Monday, May 6, 2013

Not all cells are created equal! (Shunt regulation of batteries...)

If you have ever had a device (and who hasn't?) in which several cells were connected in series to make a battery, you may have noticed something an unfortunate fact:  Not all cells are created equal!

If you have been curious enough to take a voltmeter to a battery and independently measure the voltage of each cell in a string of cells, you will likely have noticed that their voltage can vary quite a bit - particularly when the battery is old and/or nearly discharged.

This mismatch is inevitable:  Not all cells were made exactly the same when they were new and can vary by capacity (in amp/hours) and/or internal resistance.  As the battery ages, there is also the fact that each of these cells is in a slightly different environment:  Some are at the ends of the string while others are in the middle, surrounded by other cells and this can affect their temperature which, in turn, affects both their longevity and performance over time.

For the most part we ignore this problem when using conventional non-rechargeable cells such as Alkalines, ceding only to the recommendation that one uses the same type and brand of cells to best assure that they will be reasonably well matched to begin with.

For rechargeable cells, it's a different story!

In that case, we often live with that which we were given.  Since it is likely that one will be (re)-using such cells for a much longer period of time than "primary" (non-rechargeable) cells, the initial match amongst the cells will inevitably diverge over time as the cells degrade at different rates, for different reasons.  For many types of cells (e.g. NiCd and NiMH) we ignore the problem at our peril - particularly with NiCds - and it is the chemistry that is oft-blamed for longevity issues rather than the way that they are applied in the first place!

There is one place where we can easily make a difference in how "equally" the cells are charged, discharged and maintained:  Lead-Acid cells.

The "UPS" Example:

One clear example of this is a typical small-large UPS (Uninterruptable Power System).  At my work, we have a 300kVa (yes, that's well over 200 kilowatts!) that uses two parallel strings of forty "12 volt" batteries (about 542 volts, nominal), for a total of 80 batteries.  Left to their own devices, the voltages on these batteries will inevitably diverge over time from the average 13.55 volts with some batteries going down to about 13.2 volts and others rising up close to 14.0 volts!

Again, this divergence is inevitable, particularly considering that the self-discharge and leakage currents that are responsible for these divergences are typically in the range of 1-20 milliamps for the (nominally) 100 amp-hour battery!  In other words, it takes VERY LITTLE difference between batteries to cause a seemingly radical change in float voltage!

This divergence actually poses a problem:
  • Much below 13.4 volts, the battery is not fully-charged and is more prone to sulfation and loss of capacity.
  • Much above 13.65 volts, the rate at which gasses are generated increases.  Even though typical UPS batteries such as VRLA (Valve-Regulated Lead-Acid) are able to contain and reabsorb gasses to an extent, continuous/excessive gas generation will likely lead to the slow loss of electrolyte and thus shorten the life of the battery.
In either of the above cases, there will be a bias toward shortened battery life and because such a system is only as strong as the weakest cell in the string, reliability and longevity can be impacted!

Even if you don't have such an extreme case as 40 batteries in series, it is common to find small-ish UPSs that use 24 (2 batteries) or 48 (4 batteries) volt systems and these, too, can suffer inequality - particularly if one uses "pulls" (e.g. aging, but still usable batteries) from larger UPS systems.

Fortunately, there is an easy solution to part of this problem:  A shunt regulator.

While we really can't do too much about the voltage of the individual 2 volt cells within a 12 volt lead-acid battery because the interconnects between cells is (usually) inaccessible, we might as well do what we can to help mitigate the problem by regulating the float voltage of the 12 volt battery overall.

Zener Shunt regulator:
Figure 1:
Zener-based shunt regulator.
Click on the image for a larger version.

The simplest system is that shown in Figure 1 which consists of just a resistor, an LED, a Zener diode and a few "ordinary" silicon diodes.

In this circuit, the Zener diode (ZD1) has a fairly sharp "knee" at 10-11 volts such that a small increase in voltage causes a very rapid increase in current.  Included is a series LED - mostly used as an indicator of current flow - that also has a sharp diode "knee" response along with a current-limiting resistor that also serves to soften the curve of the knee somewhat.  Finally, there are several ordinary diodes in series, the tap between each can be adjusted for precise adjustment of the float voltage.  While it would be possible to put many (say, 18-22) ordinary diodes in series to achieve a similar drop, this gets gets to be awkward and cumbersome and the shunt voltage of such a system can change significantly with temperature.  The system described here - while not perfect - is a reasonable compromise between performance and simplicity.

While the resolution of the 0.6 volt taps between diodes may seem to be a bit coarse, the actual effect on the float voltage is much less than this owing to the fact that the aforementioned knee isn't ultra-sharp and the amount of current required to achieve a certain amount of voltage drop isn't necessarily a linear curve, anyway!  As noted in the above diagram, small tweaks may be accomplished by tweaking the resistor value, but I rarely found this to be necessary in practice.

As you might suspect, the above circuit is going to have the effect of clamping down the voltage of a battery that would tend to float high (above 13.55-13.6 volts) but by doing so, it will (indirectly) bring up the voltage of those batteries that would tend - because of internal leakage currents - to be on the low side.

What is surprising to many people is that just a few 10's of milliamps is all that is required to equalize batteries that are in reasonable health!  The initial reaction is often one of "What good can the current through a few LEDs do to equalize the voltage on a string of 100 amp-hour batteries" but this belies the nature of the problem in the first place!  Of course, if one of the batteries is in particularly bad shape (high internal resistance or high leakage/self discharge current) then all bets are off - and this particular battery is probably at the end of its life, anyway!

Others may question the assertion of a nominal "13.55" volt setting as this "optimum" voltage tends to vary with temperature as well as whom one might ask, but it's hard to argue that this isn't, in fact, a "reasonable" voltage for the vast majority of conditions that one might encounter:  Anyway, it would be more harmful to deviate much from this voltage for long periods of time by ignoring the problem altogether!


As can be seen from Figure 2, this circuit is quite simple:  The components are simply soldered in series with a piece of red wire and alligator clip forming the positive side.

For the negative side, a short, black jumper with a clip lead on each end is used:  One end goes to the negative terminal of the batter while the other is used to select which diode tap (and thus the shunt voltage) is to be selected.

While would could "permanently" attach the shunt regulator to the battery with a ring lug on each of its terminals, I chose not to do this since adding an extra "layer" of conductor on the battery bank of the 300 kVa UPS could contribute to added terminal resistance at the battery - a particularly important fact when it is considered that the batter current could easily exceed 100 amps!  The use of the humble spring-loaded clip lead has proven to be more than adequate and is easily removed/installed when the battery bank is maintained.  If, however, your battery bank is mobile (say, in an RV) or of modest size (say, a few thousand watts or less) then it might be more appropriate to use ring lugs as well as wire ties and electrical tape to make sure that nothing can shift and short out during transport.

Adjusting the voltages:

As noted in the diagram above, setting the precise float voltage can be a bit tricky as it can take several hours for things to settle after an adjustment.

Figure 2:
Zener shunt regulator.
Click on the image for a larger version.
It is also recommended that one actually slightly overshoot on the amount of current required to bring down the the voltage of those batteries that tend toward the "high" side and even cause those batteries that were slightly low in voltage due to higher internal leakage to come up in voltage - even rise slightly above the desired voltage.  In this case, one would also equip those "low" batteries with a shunt regulator, but one would necessarily set the shunt current commensurately lower to make the voltage equal.  In other words, it's a good idea to have at least a little shunt current across all batteries in the series string as this will help assure overall stability over time.

One should remember that the voltage on each battery should be approximately that of the average voltage of the charging system - that is, the overall float voltage divided by the number of batteries.  For example, if your UPS float charges the battery bank at 54.10 volts, you would want to set the average float voltage 13.525 volts on each battery - but, of course, it could not be higher than this!  Some UPSs have the means to adjust the precise float voltage - either with a potentiometer (sometimes unlabeled on a circuit board) or via a menu setting, and if  the average voltage turns out to be much outside the range of 13.40 to 13.65 volts, you might consider figuring out how to do this!

Observations over time:

In a number of systems - including the 300 kVa UPS - I've used this (and similar) shunt systems for over a decade and have achieved extremely good results and longevity with the battery bank.  As it happens, the 300 kVa UPS's battery bank is equipped with a system that periodically does internal resistance measurements of each of the batteries within the bank and that over time, this value of internal resistance has been correlated with the actual health of the batteries - both in terms of high-current load test capability and a longer-term amp-hour capacity test.

With good-quality 100 amp/hour VRLA batteries, it is typical to get 5-7 years of life from a battery before its internal resistance exceeds our threshold (about 5 millihoms) and it is pulled from service, at which point its capacity is typically in excess of 70 amp/hours and it will provide at least another 2-4 years in less-critical backup service.

By amassing data for each and every battery over its lifetime, I have observed a significant correlation with its float voltage and longevity:  There seems to be relatively little penalty for running the battery slightly low in voltage (e.g. 13.40 volts) but there is a statistical disadvantage to having it above 13.65 volts!

Charge/Discharge concerns:

When this system has been described, one concern often raised is that of charging batteries that may be somewhat unequal in series strings.

First of all, this is inevitable as it is simply not practical to keep all cells precisely matched and when the battery bank is being charged, it will always happen that some cells will be overcharged before others achieve full charge.  While there is nothing that we can do about this, having a degree of extra shunt current can reduce the time during which a particular battery is exposed to higher-than-desirable voltages.  Again, within the 12 volt battery, we can also do little about the state of the individual 2 volt cells within, but we can at least minimize the impact by regulating the battery, overall.

Practically speaking, it has been observed from monitoring the computer that keeps track of the state of the individual batteries that they actually achieve equilibrium quite quickly - typically within 24-36 hours - after a fairly significant discharge if equipped with such a shunt regulator system.

Other voltages:

It just happens that most of the system with which I deal use 12 volt batteries, but the shut system described could be easily adapted for 6 volt (e.g. golf cart) type batteries or, possibly, individual 2 volt cells, although the latter would require a bit more precision!

Final comments:

The Zener-based shunt system described here is about as simple as it could be in terms of construction and adjustment and still be reasonably consistent over time, under varying loads, and at different temperatures.  I did, in fact, use this system for a number of years on the 300 kVa UPS mentioned, but I gradually changed out the Zener-based shunt regulators with others based on the TL431 shunt regulator IC - but I still use the Zener-based regulators on smaller, multi-battery systems and continue to experience excellent results!


There is an update to this article:  Shunt Regulation of series-connected lead-acid batteries to equalize the voltage link That article describes a simple shunt regulator that can be used for both 6 and 12 volt batteries that will do a better job than the circuit described above.



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