Inverters In a Motorhome?

A 3,000watt Inverter can draw up to 290amps (including conversion losses) from a 12v battery, more than a car draws to start the engine.  

It doesn't take much to realise that this is going to place an enormous strain on old fashioned Leisure battery that was designed to provide just few amps over weeks. 

There is an excellent document from Victron Energy about batteries and charging, it is called Victron Energy Unlimited, which you will find on the "EHU  Full Time ? Yes or No" page. 

It notes that where a current draw of 300 amps is required, the correct size battery bank (to ensure adequate battery life) is five times the power draw converted to Ah. i.e 1,500Ah or 15 x 100Ah batteries. 


Many Starter 100Ah battery technologies will supply 300amps for a few seconds without damage, but if you drain a 100Ah AGM/Gel Leisure battery with that sort of current for more than a minute, temperatures at the plate will soar, degrading the battery significantly. 

The Exide battery guide book, which follows most other battery manufacturers guidelines, suggests that, for maximum life, a Gel battery draw and charge should be no more than 10amps per 100Ah. 

Most manufacturers say the batteries can go outside this upto 20 amps without too great a compromise on life versus usability, but go above this for either charging or continuous power draw and the penalty is a shorter battery lifetime. 


A highly regarded Victron Energy document also warns about the adverse impact to Gel or AGM batteries that prolonged charging/discharging at a higher than a 'normal' current has in raising the battery Plate temperature. Just a small rise in continous charge/discharge, raises the Plate temperature into the zone that can decimate AGM/Gel battery life, as noted above. See section 15 :

"15. Charge current -
The charge current should preferably not exceed 0,2 C (20 A for a 100 Ah battery) otherwise the temperature of a battery will increase by more than 10°C". 


This Battery Temperature chart below shows how exceeding a 20A charge rate can be done, but the penalty may be a drop in life from 7 - 10 years to just 2 years. The same heat is generated at the Plates from both charging and discharging so the same degeneration results.

Remember these are extra high quality, long life batteries, but even they are badly harmed by fast charging or continuous 'high' current draw. 



To keep below the 20 degree/20amp ideal would require a 100Ah battery for every 20amps that will be drawn, or what is now called the 5 times rule.


As further evidence that an Inverter needs a really big battery bank, go on the Trojan Battery web site. It has a web page dedicated to calculating the size of a battery bank based upon the expected current draw, see : http://www.batterysizingcalculator.com/#step3

We asked it what size battery bank we would need to run a 3000w Invertor for 15 minutes a day. The response depended on how long we wanted the batteries to last. If a 2 year life was acceptable then a 480Ah bank would work, or if 8 year life was required then over 1,100Ah of batteries was required. 

It proposed, that to achieve that 2 year life, we buy some ultra heavy duty INDUSTRIAL batteries, yet even this 480Ah bank of expensive, Industrial bemoths had a predicted life of just 2 years for our 3000 watt Inverter. 

At £200 + a battery for pretty much 5 of the Industrial monster batteries, is a 2 year running cost of £1,000 in batteries alone.  

We suspect that if more conventional batteries were used the bank size would need to be near the 1,500Ah quoted by Victron Energy?  



This all seems to suggest the general 'rule of thumb' 5 times the current drawn converted to AH (5 x 300amp load = 1,500Ah bank size), if it is for more than one minute, is the minimum required for optimum battery life.

 

I think you can guess what will happen if you use a 3,000watt Inverter off two 100Ah batteries? What we normally see is batteries become degraded by the drain, which then takes out the mains 240v charger followed not long after by the Alternator.


Consider also the load placed on the poor Mains Charger and Alternator, that has to put back all that power that was taken out by a Microwave or whatever. 

It is a load they were not designed to take, nor is the wiring infrastructure. See the bottom of the page for info on Alternator capability and why a 180amp Alternator should be run at no more than 100amps when the draw is continuous.


However, just as important is the 'additional' loss of battery capacity when connected to an Inverter that draws large currents. As the current drawn goes up so the available capacity in a battery goes down. 

So while you might get 95Ah out of a 95Ah battery if you discharge it at 9.5amps for 10 hours, if the current goes up to 190amps it will last just 0.3 hours, with a 95Ah battery only giving up just 57Ah.

Drawing high currents from a battery with an Inverter, can effectively result in the battery 'losing' 50% of it's capacity


See the chart below of a how a Varta LA95 battery performs dependent on the load for more info.




We think Inverter retailers should be more clear (honest?) on the real impact of their power units on a battery bank.



Inverter safety.

Some Inverters up to around £500 can be a risk to life. 

We are all so used to the mains being rock steady and reliable, that many of us take it for granted it is the way all 230v will be. The National Grid has sophisticated infrastructure in place to ensure voltages remain within strict limits. 

But that stability of voltage isn't guaranteed from an Inverter. 

While marketing blurb for most inverters will list lots of safety features for the 12v 'input' side, like under voltage protection (typically 11v) or over voltage protection (typically 15v) and short circuit protection, there is rarely anything listed for the 230v side. 

Yet over voltage protection that shuts down the Inverter if the voltage exceeds 255v would not be hard to include on even the cheaper units.


Some we have opened don't even have a fuse protecting the 230v output side. 

Many are unsuitable for connection to the circuit breakers because they are not even 'Earthed' due to the electronic solution limitations adopted by the Inverter designer. In our view, there are no adequate standards to govern Inverter design to ensure adequate safety.

By law our Home 230v electrics must have an RCD and Breakers protecting us from potential tragedy. The Residual Current Device (RCD), is a life-saving device designed to prevent a person from getting a fatal electric shock if they touch something live. 

A Motorhome 'fixed' Inverter installation should have the same protection, but rarely has. 


Some Inverter designs will result in the 230v output being '2 live feeds', not the live and neutral most expect. Put a 13amp 'Polarity tester' into the output or your Inverter and see how confused they can get.

Where the Inverter is wired through the RCD/breakers, it will tend to confuse the 'Reverse polarity' indicator.   


 


It is no secret that we think Inverters are not suited to installation in a motorhome, unless the motorhome owner understands, in great detail, the risk and issues.

  




MICROWAVES.

Note that a Microwave rated at Class E - 900watts is describing the energy it puts into the Food. To create that 900 watts the Electronics will consume a lot more power as they are not 100% efficient, in the case of my own microwave it draws 1,450watts from the mains to produce 900watts 'heating' energy.  

Some can drawn a 'Start Up' current higher than their running current for a few seconds of starting.



ALTERNATORS

Note that when recharging batteries or supplying power for Inverters from the Alternator,  the Alternator power rating should be regarded as 'peak' output only, it's continuous rating will be much lower. 

This website https://www.zena.net/htdocs/alternators/alt_inf2.shtml shows that for continuous power draw the Alternator should not be asked to supply more than 50% of it's rating for more than a few minutes :


Brief extract here -

 

However, it cannot produce this amount of power for more than a few minutes without overheating and potential damage.

The most power that such an alternator can be expected to provide continuously is about 1/2 the maximum rated output power -- maybe as much as 2/3 for a "heavy duty" design.