AGM Battery Bank Configuration

[Exile]

My boat came to me configured with two 8D house batteries, one 8D engine start battery, and one automotive-sized gen start battery. About a year ago I replaced all of them with Lifeline brand AGM's, and replaced my battery charger with a 50A, so-called "smart" charger made by Xantrex. The 2 house and 1 eng start 8D's are 255AH, so that equals 500AH or so to run the ship and an additional 8D that is used only to start the engine. The two house batts. are on a selector switch that allows me to use one or the other, or to use both, and I always leave the switch on both. The eng & gen batts have their own separate switches.

Thus far, everything seems to be working beautifully, but it seems wasteful to have a deep-cycle 8D devoted solely to starting the engine, esp. when it could be more efficiently put to use as a 3rd deep-cycle house battery. This seems especially the case on my boat since both the eng & gen are wired for parallel solenoids, i.e. they can both be started with help from the 2 house 8D's if need be.

My questions are:

a) Would it be advisable to use the 8D eng start batt. as a 3rd house batt. that would give me approx. 750AH, and then perhaps have the smaller gen start batt. also serve as the eng start (knowing that I have the parallel circuits as backup)? In answering this question, was there any good reason to configure the boat the way it is now (probably by the mfg)?

b) If this is advisable, should all three 8D's be wired as one circuit? As of now, my batt. charger only has 3 ports, which means that my gen batt. is charged via a relay off one of the house batts. I probably don't know what I'm talking about, but it seems like it may be preferable to only have two circuits, one for the boat & the other for starting engines. I suppose the downside is that it may not allow for easy monitoring of individual batts. like the current set-up affords.

Sorry it took me so long to describe my system, but this is all a brave new world for me. Any thoughts on those with expertise in this area?

[DeepFrz]

Sounds like you have the right idea, IMHO.
a) I would.
b) Yes, wire them all as one cct. and run the generator to the house bank. You will have to be careful setting the regulator so that you don't burn out your alternator. You don't mention the size of the alternator so it may have to be upgraded as well. Think electrical "system", not just about the components.

If I was doing it I would probably add a start battery for the engine and keep the start battery for the gen. with an echo charge to keep the start battery charged.

I hope this helps.

[motion30]

I agreee with deepfrz . Also I think at some point you may want to install a much larger charger to take advantage or the agms ability to accept a high charge rate this will reduce run time on you gen set

[Exile]

Thanks for the helpful replies. FWIW, I have a "BEP Voltage Sensitive Relay" that charges the gen batt. off of one of the house batts. It is not a diode based unit so the downstream (gen) batt. does reach a full charge. This does seem to be borne out by the meters. I suppose I could have another one of these for the additional eng batt. that deepfrz suggests. I assume this is only useful when running the eng or on shore power; when the genset is running I'm assuming it's own alternator charges it's own battery.

Getting a higher capacity charger for the reasons motion30 cites also sounds wise. deepfrz -- I'm not yet catching on to your warnings about burning up the alternator. Do you mean the engine alternators? I have 2 -- one for charging the eng batt & the other for the house banks. They are both 50A. Would these need to be upgraded as well? Or do you mean the alternator on the genset which has its own regulator?

As you may gather from my questions, I'm still pretty green on 12V electricity. Do you guys believe this modification is relatively straightforward, or best left to a pro? I'm also not sure if it is something that should be done a year into a new set of batts., or perhaps with a fresh set when that time comes due.

Thanks again for the help.

[djmarchand]

There are two reasons to separate your house and starting battery banks:

1. So you can use different types of batteries in each. But AGMS usually work ok in both starting and deep cycle service, so it probably doesn't matter in your case.

2. But much, much more importantly by having two different banks you can run your house loads off of that bank while at anchor by setting the 1,2,all switch to just the house bank. This keeps your starting battery from being discharged while at anchor so you can start your engine when you are ready to go by switching back to the start battery.

[motion30]

Exile
On my boat the gen set starting battery takes care of its self. I guess if yours sets a long time between useage it may need charging. I deepfrz is concerned that your alt may overheat as 50amps is not much to recharge 750amps. I had found a large frame 105 amp alt on ebay cheap, It has been working great Sounds to me you have a good plan

[Pete7]

Originally Posted by Exile

As you may gather from my questions, I'm still pretty green on 12V electricity. Do you guys believe this modification is relatively straightforward, or best left to a pro? I'm also not sure if it is something that should be done a year into a new set of batts., or perhaps with a fresh set when that time comes due. Thanks again for the help.

No you could do this yourself and save a few $$$ and I would do it now to gain the benefits of the larger bank. You probably have most of the cable you need but draw a diagram so you only need to buy the shortest length for any new cables. An 8D battery for engine start is way over the top, unless your have a large trawler.

We did a very similar exercise two years ago on our yacht. 2 batteries one for house and one for engine twinned up and a third much smaller engine battery fitted for the engine. VSR to join the banks together when the engine is charging and an additonal switch for emergency paralleling the batteries. this is what we used:

Power Store&

Works well, doubled our house bank capacity and no worries about running the batteries down because the engine start battery is separate. It also now means the solar charges both our house batteries even when we are away from the boat and everything is switched off, because both batteries are now joined together.

BTW this all started when our rotary 1/2/0 switch failed to disconnect despite being turned off due to being worn out after 20 years. Result one dead battery.

Pete

[btrayfors]

Exile,

I agree with most all of the above.

The reason to be concerned about your alternator is that a 50A alternator is WAY too small for a 750AH AGM battery bank. It can easily draw enough current to keep the alternator putting out at full capacity for a very long time....the alternator isn't designed for that, and is likely to burn up unless de-rated or equipped with a smart temperature-sensing regulator.

Depending on your engine setup, you might want to upgrade the charging setup by installing the largest alternator you can comfortably fit, with a smart external regulator (like the Balmar MC-612). Those AGMs can easily accept 750A charge current or more, though it's not likely you could fit anything like that capacity on your boat.

Before you do anything, though, I'd suggest you consult a competent marine electrician with experience in high-power charging systems. Could save you a lot of $$$ and headaches.

Bill

[DeepFrz]

About the alternator. AGM batteries will accept a very high charge current and your little 50 amp alternator will not be able to supply that without overheating and burning out. I would think that a 100 amp (hot rated) would be about the minimum size alternator you would want. You would still have to set the external regulator so that the alternator doesn't burn out. You would be getting much quicker initial charge and reduce your engine run time somewhat.

If you go above 100-110 amp alternator you are looking at other modifications to the engine/alternator pulley/drive belts.

Lots of information on this forum to help you out. btrayfors (among others) has posted a lot of information on the 12 volt electrical system. Look for some of his posts.

Good luck

And he posted above while I was typing this...

[savoir]

Do you have electric winches or furlers ?

[Exile]

Thanks for all the helpful replies. I now understand the concerns over the capacity of my 50A alternator that runs off the engine and charges the house banks. B'twn upgrading the alt & also possibly the batt charger, it sounds like it could get $$$ and complex to modify as needed to charge 750AH, esp. since everything I have now is only a year old . . . and is working! Yes, I do have electric furlers for the main & headsail, as well as an auto-pilot, instruments, GPS, & computer that are almost always on. No 12v refrig. though. Seems like the 2 house batts. w/500AH is more than adequate, esp. with a 8kw genset and backup Honda (eventually I'll supplement w/solar), but then again it seems a shame not to be able to put so much more capacity into service.

Given my existing set-up, Plan B could be to simply replace (when needed) the eng start 8D with a smaller, more suitable batt, modify the batt compartment space, & leave everything else alone. Would cut the expense of buying three 8D's, and reduce the hassle of getting so much wgt. out of & back onto the boat (gonna try the boom and block & tackle up through a hatch next time!).

Probably way off the mark, but here's another thought: would it be possible to use my two existing eng alts to charge the three 8D's as I am doing now, but then somehow get all three wired into service as house batts? The existing eng start 8D could perhaps do double-duty, or the existing gen start batt could also be used as the eng start. With the BEP relay set-up, my understanding is that the gen batt is always getting charged, whether it's from running the eng, or via the charger. I also have the parallel circuits as backups for starting both engines. Am I out of the ball park here??

Some good points made about thinking of this as a system rather than individual components. There are obviously several different ways to go and I will likely heed Bill's advice to at least consult with a pro before doing anything.

These forums have become a consistent source of higher education for me in pursuing my goal of becoming more self-sufficient with my boat. Thank you all!

[SV Demeter]

I would double check the alternators and confirm they are really only 50 amp units. It seems a bit odd that the factory or any good tech would go the effort of building even a 500 amp hour bank of 8D AGM batteries and only select a 50 amp alternator. If thats the case and it were me I would still opt for connecting all three 8D's but I would then upgrade the alternator to at least 100 amp and also install a smart regulator my preference being the newer Balmar 614 and use temp sensors on both the alternator and the battery bank. With a smart regulator you can inform the alternator of your battery bank size and chemistry and ensure you wont burn it out. Much lower on my list of priorities would be the AC charger but if you run the generator a lot then maybe consider choosing a much higher output AC charger first over the alternator. Either way the most important thing I can see from your posts is that you should make sure you have a decent smart regulator on the house bank alt. or you will likely smoke that thing under any serious crusing loads.

[Pete7]

Originally Posted by Exile

I do have electric furlers for the main & headsail, as well as an auto-pilot, instruments, GPS, & computer that are almost always on. No 12v refrig. though. Seems like the 2 house batts. w/500AH is more than adequate, esp. with a 8kw genset and backup Honda (eventually I'll supplement w/solar), but then again it seems a shame not to be able to put so much more capacity into service.

If he has a 8kw genny on board why would he use the engine alternator to charge the house bank on a regular basis? surely that would be the more efficient route.

Pete

[SV Demeter]

Originally Posted by Pete7

If he has a 8kw genny on board why would he use the engine alternator to charge the house bank on a regular basis? surely that would be the more efficient route.

Pete

Very good point. What would you guess the charge acceptance rate on a 750 ah bank of AGMs to be anyway, at least 500 amps no? My guess is you would want one hell of a big charger and even then I would hope he is running air conditoning or something else to pull a load or that genset wont be happy for too long. And if he does not have air conditoining would he not be better off with a DC generator?

[Exile]

When I'm underway I definitely run the genset for charging batts., refrig/freezer (110v), wtr htr, etc., as opposed to the engine. For better or worse, the boat is also loaded up with 2 a/c's (10K & 16K btu), but I generally only use those at the dock. Hopefully I'm not underutilizing the 8kw genset to the point of impairment by also not running the a/c's. The installation I got done in Annap. a year ago also included temp sensors for all the batts., although I'm not clear what role, if any, they play when I'm using the motor and both alts are therefore doing the charging vs. the A/C charger. There's also an engine drive for the frig so I rarely have both the eng & genset running at the same time. If I do, I keep the charger off just in case. My understanding is that each eng alt has its own regulator, but that's regrettably the limit of my understanding. Based on the meters, there does not appear to be any over-charging going on. I'll definitely double-check the capacity of the two eng alts per the suggestion.

Speaking purely as a layman, the ideal set-up in my mind would be to be able to leave everything alone on the charging side, but then somehow enlist the eng start 8D into service as a house batt. Installation of the smart regulator that has been suggested sounds like it may be a good idea regardless.