12V Guru – What Size Inverter – Part 2

Question to 12V Guru: What size Inverter would I need? – part 2.

Welcome back to my last month’s response to Mitch on inverters.  You may recall my response was to help provide some technical background to the right setup to ensure you could make that perfect cup of coffee from the coffee machine to start of each day.  Having covered issues such as sizing your battery bank, pure sine wave and battery style our follow-up this month will help you with more slightly technical content.

Inverters and battery chemistry

After deciding what size battery bank capacity and matching to the right size inverter, you will need to consider whether your battery bank should be multiple batteries or a single battery and which chemistry to choose – lead acid or Lithium. This is where AGM (lead acid) batteries can show their strengths, for three reasons.

  1. Firstly, an AGM battery is not controlled by internal electronics called a Power Management System (BMS). Whilst the BMS certainly has its advantages in protection mechanisms and internal cell management, as part of these features, it limits that amount of continuous or peak output current.  For example, a coffee machine that might need a current of 120Amps(A) will not operate at all with a Lithium battery which has its BMS preset for maximum of 100A. However, as an AGM lead acid battery has no such limitations you can power up the coffee machine to kick start your day.
  2. Advantage number two for AGM batteries is that internal chemical resistance is higher than that of a Lithium battery.  When you start to parallel up and add cables, fuses, connectors etc to Lithium batteries you increase the resistance making it more sensitive to requirements of the BMS.
  3. And lastly, scoring strongly for AGM batteries is that fact that if you compared a parallel system of 100Ahr AGM batteries versus a Lithium configuration, if one of the batteries in either setup became faulty the lead acid would still continue to provide a higher amperage output whereas the lithium would be reduced in maximum output.

Will an inverter damage my battery?

A question regularly asked is “Will an inverter damage my battery?”.  Not if you responsibly manage your batteries.  Unless you can set through your system a minimum threshold voltage (Low Voltage Disconnect – LVD) you will need to manage how much you use your inverter to draw from the batteries so that

  • you do not regularly discharge at too higher amperage and
  • you don’t discharge them down to an unrecoverable level. 

Most inverters will have a low voltage disconnection setting so it is important to understand this and how it matches to your battery setup.

Inverters and safety

Inverters that are used for a period will heat up as they generate AC power.  This heat is effectively the losses in converting energy from DC to AC and are lost as heat.  More efficient (and probably more expensive inverters) will generate less heat and convert the power more efficiently.  The inverter should never get so hot that it causes burns to skin.  There are strict regulations regarding electrical devices ensuring that they operate safety at 60C or less. Ensure you provide inverters with adequate airflow to allow the dissipation of heat and pay attention to mounting instructions as the inverter will have been designed with air flow in mind to cool if required.

Dodgy installs are dangerous!

Recently we have seen a couple of “homemade” set ups where RVers have used 2-3 batteries to create enough power to run a larger sized inverter to power their air conditioning system.  When we see 240V extension cords coming from the inverter under the bed, looped through the RV and plugged into the outside AC input of the van it rings alarm bells. This super inefficient as whilst it is powering the AC it is also recharging batteries with its own power.  More alarming though is the lack of safe earthing that this setup creates as it bypasses the effectiveness of the RCD fuse legally required as there is no earth.  AC appliances which fail may not trip the safety switch installed creating a potentially lethal hazard. If unsure, do not get creative! Always check with a qualified electrician or DC power expert.

Safe travels and enjoy that morning special coffee (safely)!

The article was published in the Caravan World Magazine.

Read the first part of the Inverter story here.

Understand the theory
DIY install? if unsure check with the dc power experts

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