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Battery Charging FAQ

How are “equalize”, “boost” and “fast charge” charging different?

Each of these terms describes the same function of the charger where the charger temporarily elevates the battery’s voltage above the float level. There are different uses for elevated charge voltage, as shown below:

Commonly understood meaning of the term

Equalize – Periodic “topping up” of battery capacity, and correct cell capacity differences

Boost – Can refer to “equalize,” “fast charge,” and sometimes both

Fast charge – Faster recharge of a discharged battery

What does “equalize” charging do, and why is it needed?

All batteries, even those assembled into unitized blocks, are all built of individual battery cells connected in series to obtain the required DC voltage. Like all manufactured products, there is variation between the capacities of each cell in the battery. As the battery ages this variation increases. Since the battery is a chain of cells that is only as strong as the weakest link some scheme is required to ensure that all cells stay at peak capacity.

A scheme called “equalizing” is commonly used in both lead-acid and nickel cadmium batteries. Equalizing temporarily elevates the charging voltage of the entire battery string above the normal “float” voltage. The elevated charging voltage allows all cells, including the weak ones, to accept more current from the charger than they would at float voltage. A consequence of the elevated equalize voltage is that all cells in the battery are overcharged. This is acceptable for short periods provided the battery has sufficient electrolyte.

Overcharging greatly increases the rate at which the water in battery electrolyte is electrolyzed into oxygen and hydrogen gas. Since low electrolyte level will permanently damage the battery it is important to limit when, and for how long, the battery is charged at the equalize voltage.

What is “fast charging”?

Batteries, like all electrical conductors, suffer from resistance in their conductive metals. Ohm’s law says that resistance increases in proportion to current flow through the battery (or any other imperfect conductor). This means that the more amperes of charge we attempt to apply to the battery the more will be lost due to internal heating.

“Fast charging” temporarily increases the charger’s output voltage to compensate for the battery’s internal resistance. This allows the battery to continue accepting maximum current from the charger for a longer time – instead of reducing its charge acceptance early as it would if charged at normal float voltage.

What is the correct charging voltage?

The value of both float and equalize/boost/high rate voltages is determined by the battery manufacturer, and depends on the chemistry and construction of the battery. Deviating from the recommended values, except where needed to adjust for temperature, will under or overcharge the battery – both of which will reduce the battery’s life and performance.

How do you know when the charger should operate in float or equalize mode?

Regardless of the intended purpose of increasing the charger’s voltage there needs to be a way to start and end charging at voltage higher than float.

The most common control methods are shown below.

Control method: Manual switch

  • Advantage: Simple, cheap
  • Disadvantage: High risk of forgetting unit is operating at elevated charging voltage
  • Comment: Not recommended

Control method: Manually initiated timer

  • Advantage: Simple, and automatically terminates charge
  • Disadvantage: Requires user intervention
  • Comment: No way to know when battery would benefit from elevated voltage charge. No way to know what the right time setting is

Control method: Automatically initiated timer

  • Advantage: Suited for remote sites where users do not visit frequently
  • Disadvantage: Time must be pre-programmed.
  • Comment: The correct pre-programmed time cannot be predicted since depth of discharge is likely to vary

Control method: Automatic initiation with battery-determined end

  • Advantage: Termination of elevated charge voltage is based on battery needs, not a program
  • Disadvantage: High continuous current can trick system into staying at elevated voltage too long
  • Comment: Functions like an automatically initiated timer, but can end discharge earlier if battery’s state of charge is satisfactory
Do SENS chargers offers “float”, “equalize” and “taper” charging of batteries?

Yes. Most SENS chargers employ an automatic system that provides four distinct charging phases. This provides providing the fast recharge and low possible water consumption.

Constant current phase: Upon startup the charger operates at maximum possible output in the fast charge mode.

High-rate taper charge phase: The charger stays at the fast charge voltage level while battery current acceptance falls to 75% of the charger’s rated output.

Finishing charge phase: The charger switches to the lower voltage float setting and completes the battery charge. Charger current continues to taper off to near zero as the battery charge reaches 100%.

Maintaining charge phase: The charger supplies only the very few milliamps required by the battery to maintain full charge. This small float current prevents battery self-discharge.

Why do nickel cadmium batteries need to be “boost” charged?

Nickel cadmium batteries offer the highest reliability of any battery, and are more resistant to mechanical and environmental abuse than lead-acid batteries. They do, however, need special charging in order to deliver maximum performance.

If a nickel cadmium battery is charged only at the float rate it will typically deliver only about 70% of its rated capacity. This is a more serious problem for high rate applications, such as engine starting, where even small reductions in capacity have a significant impact on performance.

The most effective way to insure full capacity is available in a nickel cadmium battery is to periodically charge it at an elevated voltage. This can be initiated either manually or automatically, depending on the charger. Automatic equalization is easier to use, and reduces the risk of forgetting to switch back to float voltage.

Do SENS chargers “force” charge batteries?

No. In contrast to “constant current” charging typical in consumer products, SENS uses a technique called “constant voltage” charging. This means that the charger produces a precise voltage, regardless of input voltage to the charger, or load placed on it. The amount of current the battery accepts depends on the battery’s state of charge. If the battery is discharged it will accept more current than the charger is able to safely deliver. As the battery’s state of charge improves it accept less current from the charger.

SENS chargers use an enhanced version of the constant voltage charge technique that includes a second, higher, voltage level referred to the “high rate”, “boost” “fast charge” or “equalize” charge level. This higher voltage level overcomes the battery’s internal resistance so that it is able to accept charging current for a longer period.

SENS chargers also modify the constant voltage value depending on temperature of the battery. This is called “temperature compensation”, and is recommended by all battery makers.

When is battery temperature compensation needed? How important is it?

It is well known that all storage batteries – vented or VRLA lead acid or nickel cadmium – require different charging voltage at different temperatures. When cold, the battery requires higher than normal charge voltage in order to deliver maximum possible performance. When warm, charging voltage must be reduced to prevent overcharging and consequent loss of electrolyte.

When the battery is located in a well-controlled environment temperature compensation adds little value. In contrast, temperature compensation is absolutely essential when batteries are located in outdoor cabinets or other areas subject to extremes of temperature. These facts illustrate the value of temperature compensation:

  • When a battery that is 90 degrees F in temperature is charged at the correct voltage for 50 degrees F it will be boiled dry in three months.
  • When a battery 20 degrees F is charged at the correct voltage for 50 degrees F it will fail to charge – and thus fail to deliver its specified performance.

Using a charger equipped with automatic temperature compensation can prevent both of these problems.

I am thinking about disabling the temperature compensation feature because the charger and battery are not in the same location, and I am worried about overcharging the battery.

Temperature compensation should only be disabled if the batteries can always be guaranteed to be at room temperature (25C, or 77F).

Remote temperature sensing (RTS) is the correct way to provide temperature compensated charging where battery and charger are in different ambients. It is always preferable to both non-compensated and locally compensated charging. Using a sensor attached directly to the battery eliminates all variables of charger temperature and different room temperatures. There is no downside to using RTS. Compared with either disabled, or in-charger temperature compensation, RTS will absolutely, positively increase battery performance to the maximum possible. Regardless of conditions, RTS causes the charger to deliver the exact voltage needed by the battery.

SENS made a provision to disable temperature compensation mainly for customer acceptance testing – to demonstrate that the voltage setting agrees with the actual output voltage. This can be difficult to determine in a temperature compensated charger.

SENS designed its RTS system so that if the remote sensor is damage or becomes disconnected the charger reverts to non-compensated operation. This change is indicated on the charger front panel. Remote temperature sensors for MicroGenius, NRG and IQ chargers are available from SENS at minimal cost, and can be retrofitted at any time.

How does SENS “autoboost” function work?

SENS chargers that include an autoboost function monitor the entire battery string and use feedback loops for both battery voltage and battery current. For a normal battery that has been fully charged, the charger will remain in float mode indefinitely. In this case, the battery current acceptance level is typically quite low (less than 1/2 Amp) and remains approximately constant until something changes, such as a battery discharge event (E.G. a breaker re-closure or engine start). When a battery is discharged, its current acceptance will become high enough to cause the charger to deliver all or most of its maximum output current in order to recharge the battery much faster than would be possible in float mode.

The autoboost feature works as follows: when the battery charging current increases from its float level up to the detection threshold (for example 90% of full rated output current) then the charger increases its output voltage from float voltage (for example 2.2 volts per cell) to boost voltage (for example 2.3 volts per cell). In this phase of the charging cycle, the charger typically goes into “current limit” which means the charger delivers a constant current to the battery. As the battery accepts charge, its voltage will increase until it reaches the charger’s boost voltage. At this point, the charger automatically transitions into the next phase of the charging cycle in which it charges the battery at a constant boost voltage. During this constant boost voltage phase, the battery’s current acceptance will decrease from its maximum “current limit” level down to the detection threshold (for example 70% of full rated current) at which the charger decreases its output voltage from boost voltage to float voltage. None of this is done based on time durations, so this technique accommodates a wide range of battery types, battery age, ambient temperatures and all other factors that affect how a battery behaves as it is recharged.

So, SENS autoboost is an effective way to quickly and safely recharge a battery by allowing the charger to automatically transition from float to boost, and from boost to float, by monitoring battery voltage and current.

How is SENS Dynamic Boost™ charging system better than other boost approaches?

Most other chargers offer simple timer-based or fixed charge programs that cannot adapt to changing DC loads, varying states of battery charge and differing battery capacities. Failure to adapt means that conventional chargers either recharge batteries too slowly or overcharge and damage them.

Enabled by microprocessor technology, SENS Dynamic Boost Charge system recharges batteries at the maximum possible rate, but without the overcharging and battery damage typical of older chargers set for fast recharge. Dynamic Boost Charge automatically and continually adjusts the charge profile. This safely minimizes charge time for each genset’s unique combination of DC load, battery capacity, depth of discharge and other real-world variables. Dynamic Boost Charge equipped chargers safely perform the work of higher amperage chargers, reducing charger size and cost, and extending battery life.

Dynamic Boost is available on SENS MicroGenius and EnerGenius IQ charger ranges.

Will your charger work on my system? I have a 60-cell lead-acid system and am confused that there are chargers offered at 120 volt, 125 volts and 130 volts nominal. What is the difference?

The charger’s rated, or “nominal” voltage is based on an industry convention where the charger is assumed to use lead-acid batteries with an open-circuit voltage of 2.08 volts per cell as shown in this table:

Number of lead acid cells
Theoretical nominal voltage
Common charger nomenclature
Typical float @ .2.22 volts per cell
6 cells
12.48 volts
12 volt
13.32 volts
12 cells
24.96 volts
24 volt
26.64 volts
24 cells
49.92 volts
48 volt
53.28 volts
55 cells
114.4 volts
110 volt
122.1 volts
60 cells
124.8 volts
120, 125 or 130 volt
133.2 volts
120 cells
249.6 volts
240, 250 or 260 volt
266.4 volts


Although for both 60 and 120 cell configurations there are three different nomenclatures, all are intended to charge the same types of batteries.

Nickel cadmium batteries would use a charger with the float voltage that most closely corresponds to the industry standard lead-acid charger. For example, 96 cells nickel cadmium at an open-circuit voltage of 1.2 volts per cell is a total of 115.2 volts, which most closely matches the charger rated “120 volts”.

I find the many different terms used with lead-acid batteries confusing. Can you clarify them?

Yes. Please refer to our battery glossary

I am using two 12-volt batteries in series in a 24-volt system. I have a 12-volt constant load and would like to simply tap one of the 12-volt batteries. Is this OK?

Doing this will shorten life of both batteries, and should be avoided. Remember that the charger regulates to a single voltage across two series-connected batteries. Drawing current from one battery and not the other will place one battery at a lower state of charge than the other. So while the average voltage produced by the charger is correct for the 24-volt battery it is correct for neither of the batteries in the series-connected string. It’s like putting your head in the oven and your tail in the freezer. On average your temperature is right, but you still hurt in both places.

The correct solution is to either use a DC converter across the 24-volt string, use a charger that independently regulates charge voltage for each battery, or use two chargers.

Can you supply info on battery gassing and what measures are taken to avoid it?

Battery gassing is a normal product of charging. Passage of electrical current through water dissociates the water into hydrogen and oxygen. These are the gases that escape an open cell battery. When hydrogen reaches a concentration of 4% in air it is explosive – thus battery gassing is a potentially dangerous problem that must be avoided.

A certain amount of gas generation during charge is completely normal, and is dealt with differently depending on the cell type. Flooded cells use vented caps to allow the gas to escape. Distilled water must be added periodically to replenish the lost water. In contrast, valve regulated lead-acid (VRLA) cells are intended to operate as a closed system in which oxygen and hydrogen are recombined back into water inside the cell. This system works well unless the amount of gas generated causes the cell’s internal pressure to exceed the limit (about 5 psi) of the cells’ pressure relief valve.

The predominant causes of excess gas generation are excess charging voltage and/or high temperature. If the user is using flooded cells he can partially compensate for this excess gassing by adding water more frequently. With VRLA batteries, however, the battery’s loss of gas each time the pressure relief valve operates is permanent. Once the gas (either hydrogen or oxygen) is lost from the battery it is no longer available to recombine into water. The battery starts to dry out. This is a particular problem in VRLA batteries because water is already in short supply in VRLA batteries. As the battery dries out its internal resistance increases until it ultimately fails.

Thus, the single most important factor to insuring the battery delivers its rated life and performance is correct charging.

Each battery manufacturer has unique recommendations regarding the correct float and equalize voltage values at room temperature for standby and cyclic service. It is important to follow the manufacturer’s instructions. Once these values are set, however, it is vital to maintain the charging values very close to the recommended setting.

According to several manufacturers’ VRLA battery user manuals, proper and adequate charging is the single most important is obtaining optimum life from a battery. The maximum allowed voltage deviation from the ideal value is between one and two percent. Even tighter voltage regulation is better.

The same manufacturers’ battery operation manuals specify that for maximum service life the battery must be float charged using a well-regulated constant-source with temperature compensated output. Higher temperatures give rise to increased current consumption at a given voltage setting. Reducing charge voltage across the battery counteracts this destructive tendency, increasing battery life and reducing the risk of thermal runaway in the battery.

This all assumes that the charger is functioning normally – that is, without a failure in its control system causing it to run away “wide-open” into the battery. Such a failure has the potential to evolve large amounts of gas from the battery. The consequences of such failure are battery damage and, particularly with a flooded battery, possible accumulation of explosive levels of hydrogen gas.

The charger manufacturer must make provision to keep the charger under control at all times, and provide appropriate alarm and shutdown systems to alert of problems and terminate charger operation in the event the charger’s control system suffers a failure. The SENS EnerGenius IQ includes two separate processors to insure safe operation. One of these processors controls the charger’s normal operation. The other provides alarm functions and watches over charger operation. Should the charger malfunction, the second processor and an independent shutdown mechanism is able to turn off the charger to minimize the possibility of runaway output voltage.

Are your chargers UL listed and why does it matter?

Most SENS chargers are UL listed to the appropriate UL standard. Underwriters Laboratories (UL) is a nationally recognized test laboratory that evaluates electrical and other products for safety from causing fires and from injuring users. Building inspectors in most US cities require electrical products to carry a UL listed label.

Could you please confirm for me that the marking and listing for SENS chargers are CSA compliant?

Yes, the SENS models LC, MicroGenius 150, NRG and IQ are fully compliant with, and listed to applicable CSA standards. There is a reciprocal agreement between CSA (representing Canada) and UL (representing the USA) in which each agency recognizes the other’s testing and vendor surveillance. These SENS chargers are “C-UL” listed, which means that UL evaluated the product to Canadian standards in addition to USA standards. Canadian authorities are bound by Canadian laws to accept the C-UL mark as equivalent to the CSA mark.

When is it OK to run a 50 Hz charger on 60 Hz AC and vice-versa?

The amount of copper and iron necessary in the power transformer to transfer energy increases as the supply frequency decreases. A 50 Hz transformer will, therefore, requires nearly 20% more material than a 60 Hz transformer all other things being equal. (This same relationship applies to electric motors and other electromagnetic devices that use AC line voltage.) If you run a transformer designed for 60 Hz on a 50 Hz supply the transformer could saturate (making the transformer a short circuit across the AC supply) and trip the input protection device. If it does not saturate it will certainly run hotter than designed. The heat is caused by transformer iron core losses – as frequency decreases core losses increase. Core loss is a fixed loss, and occurs whether the transformer is idling or transferring power. When idling this excess heat might not exceed the limits of the insulation system. When at full power, however, the additional heat load of losses in the copper windings would almost certainly cause the transformer to exceed its designed maximum temperature, creating a safety problem. All SENS chargers designed for 50 Hz operation can safely be operated on 60 Hz power, but the reverse is not true.

The site inspector says that the 70A breaker SENS installed is too big for the 49.2A input current rating on your charger.

SENS chooses the size of charger input breakers based on the National Electric Code (NEC). According to NEC, “…the rating of the overcurrent device shall not be less than the non-continuous load plus 125% of the continuous load.” (This is to prevent nuisance tripping of the breaker under worst-case conditions when the charger is operating at continuous full power at the maximum rated temperature.) Since 125% of 49.2 amps is 61.5 a 60-amp breaker is of insufficient ampacity, and we are forced to increase breaker to the next largest ampacity, which is 70 amps. While the 70A breaker is not ideal, it is sized correctly for the worst-case continuous input draw of the battery charger. The purpose of the input breaker is to protect from short circuit or other catastrophic failure in the charger, and the 70A breaker will serve just fine.

If the input circuit breaker of the charger is 70 Amps, can the contractor install a smaller circuit breaker in his panel based on the 49.2 Amps input current of the charger?

The gauge of wire feeding the charger’s input is determined by the circuit breaker on the source panel. If the installer chooses a 50A breaker then 50A wire will be safe. Depending on the jurisdiction, NEC and local codes may speak to this issue. Local codes may address this question. If you install a 50A breaker in the supply panel you run the risk of nuisance trips if the charger is called on to operate at maximum output for any length of time. These nuisance trips will become more frequent with higher ambient temperature. A breaker sized the same as the charger’s input breaker will not be subject to such nuisance trips.

One of my customers needs a charger to operate in a –40C ambient. Which of your units will do this?

SENS EnerGenius IQ is specified to operate from –40C to +50C. Our NRG, although specified to –20C, will operate without damage at –40C. The MicroGenius 150 will withstand -40C to +70C, with reduced output at higher temperatures.

What is a “crank disconnect” and when is it needed?

“Crank disconnect” relays are used on some battery chargers in engine-generator applications to protect the charger from damage when the engine starts. Crank disconnect relays are only used with old technology chargers, and are never required with more modern charger technologies. No SENS chargers require the use of a crank disconnect relay. Many old technology chargers did not include an effective current limit system. These old type chargers did not include overcurrent protection and were damaged when a heavy load beyond their rating was applied. The demand of the electric motor starting a diesel engine ranges between several hundred amps and a few thousand amps. SENS chargers include effective current limit systems that make the charger immune to heavy loads on the charger’s output. Heavy loads (up to and including a short circuit) will not damage SENS chargers. Therefore, the crank disconnect relay represents unnecessary expense and complexity when a modern current limited charger is used to charge the generator’s start battery.

Do unfiltered chargers shorten AGM battery life?

Under significant continuous load,  a non-filtered battery charger can significantly shorten AGM battery life. Under most engine-starting conditions use of a non-filtered charger is perfectly compatible with AGM batteries:

When there is a significant continuous load (defined as greater than ~25% of the charger’s rated output currentconnected to an unfiltered charger and battery, we have observed significantly reduced life in an AGM battery. We believe that the cause of this loss of battery life is that the battery is alternately charged and discharged at twice AC line frequency. Although the depth of discharge is very low, we speculate that the alternating charge/discharge causes internal heating that accelerates both battery grid corrosion and electrolyte dryout. In this case, the use of a filtered / low ripple battery charger such as the EnerGenius IQ or MicroGenius 150 is the best choice.

When there is no significant continuous load – as in most genset starting applications – we have observed no loss of life in AGM batteries. Although the battery sees pulsating current during maximum charge rate this is a very short duration event that we believe is much shorter than the time needed to cause electrolyte dryout or accelerate grid corrosion.



I am reading a DC voltage from my positive and negative DC breaker terminals to chassis, where is this coming from?

Chargers with ground fault detection alarms have a high resistance connection between both the positive and negative DC breaker terminals to chassis. When the charger is operating and ground fault detection is connected, a small amount of current flows through the resistors to chassis. This small current allows the ground fault detection circuit to detect high leakage current or a dead short from either the positive or negative rail to chassis. This occurs for all ground fault alarm sensitivity settings, including “OFF”.

When a voltmeter is connected between one of the output terminals and chassis, and ground fault is connected, the user is measuring a voltage generated by this ground fault detection circuit. However, since the circuit uses very high resistances, the leakage current is low (1mA maximum) and is not a shock or fire hazard.

If trying to trouble shoot a battery charger do not use the chassis as a reference.