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.

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).

Why might a timer equalize option be even better?

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.

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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.

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• 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.

   

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.

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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.

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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.