Location: 2 Stage vs 3 Stage Chargers

2 Stage Chargers are for Loaded Situations, 3 Stage Chargers are for Unloaded Situations

2 or 3 Stage Battery Chargers : Which One is right for your application? Battery chargers come in colors, sizes, ampacities, voltages, and for various applications. However, there are only three major types of battery chargers on the market. So the whole of the mess can be boiled down to these three groups.
Battery chargers are - One-stage, two-stage or three-stage.

In our opinion, there is a choice when it comes to battery chargers, the old 2 stage charger vs 3 stage charger discussion.
Which one is right for you depends upon the unit's intended use:

If the battery is under load while charging -->a two stage charger is best.
If the battery is isolated from loads --> a 3 stage charge regimen is best.
Many top end three stage chargers can be switched from 2 stage to 3 stage modes depending on their intended use.

If a load on the circuit is active while charging, a three stage charger is a bad idea as it will be "fooled" into pushing a higher "absorption" voltage than required. The battery charger is looking to see how much amperage is pushed through the charger. The load increases the amperage "SEEN" by the charger, resulting in a prolonged charge when the batteries are actually full. The charger "sees" the load, not the battery, and cooks the system. The result is a cooked battery or battery bank. Let's look at how the chargers work, and see why.

A three-stage charger is the most complete charger available today, cycling through bulk, absorption, and float modes. The bulk mode charges the battery at a constant current (amperage) until the battery voltage reaches a certain level. By reaching this voltage level, the charger knows the battery is roughly 70-80% full, and then switches to the next mode. Here is where things start to deviate:

On a 3 stage charger, it switches to Absorbtion.
On a 2 stage charger, it switches to Float.

So let's assume you have a 3 stage charger and it flips into Absorption mode. The absorption mode maintains the elevated voltage from the bulk phase, but adjusts the current, or amperage, throughout the phase. As the battery charge level approaches capacity, the current approaches 0. When under load, this reading can be greatly affected by the load behind the battery, not the battery itself, leading to an overcharge situation. The problem is especially present in precision model battery chargers. Precision three stage battery chargers measure this decreasing current closer to the zero mark than do the cheaper models. The precision in reading the absorption phase can dictate cost, and also pick up loads on the battery that the cheaper models would miss. Therefore a more expensive 3 stage charger will actually cook batteries more often when under load. Finally, after finishing the battery in absorption mode, the three stage charger switches to float mode to counteract the self discharge of the battery. The float voltage is roughly identical to that of the fully charged battery.

On the other hand, a two stage battery charger will bulk fill to roughly 90% and then flip directly to float mode. Float voltage allows a battery to be maintained indefinitely. Due to this ease, and lack of damage to the systems around it, a two-stage charger is usually preferred for "loaded" batteries. On the other hand, a three-stage charger is best for idle or "unloaded" battery charging.

We do not recommend 1 Stage Chargers

Ordinary one-stage chargers available from the local auto-parts store are inexpensive. On the other hand, we find that our customers complain they charge slowly and are often incapable of completely re-chargeing their large battery banks. Some can't even charge their trucks, let alone get the battery to its rated capacity. These battery chargers have no automatic control over the charge cycle, and do not vary the voltage or current during the charge process. The battery is charged without monitoring, leading often to overvoltage, water loss, or battery failure.

BD Batteries does not sell, nor do we recommend any one-stage charger available on the market today. One stage battery chargers routinely damage the batteries they are charging by simply charging. As the battery is over charged, the plates inside are warped as electrolyte is boiled away. Leaving the battery both dry, and or mechanically unsound.

Battery Charger Warnings:
Always read and follow the manufacturer's battery charging instructions prior to connecting your battery, or trying to charge a battery bank. Do not attempt to charge batteries in a confined environment. Explosive and hazardous gases are an inherent byproduct of battery charging, do think ahead. Batteries contain sulphuric acid, and lead, both of which are hazardous material if removed from the battery, or disposed of improperly, do take care to be environmentally responsible. Batteries are useful, just be safe.

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Wire Sizes & AWG
Solar Charge Controller Guide
Battery Charger Information

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