A (Working?) Battery Gauge.

Battery power is critical to the operation of a campervan, and knowing how much you have in reserve is very useful.

Lots of campervans are fitted with a battery gauge similar to the one on the left. As anybody who has used one will know they only give a very very rough indication of the state of your battery.

This type of meter just measures the instantaneous voltage of your battery and unfortunately this is a very bad guide to the state of charge of your battery.

If it says 10 volts and you are not using your inverter then your battery is probably flat. If it's been reading 14 volts for the last 10 hours then it's probably well charged.

It is possible to get a reasonable estimate of the state of charge of your battery with a volt meter but you need a fairly accurate meter, a table relating voltage to the state of charge for your type of battery and, most importantly, your battery has to unused for at least 4 hours. If during this 4 hours you have charged or discharged your battery then the result will be almost useless.

There are four methods of getting a better estimate of the state of charge of your battery.

  1. Discharge your battery at a known rate and see how long it takes.
  2. Measure the specific gravity of the electrolyte.
  3. Measure the current into and out of the battery and calculate.
  4. Measure the voltage continuously and use a mathematical model.

Discharging your battery to see how well charged it was is not a practical method of determining its state of charge, but it is the benchmark against which the more practical methods are tested.

Measuring the specific gravity of the electrolyte is really only a theoretical option in a campervan. It's messy, inconvenient and can not be used with sealed or "gel" batteries.

In principle accurately measuring the current going into and out of your battery over time will let you calculate its state of charge. For example if we start with the battery fully charged and discharge it at exactly 5 amps for four hours we will have removed 20AH (20 amp hours). If we now charge it at exactly 10 amps for two hours we will have restored the 20AH and it will again be fully charged.

Unfortunately it's not that simple. If you discharge a battery quickly you take a disproportionately large amount of charge out of it. (One amp for 100 hours is not equal to 100 amps for one hour, far from it.) Also batteries are not 100% efficient, if you put 100AH in you won't get 100AH out. To complicate matters further unless your current measuring is very accurate each time you go thru a charge/discharge cycle a small but possibly cumulative error will creep in. Sophisticated Amp Hour meters attempt to compensate for these problems.

To measure the current into and out of your battery requires the installation of a "shunt" usually in the negative feed to the battery. It is very important that all current to the battery goes thru this shunt. Installing a shunt can be a major operation requiring significant re-wiring. If your campervan already has a meter that displays the current being used and/or the rate at which your battery is being charged then you already have a shunt, but it is very probable that this shunt is not suitable for use with an Amp Hour meter. Installing two shunts gets very complicated.

Although a single voltage reading tells you very little about the state of charge of your battery if you know what type of battery you are using and a lot about the way this type of battery works and you monitor the voltage continuously you could infer the state of the battery. So for example if the voltage of your "gel" battery had been 14.2 volts for 10 hours you could be fairly sure it was fully charged. If the voltage then fell to 12.70 volts you could infer that it was being discharged at say 12 amps. So after an hour of the voltage falling slowly from 12.70 volts to 12.65 volts you could infer that say 12AH had been removed. If you updated your "mathematical model" of how the battery behaved each time you were able directly to measure the state of charge then your accuracy would improve over time. This is the way meters like SmartGauge work. It may look like digital voltmeter but it is actually a small computer running fairly complex firmware.

The big advantage this type of meter has is that you don't need a shunt (and hence no re-wiring) and in contrast to an AH counting meter (which potentially gets less and less accurate with time) this type of meter should get more accurate with use.

I have fitted a SmartGauge to my campervan (with a four year old 200AH Gel battery, a 30 amp mains charger and a 400 watt solar charger). After giving my SmartGauge a month to learn the behavior of my system (including a couple of discharge cycles down to about 40%) I ran a simple test.

With my battery fully charged (and SmartGauge reading 100%) I disconnected all sources of power and switched on all the lights (taking about 11 amps). I then recorded the state of charge given by SmartGauge over the next 11 hours.

This is not a sophisticated test (the discharge current was not measured accurately over the 11 hours, I do not know the current capacity of my battery accurately and I was not using an intermittent load like a refrigerator) but it is the sort of simple independent test I tried (and failed) to find before I purchased a SmartGauge. I found the result very reassuring. I have also recorded the SmartGauge readings whilst charging my battery at 30 amps with similar reassuring results. In this application SmartGauge seems to do what it says on the box, without fuss, and it is very easy to install.

Note that SmartGauge also works on 24 volts systems, and with minor restrictions on mixed 24 and 12 volt systems.

Stephen Stewart.

Home - This page last changed on 2009-02-09.