Alternator & Charger Simultaneously?

[Mike Sibley]

I recently had an unusual situation where the main engine and the generator were running at the same time. It later occurred to me that the alternator and the battery charger were both charging the batteries at the same time. What sort of dammage (if any) does this cause?

[James S]

I think if they’re (the charger and the alt.) are both regulated with some decent feed-back, it may be OK.

[David M]

The amount of current that an alternator or battery charger creates is determined by the system voltage. You can run both DC sources simultaneously without harm. On my boat I have two alternators and two battery chargers and have had all four running at the same time on the same DC circuit without harm to either the batteries or any of the DC sources.

What will happen is as the system voltage rises, the DC sources will start cutting back on the current (in amps) until the DC source with the highest voltage output (as determined by the regulator) will be sole source of current. The other DC sources will have cut their current output back to zero or near zero. Essentially, its something you don't need to worry about unless you have a DC source regulator that allows too high of a voltage.

Its a whole different story with AC. In order to have more than one AC source, the phase must be matched.

That's one of the advantages of DC on a boat, you can have more than one DC source. Energy storage using batteries is of course the other advantage.

[Mike Sibley]

Originally Posted by David M

The amount of current that an alternator or battery charger creates is determined by the system voltage. You can run both DC sources simultaneously without harm. On my boat I have two alternators and two battery chargers and have had all four running at the same time on the same DC circuit without harm to either the batteries or any of the DC sources.

What will happen is as the system voltage rises, the DC sources will start cutting back on the current (in amps) until the DC source with the highest voltage output (as determined by the regulator) will be sole source of current. The other DC sources will have cut their current output back to zero or near zero. Essentially, its something you don't need to worry about unless you have a DC source regulator that allows too high of a voltage.

Its a whole different story with AC. In order to have more than one AC source, the phase must be matched.

That's one of the advantages of DC on a boat, you can have more than one DC source. Energy storage using batteries is of course the other advantage.

That is good to know, but raises a second question. Suppose you have two separate battery banks (one aft and one 20' forward) that are connected with an battery integrator (that combines them when there is a charging source on either battery), there would be no benefit to have a separate battery charger for each bank, since the one with the lowest voltage (which could be the larger charger) would shut down. Is that correct?

[DeepFrz]

Why would you want separate battery chargers if you are going to use a battery combiner? That is exactly what a combiner is for. However, you might want to rethink using a battery combiner on a second bank that is 20 ft. from the main bank. You would have to supply large cables to the forward bank, as well as cct. protection (fuses or cct. breakers) at each end of the 20 ft. cable. In such a case it might be better to use a battery charger for the forward bank that you could power from the inverter, generator or shore power. That is if you have any or all of those available.

[Mike Sibley]

Originally Posted by DeepFrz

Why would you want separate battery chargers if you are going to use a battery combiner? That is exactly what a combiner is for. However, you might want to rethink using a battery combiner on a second bank that is 20 ft. from the main bank. You would have to supply large cables to the forward bank, as well as cct. protection (fuses or cct. breakers) at each end of the 20 ft. cable. In such a case it might be better to use a battery charger for the forward bank that you could power from the inverter, generator or shore power. That is if you have any or all of those available.

The current setup does not have a combiner. I do have a generator (which I tend not to use very much) and the forward bank has its own charger. However, the location of the boat (up a river) and the nature of the local cruising opportunities requires using the main engine several hours a day when we use the boat. I was hoping that there was a way that I didn't also have to run the generator to charge the forward bank. Also, since the engine and the windlass are used at the same time, it occurred to me that a current from the alternator would help with the "heavy lifting." Unused AWG 2 cables currently run to the forward area, so I thought that they could be used with a combiner to eliminate the need to run the generator to keep the forward bank charged.

[David M]

Things are getting a bit confused here. What you need is to be able to isolate your engine and generator start batteries from your house batteries. The goal here is to never be able to draw off the start batteries except for starting the engine. A combiner will allow you to draw off the house batteries and then the combiner solenoid switch will open when the house voltage gets too low, preventing you from drawing down the start batteries too low to start the engine or generator.

As far as the run between the two battery banks, the cable should be gauged according to the greatest current draw from the bank or the greatest charge to the bank...whichever is larger. Gauge it for a 3% voltage drop if you want a more efficient system.

You need to decide which will be your start batteries...the rest will be your house batteries. Do the wiring accordingly. I like to place the DC sources on the house bank, which then charges the start batteries when the combiner switch is closed. Some people like doing it the opposite. Whatever works best for you is what is best. Be sure to put in a switch that jumps (bypasses) the combiner...they do occasionally fail.

[DeepFrz]

Mike:
Yes, if the cables are already there then that is a reasonable solution. Just remember that you must put cct. protection devices at each end of the connecting cables, as well as between the batteries and the windlass. Also double check if the AWG 2 cables are capable of handling the load.