Batteries . . . Again !

[southace]

I have just added a new 21amp shore power battery charger, My thoughts are that if I added another charger to the same house batterys they will charge at double the time would this work?

[sailorboy1]

they will only charge at the rate they will accept based on size and carge state, if discharged down the batteries may accept about 25% of their capacity for a while and then slow down regardless of the rating of the charger

[sailvayu]

It depends on the charger but most will not work. Charger #2 senses the higher voltage produced by charger #1 and reduces output "thinking" the batteries are fully charged. Of course it is not quite that simple but that is pretty much the result. I have seen it done several times even by a major manufacture once. So far I have not seen a setup that worked.

[Ex-Calif]

Originally Posted by southace

I have just added a new 21amp shore power battery charger, My thoughts are that if I added another charger to the same house batterys they will charge at double the time would this work?

Depends on the batteries and the charger. Need more info to make an informed recommendation.

The specialty guys, Victron etc, make charger that can be paralled.

Your Basic Autozone charger, not so much.

Also, the really smart electrical guys hang out in the techie forums so may not have seen this post. I will ask a moderator to move this whole thread.

Cheers!

[Adelie]

Originally Posted by sailvayu

It depends on the charger but most will not work. Charger #2 senses the higher voltage produced by charger #1 and reduces output "thinking" the batteries are fully charged. Of course it is not quite that simple but that is pretty much the result. I have seen it done several times even by a major manufacture once. So far I have not seen a setup that worked.

+1

If one or both are smart chargers they will chase each other around.
If the are both dumb chargers you run the risk of damaging your batteries. The damage will probably be minor over 1 cycle but will accumulate with each cycle.

Sorry.

[delmarrey]

Besides, charging a battery too fast burns/boils the plates, shortening the life of the battery(s).

[simonmd]

Originally Posted by delmarrey

Besides, charging a battery too fast burns/boils the plates, shortening the life of the battery(s).

True, you can easilly damage then by trying to 'fast charge' them. I was wondering why you want to do this anyway?

[Ex-Calif]

Originally Posted by delmarrey

Besides, charging a battery too fast burns/boils the plates, shortening the life of the battery(s).

I am sure that 90% of the folks wanting / needing advice get frustrated by the "it depends" responses when all they want is the baby, not the pain, so to speak. Experts and there are many here, try to drag out a little more information in order to provide a precise recommendation. Then someone makes a recommendation or a comment and the thread devolves into a techie argument with pleukerts formulae and all kinds of stuff.

At great risk of that and further demonstrating my own ignorance here goes...

There are two "common" battery constructions the average boater will see - there are others. Wet and sealed. You can add fluid to the wet battery. A true sealed battery you cannot.

Wet batteries usually have three duty ranges - deep cycle, start and there are some general duty type that are supposed to be in between.

Sealed batteries are generally glass mat or gel. Sealed batteries are not generally deep cycle batteries.

Charging a battery makes heat. The faster you charge the more heat you make. Heat can boil or gas off the liquid in wet batteries and sealed batteries. When a sealed battery overcharges it may pop off the vent permanently reducing the performance or even permanently damaging the battery cuz you can't add fluid. We desire to charge "fast" especially house batteries while the engine is running, wind is blowing, sun is shining or shore power is connected. Deep cycle batteries are constructed with heavier plates inside that are more tolerant to high charge rates.

However, heat also reduces the ability of the battery to accept a charge. This shows up as the charger "thinking" the battery is charged and it stops charging. The reasons are technical and the average boater doesn't need to know why.

The wet deep cycle is a good house battery, we'd like to discharge it a lot and charge it fast when we can. It requires maintenance in terms of checking water levels. The windlass and starter take a lot of power out "quickly" but after an anchor pull or engine start the battery is really not depleted much and we aren't gonna use the starter again real quiick or pull the anchor again real soon. We can charge it slower. A sealed battery usually works well for this as it is can be cheaper, lighter and requires no maintenance.

However because they are different construction, ideally they need two different charge rates. The "heavy" deep cycle can take a faster charge.

In a big system it makes sense to have two charging rates. The batteries are very expensive and charging them wrong can deplete their lives by years! So they will have two different charging schemes.

The average small boat with 2-3 batteries will likely have all sealed batteries solving the maintenance need and eliminate the desire for two charging systems. Although with one bank deep cycling and the other not a there are a couple of other problems created but easily solved and more often than not ignored albeit at a lower total battery life.

Fnally we get to talk about charging them! Because of the heat, and the battery "tricking" the charger two stage and 3 stage chargers are now common.

Bulk charge - it is general advised that the maximum charge rate is total battery capacity divided by 10. A 200ah battery system should be charged at 20amps. This is the fastest you should go. For a depleted battery it will accept this charge and the heat will not be a problem.

Absorption - At some point (80? charge) the heat and resistance will start becoming a problem. How the charger knows this is not important. A good 3 stage charger does and starts reducing the charge current. The more expensive the charger the more sophisticated can be the sensing technology.

Float - at the "end" of the absorbtion cycle a 3 stage charge goes into float to "perfectly" top up the battery. Or so they say!

You can see if two chargers were in a system each one thinking they were in a different phase of charging it would be a problem. Really smart chargers from the same company (usually) can work together if the company advertises them to.

Conclusion:

You have to know how big your battery capacity is
You have to know what types of batteries you have
You have to know how smart your chargers are and how many phases there are and now much charge current they are gonna provide in the bulk phase. Smart chargers can be switched for when the phases kick in depending on battery type (see conclusion 2)
Don't stick two "not so smart" pep boys 2 stage chargers together - and if you have one of these with the "quick" charge position, reread the above and think about whether quick charging your $300 agms at the 200 amp setting is a good idea...

A note on solar & wind. On the average boat described above and in the amounts that the owner might install the charge these guys add is minimal compared to the alternator or battery charger. In general, hook them up and don't worry about it. The charger and alternator performance won't be negatively impacted by them.

Now when battery banks get large and solar arrays get big and electrical consumption is large and maximum charging efficiency is desired and maximum expensive battery protection is rerquired the systems get very complex quickly.

Let the technical quagmire begin!

[savoir]

There is a figure for the max output a charger can safely send to a battery bank. My recollection is 1/4. That means if you have 300 ah worth of batteries then the biggest charger they can handle is 75 amp. Any more and they will cook.

No doubt the experts will confirm or deny this figure.

[goboatingnow]

In general paralleling charge sources will do no damage. During bulk mode all charge sources will contribute, during absorption only some may contribute but then lots of current isn't needed.

They will not " chase each other round "

This was comphrensively debated on another thread recently

Dave

[southace]

So by adding many solar pannels and wind gens what you are all saying is you can only put so many amps at a time back into the batterys??
Most battery charges I see are a max of 21amps in the shops, yet while running a 40 amp alternator plus a wind gen plus solar you would be adding say 80 amps per hour so that got me thinking you could add 2 x 21 amp battery chargers on shore power so your batterys would charge in double the time?

[southace]

Originally Posted by savoir

There is a figure for the max output a charger can safely send to a battery bank. My recollection is 1/4. That means if you have 300 ah worth of batteries then the biggest charger they can handle is 75 amp. Any more and they will cook.

No doubt the experts will confirm or deny this figure.

Ok that explains why my charger says it can charge 400 amps of battery in 24 hrs

[Pete7]

Originally Posted by southace

So by adding many solar pannels and wind gens what you are all saying is you can only put so many amps at a time back into the batterys??
Most battery chargers I see are a max of 21amps in the shops, yet while running a 40 amp alternator plus a wind gen plus solar you would be adding say 80 amps per hour so that got me thinking you could add 2 x 21 amp battery chargers on shore power so your batterys would charge in double the time?

Yes, except you don't get 40 amp hours from an alternator for long. If you watch an ammeter the current quickly drops down to a point were you wonder why you are running the engine because its doing so little.

Same applies to my 40AH 4 stage battery charger. Starts with 42 AH, but it will be reading in the high 20s within an hour.



Pete

[btrayfors]

Some of the posts above suggest that using "too large a battery charger" will cook your batteries.

It won't.

Batteries will accept what they're going to accept depending on their type, age & condition, and state-of-charge (SOC), period. So long as the charger doesn't exceed the normal charging voltages for each stage, it's perfectly OK to use "too large" a charger.

Example:

A 200AH battery bank is now on charge with a 40A charger and is now accepting 23 amps @ 14.4 volts.

How much will it accept if you use an 80A charger @ 14.4 volts?
It will still accept 23 amps.

How much will it accept if you use a 150A charger @ 14.4 volts?
It will still accept 23 amps.

How much will it accept if you use a 250A charger @ 14.4 volts?
It will still accept 23 amps.

It's not the size of the charger which can cause problems with boiling and overcharging. It's the voltage level at any given stage of charging.

That's why you can't just look at the amperage being delivered. Your 100A alternator putting out only 12A may be just fine....the batteries wouldn't accept any more if you had a 250A alternator (at the same voltage and stage of charging).

Try as you might, you can't beat Ohm's Law.

In an active circuit, amperage = voltage/resistance.

So long as the voltage is held constant, the amount of amps in the circuit will be directly affected by the internal resistance of the battery.

The battery doesn't know it's connected to a small or a huge capacity charging source. It just knows that it's internal resistance at any given state of charge is X, and that factor plus the applied voltage will totally determine the amount of amps flowing.

Bill

[simonmd]

Originally Posted by southace

So by adding many solar pannels and wind gens what you are all saying is you can only put so many amps at a time back into the batterys??
Most battery charges I see are a max of 21amps in the shops, yet while running a 40 amp alternator plus a wind gen plus solar you would be adding say 80 amps per hour so that got me thinking you could add 2 x 21 amp battery chargers on shore power so your batterys would charge in double the time?

Dont forget that wind gens and solar arays are fitted with regulators that stop the batteries being over charged. If they detect the current from an alternator going in or if the battery is full, they will shut off.