Inverter capacity Vs Battery capacity

Submitted: Sunday, Apr 07, 2013 at 09:16
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Hi Electronic Experts,

Does anybody know of an old Rule of Thumb for the maximum capacity of Inverter one should not exceed with respect to Capacity ( amp.hours ) of ones Battery(ies).

Obviously one normally looks at Inverter capacity with due regard to the list of 240 v equipment one wants to power. But there must be a point at which the high 12v current draw from the batteries must be bad for them given that virtually all van/motorhomes house batteries are " deep cycle" style with fewer number of thicker plates and are thus designed for only modest current draw over a longer period of time. (Whereas cranking batteries with a higher number of plates and thicker plates are designed for high current draw over a short period of time to only modest levels of discharge.)

Cheers from Martin B.
Fuso 9.8 m 6.9L turbo motorhome.
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Reply By: The Bantam - Sunday, Apr 07, 2013 at 10:21

Sunday, Apr 07, 2013 at 10:21
Martin mate battery drain is the single biggest practicality limiting factor for inverters.

consider that whatever you draw from the 240V side in the way of current you are drawing 20 times that pluss losses from the battery on the DC side.

A 1500 watt inverter fully loaded will be drawing more than 130 amps from the battery.......that is about what the starter motor on a 4 cylinder diesel draws in normal starting.

So how long do you expect to crank that motor?

Inverters themselves have got very much cheaper and easier to obtain in the last few years, but the findemental pracitiality issue of......"where is the energy comming from" remains.

Now remember that ALL batteries work less efficiently the heavier the current drain.

You may have a 100AH battery when measured at the 10 hour rate, but when you hit it realy hard its actual capacity may be half that...then remember we don;t need to be deep cycling our batteries if we want them to last.

So that 100 AH battery realy has a practical capacity of may be 25 amp hours at high currents.

So that 1500 watt'll get about 10 minutes out of it before you start to damage ya battery.

there are lots of people buying very large inverters for portable use......sorry the practicality is just not there unless you are carrying huge battery banks.

Th real practical case for 240V devices in campng / touring/ caravanning is getting less and less, as so many things run native on 12 volts now.

AnswerID: 508397

Follow Up By: 08crd - Sunday, Apr 07, 2013 at 20:05

Sunday, Apr 07, 2013 at 20:05
+1 Great answer.
FollowupID: 785964

Reply By: Peter_n_Margaret - Sunday, Apr 07, 2013 at 10:26

Sunday, Apr 07, 2013 at 10:26
It is more about how long you wish to maintain a high draw via the inverter.
Your battery can deliver cranking amps (maybe 600 or 1,000A) for 30 seconds, so it could deliver 2,500W via an inverter easily, but not for 10 minutes.
Lead acid batteries also deliver less total power when it is delivered at higher rates too.

Dig out the specs for the battery involved (they should be available for a good deep cycle battery) and calculate the total power you want to draw over time.

OKA196 motorhome
AnswerID: 508401

Reply By: Member - Jack & Lorraine B (QL - Sunday, Apr 07, 2013 at 12:07

Sunday, Apr 07, 2013 at 12:07
Preferably do not use an inverter!
If you have to use one use it only for very short time and on low power items.

As previously mentioned it is better to use gear that runs on 12v instead, especially if you need to use it a lot.

If you have power tools you want to run on an inverter, then do not run it of the battery , run it of the running car engine or run a generator that produces 240V,

and if you want to run an aircon unit , definitely use a large generator over 2.5kW

AnswerID: 508406

Reply By: Racey - Sunday, Apr 07, 2013 at 15:17

Sunday, Apr 07, 2013 at 15:17
Data from 1 inverter manufacturer suggests minimum requirements for 12 volt system :-

1500 Watts 200ah, 3000 watts 500ah. The ah ratings are C10, in other words 10 hour ratings. So using usual 20 hour ratings you will need approximately double the capacity.

AnswerID: 508416

Follow Up By: The Bantam - Sunday, Apr 07, 2013 at 16:20

Sunday, Apr 07, 2013 at 16:20
Just at a reality check a 200 amp hour battery weighs around 60 to 70KG...that is more than a not so skinny girlfriend or about 8 slabs of beer.

A 500 ah battery is going to weigh the best part of 200KG or about the weight of ya not so skinny girlfriend AND her mother AND her little sister or about half a pallet of beer.

FollowupID: 785946

Reply By: Martin B - Sunday, Apr 07, 2013 at 15:32

Sunday, Apr 07, 2013 at 15:32
Thanks Bantam and others.

The reason for my query post starts with the "Navigator " requiring her cappuccino immediately upon arising before any thought or action could be contemplated for the day.

So we have a DeLonghi 1250 watt 240v cappuccino machine supplied by a Victron 24v 1200 watt(cont.) Inverter/Battery Charger attached to 2 x 200 AGM VRLA batteries in series and 3 x 165 watt 24v Sharp solar panels. Probably 4 cups of coffee are made each day with say 4 x 1.5 minutes each of current draw.

The house batteries, new in Dec 2009, have died over the last couple of weeks - auto elec in Goondiwindi advises they are down to about 35% capacity. I would have expected a much longer battery life given that the batteries are always fully charged via the solar when the motorhome is not in use. I was wondering whether drawing 1,200 to 1,300 watts 4 times per day for 1.5 minutes each was too high a draw for these batteries ( cyclic or deep cycle batteries having fewer and thicker plates than high discharge rate = cranking batteries) and this may have need a contributor to what I consider a too early failure.

Probably drawing about 60 amps (at 24v) from the batteries whilst making coffee so is this likely to cause premature? battery failure ?

Cheers from Martin B.
AnswerID: 508417

Follow Up By: Razerback - Sunday, Apr 07, 2013 at 17:33

Sunday, Apr 07, 2013 at 17:33
Martin a couple of questions, perhaps you need a battery monitor to ascertain exactly where your batteries are at any point in time.? How many amps does the charger deliver? If you have been inadvertently drawing down the batteries without getting them back up to full charge then you will be shortening their life span. You are assuming the solar system is charging the batteries without an accurate battery monitor. You say the auto elec says the batteries are at 35% and at the same time you are charging them fully via the solar panels, can you say for sure they are being fully charged each day? Food for thought and apologies if I have missed anything.
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Follow Up By: Mick O - Sunday, Apr 07, 2013 at 19:01

Sunday, Apr 07, 2013 at 19:01

as an aside I'd be keen to know just what you are using as a battery management system over the two large house batteries. Given their cost, I'd be very disappointed if my batteries were deteriorating at such a rapid rate. Do you have a system that will give you a graphical readout or record your batteries State of Charge over a period of time say a week or even better, a month.

I'm presuming that your vehicle is custom and/or professionally built so would imagine it has a good system. Could you enlighten us as to what components are involved particularly for regulating and charging the batteries.

Cheers Mick
''We knew from the experience of well-known travelers that the
trip would doubtless be attended with much hardship.''
Richard Maurice - 1903

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Follow Up By: Robin Miller - Sunday, Apr 07, 2013 at 19:10

Sunday, Apr 07, 2013 at 19:10
Our Cappicino machine - designed in the 50's and requires no electricity.
Robin Miller

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Follow Up By: The Bantam - Monday, Apr 08, 2013 at 00:16

Monday, Apr 08, 2013 at 00:16
I agree that the OP needs some sort of state of charge monitoring..even if it is a simple volt meter.

More importantly we need to know what else is being drawn form these batteries.

I'd be less concerned about getting the batteries "fully charged" and more concerned about the batteries being deep cycled, because the solar system is not keeping up day after day, and the batteries getting deeper and deeper cycles.


FollowupID: 785989

Reply By: The Bantam - Sunday, Apr 07, 2013 at 16:41

Sunday, Apr 07, 2013 at 16:41
Firstly you have to remember that batteries do not live for ever...and every high current drain ocurance takes a little off their life.

NOW.....contraversial statemant here......Deep cycle and in particular AGM is being pushed very hard onto travelers.....they may not be the best choice where batteries are hit hard and heavily recharged every day.

That is more a cyclic application like a cranking battery.

Remember a sealed marine cranking battery will cost half what a deep cycle AGM will......if you are hammering a deep cycle It will last no longer than a cranking battery.

Get the numbers off the existing batteries and google up the specifications..and consider their age.

It is documented fact that people are getting far less than the designed life span out of AGM batteries for a number of reasons....or many other batteries for that matter

I was reading a research paper the other day that claimed that long life stationary AGM batteries could have the life reduced from 15 years design to 60 days by excessive heat and as few as 10 deep discharges to 10.5 volts...while otherwise maintaining to spec operation.

I doubt very much that the coffee machine is killing ya batteries all on its own....but 60 amps will be a kick in the balls after supplying the ovenright load.

Have ya thaught about one of those little stove top perculators, and a $2 milk frother from Ikea.
A coffe machine might be comnvienient, but it does not make the smells and sounds that a little perc does...AND ya get a better choice of coffees.

AnswerID: 508420

Follow Up By: Member - bill f (QLD) - Sunday, Apr 07, 2013 at 17:49

Sunday, Apr 07, 2013 at 17:49
Hi, My cook is particular about her coffee. Loves her espresso. After trial & error seems satisfied with using Nescafe instant espresso. Tried all the others but reckons that's the best. Froths like the real thing. Bill
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Follow Up By: Member - PJR (NSW) - Sunday, Apr 07, 2013 at 19:12

Sunday, Apr 07, 2013 at 19:12
Same here.
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Follow Up By: Martin B - Monday, Apr 08, 2013 at 13:29

Monday, Apr 08, 2013 at 13:29
Hi Bantam,
Could you please email me at
I would like to persue a couple of your valid points directly with you (off-Forum).

Thanks, Martin B.
FollowupID: 786022

Reply By: Member - PJR (NSW) - Sunday, Apr 07, 2013 at 18:39

Sunday, Apr 07, 2013 at 18:39
Like Bill above my wife is also particular about her tea and coffee.

Maybe that cappaccino monster should be replaced with a billy on the fire and a packet of Nescafe Cafe Menu /Cappuccino at 63c a cup. It's not new it's been around for a while.

And ditch that monster or leave it at home. You will very quickly hate it after replacing batteries.

Or camp at a powered site.

AnswerID: 508425

Reply By: Member - J&R - Sunday, Apr 07, 2013 at 18:44

Sunday, Apr 07, 2013 at 18:44
They are probably the most expensive cups of coffee in the world.

I thought the concept of 'getting away from it all' was leaving this sort of stuff behind?

A stovetop coffee maker will probably make a good cup, albiet not a 'cuppacino' for about 50c.

Do you think a reality check may be required?

AnswerID: 508426

Reply By: 08crd - Sunday, Apr 07, 2013 at 20:11

Sunday, Apr 07, 2013 at 20:11
Sounds a bit like too much pandering going on. What do you get when the day starts?
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Reply By: Peter_n_Margaret - Sunday, Apr 07, 2013 at 21:14

Sunday, Apr 07, 2013 at 21:14
A 120Ah Fullriver DC AGM has a CA rating of 900A for 30 seconds and can deliver 75A for 50 minutes.

90 seconds at 1200W (100A) is 2.5Ah at 12V.
That should not bother any reasonable battery.

OKA196 Motorhome
AnswerID: 508443

Follow Up By: The Bantam - Monday, Apr 08, 2013 at 00:11

Monday, Apr 08, 2013 at 00:11
Yes but those figures assume that the battery is new and fully charged.

If said battery has some age under it and it is at a low state of charge its ability to delivery and withstand a 100amp load may not be so good.

Remember there is a coffee made first thing in the morning..what else has been running of this inverter since sundown the night before.

FollowupID: 785988

Reply By: Travis22 - Sunday, Apr 07, 2013 at 23:30

Sunday, Apr 07, 2013 at 23:30
My friend has just fitted a 1500w contentious 240v inverter into his ute.

While camping over Easter we decided to give it a try out powering our Baby bottle etc. Sterilizer as we had our baby with us. We would normally use our little Honda genny for the 5 or so minutes it takes to run the Sterilizer but given his new inverter we thought why not give it a shot.

The Sterilizer draw's around 700W at 240V.

This equated to a 70Amp (measured by my Fluke true RMS clamp meter) load from his Inverter while the Sterilizer was running (5-6minutes max) with the car engine running and the alternator supplying the load. Without the engine running id hate to think what the load might have become as voltage drop started to kick in from the batteries.

The inverter didnt break a sweat and the housing remained stone cold.

There is no way i would run the inverter with any sort of load above a camera battery charger or phone charger connected without running the engine.


AnswerID: 508451

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