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The three stages of charging with solar

Here we detail some more info on the three stages of charging your batteries with solar. Don't forget the Victron MPPT solar charge controller is the box of tricks that regulates the flow of power from the solar panels to the battery bank. The MPPT controller is always 'talking' to your solar panels to see what solar power is available and 'talking' to your battery bank to see what it needs. Importantly your MPPT controller prevents overcharging of the batteries.

Battery charging takes place in 3 basic stages: Bulk, Absorption, and Float.

Bulk Charge - The first stage is when current (Amps) is sent to batteries at the maximum safe rate they will accept until voltage rises to near full (80-90% charge). Voltages at this stage typically range from 10.5 volts to 15 volts. There is no "correct" voltage for bulk charging, but there may be limits on the maximum current that the battery and/or wiring can take.

Absorption Charge: The 2nd stage when Voltage remains constant and current (Amps) gradually tapers off as internal resistance increases during charging. It is during this stage that the charger puts out maximum voltage.

Float Charge: The 3rd stage is after batteries reach full charge, charging voltage is reduced to a lower level (typically 12.8 to 13.2) to reduce gassing and prolong battery life. This is often referred to as a maintenance or trickle charge, since it's main purpose is to keep an already charged battery from discharging.

In the Victron Connect app, in particular in the history tab, you see a lot of detail on how you have been charging your batteries each day with your solar panels. Click on any of the days bar chart and you will see accurate numbers for how much time you spent charging in bulk (white - bottom), absorption (greyish - middle) or float (blueish - top).

Temperature Compensation

Temperature Compensation is important for all battery types. Keep in mind that even if you are in an area where temperature does not fluctuate much, where your batteries sit may have significant temperature swings. This can be caused by charging and discharging the batteries, which they themselves will create heat and also battery compartments can get hot from residual heat from sunny days or the engine running.

We recommend the addition of the Victron Battery Sense, essentially a low-cost battery temperature and voltage sensor. It literally sticks to your battery and transmits info via bluetooth to your MPPT charge controller(s). In essence it gives more accurate readings for improved battery charging and help ensure longer life of your battery bank.

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