Control Options

NOW THAT YOU’RE thoroughly alarmed, let’s look at some control options. We’ll look at automatic controls for equalizing batteries, forcing two chargers to share the dc output current, and, most importantly, adjusting the output voltage for temperature variations.


Since automatic equalize controls are standard in the full AT series charger line, this section deals only with the SCR/SCRF.

Why do you recommend indicator lights if you control float/equalize manually?

If you have a manual float/equalize switch (the default), it’s smart to have float and equalize indicator lights. These indicators are mounted on the front panel, next to the float and equalize adjustment potentiometers. They allow you to see the operating mode from across the room (green for float, red for equalize).

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Why is this smart? If you want to manually control equalize charging, you can get by with a manual switch. But what if you were to set the charger to equalize, and then leave for a weekend, or even vacation? The indicators can remind anyone that the charger is in equalize mode – which is especially important with valve-regulated lead-acid batteries.

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A timer equalize option will return the charger to float mode at the end of a predetermined equalize period. There are manual timers, automatically started timers, and cycle (or percent) timers. Enjoy your vacation with a peaceful mind.

Manual Timer with Indicating Lights

The manual timer is run by a clock motor and can be set from one (or less) to 72 hours. Equalize charging begins as soon as you rotate the front panel dial to select the time. When the time expires, the charger returns to float, and stays there. This is a one-shot deal – to equalize again, you turn the dial again.

Advantages: Simple, reliable, guaranteed maximum equalize time. If the ac power fails during equalize, the timer picks up where it left off when the power returns.

Disadvantages: Someone must be at the site to touch that dial. A good choice for a staffed site, or one with a comprehensive battery maintenance program.



OK. I have a manual timer. How long should I equalize my batteries?


Methinks this is a loaded question. You didn’t say anything about your battery type, its size, or the site application. Here are a few guidelines.

  • If you have a VRLA battery, an equalize period of zero hours is about right. In other words, don’t equalize. You might think about setting the equalize voltage to the same value as the float voltage, so that if anyone tampers with the controls, the battery won’t be damaged.
  • In the AT charger, if you set the equalize timer to zero hours, the equalize mode is inhibited, so that even the manual equalize setting won’t work.


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  • For a flooded lead-antimony or lead-selenium cell, you can equalize for about 8 to 24 hours, if you’ve had one or more deep discharges. If equalize charges are infrequent, aim for the upper number. If you’ve set the equalize voltage to the maximum value specified by the manufacturer, go for the lower number. Remember, in a series string, it’s the higher capacity cells that are lower in voltage than the lower capacity cells. You want to equalize only long enough to bring up the charge in the higher capacity cells because you’re simultaneously overcharging the lower capacity cells. When the battery reaches the equalize charging voltage, the charge current starts to decrease. Terminate the equalize charge when the current to the battery falls to the C/20 rate.
  • For flooded lead-calcium cells, equalize sparingly. If possible, measure individual cell voltages to determine the need for equalization. If possible, you should measure the voltages with the battery on open circuit (no charger, no load).

I have a UPS battery that’s a VRLA. Shouldn’t that be equalized, since it’s cycled a lot?


You probably have a thin plate, lead or lead-tin battery designed for high rate deep discharge. Although this battery type has some cycling capability, you probably aren’t cycling it as much as you think; UPS is basically a standby application. With a healthy recombinant battery, the cells should be equalized after an extended time on the manufacturer’s recommended float charge.

Auto-Equalize Timers

If you don’t want the hassle of manually initiating an equalize charge, or if you have a remote site with flooded batteries, you might consider an auto-equalize timer. The most common such timer is initiated by an ac power failure, on the theory that an ac failure results in a battery discharge. To guard against an equalization for every glitch in the ac power, most timers have a delay (10 seconds in the case of the SCR/SCRF charger) before “arming” the timer. The equalize charge starts when the ac power returns, and continues for the time selected (by the user) on the timer.

Advantages: Simple, reliable, guaranteed maximum equalize time. Hands-off operation. Useful for an unmanned site.

Disadvantages: If the ac power fails again during equalize (for the duration of the time delay), the timer is reset to the full equalize time. Multiple power failures can result in excessive equalize charging. Since equalization causes electrolyte loss through electrolysis, battery maintenance requirements are actually increased over a manual timer. You should not use an automatic timer with VRLA batteries.


Other Auto-Equalize Timers

You can also get an auto-equalize timer triggered by the charger’s current limit signal. This is a second-order effect. Presumably, if the charger is in current limit, that means that the ac power previously failed, and the battery was discharged and should be equalized. The problem is, the battery could also be discharged slightly by a transient load that exceeds the capacity of the charger, and the charger will go to current limit for a short time to recover that discharge. The current limit-activated timer has about a 20-second fixed time delay that helps to overcome this disadvantage.

A percent timer, or cycle timer, equalizes the battery periodically, irrespective of any external conditions. Typically, the battery will be charged on equalize for a few hours each week. This might be good for NiCd or nickel-iron batteries, since they have a higher self-discharge rate than lead-acid. However, you shouldn’t use it with recombinant NiCd batteries (designed for reduced maintenance) because of – you guessed it – the increased battery maintenance that will result.

Any two battery chargers with the same output voltage can be operated in parallel. They don’t even have to have the same current rating. Each charger will provide output current, as needed, up to its current limit setting. But the chances are that they will have unequal output currents, even if they’re the same current rating. This is random load sharing, which means it isn’t load sharing at all.

The unequal output currents are due to natural differences in the output voltages of the chargers over the load current range. For any given load current, you could adjust the two chargers so that they have the same output voltage, but as soon as the load current changes, the output voltages would no longer track. Technically, we say that the output transfer characteristics of the two chargers have different slopes.

What if you wanted the two chargers to share the load equally, no matter what the total load current is? For that, there is the Forced Load Sharing option (for the SCR/SCRF charger; it’s standard in the AT1.1 and AT30 charger, but requires an optional connection cable). With forced load sharing, the two chargers will have the same output current, within 2%, all the way up to their current limit settings. You can see that the total available output current is twice the rating of each charger.

And what if the chargers don’t have the same current ratings? SCR/SCRF chargers will still share proportionally. Here’s an example: Suppose you have one 50 Adc and one 100 Adc charger, and the total load is 100 A. The load is 2/3 of the total charger capability, so each charger contributes 2/3 of its rating. The 50 A charger provides 33.3 A, and the 100 A charger provides 66.6 A, for a total of 100 A. (Alright, 99.9. So sue me.)

One caveat: The AT forced load sharing requires the chargers to have the same current rating. The control program simply ignores your wishes if the chargers have different ratings.

Whoops, another caveat: In the SCR/SCRF charger, you can’t successfully use forced load sharing and temperature compensation together. With temperature compensation active, the chargers won’t share the load within 2%, and probably not even within 10%. See the next section for a description of temperature compensation.

Uh, we aren’t done yet. For both SCR/SCRF and AT chargers, if you are sharing three-phase chargers, the phase rotation of the ac input voltage for both chargers should be the same for maximum stability.


If you’ve read Section 1.5.6 which discussed the temperature effects on battery charging, you know that the charging voltage of a battery decreases as the temperature increases. A charger that does not compensate for temperature will, at the least, have more maintenance requirements and possibly reduced battery life.

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Temperature compensation options, available for both the SCR/SCRF and the full AT series charger line, help to offset the effects of temperature changes by adjusting the charger output voltage appropriately. Temperature compensation is helpful for almost any battery installation, but is critical for VRLA batteries, especially in an environment that isn’t temperature-controlled.

For the SCR/SCRF product, the standard temperature compensation option uses a temperature sensor (probe) inside the charger, but an external probe is available for mounting on the battery. The AT charger option uses only an external probe. Full instructions for mounting the probe and operating the charger with temperature compensation are included with each option. If you specify the option at the time that you order the charger:

  • SCR charger – For the internal probe, you don’t have to do anything. If you get an external probe, mount it preferably on a battery intercell connector. Run the connection cable back to the charger and connect the wiring to TB45 as shown in the instructions. IMPORTANT: The wiring carries the full battery voltage. Run the wire carefully in its own conduit. Do not run it next to other power wiring. The polarity of the connections is unimportant. Please note, though, that in the SCR/SCRF series, temperature compensation probes aren’t interchangeable.
  • AT10.1,AT30 & ATevo charger – Install the probe, preferably on a battery intercell connector. Run the connection cable back to the charger and connect it to TB8 as shown in the User’s Manual. The cable carries only a signal level voltage, but for safety, and to avoid noise pickup, run the cable in its own conduit. The polarity of the connections is unimportant. AT series temperature compensation probes are interchangeable.

There is an application note, JD5003-00, for the AT temperature compensation, which you can download from HindlePower’s web site.

For a discussion of temperature compensation in excruciating detail, see Temperature Compensation in SECTION 5.3.


William K. Bennett

Former VP/Chief Engineer

HindlePower, Inc.

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