A rational, fair, comparison between SmartGauge and Amp Hours counters

This document attempts to fairly, and honestly, compare the installation and bare performance of SmartGauge with that of the more conventional Amp Hours counter when both being used for the purpose of monitoring the state of charge of deep cycle lead acid batteries.

Note that this document relates only to the SmartGauge installed alone. It does not relate to SmartGauge installed in conjunction with the SmartBank split charge system.

SmartGauge Operation

SmartGauge uses "computer models" of different types of lead acid, deep cycle, batteries. A "computer model" is a set of numerical values which attempt to represent the various parameters of a real world item. In this case a deep cycle, lead acid, battery. This model is then used by an "algorithm" in SmartGauge to calculate the state of charge. An algorithm is basically nothing more than a series of calculations, but rather than just performing a fixed calculation on a fixed set of figures, the algorithm continually calculates results, and some of these results are fed back into future calculations giving an ever changing, and self correcting, result.

This is a vast over-simplification but it does serve the purpose of explaining the basic operation of SmartGauge. For instance, the "models" used in SmartGauge contain 408 different numerical values, representing the various different parameters of the battery types. The "algorithm" performs in the region of 1.2 million calculations every minute in order display the state of charge.

There are certain measurements SmartGauge can make (and this is what makes SmartGauge so unique and accurate), but only at certain times in the charge/discharge cycle, where it is possible to actually measure the state of charge of the battery rather than relying upon the calculated state of charge. At these times, SmartGauge compares the results of the measured state of charge with the results of the calculated state of charge and uses this comparison to automatically adjust both the battery "model" and the "algorithm" so that future calculations will become more accurate. This is why it is so important to keep SmartGauge permanently connected to one battery bank. In addition, there is an advanced setup menu (totally hidden from the user), which enables a totally new battery model to be programmed into SmartGauge or an existing model to be "tweaked" for special purposes or new batteries that may be introduced in the future.

SmartGauge Programming

The only parameter to be set by the user in SmartGauge for the purpose of battery state of charge monitoring is the battery type.

Incorrect setting of this parameter will result in an erroneous state of charge display (by up to 25%) that remains at the same level of error i.e. it will not get worse with time.

SmartGauge Installation

For monitoring a single battery bank state of charge, SmartGauge requires 2 light duty cables to the battery posts.

Incorrect installation (i.e. reverse polarity) will simply prevent SmartGauge operating. It will not damage it. Correction of the reverse polarity will see SmartGauge fully functional.

Amp Hours Counters Operation

Amp hours counters operate in a completely different manner. Amp hours counters measure the charge current and the discharge current, integrate these over time (effectively adding up a running total) then subtract one from the other and, hopefully, the result will be the state of charge. This, again, is a vast over-simplification so a more detailed explanation is really required. An explanation by way of example is probably the best way to do so.

Assume the battery to be fully charged. The amp hours counter is set to zero by the user. A load of exactly 10 amps is now placed on the batteries. The amp meter measures -10A. After 6 minutes, the amp hours counter will have accumulated (integrated or "added up") -1Ahr, after 12 minutes it will have accumulated -2Ahrs, after 18 minutes it will have accumulated -3Ahrs and so on. After 1 hour it will have accumulated -10Ahrs (10 amps for one hour = 10Amp hours [Ahrs]).

Now when a charger is switched on it performs exactly the the same thing in reverse. Assume a highly accurate 10 amp charger. The amp meter will measure +10A. Remember we are starting from an accumulated figure of -10Ahrs so after 6 minutes the accumulated figure will be -9Ahrs, after 12 minutes it will be -8Ahrs and so on until after 1 hour the accumulated figure will be 0Ahrs, back where we started.

Well, that's the theory anyway. Several problems almost instantly rear their heads.

1. Firstly, there is the problem of a certain Mr Peukert (see Peukert's Equation). Peukert's equation quantifies how heavier discharges actually remove more power from the batteries than a simple "Amps X Time" calculation would show. What this means for the amp hours counter is that, assuming the example given above worked, then doubling the discharge current would give erroneous results.

For instance, discharging at 20 amps for one hour from a 100Ahr battery, the amp hours counter would have accumulated -20Ahrs. However, according to Peukert's equation the true figure would be much higher. Around 30 amps with a typical Peukert's exponent for a deep cycle battery. So now, after one hour of dishcharge at 20 amps, the amp hours counter reads -20Ahrs but in actual fact the battery is at -30Ahrs. When the charger is switched on (the same 10 amp charger as before), after 2 hours the amp hour counter will have accumulated 20 amps total charge so will be reading 0Ahrs, indicating that the batteries are full. In actual fact the batteries are at minus 10Ahrs.

2. Assume the amp hours counter is set up for a 400 amp hour battery bank. The 50% rule tells us we should be discharging to -200Ahrs on average, then recharging. (see the 50% rule). Regularly discharging below this level unacceptably shortens the life of the batteries and reduces their capacity. Just one or two discharge and recharge cycles below this level can reduce the capacity of a battery bank by 1%. So after 5 discharge and recharge cycles, this 400Ahrs capacity battery bank could actually be a 395Ahr battery bank. Discharging to -200Ahrs is now below a 50% discharge. So this has more and more effect on reducing the capacity of the battery bank each time it is cycled.

After a few months the owner of these batteries could actually be discharging to -200Ahrs, thinking they are doing the right thing by discharging to 50%, whereas in fact the battery bank is now 300Ahrs capacity and the owner, unwittingly, is discharging this to 33% thus greatly accelerating the death of the batteries.

3. A 1% error in readings is quite an acceptable figure for an ammeter (in fact it is much better than the specification of most amp hours counters - don't confuse resolution and accuracy). Let's assume the ammeter part of the amp hours counter is in error by minus 1%. In practice this means that during a discharge at 10 amps the meter will actually be measuring -10.1 amps. During charge at 10 amps the meter will be reading +9.9 amps. After a total time of 20 amps (average) discharge for 100 hours and 20 amps (average) charge for 100 hours (a typical weeks cruising on a 30 foot yacht or driving a medium sized RV) the amp hours counter will have accumulated a total discharge of 2000 amp hours and with the 1% error this will actually be read as 2020 amp hours. The total charge will have accumulated 2000Ahrs charge, which with the 1% error will be read as 1980 amp hours. The amp hours counter will be reading -40Ahrs even though the batteries are fully charged. Continue this for another three weeks (a one month holiday) and the amp hours counter is reading -160Ahrs indicating that the batteries are nearly discharged down to 50% and in need of recharging. In actual fact the batteries are fully charged.

Take this error the other way round and the effect becomes the opposite. i.e. The amp hours counter tells you the batteries are at zero Ahrs (fully charged) when in actual fact they are at minus 160hrs (nearly half discharged).

4. Batteries are not 100% efficient. A typical figure for the efficiency (known as charge efficiency) of a deep cycle, wet cell battery might be quoted as being around 90%.This means that for every 9 amps taken out of the battery, 10 amps have to be put back in to get to the same level of charge. Some amp hours counters have a user adjustable setting for this. The problem is that if it is set incorrectly, then clearly it will not work. Even if it is set correctly, the charge efficiency falls as the battery ages so it won't be long before this figure is, once again, wrong. Further, the charge efficiency changes depending upon the rate of charge and discharge and there is actually no way to calculate this effect. If it was measured, the result would actually be different for each and every discharge/recharge cycle (see the Charge Efficiency anomoly).

Some amp hours counters attempt to calculate their own figure for charge efficiency and show this to the user as a factor on how well the batteries are performing. They also attempt to use this figure to predict how many amp hours will have to be returned to the battery, following discharge, to return to a fully charged state. This wouldn't work, even if it was possible to track the state of charge of a battery by amp hours, however, as we have shown here, it isn't possible to do so, and therefore this calculated figure will be even more incorrect than it would be to start with.

For a more detailed explanation of charge efficiency take a look at the Charge Efficiency anomoly.

5. Amp hours counters require the user to input the battery bank size. This is all well and good with a new battery bank. Each battery states, for example, "110 Ahrs", four are installed so the battery bank is 440 Ahrs. Well it is when it's new. After 6 months it could be substantially less than this. After 12 months of very heavy use it could be down to around 200 Ahrs. Yet some amp hours counters continue to attempt to show the state of charge as a percentage of available capacity. They do this simply by dividing remaining amp hours by the declared battery bank size and multiplying by 100. So in this example, once 220 amp hours had been discharged the meter would show 50% charge status. This is, of course, correct when the batteries are new. After 12 months, when they are down to a total capacity of 300 Ahrs the meter will still show 50% at -220Ahrs. In actual fact -220Ahrs from a 300 Ahr battery bank is a state of charge of 26%. This could easily give the user a nasty shock. Not to mention the fact that the batteries are being severely overdischarged.

6. Batteries have a certain internal self discharge. That is to say, a fully charged battery, with nothing connected to it, will still run down in charge status. It will eventually go flat. The self drain of deep cycle batteries varies from type to type with wet cell, antimony/antimony deep cycle being about the worst at around 1% per day, the best being AGM at around 0.5% per week. These figures vary widely between manufacturers and increase as the battery ages. Amp hours counters simply cannot detect this self drain. A battery left with nothing connected (except the amp hours counter) for 6 months would still show the battery to be fully charged. In actual fact it would probably be very deeply discharged. SmartGauge would have detected this self drain.

These are the main problems with amp hours counters (there are many more). In effect they are useless for monitoring the state of charge of deep cycle batteries. We may have given the impression we do not like them and see no use for them. This could not be further from the truth. The engineer who designed the SmartGauge uses one on his own boat. But for completely different purposes.

The amp hours counter is handy in that it shows the actual current draw of a certain piece of equipment. This is something the SmartGauge cannot do.

A fantastic use for an amp hours counter is to get an idea of how much power certain equipment uses. For instance a light bulb is easy, just switch it on, measure the current, and that's it. Something that cycles on and off or that has varying power levels is not so easy. A fridge, for example, would use nothing when the thermostat has switched it off, but could be quite high when running. Without sitting there all day, monitoring when it switches on and off, there is no way of knowing how much power it uses. One could be left running for say 24 hours with an amp hours counter monitoring it's current draw. After 24 hours, the total amp hours consumed, divided by 24, gives an average current draw for that equipment in amps. Again this is something SmartGauge cannot do.

Amp hours counters are excellent for getting an idea of the performance one is getting from auxiliary charge sources such as solar panels or wind turbines. An amp hours counter could be set up on a wind turbine then checked after say one week. The total charge provided by the wind turbine will have been accumulated on the amp hours counter. The wind turbine could then be moved to a different position and tried for another week. The same idea applies to solar panels. Without averaging their performance over a long time, it is almost impossible to monitor their performance. This is one of the best uses for an amp hours counter and in all honesty, there is probably no other way of doing it. Again this is something that SmartGauge cannot do.

So, whilst we can praise amp hours counters for certain functions, the simple fact is, they are all but useless for the puspose of monitoring the state of charge of deep cycle batteries. They simply do not do what is required.

Most modern amp hours counters (not the basic ones and not the old ones) now incorporate full Peukert calculations in an attempt to compensate for the enormous errors this effect can introduce into the accumulated amp hours figuers. As an example, discharging a 400Ahr battery bank at 200 amps for 1 hour would put the amp hours reading out by roughly 200 amps without correction for Peukert's effect - that is half of the total battery capacity, an absolutely enormous error. Doing this would actually, almost, completely flatten the battery bank, but the amp hours counter would only have registered -200Ahrs indicating that it was still at 50% charge state.

The problem with compensation for Peukert's effect is that there is no way to get it to work reliably. The first drawback is that Peukert's exponent has to be entered manually. There is no way to measure it when relying on amps measurements. So it has to be entered manually from the data sheet for the batteries. Actually getting hold of this information can be difficult. If it cannot be found then it either has to be guessed at (rather pointless) or measured by discharging the battery several times at various discharge rates then calculating Peukert's exponent from the results.

The second problem is that Peukert's exponent changes quite drastically during the life of the batteries. So what worked reasonably well in the first month of use, will run further and further adrift as time goes on.

Finally, adding (an attempted) correction for Peukert's effect into the amp hours counter now means that the amp hours counter no longer reads correctly when being used as an "accumulated amps register" for monitoring equipment current draw or wind turbine/solar panel performance. At discharges around the 20 hour rate (i.e. 5 amps on a 100 Ah battery) the error won't be too bad. But a piece of equipment that draws heavier currents will accumulate enormous errors. For example, an item that draws 80 amps from a 400 amp battery bank should, after 2 hours, have accumulated 160Ahrs. With a typical Peukert's exponent, it would have actually measured around 240 amp hours. That is a 66% error.

Admittedly, the user could manually set Peukert's exponent to 1.0 so as to remove it's effect, but then the amp hours counter would be back to it's enormously erroneous readings as far as state of charge of the batteries is concerned.

As shown here, amp hours counters do not work as state of charge meters (even with correction for Peukert's effect there are too many other errors that accumulate with time), so actually setting Peukert's exponent to 1.0 and using the equipment to monitor current draw makes far more sense. In fact, that is the proper use for an amp hours counter. To monitor power usage and power return. Not to monitor the state of charge of the batteries.

Amp Hours Counters Programming

Amp hours counters vary between manufacturers in the parameters that need to be set by the user. Typically the parameters to be programmed would be:-

Battery bank size
Charged voltage
Charged current percentage
Peukert's exponent
Battery type
Charge efficiency
Battery temperature (for those that do not incorporate a temperature sensor)

Incorrect setting of any of these will introduce enormous errors that accumulate and get worse with time.

Also consider that 5 of the above parameters change as the battery ages and, unless these are corrected by the user, further enormous errors will be introduced.

Amp Hours Counters Installation

For monitoring a single battery bank state of charge a typical amp hours counter will require:-

Breaking into the heavy duty main battery cable (sometimes +ve, sometimes -ve, sometimes either) to install the shunt.
Ensuring that all other connections to the batteries are removed from the battery terminals and connected to the load side of the shunt.
Typically 5 small cables to a combination of the battery posts and shunt.

Incorrect installation can result in wildy inaccurate readings, accumulated errors getting worse with time or damage to the meter.


I hope you have been convinced of three things.

1. Amp hours counters are incredibly useful pieces of equipment for monitoring power usage and power return (charging).
2. However, they are useless for the purpose of monitoring the state of charge of batteries and a SmartGauge does a far better job, more reliably and is simpler to understand.
3. The simplicity of installation of SmartGauge.

See also the explanation of the difference between amp hours remaining and energy remaining.

See also why amp hours counters are really guessing during the charge cycle.


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Page last updated 02/04/2008.
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