Arranged, as a simple question and answer section.

At the present time there is no measurement one can make on a battery to determine it's state of charge when the battery is being used other than an SG (Specific Gravity) reading. Even that method is not foolproof requiring the full battery data, the condition of the battery must also be known and it requires a certain amount of human interpretation. Also, the battery has to be shaken vigorously in order to fully mix up the electrolyte

otherwise very misleading readings will result. Further, many batteries are now totally sealed meaning an SG reading simply cannot be taken.

There are ways of measuring the state of charge but they involve discharging the battery. Hardly what is needed. That just tells you how much power was in the battery before you discharged it in order to measure it. It still doesn't tell you how much power is in the battery now.

Battery voltage alone will NOT tell you the state of charge of a battery. Period. There is no getting away from this fact.

An SG reading can help determine the condition of the battery but not it's state of charge unless the actual condition of the battery is known. And the only way to determine the condition of the battery is to charge the battery up fully (i.e. until it accepts no further charge current), then take an SG reading. This will tell you the condition of the battery. But you already know the charge status, it's 100%, because you had to put the battery in this state of charge in order to determine the condition of the battery in the first place.

By way of clarification: An SG reading of 1.250 could mean the battery is fully charged and getting old and tired. This is clearly 100% charged state as that battery cannot be charged any more. The same SG reading could also mean the battery is at 60% charge state and is in perfect condition. If we take the SG even lower things get worse. An SG reading of 1.20 could mean the battery is 100% charged and getting very old and tired. It could also mean the battery is brand new and at about 25% charge state. So with the battery in any condition between useless and perfect, a certain SG reading could mean any charge state from flat to fully charged. Unless the condition is known, SG will not tell you the state of charge. And unless the state of charge is known, an SG reading will not tell you the condition. Catch 22.

An SG reading will NOT tell you the state of charge of the battery.

This leaves us with the unfortunate situation that the only way to determine the state of charge of a battery that is in dynamic usage is to measure certain parameters and then calculate the state of charge from those measurements. This is what amp hours counters do. This is what SmartGauge does. They do it in completely different ways, but they both calculate the final figure. This is why SmartGauge, and amp hours counters, MUST be permanently connected to ONE battery and cannot be switched between them. Typically these meters take measurements every second and perform many tens of thousands of calculations on these measurements every second. SmartGauge does exactly this. Disconnecting SmartGauge from the battery for just, say, 10 minutes means it will have missed in the region of 12 million calculations.

Remember, other than an SG reading (and knowing the exact condition of the battery), there is no measurement that can be made on a battery under dynamic usage conditions that will show the state of charge. And without knowing the state of charge, there is no measurement one can make to determine the condition of the battery. There simply isn't one.

When is a battery "flat"?

This isn't such an obviously stupid question as it may at first appear. The easiest way to determine this fact is to ask 10 different people. You'll probably get 10 different answers.

A common answer is "when the voltage is 10.5 volts". Without actually specifying whether that is on load or off load it's a rather meaningless thing to say. A battery discharged so that it's voltage is 10.5 volts with no load connected is extremely (in fact probably dangerously) discharged. It is unlikely it will ever recover fully from such a deep heavy discharge. A battery that has a terminal voltage of 10.5 volts with a very heavy load on it could in fact still be 95% charged. So clearly, the voltage measurement cannot be used.

Another answer might be "when the battery will no longer power it's intended load". This is a very common answer and totally incorrect. It may not be able to power it's intended load simply because the battery is too small. For instance a fully charged 10 amp hour battery will not start a 10 litre diesel engine. It doesn't mean the battery is flat. And "too small" doesn't mean a battery that was too small was bought for the intended application. It could be that the battery was correctly sized inititally, but due to ageing it has lost capacity and is now too small (even though it wasn't too small initially). Due to the fact that the battery once successfully powered the load, but now it will not, this can give the false impression that the battery is flat, when it actual fact it is fully charged. It is simply that the battery is now too small due to reduced capacity as a result of old age.

So it's actually not such a simple question. SmartGauge considers flat to be when it's own internal calculations show that an SG reading would be what the manufacturer states as being the flat SG reading. Each manufacturers flat SG readings vary slightly for each battery type so SmartGauge takes an average of several manufacturers flat SG readings for each battery type and uses that.

Does "flat" mean there will be no power left in the battery?

Not at all. Even when SmartGauge first reaches 0% charge status (or when an SG reading showed the battery to be flat by the manufacturer's standards) , it would still be possible to extract some more power from the battery. This would be possible right down to the point of totally destroying the battery by flattening it to zero volts. Totally flat is the level at which you are about to cause immediate permanent damage to the battery. Acceptably flat is the level at which the battery will recover to at least a usable state. SmartGauge works on the "acceptably flat" principle. 0% according to SmartGauge does not mean "no power left whatsoever", it means "no safe power left". Discharging below this level will, in all probability, cause immediate, permanent damage to the battery. Probably by reverse charging the weakest cell in the battery. This is almost impossible to recover from.

What is a safe discharge level?

Again a difficult question. There is no safe discharge level. Batteries are consumables. Every battery has a finite life. Each discharge and recharge cycle uses up some of it's life. The deeper it is discharged, the more it shortens the battery's life. An accepted best compromise is to discharge to 50%. This is known as the 50% rule. The reasons for it are quite involved but it is a well accepted and well calculated figure. It works. Stick to it. Occasionally discharging deeper than this is fine. Just don't over do it. The usual trick of running lights until they go orange, then recharging the batteries is one of the main causes of premature battery death. The other main cause is not recharging fully.

Ok, so when is a battery fully charged?

This is actually even more complicated than the question of "when is a battery flat?"

The problem is as follows. Let's take the example of a wet cell deep cycle battery as this type shows this characteristic more than the others.

In this example we will assume the battery has already gone through it's first 50 or so discharge and recharge cycles (lead acid batteries do not reach their full capacity until they have been cycled a few tens of times)

Start with the battery fully charged, let's just take it as read that the battery is at 100% charge state. Now discharge the battery to 50%, then recharge. No matter how long you charge that battery for, no matter what voltage you charge it at, it will never reach the same 100% charge state it was previously at. It will be slightly lower. This is perfectly normal and part of the life cycle of lead acid batteries. They wear out. It is that simple.

So this leaves us with the question of do we now say the battery is at 99.7% (or whatever) charge state? Or do we say the battery is again at 100% charge state because that is now the highest charge state it will ever be at? SmartGauge takes the latter view. That is, SmartGauge will now show that battery to be 100% charged. SmartGauge therefore shows you how much power you have left as a percentage of your currently available battery capacity. Other meters show you how much power you may have left as a percentage of the available battery capacity when the batteries were new. Clearly the two could be completely different. Clearly the two will be completely different once the batteries have been cycled a few times.

The phenomenon of reduced capacity (no matter how small) with each discharge and recharge cycle is just one of the many effects that fools amphours counters into giving the wrong results.

SmartGauge recalibrates with each and every cycle and automatically compensates for the reduced capacity due to cycling. This is one of the reasons that SmartGauge cannot run out of synchronisation with the batteries.

If an equalisation charge is carried out (basically a controlled overcharge at a higher voltage) this will increase the overall capacity of the bank by removing some of the hardened sulphate and converting it back into useful electrolyte. If this is done, SmartGauge will sense this and automatically correct it's battery model to account for the new increased capacity. It can make a very good estimate of the new increased capacity by the length and type of equalisation carried out and will then continue in its normal self-correction mode during the next few discharge and recharge cycles thus resynchronising itself with the batteries' health as well as their state of charge.

Note that an equalisation cycle will usually trigger an E 03 error if these have not been disabled. Do not be concerned about this. SmartGauge continues to track correctly during an equalisation cycle and during an E 03 error.

This problem of reduced capacity with each cycle is reduced drastically with the use of constant voltage chargers or constant current chargers, both of which use much higher charge voltages. With typical 3 stage chargers it is imperative that the batteries are periodically equalized. This will greatly reduce the effects of this problem and increase the life of the batteries.

This effect is more fully explained here

Why doesn't SmartGauge ever reach 100% charge status?

This is a very common problem. And the answer is very simple. It isn't SmartGauge that is at fault. It is the charging system. SmartGauge isn't reaching 100% charge status because your batteries aren't reaching 100% charge status. The solution is to sort out your charging system.

The most common reasons for this when using intelligent 3 stage chargers are insufficient acceptance charge voltage (say 14 volts on a wet cell - not high enough unless you intend charging for 24 hours or so) or insufficient acceptance times before switching to float voltage (1 hour is common, it isn't long enough - especially for Gel cells or AGMs) or poor connections or insufficient cable size between the charger and the batteries. These last 2 can make an enormous difference to the efficiency of a charging system.

Another possible charger problem is that of using a charger that is too large. This can cause 2 distinct problems which lead to the same effect......

Firstly the charger may have an "acceptance to float transition current" ("transistion current") that is too high. This may be based on an ideal figure for the size of battery bank the charger is suited for. For instance a 200 amp mains powered charger would typically be used on a battery bank of around 1500 amp hours. So this charger might have a "transition current" of around 30 amps (2% of the battery capacity). This means the charger will go into float charge when the acceptance current has dropped to 30 amps. This is fine for a 1500 amp hour battery bank but it is far too high for a bank of, say, 400 amp hours. On some chargers this setting is adjustable but on many it is not. Instead it is fixed for a typical bank for that particular charger.

If this setting is adjustable on your charger then often it is recommended to set it to between 2 and 4% of the battery capacity. This really is too high. Between 0.5 and 1% is a better figure. The only problem is that other loads can prevent this low charge current ever being reached, which is why the chargers also (usually) have a time limit on the acceptance cycle.

The other problem relating to a charger that is too large is that the batteries may be very strongly surface charged which can cause the charge current to drop to a low level before the batteries are actually fully charged. This can cause the charger to go into float charge too early, i.e. before the batteries have reached 100% charge state.

The other most common reason is using what can best be described as toy chargers. Those cheap things available at car accessory shops. They're a joke.

Finally there are engine driven alternators charging at 13.8 volts. It is true that this voltage will fully recharge a battery. But you'd be amazed at just how long it will take. Like about 72 hours for a wet cell battery! Yes 72 hours. That is not a typographical error. So your little jaunt to the next port and back isn't really up to the job of recharging your batteries.

It is interesting to note that in the case of the charge acceptance voltage being too low, most amp hours counters will actually detect the batteries as being fully charged well before they actually are. This causes the amp hours counter to run further and further out of sync with the batteries. Finally the amp hours counter tells you the batteries are fully charged when in actual fact they are totally flat. SmartGauge is not so easily fooled.

Applicable to all the possible reasons above for SmartGauge not reaching 100% charge status is this very important point...... In general, batteries don't die of old age. Instead, they are murdered by being continually undercharged. i.e. not charged to 100%. Continually undercharging batteries is far more detrimental to their health than continually overcharging them.

Overcharging in this context refers to holding them at the acceptance voltage for longer than usual - it does not refer to charging at too high a voltage. Undercharging them means not reaching 100% charge state which could be the result of either too low an acceptance voltage or too short an acceptance time.

Obviously charging them exactly to 100% is preferable (with the occasional controlled overcharge in the case of wet cells) but this is rarely possible. If this is the case, err on the side of overcharging them rather than undercharging them. Continually overcharging by a certain amount might shorten a battery's life by say 20%. Continually undercharging the same battery by the same amount might shorten it by 90%.

What condition of batteries will SmartGauge work reliably with?

This opens up a rather complex question. The question needs to be more clearly defined. Most battery manufacturers and professional users consider a battery to be unserviceable when it's capacity has fallen to 80% of it's original capacity. Most people seem to happily use batteries in a much worse condition than this and manage to get useful life out of them. SmartGauge will work reliably with batteries down to 50% of their original capacity.

When a battery is in a condition above 50% of it's original capacity it can simply be regarded as a smaller battery.

Below this figure the battery cannot really be considered as tired but more as broken. All sorts of odd things start to happen such as the battery holding acceptable terminal voltage down to about 80% charge status, then suddenly the battery voltage collapses by about 2 volts as one cell gives up the fight. As we said, this can really be considered a faulty or broken battery as opposed to just tired. SmartGauge, like any other type of battery state of charge monitor, will not operate reliably with a broken a battery.

What other battery conditions will prevent SmartGauge from operating correctly?

Any form of battery fault will prevent correct operation. Common battery faults are:-

Shorted cells. A single shorted cell turns a 12 volt battery into a 10 volt battery. SmartGauge expects a 12 or 24 volt battery. Instantly you will see that all readings will be completely wrong. The battery will charge much faster than usual due to each of the remaining 5 cells being presented with roughly 20% higher charge voltages. SmartGauge cannot track this. During discharge, the off load voltage will be 17% too low. SmartGauge will interpret this as the battery being in a much lower charge status or that it has a very heavy load connected. The result is that it will not track the charge status correctly.

High resistance in Gel Cell and AGM batteries. Usually caused by high charge voltages. These batteries are very susceptible to damage from high charge voltages. Much more so than standard wet cells. It results in the no load battery voltage remaining perfectly normal but the voltage collapses drastically under even the tiniest load. Recharge times also take much longer. Although the power is still available in the battery there is no way to get at it at anything other than very slow discharge rates. Again this can only be considered a faulty battery, not simply tired.

In short SmartGauge will work reliably and track the charge status on batteries that are down to 50% of their original capacity. It will not work with faulty or broken batteries. SmartGauge is a battery state of charge meter. It is NOT a battery tester.

I have an amp hours counter installed too. Initially it agrees with SmartGauge but with time they start to disagree. Why is this.

Quite simply because SmartGauge was designed from the ground up as a battery state of charge meter and amp hours counters were not.

It really is that simple. To prove it to yourself, just carry on using the system. You'll see that SmartGauge tells you the batteries are fully charged when you know they are. If you are daft enough to run the batteries this low you will see that SmartGauge tells you they are flat when they actually are flat. You will also see that the amp hours counter tells you they are full when they are empty, empty when they are full and all sorts of other rubbish unless you continually pay attention to the amp hours counter, reprogram it, reset it, test your batteries and reset Peukert's exponent etc.

On my amp hours counter I have to manually set Peukert's exponent according to the battery data and then keep adjusting this by guesswork as the batteries get old. Also, as the batteries get old, I have to reset the battery capacity amp hours to a new lower figure that I also have to guess at. I also have to periodically reset the amp hours counter as it runs out of sync with the batteries. Unless I do this once a week or so it can be up 200% in error within a month. Why don't I have to do any of these on SmartGauge?

Because SmartGauge works on a completely different principle and makes certain measurements and calculations, and performs certain algorithms that allow it to track these changes, on the fly, and adjust it's own record of the condition of the batteries as they age. All this is done totally automatically, totally transparently and carried out continuously. It results in a battery state of charge meter that works like no other. In fact SmartGauge just works, whereas the others don't. Remember, SmartGauge was designed, from the outset as a battery state of charge meter. Amp hours counters were designed as devices to track power consumption and return (recharging). That was their initial design purpose. Attempts have been made to transfer this funtion to monitor battery state of charge. It, quite simply, does not work.

Why does the voltmeter reading alternate between say 12.35 and 12.40 volts?

Either because the battery voltage has ripple on it and SmartGauge is displaying the measurement from different points in the ripple on each measurement or because the battery voltage is directly between these two readings. A normal DC voltmeter can severely filter the input to remove any ripple thus giving the illusion of continuous, identical, readings, however this ripple is very important as far as it's effect on the battery is concerned therefore SmartGauge needs to be able to measure it and therefore displays the actual voltage at the time of the measurement as opposed to an average over several measurements.

So, finally, why are there so many amp hours counters out there?

We may have given the impression that we consider amp hours counters as being worthless. This really isn't our view. They can be extremely useful pieces of equipment. They allow the user to track their power usage and the accumulated consumption of various items of equipment, this can allow better estimates to be made of the size of charger required, the size of battery bank needed, how often a generator needs to be run and a whole host of other useful information.

Unfortunately. Despite their sales literature, battery state of charge is not one of the functions they are very good at displaying.

For the uninitiated, an amp hours counter attempts to show the amount of power left in a battery by measuring how much goes into it (during charging) and subtracting from that, how much comes out (during discharging) (admittedly this is a great simplification - but it is essentially how they work). This is never going work! It may work for a short period, but it won't be long before errors start to creep in. And that is exactly what happens. Over time they run further and further out of synchronisation with the batteries. Ask anyone who has ever attempted to use one as a battery state of charge meter. They simply do not work.

Some manufacturers and suppliers of amp hours counters do not even pretend that they are any good at tracking battery state of charge. They make no mention of it in the manual. They state, quite clearly, that the amp hours counter is for measuring current consumption and return. They make no mention of battery state of charge. These manufacturers are clearly being totally honest about what an amp hours counter can, and cannot, do.

If I don't fully recharge then why does the charge status, as shown by SmartGauge, continue counting up even after the charger has switched off?

There are 2 possible reasons for this.

The first is that SmartGauge uses the transitional period between charge and discharge as one of it's automatic synchronisation periods. It may be self-correcting it's battery model and algorithm.

If SmartGauge only does this on the first few charge cycles after which the effect gradually diminishes then the above is the answer.

If SmartGauge continues to do this then the following will offer an explanation....

It is related to surface charge and Peukert's effect. If the charge is terminated at, say, 90% charge state then a proportion of this charge will simply be "surface charge". This means that if we start to discharge at that moment, we will effectively be discharging a smaller battery bank (we will only be discharging the plate surfaces as opposed to the full plate depth). This means that the Peukert corrected amps will have a higher figure (as the discharge current represents a higher fraction of the plate surface capacity). This means less total energy will be available. If we wait a while, for the surface charge to permeate deeper into the plates, then we will be discharging the full battery and so more total power will be available during the discharge cycle (due to a lower Peukert corrected amps figure). The longer we wait (up to a limit) the more total power will be available. SmartGauge is counting up the charge status to track the increased total energy available for the next discharge cycle as a result of surface charge permeating deeper into the plates.

The lower the charge status, when the charger is switched off, the greater this effect will be. This is because the effect of "surface charge" is greater towards the beginning of the charge cycle than towards the end.

If discharging is commenced immediately the charger is switched off, then the effect will be greatly reduced as the batteries will not benefit, to the same degree, from the increased available capacity due to a more evenly distributed charge.

Finally, if the acceptance cycle of the charger is long enough, then when it finally goes into float charge, the batteries will be evenly charged, there will be no excess surface charge and the effect will disappear completely. i.e. SmartGauge will not continue counting up. Which it probably won't be able to anyway as if the acceptance cycle was correctly terminated, SmartGauge would be showing 100% charge status anyway.

Note that "surface charge" is completely and fully simulated and accounted for in SmartGauge (as is "surface discharge" and "battery recovery") which it is not in amp hours counters.

 SmartGauge FAQ