Droop and Sag - Age Old Partners

My first dabble with installing air conditioning on boats was in the mid ‘80’s. I was new to the game and was an eager gofer helping to install a system on a 44’ sailboat. When I mentioned this to my buddies at the bar, they were amazed to hear that you could actually install air conditioning on a sailboat, especially on one that small!

Fast forward a decade or three and we now have owners of much smaller boats not just wanting, but expecting to have air conditioning on their vessels. Owners of 25’ sailboats want to be able to sleep in comfort, and even those with 18’ walk-around powerboats with outboards are asking how they can get a blast of cold air on their faces as they zoom around having fun. Well, it can be done and it is being done, but there is some debate about how best to power the thing.

We offer the Climma 4,200 Btu air conditioner for just those applications, as well as for individual small cabins on larger boats. This a 110v mains-powered unit, so when the vessel is plugged in to shore power or a small generator it can run, but it can also be powered through a small inverter from a DC source such as a battery bank or an engine alternator.

The trick here is to install the inverter as close as possible to the battery to reduce the size and length of cables required. With DC power cables, a long run with small gauge cable and a high current draw will result in significant losses which manifest as reduced voltage at the business end. By contrast, there are negligible losses when running AC current through cables, so the 110v AC cable from the inverter to the unit need only be small gauge and of unrestricted length.

At recent shows we saw a resurgence of interest in small air conditioners powered directly by 12v or 24v DC only. There are one or two around, but they use highly specialized and complicated compressors and other components that results in a hefty price tag; a 5,000 Btu DC unit costs three to four times the price of the Climma 4,200 Btu unit. But that’s just for the unit. Now you’ll have to run big, heavy, and expensive battery cables in order to reduce the resultant drop in voltage to an acceptable level.

For example, let’s say that you have a 12v (nominal) DC air conditioner that consumes 40 amps, which is not unusual. And let’s say that you have to run the two (positive and negative) cables for 15’ each. The tables we use to determine wire sizing say that for an expected volt-drop of 3% we must use 4 AWG cable, and that’s around $100 worth of cable excluding terminals, etc. Now, unless you have super-dooper lithium batteries, the voltage at the battery will droop considerably as soon as the 40 amp load is applied.
This phenomenon is known as “voltage sag”, and the amount of sag will depend on the size of the battery and the load. On a good 200 Amp/Hr battery, the voltage at the battery itself can easily sag almost one volt initially, so although the battery voltage may be 12.8v with no load applied, it could then sag to around 12.0v when the air conditioner is turned on. But keep in mind that we are expecting a 3% volt drop, and so that reduces it further to 11.6v at the air conditioner. So that 12.8v in our fully charged battery is then only 11.6v at the 12v DC air conditioner 15’ away the moment we turn it on.

Now, let’s say that that the specifications of the 12v DC air conditioner state that the 40 amp current draw was originally measured with a voltage of 12.5v. As power consumption in watts is the result of volts x amps (W = V x A), we have to take the manufacturer at their word and say that the air conditioner consumes 500 watts. But in our example we don’t have 12.5v, we have only 11.6v at the air conditioner, so the current consumption must increase to 43 amps to compensate for the 500 watt power draw. And as the battery becomes depleted and the voltage decreases further to 11 volts at the air conditioner, the current draw must increase to 45 watts.

Now stay with me for this next bit; with higher current and lower voltage, the volt-drop will increase, and increased volt-drop leads to lower voltage which leads to increased current draw which leads to lower voltage which … well, you get the picture. It’s a downward death spiral that accelerates as the battery becomes depleted, and only stops when the low voltage limit is reached. This low limit is commonly set at 10.5v, but due to the cable losses described above, the electronics in the 12v DC air conditioner will be seeing that 10.5v sooner and stopping the system much earlier than the inverter will.

What goes down must come up, so said Sir Newton Isaac, the patron saint of bungee jumpers, and when the compressor shuts down and the bulk of the load is removed, the battery voltage will bounce back up again and recover most of the voltage that was lost on power-up due to voltage sag. However, due to the higher cable losses, the bounce will be higher with the 12v DC air conditioner, and so less of the available battery capacity will have been used than in the 110v/inverter set-up before the low voltage cut-out activated.

The clever ones among you, so much cleverer than me, will no doubt be twiddling around with your slide-rules right now (no batteries required) and squawking on about Peukert’s law and such. But I’m simple, and I’ve tried to keep this simple so that even I can understand it.

When powering air conditioners from a battery, voltage sag is unavoidable whether you use a dedicated 12v DC air conditioner or a mains powered unit with an inverter. But in the mains powered option, by mounting the inverter as close as possible to the battery and using very short cables, the volt-drop menace can be all but eliminated. That, together with far lower cost, easier installation, better performance, and mass-produced components makes the Climma 4,200 Btu 110v unit running through an inverter the easy winner when considering a small battery-powered air conditioning system.

Droop, sag, bounce, recover. Sounds like a four-day Thanksgiving weekend. Gobble on!