Engine (alternator) and AC charger at same time ?

[PKFK]

Hi - I was wondering if it would be ok to do the following at the same time :

- run engine to charge batteries
- also run shore power charger (via generator) to also charge same battery bank

The shore power charger is a 60 amp unit, and the Yanmar 3xM30 has the stock (50? ) amp alternator

This all feeds into a 450AH bank of Trojans.

Would running both at the same time result in 60 + 50 amps charge current ? (Less the inefficiencies of hot alternator, voltage regulators, etc) - but basically will this give me a faster charge, or cause problems ?

25% charge acceptance rate of the bank should mean it can take 112 amps - but do the 2 sources combine or not ?

Thanks !

(If I gotta listen to the engine, then listening to the genny at the same time makes sense if it reduces overall charge time)

 

[Chief RA]

Go directly to Maine Sail on this as this is his type of specialty. Your dealing with so much charge with using both sources I would be reluctant to put in so many amps at once, but Maine Sail can address this. Chief

 

[Maine Sail]

Not an issue at all.. eventually one device will drop out and you will finish with the device with the highest voltage regulation set point...

 

[Rich Stidger]

I did this once and with both the charger energized from the genset and the engine running, an alarm was generated on the engine panel. I think it was a battery voltage alarm but I'm not 100% sure. It was some 16 years ago.

Anyway I agree with Mainesail that the two sources will combine, but somehow my engine control didn't like the second source feeding at the same time. No harm was done tho.

However, if you have just the stock alternator, it will taper off the charging current very quickly. And the 50A is peak amps when cold- probably only 25A rms (average) when hot. I suspect that you won't gain much with both units running.

And the batteries will only accept just so much at a given charge voltage. You really need 14.8 or even 15.0 volts to really charge quickly. I'm pretty sure your alternator max voltage is about 14.4V and I have no idea about your 60A charger.

Case in point, I am currently running my genset with a 80A charger set to 14.6V and my batteries are accepting 40A at about 75% state of charge. Just because you have a 60A charger doesn't mean that it will recharge your batteries at 60A.

Read the stuff that Mainesail has published. It is a college education's collection of information.

 

[artboas]

Not reccomended

I did this once and with both the charger energized from the genset and the engine running, an alarm was generated on the engine panel. I think it was a battery voltage alarm but I'm not 100% sure. It was some 16 years ago.

Anyway I agree with Mainesail that the two sources will combine, but somehow my engine control didn't like the second source feeding at the same time. No harm was done tho.

However, if you have just the stock alternator, it will taper off the charging current very quickly. And the 50A is peak amps when cold- probably only 25A rms (average) when hot. I suspect that you won't gain much with both units running.

And the batteries will only accept just so much at a given charge voltage. You really need 14.8 or even 15.0 volts to really charge quickly. I'm pretty sure your alternator max voltage is about 14.4V and I have no idea about your 60A charger.

Case in point, I am currently running my genset with a 80A charger set to 14.6V and my batteries are accepting 40A at about 75% state of charge. Just because you have a 60A charger doesn't mean that it will recharge your batteries at 60A.

Read the stuff that Mainesail has published. It is a college education's collection of information.

Ok, here is why I wouldn't do this.
It has to do with the theory of current flow.
If you have 2 voltage sources of different potential connected together then there will be current flow from the higher source to the lower source.
Look at it this way;
If you have 12v hooked up to a 12 ohm lamp, which is grounded at the low side, you have 1 amp of current through the lamp.
Just to complete the formula, you have 12 watts of power being consumed bulb the bulb.
Now, assuming your boat charger is putting out 14v, and the alternator 13v
You have a 1v potential difference between the 2 sources, and current will flow. And as chargers are meant to output high current, this could generate high backfeed currents.
Now, it is possible that either the alternator charger, or shore charger shut down, but this is very defendant on the output stage design & monitoring.

Short answer,,, you could damage one of the 2 charging sources. It just isn't worth the risk.

 

[Maine Sail]

Ok, here is why I wouldn't do this.
It has to do with the theory of current flow.
If you have 2 voltage sources of different potential connected together then there will be current flow from the higher source to the lower source.
Look at it this way;
If you have 12v hooked up to a 12 ohm lamp, which is grounded at the low side, you have 1 amp of current through the lamp.
Just to complete the formula, you have 12 watts of power being consumed bulb the bulb.
Now, assuming your boat charger is putting out 14v, and the alternator 13v
You have a 1v potential difference between the 2 sources, and current will flow. And as chargers are meant to output high current, this could generate high backfeed currents.
Now, it is possible that either the alternator charger, or shore charger shut down, but this is very defendant on the output stage design & monitoring.

Short answer,,, you could damage one of the 2 charging sources. It just isn't worth the risk.

Won't happen and is one of those urban myths that I don't understand why it persists. This is done all the time on cruising boats. I have multiple boats that have wind, solar, alternator and gen set and all of these items speed bulk charging. Once out of bulk and into absorption devices will simply turn on or off depending upon their voltage set points. I also have a few boats that have two or three AC chargers that can be fired up on top of the alt to speed charging.

This was standard practice aboard the 50+ foot sport fishing boats I used to work on. Genset fed a large charger which fed a massive bank of batts. Alternators also fed the same bank of batts. These boats were burning 45-60A just to keep pumps and fishing equipment running. These boats wracked up 1000+ hours per season doing this with zero issues.

Most of the long range trawlers also do this as do a ton of long range cruising sailboats. Weekend warriors can also speed bulk charging in this manner.

All these devices know is voltage. Either the voltage of the "system" is below their limit or above their limit. If below they provide current, if above they simply shut off and wait for the voltage to drop below the set point.. Simple stuff and no damage is done to any of the devices...

 

[Scott B]

Hi - I was wondering if it would be ok to do the following at the same time :

- run engine to charge batteries
- also run shore power charger (via generator) to also charge same battery bank

The shore power charger is a 60 amp unit, and the Yanmar 3xM30 has the stock (50? ) amp alternator

This all feeds into a 450AH bank of Trojans.

Would running both at the same time result in 60 + 50 amps charge current ? (Less the inefficiencies of hot alternator, voltage regulators, etc) - but basically will this give me a faster charge, or cause problems ?

25% charge acceptance rate of the bank should mean it can take 112 amps - but do the 2 sources combine or not ?

Thanks !

(If I gotta listen to the engine, then listening to the genny at the same time makes sense if it reduces overall charge time)

The 3YM30 came stock with a 60 amp alt. I typically see 25- 30 amps max. at first when charging with the engine.

and just the annoying part, the 3YM30 now comes stock with 120 amp alt..... :cussing:

 

[RHUMB RUNNER]

You don't want to run your engine with out a load, you can glase the cylinders. A battery charger is not enough of a load, rpm has nothing to do with it. If you are motor it's fine but not at anchor.

 

[spraygun]

I run my engine prior to leaving my marina with the charger on.Then I unplug from shorepower.Or I just fire up the engine for any given reason at the slip.Never had a problem.

 

[PKFK]

Thanks guys for the inputs (esp. Maine for the expert input).

Bulk charging is what I am thinking of - so this sounds reasonable to run both the engine and generator (to power the 60a xantrex charger) at the same time when away to speed up the bulk charging. I never expect to top up the batts when away form the dock and have read most of Maine's tutorials. But sometimes we still need some clarification - so thanks for the info - much appreciated.

FWIW - I have 4 T105s in a house bank - but they seem to not be giving me the range expected (I use about 100 AH per day - estimated) - but after 2 days (should be 50% discharge) - the inverter complains when making the morning perked coffee - and I need to run the engine or generator.

I in the short term trying to speed up bulk charge when away from the dock to compensate for reduced capacity - longer term I suspect the bank is not too healthy. They are from 2007, and when we got the boat 2 yrs ago, the batteries almost had exposed plates (put in almost 1gallon of distilled water into the 4 T105s )

 

[Bob 04 H260]

Hi Paul, you are probably drawing 100 amps from your batteries to make the coffee. If the bank is down to 50% and is hit with a big draw like this the battery voltage probably drops enough to trigger a low voltage alarm on the inverter. I don't know how unusual this is. I've heard others complain of this same situation. I guess the batteries internal resistance goes up as they discharge ?

Our solution is if the bank is down to 50% in the morning we fire up the generator to charge the batteries and run any 110 V equipment we need at that time.

I've run our 140 A alternator and the 50 A battery charger off the generator at the same time with no problems.

Bob

 

[PKFK]

Thanks for the input Bob - sounds plausible - though it's a drag if the batteries won't run the inverter for coffee if they are 50% or lower.

I also notice that the house bank goes from 12.4v to 12.0v in 24 hrs - which is a drop from 75% to 25% charge. This doesn't add up as the bank is supposed to be 450 AH and we burn thru about 100AH per day.

I'm guessing this is because the bank is old and doesn't really have 450AH capacity.

 

[Bob 04 H260]

Paul, did you ever measure the voltage drop between the battery terminal and inverter input when you turned on the coffee ? If the connections are not great it might be the straw that breaks the inverters back
Bob