The standard SLAC was designed to be used in a portable instrument that Sigmatronics manufactures. We looked for a rechargeable battery system for this instrument and decided, based on our experience with various rechargeable battery types, that a sealed lead acid system would be the most reliable and easiest to use for the customer.
We like sealed lead acid batteries because they can be continuously float charged and kept in a fully charged condition so that the portable instrument would be ready to go at a moment's notice. This is a real advantage to the customer since he or she doesn't have to worry about overcharging their rechargeable battery and yet they can always know that their instrument is fully charged and ready for field use.
We're going to discuss a few battery types here just to help explain
why we settled on sealed lead-acids for our instruments.
We (and most others) use the word CELL to mean a single electrochemical unit. The voltage of that cell (often called the "cell voltage" will depend on the chemistry of that cell type. An alkaline cell will have a voltage of around 1.5 volts, a NiCad cell will have a voltage of around 1.2 volts, a Lithium cell will have a voltage of around 3 volts, and a lead-acid cell will have a voltage of around 2 volts.
A single "D" battery is an example of a cell. So is an "AA" or a "C" cell.
So what's a battery then? A battery is a group of cells which are connected in series or possibly in parallel, or maybe series parallel. The word battery implies a group of something. Much like a battery of guns, a tank battery (for those of you in the oil business), etc.
Thus, when we talk of a battery in this discussion, you will know that we mean a group of cells. In our case they are connected in series to achieve a higher voltage than the individual cell.
A standard automotive battery is a group of six 2-Volt lead-acid
cells
connected in series. This gives us a nominal voltage of 12.
A 9 volt "transistor" battery contains six 1.5 volt alkaline cells
connected
in series. Most electronic devices require a battery in order to
have a high enough voltage to operate. This battery may be in the
form of a single "battery pack", a single battery unit, or a battery
holder
into which the consumer places multiple cells.
On the downside, they do not tolerate overcharging very well. One must be careful not to leave NiCads trickle charging continuously, since they tend to develop dendrites which grow through the plate separators and cause internal short circuits within the cells themselves.
This dendrite formation and the resulting internal short circuiting of the cells is what is most often mistaken for "memory effect". In reality, true memory effect in NiCads has never been demonstrated in any scientifically controlled experiments. There is a "voltage depression effect" which NiCads can develop when they are carefully discharged to the same depth of discharge repeatedly. This almost never happens in any real-world application, and the result is not a lowered cell capacity as the legend would have you believe, but a slightly depressed cell voltage. This problem is very very rare (to the point that scientists have not had much success recreating it in the lab), and can easily be reversed by merely charging and discharging the battery a few times.
Interestingly, many companies have made and continue to make money selling charger systems for NiCads which intentionally discharge the battery. This is almost always a very bad idea since intentionally discharging any rechargeable battery wastes some of the charge-discharge life of that battery. All rechargeable batteries have a finite number of charge-discharge cycles available. It's not wise to throw away any of that battery life, especially for the sake of an untrue battery "legend".
If you ask the engineers who design the power systems for satellites (probably the most expensive NiCad application we can imagine), they will tell you that they do not routinely discharge their NiCads. Sometimes (about once per year of operation of the satellite) they will take one bank of cells off line and do a complete discharge/recharge cycle in order to keep the battery in good condition, but they certainly don't do it every time they recharge the battery. Satellites cost billions of dollars to build and put into orbit. If the folks responsible for the reliability of these systems don't like the idea of intentionally discharging their NiCads, then we don't think your application will benefit from it either. We suspect that the satellite people have researched this quite well.
In general, to get the most life from a NiCad system, you should use the product until the battery is low (not dead!), then recharge it. That way you're getting the most use from the battery.
One should never discharge a NiCad battery fully. The reason is that the cell capacities within a NiCad battery are never exactly matched. As the battery is discharged, the cells with less capacity will be exhausted first and the cells with higher capacities will reverse charge the already depleted cells. Although this will not ruin the battery, it will require that the battery be fully charged, then carefully discharged to near exhaustion and then recharged fully a few times in order to restore the full capacity of the battery. This is one of the problems with the false belief in non-existent "NiCad Memory Effect" legend. It leads people to believe that they should, for some reason, be fully discharging their NiCad batteries. Some ingenious devices have been built to discharge the battery only to a safe cell voltage. These are still generally a waste of money, since the NiCads don't benefit from being discharged anyhow.
Once again, the best way to keep NiCads going strong is to use the appliance until the battery is nearly dead, then recharge it fully. Most things that run on a NiCad battery will stop functioning properly long before you've discharged the battery to a dangerous level. So, just use the device till it dies, then recharge the battery. Don't leave the battery trickle charging for more than 24 hours. Just charge it up for the recommended period, then take it off the charger. The bad thing about this is that it means that if you leave the device sitting for months on end, there may be some self-discharge of the battery, and you won't be sure that it's charged fully when you need it.
It also means that in order to get the maximum life from the battery, you don't want to either intentionally discharge it or to charge it until it really needs it. That puts the user in a bad spot, since, for maximum use of the battery, he wants to wait until the battery is low before charging. What happens if you suddenly need to use that piece of equipment but the battery is about half discharged? Now you wish you had charged it, but if you charge it after every use, you're probably overcharging it. What a pain!
Of course the simple answer is that you should have several batteries for the unit. When one is fully used, you set it aside and put in a fresh one. Then you charge the depleted battery when you have the chance. That's what we like to do with our NiCad operated devices. We keep three or more batteries or sets of cells for the unit. We mark the batteries or cells with numbers so we know what order we're rotating them in. With groups of cells (such as penlight cells for a digital camera), we put the cells into groups of how many the unit uses. Then we mark all of the cells in each group with a number.
For example: A digital camera uses four cells. We have three sets of four high capacity NiCads for this camera. One set has each cell marked with a number 1. One set has each cell marked with number 2, and the final set are all marked with a 3. We never mix cells from the sets. We use them together in the camera, and we charge them together. This way we are always charging a set of cells who are equally discharged together and we're always using a set of cells who are equally charged. We also rotate the cell sets (batteries?) into the camera in the order of the numbering. This keeps the usage on each of the sets equal to the usage of each other set. I suppose they'll all die at the same time.
This difficulty in properly charging and using NiCads is the main
reason
we decided against NiCads for our system.
As with any battery, one should be sure to use the proper battery
charger
for these units. A "smart" charger for NiCads will not
properly
recharge NiMH cells. Most of the chargers we've seen for NiMH
batteries
are of the trickle charging type. It has been written that NiMH
cells
charge more fully when trickle charged than when quick charged.
We
haven't had much experience with these types and will refer the reader
to other experts for better information.
Being a lead-acid cell type, these batteries did not tolerate deep discharge well. Newer SLAs are much more forgiving of deep cycling, but it is still something to be avoided if possible. This intolerance of deep discharge has been the chief disadvantage of SLAs. What this meant was that the customer had to avoid running a SLA down too far. It's much like leaving your headlights on in your car. You can get away with it if you only let the battery discharge partially. However, if you leave your car sitting with the lights on for a few days, the battery will never be the same. It might accept a charge when you come back to it, but then again it might not, and in any case, you won't get the life out of that battery that you would have had you never run it down dead. Once again, newer car batteries are becoming more forgiving of this and can tolerate it much better than they ever did before.
With this newer tolerance of deep cycling, SLAs are more attractive than ever.
SLAs prefer to be kept fully charged at all times. If you want to see a SLA last for ten years, just keep it constantly float charged at its proper float voltage and occasionally equalize the cells. This is what makes SLAs the preferred choice for standby power systems such as uninterruptable power supplies, and for such applications as emergency lighting, solar or wind powered telemetry systems, etc.
The reason that SLA batteries sometimes do not last as long as they should in some systems (mainly UPS units and emergency lighting) is that the float voltage is not controlled carefully enough, and manufacturers almost never incorporate any facility for equalizing the cells in the battery.
The manufacturers of these systems often don't care enough about
their
battery life to put the required effort into properly charging
them.
This is a shame for the end-user since they are getting only one fourth
to one third of the life these batteries should give. When we
purchase
UPS units or emergency lighting, the first thing we do is to open the
units
and adjust their float charging voltage to match the type of battery in
the unit. Of course, this should only be done by qualified
personnel
observing the proper safety precautions.
Since we had a very specific application for the SLAC, it was designed around that application. In this case, the battery we were using was a 12 Volt, 1.2 Amp-Hour sealed lead acid battery. This is a reasonably small battery, but it had more than enough capacity to run our portable unit for at least 24 hours of continuous operation.
The SLAC was designed to charge a 12V, 1.2AH battery. However, we also use them in another device where it is set up to charge a 6V, 2.4AH battery.
By changing the values of some of the components, the standard SLAC board can be configured to charge different voltage batteries as well as accommodate different battery capacities.
There are limits, however. We don't generally set up the standard SLAC to charge batteries of capacities greater than 5 Amp Hours, nor do we like to go above 18 volts nominal battery voltage with this unit.
That doesn't mean that a charger can't be designed for nearly any battery capacity. We welcome requests for new designs based on the standard SLAC charging methods.
The SLAC operates by float charging the battery to 13.65 Volts. This is the middle of the proper float voltage range at room temperature for most 12 Volt SLAs. When the charger detects a charging current of greater than about 165 milliamps, it switches automatically into the "bulk charge" mode. In this mode, the charger attempts to pull the battery voltage up to 14.45 volts. This voltage is great enough to equalize the charge in the battery's cells. At all times, the charger limits the current output to 300 mA. When the battery becomes fully charged, its current drops off. When that current drops to less than 12 mA, the charger will switch back from the bulk mode to the float mode where the voltage will be maintained at 13.65 volts.
Thus, when a battery which is slightly discharged is first connected, or power is applied to the charger, the battery will draw enough current to put the charger into the bulk charge mode. The battery will be charged, and it's cells will be equalized. Then the charger will switch automatically to the float mode to maintain the battery's state of charge at precisely the correct voltage.
This means that the end user can just "plug in" the charger whenever he's not using the instrument in the field. The battery will be charged and kept at it's maximum state of readiness until the next time it needs to be used. The battery can be left charging indefinitely. The user always knows that the instrument is ready to go when it's needed. There's no need to wait until the battery is discharged before recharging. The sealed lead acid prefers to be float charged constantly, or recharged whenever it's not being used.
Another feature incorporated into the SLAC is a cut-off relay to disconnect the charger completely from the battery when mains power is not applied. This prevents the charger from putting any load or drain on the battery when it is not powered up and charging.
One additional safety feature is the use of a self-resetting thermal "circuit breaker" device which protects the charger from short circuit loading and limits the amount of current which could be drawn from the battery in the event of a failure of some sort. No system incorporating a battery with low internal resistance such as a SLA should be unprotected. The closer the fuse or other protection is to the battery, the better.
Since the charger detects the charging current and uses that information to decide when to switch in and out of the bulk charge mode, it places some restrictions on the use of this charger. The main thing is that if the load to the battery is continuous, and it is greater than 12 mA, then the charger will never switch out of the bulk mode and the battery will be kept at too high a voltage. In these applications, the battery must be charged when the load is switched off. This is often no problem since many applications only require that the battery be recharged between uses of the device.
For instance, the SLAC would operate an emergency light system quite well. It would also be suitable for a rechargeable lantern or light. The SLAC would work well in any portable instrument that will not be charged at the same time it is being used.
In our application, the instrument doesn't draw current constantly from the battery. Thus, if the customer wants to use the instrument while the charger is powered from the mains, this is quite acceptable. The current drain of the system is less than the 165 mA threshold which would put the charger into the bulk mode, and the current drain is intermittent so that the drain periodically drops below the 12 mA threshold to let it fall back into the float mode. That means that the battery will only be equalized when the battery has been discharged and the charger is initially powered up again.
In other applications, where the total drain was very low, but constant, we have been able to use the SLAC since the device doesn't draw enough to keep the charger locked into the bulk mode.
We realize that the standard, off-the-shelf SLAC may have limited application due to the specific battery it was designed to charge. However, the same charging principals apply to all voltages and capacities of SLAs, so virtually any application can be accommodated with value changes or with a custom design.
If you have any questions about a particular application, don't hesitate to email or call.
Contact Sigmatronics Contact us via email.
The specifications for the standard SLAC are as follows:
Circuit board dimensions: (Sorry, we're looking this up today. Check again and hit the reload button on your browser)
Nominal float voltage: 13.65 Volts
Nominal equalize voltage: 14.45 Volts
Nominal current limit: 300 milliamps
Switchover current - bulk to float: 12 milliamps
Switchover current - float to bulk: 165 milliamps
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