Battery Charging

The success when recharging a battery directly depends on controlling the charge that is applied to the batteries.

When we bought the Maxi 95 it came with 2 Wet Acid batteries which were connected directly to the engine alternator. When we replaced the engine we also renewed the batteries and added some FET splitting diodes to maintain the correct charging voltage. The HR 41 came to us with Gel Cell batteries which require a specific charging regiment, consequently they came with 3-stage chargers for both the shore power and alternator output.

Some boats hook up the battery directly the alternator output (as the Maxi 95 when we first bought it) and also hook up solar panels and wind generators directly to the battery banks. This is both inefficient and dangerous.

Batteries are ultimately chemical devices, the power they output is from a chemical process and any charge they may accept (i.e. recharging the battery) is also a chemical process. Various parameters control the chemical process: the amount, concentration and distribution of various ingredients as well as the temperature. Heat through the charging or discharging processes can raise the temperature of the battery while it is being used or recharged. Incorrect control of the temperature, discharge rate, or charging rate can damage or destroy batteries. Controlling these factors is what a proper battery charger or charge regulator does.

Multiple Step Charging

Old style battery chargers simply supply a voltage and whatever current can be forced into a battery. This almost always tends to shorten the life of the battery. Due to numerous factors batteries are quite limited as to how much current they can accept. In normal electrical circuits the amount of voltage determines how much current will flow in the circuit. When charging batteries the optimal solution is to use electrical circuits in the charger to independently control the current and voltage. The exact relationship between the voltage and current, and the duration of each phase differs depending on the battery type. The following graph show how the voltage and current relate in a 3-stage charger:



The 3 stages are:

  • Bulk Charge
    Current is supplied to the batteries at the maximum safe rate they will accept while the voltage rises to near full charge level.
  • Absorption Charge
    The voltage remains constant while the current gradually tapers off.
  • Float Charge (AKA Trickle Charge)
    Batteries left unused will slowly discharge through internal chemical processes. This stage compensates for the natural internal discharge.

There are now 4 and 5 step chargers, the specifics differ but the overall function is similar. The charge parameters (time, amperage, voltage) are different depending on whether a Gel Cell or AGM or Wet Acid battery is being charged. Also the parameters are affected by the temperature (all chemical reactions are affected by temperature). The charge voltages are set by the type of battery (i.e. what chemicals are in the battery) while the amperage is determined by how big a battery bank is being charged and what the temperature is. This means that the time to fully charge a battery can be reduced to a point but no further. The battery will only accept a specific amount of current and increasing beyond that value simply damages or even destroys the battery. The green line on the graph shows how the amp-hours (i.e. level of charge) are restored to the battery during charging. As the length of the first 2 phases is determined by the battery type, size and temperature the time to reach Phase II “float charge” is fixed. The more batteries being charged, the more current can be used, but the amount of current per individual battery is fixed and can not be exceeded without damage occurring.

Charging Regulators

The previous section covered charging batteries from either the engine alternator or shore power. Multiple step chargers are used to optimally recharge the battery. With solar and wind power a similar situation exists, a power source needs to be applied  to  the batteries in a controlled and optimized manner. For some reason multiple step chargers are not that common for regulating the charge from the wind and solar sources. This is probably because typical solar and wind energy sources on yachts tend to be relatively small and to fluctuate as the sun and wind fluctuate. Quite often the current produced by solar and wind on a yacht will be about equal to the trickle charge that would be provided by a multiple step charger. Obviously as the size of the solar and wind generation increases the need to regulate that charge also increases. The units that manage the charging of the batteries from wind and solar power are known as “regulators.” Their function is to regulate the amperage coming from the wind and solar and to apply it optimally to the batteries without letting the batteries take damage.

A couple of types of regulator approaches that avoid the expense and complexity of multiple step charging units are:

  • dump resisters
  • PWM

Regulators with dump resistors monitor the voltage of the batteries, when a threshold (usually adjustable on the unit) is reached then the current is diverted from the battery to a “dump resistor.” The dump resistor is simply a heating coil that turns the electricity into heat which is then radiated into the air. If someone was to work it out then the heating coil could be applied to a water heating tank and the electricity used to heat water rather than simply heating the air and drifting away. But the primary action is to remove the charging amps from the battery to prevent damage to them. Dump resistors are usually fitted to the regulators of wind generators as they can quickly overcharge the battery if the wind increases dramatically. Older models of wind generators would damage themselves if they spun and generated power without a load to apply that power to. Modern units now have either manual stops or automatic circuitry that stops the unit when the charge gets too high. Obviously if enough solar power was being collected then the same situation could occur where excess power needed to be diverted away from the batteries, the difference is that solar panels do not mind being “open circuited” (having the load removed from them when the are delivering power) and so do not need a dump resistor.

PWM, or “pulse width modulation” is a different approach to regulating the charge applied to a battery. The PWM regulator constantly monitors the battery voltage, when it senses a tiny voltage drop in the battery it sends very short charging cycle (pulse) to the battery. The voltage of the battery and the level of charge are used to determine how frequently a pulse is sent (it may occur multiple times per second) and how long that pulse should last for (the width of the pulse may range from a few microseconds to several seconds). Potentially each pulse is a different width (i.e. length) which gives the name Pulse Width Modulation.

The charging regulators are typically used to manage the charging from solar and wind power. We have not seen any regulator that implement a multiple stage charging approach. As more and more complexity that used to be in discrete hard wired components moves into software via microprocessor chips it shouldn’t be long before such small scale multiple charging units appear.

  • you can not get the full potential from your batteries without a matched charging system

We have been very pleased with our alternator controller and shore power battery charger, both of which are from Sterling Power. And our solar panel regulator which comes from FlexCharge.

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