Marine battery group size whats the difference???

okay as for charging the general rule i have heard is with wet batteris you want around 20 to 25 % in you alt size to the battery bank. ie if you have a 200 amp hour bank you want around 50 amps of alt, if you have 400 you want 100 amps from the alt. going bigger is not a problem due to the way the batteries accept the amps.

the advantage of bigger batteries is its normally cheaper to get 2 batteries that add up to say 200 amps than 3 batteries that do. if the boat can fit bigger batteries then go for it.

as for having more amps that another discussion entirely. unless you do nothing but single day sails with motoring or shore power time with a charger you want more amps than what a group 24 gives you.

the deep cycle thing applies to this as well. the less % of discharge you do the longer the battery will last. ie if your day sails use 50 amps, and your bank is 100 thats a 50 % discharge. then if the battery is say 125 amp hour and you use the same 50 amps its only a 40 % discharge which will mean you will get more discharge cycles out of the battery before it needs to be replaced

moving from a 24 to a 27 is only like 20 to 25 amp hours difference. ie about 5 % if you had a 55 amp alt, so no that is fine. now going to say a 300 amp bank from a 100 amp one on a 55 amp alt could be a problem. although it just means to charge by engine you just would have to run it longer.

a 300 amp bank, at 50 % discharge would take around 4 hours to get to 80 % charge with a 55 amp alt, vrs with a 100 amp it might take 2 to get to 80%.

the old saying is you get 75 % of the charge in 25 % of the time. so anything over 80% charge will take two or 3 times as long as the first 30 % from a 50 % charge.

i run a 55 amp alt on a 230 amp hour bank, yes a little under sized for the alt but not bad.

btw all of this goes out the window when you add solar or wind, which is great for the last 10 to 20 % of charging, due to cost for watts. ie why run the motor, for the last 30 amps of charge when 160 watts of solar will do it during one sunny day with room to spare ( 160 watts will give you about 10 amps per hour )

I installed a double 31 group 105 amphr AGM's as my house battery and swapped out the 35 amp with a 55 amp on a 2GM20 Yanmar. After a long night out, the engine will still start from the house but I have an engine start battery as backup. The amp meter does display close to 50 amps for about 15 minutes and then starts to taper off as it charges. I'll use shore power on the house charger to keep the batteries topped off should I not run the engine enough to do so.
I've had this system for 5yrs now and the batts are still going strong. The AGM's are great but someday they will need to be replaced.
I read a reference somewhere that for an 100 amp alternator it should have 2 drive belts due to the loads being too great for a single drive belt. Cars have high output alternators on single belts, but I can remember having to change them to keep them from squealing as they started to age. Is there any real rule of thumb of alternator capacity and # of belts required????

Charging Batteries

This talk of alternator sizing is a little bit misleading. Some seem to think that you have to have a bigger alternator. You can charge a 400 ah battery bank with a 10 amp alternator. Might take the rest of your life, but you can do it.
The primary difference the alternator size has it charging time required. The 25% of capacity charging rate is the ideal number, but it is not the required number. Keep in mind that a 400 ah battery bank, which should accept 100 amps at 25%, is only going to accept that 100 amps for a very short period of time. As soon as the battery starts to come up the charge rate is going to go down. Above 80% charge, the acceptance rate is pretty low. Many, including myself, try to maintain between 50 and 80% charge. Getting that last 20% into a battery just takes too long, unless plugged up to shore power. Just put in the biggest things you can get in there and can afford. You can never have too little of a battery bank.

Flat,

That recommendation is based on typical sailboat engine usage -- which is MUCH less than motorboats. The idea with sizing alternator output approximately 25% of the battery bank reserve capacity is to hopefully ensure that the batteries get as close to fully charged as possible in the limited time that sailboat's typically run their engines. Chronically undercharged batteries will offer poor performance and typically have a shorter lifespan.

In other words, if someone were to put in a 400 amp battery bank and charge it sporadically from a 50 amp alternator, they could be worse off than if they put in a 200 amp reserve capacity battery bank and charged it for the same amount of time from the same alternator. Assuming typical sailboat usage and charging profile, the 200 amp battery bank generally speaking could have a longer life and offer better performance.

In a motor boat, where the engine is running the entire time that the vessel is in transit, you could get away with a smaller alternator. Funny though, motorboats tend to have comparatively high-amp alternators -- I guess the theory being that you have the power available so why not use it.

I reread all the posts on this thread and it seems to me the discussion is about both. But it's a problem that can't be beat with engine charging alone. That's why many long term cruisers away from the dock for weeks on end exist between the 50% and 80% level with occasional full charging with ac charger when plugged in. Solar panels and/or wind gen will allow full charging when away from the dock. As long as you have a good 3 stage regulator (Balmar or similar) on your alternator you should be maximizing its output between 50% and 80% levels (bulk charge).
For example take roline's system, 210 amp/hours of AGM batteries, 55 amp alternator. A 110 amp alternator should half his charging time between 50% and 80% charge - as long as he has a 3 stage regulator that will not allow his alternator to lower its output when the battery voltage rises a bit.
flatracker - yes a 10 amp alternator will work eventually, but remember we are talking of sailboats where any engine use is an intrusion with the exception of motoring in and out of the harbour and powering through calms. Time is the goal and since there is no magic way to go from 80% to 100% the best we can hope for from our engine is the fastest way to get from 50% to 80% charge level.
Brian