Adaptive charging - How does it work?
Adaptive Charging: how it works
The text below closely follows the explanation given in the Phoenix battery
charger info sheet, but with additional detail.
For general knowledge about batteries and battery charging, please refer to
the text Electricity on Board, available on the website of Victron Energy.
1. The right amount of charge: variable absorption time
To fully charge a battery, a period of charging at a relatively high voltage is
needed. This period of the charging process is called absorption charge. A
battery that has been deeply discharged needs an absorption time of several
hours, whereas a battery that is only slightly discharged requires a much
shorter absorptoin period.
Classic 3-step chargers nevertheless have a fixed absorption time, for
example 4 hours. Charging a battery with a fixed absorption time works well as
long the battery has been, on average, substantially discharged before a
recharge cycle is started.
In several applications however a fixed absorption time can lead to
overcharging, which will reduce service life. In case of flooded batteries
frequent topping off of the electrolyte will also be needed, due to increased
Consider for example a typical boat, or a coach, connected to a shore-side
supply with limited output. Household equipment like a microwave, coffee
maker, washing machine or an electric stove might trip the shore supply circuit
breaker. The solution is to run this equipment from the battery with help of an
inverter. A charger is connected to shore power to charge the battery. The
battery in this case is used as a peak shaver, with short discharges every time
there is a high current draw due to household equipment being used. Chances
are that, in case of a fixed absorption time, the battery will be subjected almost
continually to absorption charge. The result is overcharging, which will
substantially shorten battery life and might even result in thermal runaway of
the batteries.
An adaptive charger will also execute a recharge cycle after each shallow
discharge, but the absorption time will be much shorter, thereby increasing
battery life.
The absorption time of a Phoenix Charger or Phoenix Multi will adapt itself as
after each period of bulk charge (= the charger has reached its maximum
current) an absorption period of 5 times the the bulk charge period will follow,
with a maximum set at, for example, 4 hours.
- A bulk charge of 5 minutes (battery was nearly fully charged) will result in an
absorption charge of 5 x 5 = 25 minutes.
- A bulk charge of 30 minutes will result in an absorption charge of
30 x 5 = 150 minutes, or 2,5 hours.
- A bulk charge of 2 hours will result in the maximum absorption charge time of
4 hours.
- When connected to a fully charged battery and switched on, no absorption
charge will occur because the charge current will almost immediately drop off
to a low level.
2. Preventing damage due to excessive gassing: the BatterySafe mode
Often the absorption charge voltage of a battery does not exceed the gassing
voltage limit (approximately 14,4 V for a fully charged 12 V battery). Some
batteries however need a higher absorption voltage to fully charge them
(tubular or thick plate deep cycle batteries for ex.), and open, flooded,
batteries in general can be charged faster by not only increasing the bulk
charge rate, but also the absorption voltage.
A high charge rate will heat the battery (temperature compensation needed!)
and will also increase gassing, in extreme cases up to the point that the gas
bubbles will push the active mass out of the plates, destroying the battery. The
BatterySafe mode limits the rate of voltage rise of the charger output after the
gassing voltage has been reached. The effect is a sharp drop in charge
current which prevents excessive gassing (see fig. 2 on the Phoenix battery
charger info sheet).
3. Less maintenance and aging when the battery is not in use: the Storage mode
After completion of the absorption period, a battery charger in general
switches to the float charge mode. In case of a 3-stage charger the float
voltage should be sufficiently high to compensate for self discharge of the
battery, but should at the same time be as low as possible in order to limit
corrosion of the positive plates and gassing. In practice the balance isn’t
perfect: flooded batteries will gas substantially more than when left open
circuited and will need regular topping up.
We have therefore introduced a fourth stage: the Storage mode. The Storage
mode kicks in whenever the battery has not been subjected to discharge
during 24 hours. In the Storage mode float voltage is reduced to 2,2 V/cell
(13,2 V for a 12 V battery), which is close to the open circuit voltage of a fully
charged battery.
Corrosion and gassing are reduced to absolute minimum, but self-discharge is
not compensated. To compensate for self discharge, and to stir up the
electrolyte, the voltage is raised back to absorption level once every week.
Note: although sealed (VLRA AGM or gel) batteries can be float charged at
13,5 V to 13,8 V during long periods of time (no topping up needed!), some
studies have shown that the Storage mode will increase service life of sealed
batteries (see for ex. ‘Batterie Technik’ by Heinz Wenzl, Expert Verlag, 1999).
Adding a forth charge stage, the Storage mode, also provides the option to
increase the voltage during the third, ‘float’ stage to 2,33 V/cell (14 V for a
12 V battery). This is the charge voltage generally used for starter batteries in
vehicles, and is ideal to ‘supercharge’ an already charged battery.
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