Running a big inverter from a small battery just doesn’t work – here’s why

If you’re planning a van conversion and you want to run some beefy 120 volt appliances, like a microwave or (heaven forbid) a 120v air conditioner without a shore connection, be prepared to spend a lot of money on your batteries as well as on your inverter.

What size and type of inverter should you get?

Even a 600W rated microwave draws around 1000W of power. A 120V AC air conditioning unit may be rated at 2Kw running power, but it takes a massive initial kick to get it working, often more in the 4Kw range. There are only so many devices you can run without plugging in to a generator or a shore power outlet.

Once you’ve worked out your 120v requirements, and how many of these devices you’ll use at the same time, you’re ready to choose an inverter.

The larger devices – especially those with motors in – will probably want to be fed with pure sine wave power, and inverters that provide it cost more than those that only produce square wave output. We have a 2KVA (around 1700 Watt) pure sine wave inverter. There are also 3KVA models and larger on the market.

If you have the cash, there is always the temptation to just go with the largest pure sine wave inverter you can afford, and call it good. But there are issues with that approach.

There’s a nice article on the SolarPanelTalk forum suggesting nobody should use an inverter larger than 1000 Watts. It was written by somebody who obviously truly cares about your safety. It’s worth a read because it’s quite sobering, but remember it was written in 2010. That was before you could buy quality welding cable at a reasonable price, hydraulic cable crimpers for under $50 and lithium battery systems for less than $500 per kilowatt-hour.

Why do I mention welding cable, crimpers and lithium batteries? Let’s take each in turn.

How fat do your cables need to be?

Like the SolarPanelTalk posting mentions, pulling a high current at a low voltage means you have to battle with cable resistance, which basically means battling heat. A 3kW load at 12 Volts is 250 Amps. To avoid the resistance this could create, you have to use really thick cables.

Let’s say you planned well and put your 3kW inverter just four feet away from your battery, for a total 8 foot cable length when you account for the positive and negative leg of the journey. If you use a cable size calculator, you’ll notice that to minimize voltage loss to 1.5% (0.2V) you’ll need 4/0 AWG cable. That’s four OVER zero gauge (“four ought”), not four gauge. Eight feet isn’t very much. Even if the battery and the inverter are mounted in the same location, you have to get cable from one to the other and that often means bends and loops.

Every other component in the system – your buss bars, your shunt, your battery fuses and disconnect, and so on – must also be able to handle at least the 250 Amp load, and preferably one much higher than this to account for peak current spikes.

How will you attach your cable to your components?

Let’s say you are using 4/0 AWG cable. The next issue is how to connect it to your battery and your inverter. Typically you need to attach lugs to the cable ends. Those lugs must be crimped on to the cables.

For lugs that big to be that well compressed, you’ll need a hydraulic crimper. Back when the SolarPanelTalk article was written, those things were just not available to the general public. Now, you can buy one that’s sufficient for DIY use on Amazon for less than $50 (the price fluctuates – look around on the site if it’s higher than $50 when you click the link).

Cable crimper

Of course, you can still completely mess up the crimp connection even with a hydraulic crimper, but at least you have the opportunity to succeed!

How much battery do you need to support that inverter?

The other consideration is how much battery you need in order to run your inverter. We’ve already discussed how your lead acid battery bank is only half usable, and how the Peukert effect means that a heavy load draw will reduce its usable capacity still further. When you suck power from a lead-acid battery too quickly, its internal resistance builds up. Again, resistance = heat. Hot acid is bad.

Let’s do some math

The safe discharge rate for good quality lead-acid batteries is between C/8 and C/4 (one eighth and one quarter of total capacity). So for a 100 Amp-hour battery discharged at C/4, that’s 25 Amps current. The Peukert effect means the battery won’t last as long as its rated (C/20) discharge rate, and you can only safely discharge to 50%, so you’re probably looking at 90 minutes of power at this 25 Amp rate before the battery needs to be recharged.

Your 3kW inverter is drawing 250 Amps. That’s ten 100 Amp-hour batteries, just to give you 90 minutes of power.

Of course, you’d probably be using larger capacity batteries in reality. Let’s go to the largest you can usually find – 8D size, 255 Amp-hours. A Lifeline GPL-8DL battery will give you 550 minutes (9 hours) of discharge at 25 amps, based on their statistics. That’s the highest discharge rate they provide, and it doesn’t say to what level of discharge. I’m guessing 100%.

So to run your air conditioning overnight, you’d need ten 8D batteries for 2550 Amp-hours, at a weight of 1620 pounds and a cost of $6,600 (although you might get a discount for buying that many at once!).

Even if you discharge the 8D batteries at C/4, you’re only getting 64 Amps from each battery. To reach 250 Amps we’d need four 8D batteries. If we’re happy discharging them to 50%, then when we calculate the Peukert effect and take that into account, we’d end up with one and a half hours of usable inverter time.  Living life on the edge with an 80% discharge gives us just over two hours. Four 8D batteries for a maximum of 2 hours use.

Let’s get real again

You obviously won’t be installing half your payload and taking up 11 cubic feet with ten 8D batteries. So, if you’re using lead-acid batteries to power your inverter, you will be pretty much guaranteed to over-work the batteries.

The answer lies in lithium batteries. They are smaller, lighter, can discharge at C/2 or even C/1, can be discharged to 20% rather than just 50% of their total capacity, have at least twice the lifespan, can be stored in any state of charge, charge faster, and most importantly for this discussion they don’t suffer in the same way from the Peukert effect. That means you can use a smaller capacity battery because it doesn’t have a capacity down-rating at high discharge rates. Lithium batteries’ only downside is that they cost more up front than lead-acid batteries do.

How are you going to recharge?

That massive battery bank you need will have to be kept topped off. We have nearly 800 Watts of solar panels. Other people use second alternators attached to their engine. You will NOT be able to use the stock alternator or a relay attached to the vehicle starter battery, because the current draw to recharge the large house battery is just too much for those systems.

So what does this all mean?

Sure, you *can* connect your 2000W inverter to your 150 Amp-hour lead acid battery, but don’t expect it to power all of your 120v devices, and don’t expect your battery to last very long. The battery will run down within half an hour, will be worked very hard and will most likely start to sulphate pretty fast. Soon, it won’t even be able to hold a regular charge. You could also damage your cables and potentially create a fire risk.

There are two real options available to you. One is to choose an inverter that is more sensibly sized for your battery capacity, and then use propane rather than an induction cooktop, microwave, and electric water heater. The other option is to get a battery (and all the other stuff that goes along with it) that can handle your inverter load. To do that sensibly with a 2kW or 3kW inverter, that’s probably going to mean at least 400 Amp hours of lithium batteries and some way of recharging them on-the-go like a large solar array or second alternator.

6 Replies to “Running a big inverter from a small battery just doesn’t work – here’s why”

  1. I just had this conversation with a solar retailer regarding my plans for a van conversion. I had no idea regarding this effect of drawing power to quickly from batteries.

    Your article explains this quite clearly. thank you for posting.

    Wes

  2. DF, question in regards to inverter size. Originally I purchased a 2000w p sine xantrex. Come to find that I cannot charge with shore/generator power because it does not have a built in charger. Now I’m considering another xantrex with built in charger rated at 1800w. I have 240 watts of solar. Although based on your explanations it seems like the 1800w is too robust, I only plan to draw large AC loads for short periods of time (i.e. blendtec blender (1500w 1-2 minutes daily / 600w micro – 5-10 minutes daily). Does this seem acceptable?

    1. Ryan, the short answer is, it’ll work if you have a reasonable size battery but it will put a big stress on the battery.

      You don’t say what size your battery is, and that’s the limiting factor on how large your inverter can safely be. While it’s true that shorter run times will allow you to use a smaller battery, there’s still only so much juice you can safely pull from a lead acid battery. If you’re pulling power out too fast, the battery won’t be able to provide its rated number of Amp-hours. Instead, it’ll run down much faster. The smaller the battery, the larger the impact of the Peukert effect.

      A 600W microwave is probably going to draw at least 1000W from the inverter, if not more. The 600W is output power, not input power. Quick calculation: 1200W at 120v is 10A. On the 12v side of your inverter, with inefficiencies taken into account, that’s more like 100A. So if you had a 200Ah lead-acid battery, that’s theoretically 1 hour of use to run it down to 50%. But because you’re pulling the power out so fast, you don’t get that hour. You might get closer to 30 minutes. If you were just using the battery for your smoothies, you’d probably be OK. If it’s trying to power other things too, you won’t be so OK. Pulling that type of power out that fast will also reduce the battery’s useful lifespan.

      Of course, folks do it all the time. And they replace their batteries regularly too. Good deep cycle batteries aren’t cheap, so it’s a bit of a gamble.

Leave a Reply