Charging Batteries from an Alternator

Last month we wrote about the different battery charging options available for boats.  This month we are going to take a look at how you determine if your batteries are actually being charged from an alternator.  Let’s recall that in 99% of boat systems, the alternator is directly connected to the starter solenoid. Therefore, whatever battery is connected to the starter is also the battery that will get a charge from the alternator. To make matters even more challenging for recharging multiple battery banks, alternators are single output devices and therefore are meant to only recharge one battery bank. In order to share an alternator with more than one battery we need 1) a parallel battery switch, e.g. Off-1-2-both, 2) a battery combiner or 3) a battery isolator.

Dedicated Battery Banks. Nowadays, separate battery banks, for different purposes, is the norm. It’s typical to see batteries dedicated for starting engines, running house loads or inverter loads, and even thruster batteries. A simple electrical system on a sailboat or boat with a single engine might have one engine battery and one house battery. On a large boat, you might have four or more battery banks: engine, house, aft thruster, and forward thruster. The challenge for boaters is how to  recharge so many battery banks with only one or two alternators?

It is not uncommon for PYS to see a setup where the alternator is only charging the starter battery and not the house bank or even a thruster battery bank.  Another common scenario is a house bank that is charged by the alternator only when using the Off-1-2-Both switch in the Both/All position.  Unfortunately, on many boats, charging all the batteries from the alternator is a manual system and many boaters simply do not know or forget what position the battery switch should be set to, when and to charge which battery. 

An easy way to figure this out  is through a process of cause and effect using the voltmeter gauge you have onboard.  First, determine a baseline battery voltage, i.e. what is the resting battery voltage without loads or charging.  Ensure there are no other charging sources to your batteries, no shore power or generator running to power the battery charger, blankets on the solar panels, EFOY turned off. Wait about 30 minutes for the battery voltage to settle and write down this resting voltage for each battery onboard your boat. Also make sure that none of the battery switches are on Both/All or that the parallel battery switch is in the on position.

Look at the voltmeter gauge as you are about to turn the key to start one engine. If you are looking at the correct battery, the voltmeter value should go down when the engine is turning over. Once the engine is started and the engine is in fast idle, say 1400 rpm, you should see the voltmeter reading increase. This test sequence confirms three things; 1) the correct battery is starting the engine, 2) the alternator is working and 3) the alternator is connected to said battery. Remember to only start and run one engine at the time.

The next step is to observe the other battery voltages onboard. For instance, the house or thruster battery. With the single engine running at 1400 rpm, what are the voltages of the other batteries, did those voltages go up from the baseline voltages you measured prior to running the engine? If any battery voltage is now higher, you’ve confirmed there is either a battery combiner or battery isolator to share the charging voltage from the alternator.

As boaters, we want to ensure that all of our batteries are getting a charge from the alternators when the engines are running. The testing procedure, outlined above, will allow you to confirm which alternator charges which battery bank.

What rate are the batteries being charaged? Once you have determined that the alternator is working and which batteries are being  charged from the alternators, you need to figure out at what rate the batteries are being charged?  A typical question we hear is “If my alternator is rated at 55A, does that mean it is putting 55 amps per hour into my batteries?”  Let’s recall that amps refer to the rate of current flow and amp-hours refer to the capacity.

The charging rate of your battery is dependent on the state of charge of your batteries. If your batteries are full the alternator output will be limited and if your batteries are drained the alternator will produce full output. The alternator regulator's job is to make sure that your alternator doesn’t overcharge your batteries and also charges them as fast as possible when the battery voltage is low.

All alternators have a voltage regulator which controls the alternator charging voltage and, in turn, the current produced by the alternator.  When the battery voltage is low the alternator regulator will allow the alternator to produce more amps which triggers the voltage regulator to turn the alternator on. 

Many boaters often wonder if their alternators are working because the battery voltage isn’t instantly 14.4 volts once the engine is running. For most boaters, the ratio between alternator size and battery bank won’t allow an alternator to get the charging voltage to 14.4 quickly for a heavily drained battery. We are looking for a change in battery voltage. What was the battery voltage prior to starting the engine and after, when the engine and alternator are running? As long as the battery voltage rises when the engine is running, you know that your alternator is working. The ratio between the battery bank and the alternator will dictate how long it will take for the charging voltage to reach 14.4 volts.

Next month we are going to look at some ideal charging scenarios using battery combiners and isolators to automate and simplify the charging process. 


About the author: Jeff Cote is the owner of Pacific Yacht Systems, a full service shop delivering marine electrical and navigation solutions for recreational boats. Visit their website and blog for info and articles on marine electrical systems, projects and more: