Confused..What size charger?

[Sid at SailAway]

Decided on a charger only and I'm a little confused as to what size to buy. I have two Trojan starting batteries and one bank of house batteries consisting of 6 volt Trojans connected together to produce 600 amp hrs. I will power the charger with either shore power or a Honda 2000.

Do I need a 40 amp, 60 amp or larger? What does it matter about the amps? Is the difference in how fast the charger can charge the bank? Can the Honda power a larger charger?

I found a Mastervolt that is rated 40 amps and is recommended for a 120 to 400 amp hr bank. Does this mean it won't charge a 600 amp hr bank?

I know, lots of questions but I do appreciate all replies!

[skipmac]

To some degree the size charger will depend on your daily usage, boating style and source of power for the charger.

Rough rule of thumb buy a charger that puts out about 25% of your total battery capacity. Since the starting batteries should not lose much charge at all in normal usage you can pretty much ignore them and base the calculation on the total amp hours of the house bank.

The 25% assumes standard wet cell, lead acid batteries.

[Dsanduril]

Really depends on how you use the boat and power the charger. A 40A charger will charge your 600Ah bank just fine, it will just take a while. In theory a fully discharged battery would take 600/40 = 15 hours to charge, although it will be longer than that as the charger taper off. A 60A charger would take 10 hours.

If you go out for the weekend, put the boat back on the dock Sunday night for the week, and plug in the charger then pretty much anything will do. If you drain the batteries down 30 or 40% every day and need to fill them back up then you'll want a bigger charger, particularly if you are running the little Honda, do you want to listen to it (quiet, but still...) for 4 hours every day? If you are using the Honda don't get a charger that is too big (or that doesn't have the sophistication to allow you to limit the output). A 100A charger will require about 1400W on the output side, which runs to about 1700W on the input side (depends on efficiency), so anything larger than that will likely overload the Honda. Depending on conditions you might even overload under 100A, depends on too many unknowns.

[Stumble]

Sid,

I can't imagine the size engine you have that needs two starting batteries. My 450hp diesels only need one.

That being said, a battery charger needs to be sized to the batteries, but the information you are leaving out is how quickly you want the batteries to charge. You also need to be aware that the faster you charge a battery the shorter it's life span will be. So a long slow charge will extend the life of the battery compared to a quick, fast charge. With fast charging there is also a problem called surface charge. Where the batteries will show higher charge states than they really have when charged at high speeds.

You also need to account for permanent power draw. For instance if you have a 12v fridge that is left on, this draw needs to be taken into account, or a small charger may never be able to get the batteries up to full charge, because they are being drawn down as fast as they are charged up.

Realistically I like about 25% of the ah of the bank in a charger. But with large banks (like yours) this becomes impractical. In which case I would recommend the largest charger your wiring and source can support. Which for you would probably be a 100amp smart charger (always, always buy smart chargers btw). This will draw about 15amps at 110v so a standard 20 amp fuse will work.

The Honda generator will be a little under this power generation (they are rated at 13.3amps) so you may need to step down to a slightly smaller charger, but this will significantly extend your charging time on shore power.

[Sid at SailAway]

Just a little more info..I'm on a cat so have twin engines, thus the two starting batts. We are full time live a board but do not plan on spending much time at the dock. Most of the time we hope to spend on the hook. This first month or so we will be docked as we just bought the boat and will be getting her ready. I will be adding more solar in the future, but that's maybe a year off.
Hope this hopes....Thanks!

[hellosailor]

You might want to consider using the charger to charge the 600A house bank, and then using two echochargers, hellroarers, or yandina/west battery isolators so that once the house bank is seeing full charging voltage, the starting batteries will get joined in and charge as well. Makes for a faiarly simple way to keep everything charged, but also isolated when not charging.

If the total capacity is 750? 800? Ah, and in theory you are a kind master and never pull them down below 50%, then you would never need more than 400Ah put back into the batteries. (An even kinder master might only pull them down 30% and never need more than 300Ah.)

Wet lead batteries as a rule of thumb can charge at 1/4 of their capacity rating, so if you need to put in 400Ah, a 100A charger would do it in four hours. Or a 40-50A charger would do it overnight on shore power. You have to decide, how much capability do you want, how much will that cost you, do you have space to install it, etc. And of course having some extra capacity in the charger means it will probably run cooler and last longer as well.

[Dsanduril]

Sid,

That helps a lot. The remaining question; do you have (or are you installing) a high output alternator on one of the main engines? Kind of boils down to personal choice.

Either way I'd probably install the maximum charger that your Honda can power. That's probably 80A or 100A. If you go for 100A or larger make sure it has the ability to limit output power. When on the Honda you can set it for 80 or 90A, when you do have shore power you can let it go full bore. With a big battery bank you'll want it to be able to charge as fast as possible if you are using power every day.

Go for the solar sooner rather than later. Once your start listening to the generator/engine every couple of days you'll be happy with the silent investment.

[Stumble]

It does Sid, since it explains the two starting batteries.

I still would stick with my recomendation of a 100a charger if you can swing it. If you are going to be reliant on the 2kw generator though I would go with a 75amp charger. It will eat up almost all of the generator though, since it should draw about 12amps at 110v. Frankly this would mean leaving the generator on for long periods of time, so those solar panels become pretty important if you ever want fully charged batteries.

I might seriously recommend switching to AGM batteries that can take much higher charging amperage, and getting a second 2kw Honda. This way you could parallel them and use a 150 amp charger. It would cut your charging time in half, and actually allow you to turn off the generators some during the day.


On a bigger picture, do you have a power budget? I am concerned that as a live aboard you may be drawing down so much power your small generator won't be able to keep up, or would require really long run times.

[hlev00]

Originally Posted by hellosailor

You might want to consider using the charger to charge the 600A house bank, and then using two echochargers, hellroarers, or yandina/west battery isolators so that once the house bank is seeing full charging voltage, the starting batteries will get joined in and charge as well. Makes for a faiarly simple way to keep everything charged, but also isolated when not charging.

If the total capacity is 750? 800? Ah, and in theory you are a kind master and never pull them down below 50%, then you would never need more than 400Ah put back into the batteries. (An even kinder master might only pull them down 30% and never need more than 300Ah.)

Wet lead batteries as a rule of thumb can charge at 1/4 of their capacity rating, so if you need to put in 400Ah, a 100A charger would do it in four hours. Or a 40-50A charger would do it overnight on shore power. You have to decide, how much capability do you want, how much will that cost you, do you have space to install it, etc. And of course having some extra capacity in the charger means it will probably run cooler and last longer as well.

Good idea on the echocharger. However I beg to differ on the estimate on charge time for lead acid. The charge acceptance rate tails off so much towards the end that for the last 10% you are only charging from somewhere around 10A down to 1-2 A. You'll have to run the generator for a long time (10-15 hours) to get to a full charge.

[Sid at SailAway]

I forgot to add that the Trojans are AGM's...

[David M]

A Honda EU2000i will not power a 100 amp charger at full charge. I have tried it and it knocks the generator offline. What you can do is to get a 100 amp charger which allows you to control the maximum output, say for example limit it to 80 amps for when you are using the generator and set it back to a maximum of a 100 amp charge for when you are hooked up to shore power. Although it is called a 2000 it is actually a 13.3 amp (1596 continuous watts of power) generator.

I wouldn't waste my money on a second generator when all you have to do is to get a 100 amp charger that you can throttle back a little so you do not overload your generator.

http://powerequipment.honda.com/gene...models/eu2000i

[sailinglegend]

Originally Posted by Sid at SailAway

I forgot to add that the Trojans are AGM's...

AGMs change things a lot!!!! You need to give them as much current as you can, so go for the largest charger you can find, and downsize the charger output when using the Honda on the hook. You might want to consider a DC generator that will charge at about 250 amps and sell the Honda. Also do look at high output alternators for each engine.

The bottom line is when going into a marina - at least once a month to FULLY charge your batteries - you only want to do this for one night. If you come in at night and want to leave in the morning a 600 Ah bank may not get back to full charge even if you have a large charger.

If you go for a 120 amp charger then also consider the extra cost of an Inverter/Charger like the Victron.

I also agree that Solar is the way to go - try for 600 watts - easier on a Cat than a monhull.

[Dockhead]

Victron, MasterVolt, and probably some other charger/inverters allow you to regulate the power draw and turn the battery charger down to match the source, in order not to overpower your Honda generator.

There have been a few calculations about how fast you can reach a 100% charge by using a charger rated at 25%, say, of battery capacity. These calculations are fundamentally wrong because battery acceptance rates fall off rather sharply after about 70% charge is achieved. By the time most regular lead acid batteries are nearly fully charged, they are only accepting about 2% of C -- the last 10% or 15% is extremely slow whatever your battery charger capacity is. So if you are drawing down to 50% and charging to 100%, it will take many times more time to do that, than would be predicted by just dividing the charger theoretical amps by battery capacity.

One result of this fact is that bigger battery chargers don't always make such a big difference in practical charging times. I think for most people 15% of C is enough.

[sailinglegend]

Originally Posted by Dockhead

...... battery acceptance rates fall off rather sharply after about 70% charge is achieved. ....

One result of this fact is that bigger battery chargers don't always make such a big difference in practical charging times.....

Your quote is only true if you take into account the TOTAL charging time to 100%. Up to 80% - especially if you are running the engine or a generator - then there is a big difference. Above 80% let a big solar array take over.

If I change from my 100 amp engine alternator which is charging at 80 amps, to my DC generator, I get 250 amps going into my batteries. Only when they reach about 80% charge do they both put in the same current. So you will get from 50-80% of charge much quicker. That's 60% of your usable capacity.

If you have AGMs then their faster charging time with a higher current WILL get the bank up 80% much quicker. To put 100 Ah into an AGM will only take 102 Ah. To put 100 Ah in a flooded battery will take 120 Ah or more because of the difference in their charge efficiency.

With AGMs it makes sense to optimize your charging systems to justify their increased cost. Lifeline say do NOT charge their AGMs at less than 20% of C.

[Exile]

Originally Posted by sailinglegend

Your quote is only true if you take into account the TOTAL charging time to 100%. Up to 80% - especially if you are running the engine or a generator - then there is a big difference. Above 80% let a big solar array take over.

If I change from my 100 amp engine alternator which is charging at 80 amps, to my DC generator, I get 250 amps going into my batteries. Only when they reach about 80% charge do they both put in the same current. So you will get from 50-80% of charge much quicker. That's 60% of your usable capacity.

If you have AGMs then their faster charging time with a higher current WILL get the bank up 80% much quicker. To put 100 Ah into an AGM will only take 102 Ah. To put 100 Ah in a flooded battery will take 120 Ah or more because of the difference in their charge efficiency.

With AGMs it makes sense to optimize your charging systems to justify their increased cost. Lifeline say do NOT charge their AGMs at less than 20% of C.

If I'm understanding this correctly, the relationship b'twn batt capacity & charger size is only really pertinent to the "bulk" charging phase, i.e. up to 80-85%. After that, the batts are then into the "absorption" & finally the "float" stages so the charger capacity -- whether a charger or solar, etc. -- no longer really matters since it's all about the batt acceptance rate vs. the charging source output. Is this right?

Re: your comment that "Lifeline say do NOT charge their AGMs at less than 20% of C," is this total batt capacity or each batt that the smart charger is charging at any particular time. In my case, for e.g., I have 3 Lifeline AGM's with 225ah each, but my smart charger will only charge one at a time based on need. So in my case is the 20% calculation based on 225ah, or the combined 675ah?

Thanks for all the useful info.