 50% of its capacity... So that means you only have usable capacity in the battery of 48Ah. An inverter of 90% efficiency will require 155W from the battery. At 12 V that means a draw on the battery of about 13A... =3.5 ish hours pushing your battery hard.
 
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I went the whole hog and fitted a SolaX 3.6kW inverter and 11.8kWh of SolaX batteries,
24hrs off grid if need be. There is also 4.3kW of solar panels to keep it charged and if that's not enough a variable electric tariff to charge from very cheap electricity 😎
Should make my outlay back in 8 years from savings in electricity and export πŸ€”
 
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The basic issue with the fiirst calculation is that an AGM or any other lead acid based battery shouldn't be discharged below 50% of its capacity... So that means you only have usable capacity in the battery of 48Ah. An inverter of 90% efficiency will require 155W from the battery. At 12 V that means a draw on the battery of about 13A... =3.5 ish hours pushing your battery hard.
Or if it's a pure sine wave inverter 140W / 65% = 215W
215W / 12V (nominal) = 17.9A current draw
50% battery capacity 48Ah /17.9 A = 2.68Hours.
 
Oct 8, 2006
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Pulling the sort of currents at 12V that have been discussed needs some decent cables as well. I would suggest an absolute minimum of 4mm and preferably 6mm, and it should be multicore flex - not solid single core - to allow for vibration when in transit.
 
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Pulling the sort of currents at 12V that have been discussed needs some decent cables as well. I would suggest an absolute minimum of 4mm and preferably 6mm, and it should be multicore flex - not solid single core - to allow for vibration when in transit.
Buckmans is talking about using the inverter for a TV at home, during his frequent power cuts, NOT in a caravan.
 
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Do any of the TV have a separate power brick that gives out 12v ?
If so you could run directly from the battery with a suitable fused cable.
 
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Do any of the TV have a separate power brick that gives out 12v ?
If so you could run directly from the battery with a suitable fused cable.
Whilst many smaller TV sets (like those used in caravans) that may use a power brick providing 12V dc, I have seen different sets that need requires almost every voltage up to 20V. Obviously if you have one of these higher voltage rated sets the 12V from a battery will not be enough.

However if you do have a TV that uses a power brick, then there are" variable DC to DC voltage boosters available from Ebay for about Β£20 that can be adjusted to the correct DC output voltage and handle the necessary current when correctly installed. These would be a much more efficient method of deriving a higher dc voltage from a battery than using a pure sine or even a quasi sine wave inverter to run a power brick.

But most large screen domestic TV's have built in power supplies that need mains voltage.
 
Jul 18, 2017
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Or if it's a pure sine wave inverter 140W / 65% = 215W
215W / 12V (nominal) = 17.9A current draw
50% battery capacity 48Ah /17.9 A = 2.68Hours.
Going by the two posts, if using a modified sine wave inverter instead of pure sine wave, it seems that the modified sine wave inverter would last a lot longer than the other.

Just to add that the village had another power cut last night, but this time we were not affected.
 
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Going by the two posts, if using a modified sine wave inverter instead of pure sine wave, it seems that the modified sine wave inverter would last a lot longer than the other.
...

Just to be clear when I have referred to Quasi Sine Wave Inverters, it includes "Modified Sine Wave Inverters - they are the same thing i.e not Pure Sine Wave.

Yes QSWI is more efficient than a Pure Sine Wave inverter at converting the power which will mean it will drain a battery less quickly. But there can be other effects, which means the appliance you connect to the QSWI may not work as efficiently.

I have already mentioned transformers and motors getting hotter compared to pure mains, Some can also sometimes make extraneous noises as the stepped changes in voltage excite electromechanical resonances or phase reversals in windings. In the worst instances motors may run slower, I have witnessed a record player [- do you remember them? o_O ] synchronous motor that ran slowly and mechanically buzzed and got hot when it was run on a QSWI.

There is no simple way of predicting which appliances might react badly to a QSWI, and it also could depend on the make/model of inverter.

With the advent of low cost and wider range of available high voltage components there has been a rash of cheap QSWI's mainly from the far East. They often exaggerate their output capabilities and produce poor output voltage regulation and waveform smoothing. I strongly suggest if you are looking at such a device you would do well to go to reputable supplier and avoid the cheap imports which could damage your probably expensive TV.
 
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Just to be clear when I have referred to Quasi Sine Wave Inverters, it includes "Modified Sine Wave Inverters - they are the same thing i.e not Pure Sine Wave.

Yes QSWI is more efficient than a Pure Sine Wave inverter at converting the power which will mean it will drain a battery less quickly. But there can be other effects, which means the appliance you connect to the QSWI may not work as efficiently.

I have already mentioned transformers and motors getting hotter compared to pure mains, Some can also sometimes make extraneous noises as the stepped changes in voltage excite electromechanical resonances or phase reversals in windings. In the worst instances motors may run slower, I have witnessed a record player [- do you remember them? o_O ] synchronous motor that ran slowly and mechanically buzzed and got hot when it was run on a QSWI.

There is no simple way of predicting which appliances might react badly to a QSWI, and it also could depend on the make/model of inverter.

With the advent of low cost and wider range of available high voltage components there has been a rash of cheap QSWI's mainly from the far East. They often exaggerate their output capabilities and produce poor output voltage regulation and waveform smoothing. I strongly suggest if you are looking at such a device you would do well to go to reputable supplier and avoid the cheap imports which could damage your probably expensive TV.
Thanks for informative post as it is a big help. Biggest issue is recognising the "cheap imports" and the reputable suppliers. Taking into account our needs, I am now looking for a reputable modified sine wave instead of the more expensive pure sine wave.

Our current 20 year old Ring inverter is only 150w which was ample at the time. However for the Freesat box (10w) and 32" TV (36w) it may be ample. Maybe a 300 or 500w would be better and more future proofing? The router now has a UPS that should last a couple of hours before requiring a recharge.

Better than sitting in the dark for hours on end! LOL
 
Jul 18, 2017
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Doing some calculations and it seems a unit drawing 65w off through a Victron 500w pure sine inverter a 95ah battery could last up to 17 hours of the battery is in good nick. Battery capacity = 95Ah x 12V = 1140 Wh. Using the formula to calculate the battery life: Battery life = 1200Wh / 65W β‰ˆ 17.53 hours
 
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Doing some calculations and it seems a unit drawing 65w off through a Victron 500w pure sine inverter a 95ah battery could last up to 17 hours of the battery is in good nick. Battery capacity = 95Ah x 12V = 1140 Wh. Using the formula to calculate the battery life: Battery life = 1200Wh / 65W β‰ˆ 17.53 hours
Enough time then to succumb to hyperthermia? Might be better to use the battery power for a low wattage fleece over. πŸ˜‚
 

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