Uncommon (But Useful) Options for SCR/SCRF Chargers?


1% Analog Meters

The standard front panel dc meters (1 voltmeter, 1 ammeter) are 3½ inch analog meters with 2% accuracy. Accuracy is rated at the full-scale deflection of the meter. So, for example, in a 130 Vdc charger, with a 200 V meter, the reading at any point has an uncertainty of ±2 volts.

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You can improve on that by ordering 1% accuracy meters. The meters look identical, but now reading uncertainty is ±1 volt.

In either case, the displayed voltage isn’t accurate enough for adjusting the float or equalize voltage. As noted in the charger instruction manuals, you should use a portable digital meter with at least 0.25% accuracy.

Switchboard Meters

Or, you could order switchboard meters. These are optional 4.5 inch analog meters with a 240 degree dial and 1% accuracy. Because of their larger size, they aren’t available for chargers in the Style 1A enclosure (refer to the SCR/SCRF sales brochure to see what charger ratings are affected).

The dial is round, although the front bezel of the meter is still square. To use switchboard meters, you need a lot more panel space, but you pick up a longer dial, which makes it easier to eyeball the voltage or current within 1%. But even with that accuracy and improved resolution, you still need a portable digital multimeter to calibrate float and equalize voltages properly.

Digital Meters

Or, you could order digital front panel meters. This option provides 3½ digit LED meters for both the dc voltmeter and dc ammeter. The display is a red LED, 0.56 inch high. Sorry, you can’t order one digital and one analog meter. Also, digital meters aren’t available for the small chargers normally supplied in Style 1A enclosures. 

The option specifies an accuracy of ±0.1%, so for the 130 Vdc charger, where the meter has a full-scale rating of 200V, the reading is within 0.2 V. For the ammeter, the full-scale reading depends on the current shunt used in the option. One example: For a 100 A charger, a 150 A/50 mV shunt is used, so the accuracy of the meter display is within 0.15 A.

But as with any digital meter (including your portable meter), the last digit is always fuzzy. Allow another ±1 digit for any reading. Practically, this means that the ammeter in the 100 A charger, given in the example above, really is accurate within 0.2 A instead of 0.15A.

Digital meters are normally supplied for connection to 120 Vac power, which comes from the charger’s main ac power input. This means that the meters go dark during an ac power failure, a difficulty not shared by analog meters. You can provide your own 120 Vac from a backup source, however, so that meter operation is continuous.

AC Voltmeter & AC Ammeter

These meter options, available separately, indicate the primary ac voltage supplied to the charger and the line current to the charger. For three-phase chargers, a phase or line selector switch is provided so that the user can read each phase individually. The standard meters are 3½ inch analog meters, with the same “look and feel” as the standard dc meters. As is true for the digital meters, the options aren’t available for chargers in Style 1A enclosures because of the lack of usable panel space.

The ac meters are of the moving-iron type; they display the true rms value of the voltage or current with 2% accuracy. For chargers with ac input current over about 35 Aac, the ammeter uses a current transformer; three-phase chargers have three current transformers.

These options may be useful for sites that are cramped for space, since you can put ac instrumentation right into the charger. But the ammeter is of limited value in a standby application because the ac input current would spend most of its life near zero. You don’t get much useful information from a 50 Aac meter, with an accuracy of 2%, when it’s indicating only 1 Aac.

Remote Meters

Sure, you can have remote meters. This option doesn’t actually supply the meter(s), just the connection points in an SCR/SCRF charger. You can connect a remote dc voltmeter or remote dc ammeter to the terminals provided (optionally) on TB3. You want the meter, too? Just ask.

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Just remember, please, that the wiring to a remote voltmeter carries the battery (or charger output) voltage, and the wiring to a remote ammeter carries the shunt signal that feeds the charger’s front panel meter and controls current limit. If you short-circuit either one of these signals, there will be big trouble. We recommend that you install a 1 A, 250 Vdc fuse in series with the remote voltmeter.


Battery discharge occurs, of course, during an ac power failure, but may also take place if the instantaneous dc load exceeds the capability of the charger – not an unusual occurrence, for example, when starting a pump motor at a site.

You have two options: You can add a front panel ammeter to display battery discharge current, or an alarm circuit to send a contact closure when discharge occurs. In other words, in the spirit of Heisenberg, you can see it or send it, but not both.

To answer your next question, yes, some users have ordered both, and Engineering will do it.

Zero Center Ammeter

The ammeter displays battery charge or discharge current. A separate current shunt is installed, and the battery and load wiring are arranged so that the current shunt measures only the battery current. Positive deflection of the meter (to the right in the Northern Hemisphere) indicates charge current; negative deflection indicates discharge.

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The key to ordering a battery discharge ammeter is to specify the maximum discharge current that the battery will see. This is usually more, sometimes a lot more, than the charger output rating. If the ammeter option has the same rating as the charger, the meter movement will probably be pegged (deflected to maximum) during battery discharge, and there is a risk of damage to the current shunt.

Battery Discharge Alarm

The Battery Discharge Alarm uses the same current shunt as the discharge ammeter (and the alarm should be specified for the same maximum discharge current, as discussed above). The shunt signal drives an electronic detector that can be adjusted from 2% to about 50% of the shunt rating.

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Example: If you know that your battery will see a 150 A discharge, order a 200 A discharge alarm. It can be adjusted to send an alarm (transfer the alarm relay contacts) on a discharge current as low as 4 Adc, or up to about 100 A. This adjustment can be made either at the factory or in the field.



We briefly described the common alarm circuit in the CASM on PAGE 88. If you have one or more legacy alarms, such as those described in this section, you may want to group their relay contacts into a single alarm contact. Here’s the menu:

Common Alarm Buzzer

The Common Alarm Buzzer provides an 80 dB(A) horn, about 2,900 Hz, to alert you that any one of the included alarms has been triggered. Fine for an attended site, but if no one’s there to hear it....

The signal is loud enough and high enough to be heard above most industrial noise, without quite being at the level that would damage hearing. If you can’t hear it, of course, then there is already too much noise at the site.

An optional lightning arrestor (secondary arrestor) is available that increases the surge current capability to at least 10,000 A peak for the 8 x 20 µsec waveform. This offers substantial extra protection for induced surges. If a user wants primary surge protection (that is, protection against a direct lightning strike), he must install lightning protection at the building service entrance or distribution panel for his location. But note that ANSI C62.41 allows the use of secondary arrestors at these locations.


Common Alarm Buzzer for CASM

If you have a CASM, and you don’t want any other alarms, you can also get a common alarm buzzer for it. It doesn’t take the place of the common alarm relay on the CASM; you still have that. The option uses the same buzzer as in the legacy common alarm. The option includes a toggle switch to disable the buzzer, which also acts as a “call-back” to remind you to re-enable the buzzer when the fault condition has been cleared.

Common Alarm Relay with Buzzer

If you have a group of legacy alarms, you can get a Common Alarm Relay either with or without a buzzer included. Incidentally, are we stretching the meaning by calling a 2,900 Hz tone a buzz?

The common alarm relay works just as it does in the CASM but includes any legacy alarms you may have ordered. It can also include the CASM, of course, by wiring the CASM common alarm in with the legacy common alarm. That’s a lot of commonality.


Internal cabinet heaters are useful if the charger is to be stored for an extended period in an unheated location. The heaters provide just enough energy to keep frost from condensing on critical components, or for that matter, any components at all. Of course, you need to provide power for the heaters: 120 Vac or 240 Vac, 50 Hz or 60 Hz, up to about 450 W, depending on the cabinet size.

The option includes a thermostat control, set to 70 °F (about 20 °C), and a separate single-pole circuit breaker.


Here is a quote from HindlePower’s Charger Specs FAQ:

All chargers are protected by MOVs (metal oxide varistors), on the ac input and dc output, that meet the requirements of IEEE/ANSI C62.41 for Category B locations.

The IEEE standard defines test methods for protection devices, such as MOVs. The test waveform for induced surges in indoor locations, Category B, is:

6 kV open circuit, 50 ohm source impedance, 1.2 x 50 µsec unipolar

3 kA peak, 8 x 20 µsec unipolar

What the heck does all this mean? It means that the charger ac input and dc output are protected by MOVs that have been tested to withstand 3,000 A peak current from a lightning surge that lasts 20 microseconds. Once. Maybe several times. These are called induced surges. That is, they are a secondary effect – a transient caused by current flowing in power lines, or even the ground, due to a lightning strike elsewhere. A direct lightning strike can be much more damaging.

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If you think you’re in a location that needs more protection, the optional lightning arrestor can take a 10,000 A beating and keep on ticking. But note that the onus is on the user to ensure that the primary service entrance is protected against a direct light-ning strike. So if you’re in a location with frequent, severe thunderstorms, the lightning arrestor offers good secondary protection at the charger location, but make sure your service entrance protection is up to snuff.


How do I know if the MOVs in my charger are still OK?


The MOV failure mode is a helpful indication. After several surges, the withstand voltage gradually deteriorates, and eventually the MOV fails when a surge overwhelms it, and it vaporizes, usually with a loud report. It may leave its pigtail leads behind. If you find that an MOV is missing, it should be replaced as soon as possible. We don’t recommend elective replacement of MOVs, but I’ve read that some advisors recommend that for consumer equipment.


Time for another excerpt from HindlePower’s standard documentation:

Oscillatory surges (IEEE 472/ANSI C37.90 Surge Withstand Capability (SWC)) Test Waveform: 2.5 – 3.0 kV initial peak, ringing at 1.0 – 1.5 MHz,

decay to 50% in 60 µsec

Repetitive at 50 Hz minimum

Apply for 2 seconds minimum

NOTE: There are two levels of this requirement:

Level 1 (NEMA PE 5-1991) The equipment must survive the test waveform without damage or any degradation in performance.

Level 2 (ANSI C37.90) The equipment must produce no erroneous indications during the application of the test waveform. We interpret this to mean no erroneous alarms, and no more than a 10% change in output current.

The SCRF Series meets Level 1 with no additional filters. The optional SWC filters must be added to meet level 2.

The full AT Series line meets all requirements with no additional filters required.

Note that the AT series meets the SWC surge requirements with no added filters, but it was tested to the 1991 version of NEMA PE 5. The 1996 version requires the equipment to pass at Level 2.

The SWC filter option applies to the SCR/SCRF charger and provides the necessary immunity for the original oscillatory surge waveform. ANSI C37.90 also includes a “fast transient” test. The AT charger (but not the SCR/SCRF) meets this requirement.


These are covered in CHAPTER 5, Temperature Effects.


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You know that dc filters use electrolytic capacitors, and that, traditionally, they have required eventual replacement because of gradual loss of capacitance. This is the only charger component that we suggest you might consider electively replacing after ten years. First, though, there is nothing sacred about ten years. Capacitor manufacturers insist that their products are more durable than that. Second, we also say that the best indicator is to measure the output ripple voltage during routine maintenance, and if it starts increasing, it’s time to act.

The High Ripple Alarm is an option that could help keep an eye, figuratively, on your filter capacitor life. If the output ripple out of the box is 100 mV, and you set the ripple alarm to trigger the alarm relay at 200 mV, you have a good chance of nipping any problem in the bud. You can set the alarm threshold higher, up to what you think is tolerable for your system. A level of 500 mV on a 130Vdc bus is probably good.

There’s a cost associated with this, of course. You have to trade off the purchase price against the likelihood that you will use this only once or twice during the life of the charger, and factor in any degradation in system performance due to undetected high ripple voltage. You must also consider the ease of measuring ripple voltage manually during routine maintenance.

It seems obvious that you wouldn’t order this option with an unfiltered charger. But think about this: Since ripple voltage on the battery is a function of battery impedance, a High Ripple Alarm might detect a rise in battery impedance. There are a lot of other factors to consider, though. Other loads on the dc bus (UPS, for example) can contribute ripple voltage to the bus, and affect alarm operation. The bottom line? High ripple is usually caused by the charger, but not always.

We get quite a few requests for this option, and it mystifies me. First of all, you shouldn’t be alarmed that the charger is in current limit. That’s a normal operating mode. You should only worry if the charger isn’t in current limit when it’s supposed to be because then something is wrong.

It’s good to remember that in a dc system, the battery sets the voltage. Not the charger. The battery. (Refer to Battery Chargers: How Different from Power Supplies? in SECTION 2.1.4.)

The charger feeds current to the battery, and the battery decides what voltage it’s going to show to the world, based on the charge current and its own state of charge. The only time the charger thinks it has the upper hand is when the battery is fully charged, sitting at float voltage, and the charger has to provide only enough current to maintain the float voltage.

This means that any time the battery is less than fully charged, it will present a terminal voltage less than float voltage. When a charger sees this, it says, “OK, I have to send more current to try to bring that voltage up,” and will wind the output current all the way up to current limit, if necessary.

So, the Current Limit Alarm is, in essence, a “battery isn’t fully charged” indicator. In the AT10.1, AT30, and ATevo charger, a current limit signal is available through the communications option, using MODBUS or DNP3. In the SCR/SCRF charger, the alarm is an add-on circuit board that has to be calibrated to a specific current limit value. If you change the current limit setting in the field, you have to recalibrate the current limit alarm also. Is it worth it?



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Flooded batteries are occasionally equalized. During equalization, some hydrogen is inevitably evolved, and it’s a good idea to try to get rid of it before it turns your facility into a famous dirigible. Usually, with proper charging and proper site design, hydrogen will not build up to an explosive level.

But your site plan might call for forced ventilation of the battery room during equalize charging. An optional relay contact that closes on equalize can be provided to control an external contactor. You can also order a Ventilation Fan Control option, complete with a contactor rated for 10 Aac or 20 Aac at 240 Vac. Breathe easy.


Back in SECTION 1.5.5 on Equalization Charging, we mentioned counter-EMF cells, used to standardize the voltage applied to loads on the dc bus during equalization. When a battery is equalized, the terminal voltage may rise to a level higher than the dc loads can tolerate. A counter-EMF cell can be inserted in series with the load to reduce the voltage to an acceptable level. 

Today, counter-EMF devices are solid state, using silicon diodes. There are many decisions you need to make when specifying a device, depending on the load current level, the voltage to be dropped, and the control scheme, which may create multiple voltage drops.

Because there is heat to be dissipated, such devices take up a fair amount of room. But once installed, they need no maintenance or electrolyte replacement, as was historically required for electrolytic counter-EMF cells.


William K. Bennett

Former VP/Chief Engineer

HindlePower, Inc.

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