Batterie(s); 2 flooded or 1 big AGM, help me decide which way to go

[ajgraz]

Long post, I hope I have provided all the detail needed to help you help me decide...

15' 1978 StarCraft trihull with 1978 Merc 70hp (rectified, but unregulated charge circuit, hooked to start battery). Also run a Motorguide SW46 12v trolling motor (barely adequate, but does the job in the Bay; I mostly just drift with the wind, tide and current). Have owned this rig about 6 years, will probably own it 3 or 4 more years.

Have been using: 1) a group 24 lead-acid flooded starting battery (525 CCA, 80min RC), all boat electricals tied to that battery; and 2) a group 27 lead-acid flooded deep cycle battery (182min RC), trolling motor only. Onboard charger Motorguide MAXI 26 2-bank, 6amp per bank.

Here's my issues:
1) Charger took a crap, so I'm in the market for something new (looking at ProMariner Gen2 or the Cabela's brand or Bass Pro Shops XPS, all the same thing I believe) I only take the boat out every two or three weeks or so and I always charge right after I get home and then again a day or two in advance, so I really only see needing 4-6 amps/bank.
2) Deep cycle battery took a crap (3rd one in 6 years), so I've got to do something there. I think maybe I need to charge more regularly, because I can go three or four months without using (charging) the boat if I'm travelling or working on other things.
3) Starting battery still good after 6 years, but surely living on borrowed time.

When I have nice, working batteries, I get from the above:
-Can run all my required accessories while fishing (VHF, fishfinder, bait tank pump, anchor light, figure about 7amps total draw) for 2 to 2.5 hours at a time off the start battery, calculating using up about 50% of the RC (not counting any motor re-charging).
-Can run the trolling motor (figure 40 amps draw, probably an overestimate) off the deep cycle a solid 60-90 minutes, calculating using up about 80% of the RC.

IF I was to go to a Group 31 deep cycle (say 205min RC), AND switch the bait tank pump (about 3 amps all by itself) to the deep cycle, then theoretically I should be able to pull 4 hours of accessory off the starting battery (using 50% RC), and 4 hrs bait pump and 2 solid hours trolling motor off the deep cycle (using 80% RC). I like this idea because I want to hit the water longer.

Now, I also have an AGM battery which I picked up super-cheap off EBAY a few years ago, and was using in a popup camper I no longer have. It is in all respects the same size and specs as a Lifeline GPL-30HT (700 CCA, 315min RC):
http://www.lifelinebatteries.com/marineflyer.php?id=4

Based on my calculations, the AGM battery alone should be able to do exactly the same job as the two batteries I was using (start, pull 2hrs acc and pull 90 min trolling), while using up only about 60% of its RC (not counting any motor re-charging). If I was willing to use up 80% of the RC, I could theoretically get 4 hours of accessories and up to 2hrs solid trolling motor use (just like if I upgrade to a group 31 deep cycle).

So, which option should I choose? And why?:
1) Two flooded lead acid batteries, Group 24 starting and Group 31 deep cycle. Would need to buy a deep cycle (say $130) and a 2-bank charger (say about $110-$130) now, and probably need a new starter battery (say $80) later this year.
2) Switch to the AGM battery alone. Would need to buy a 1-bank charger (say about $100) and refit the OB with a universal regulator (say about $40-50).

On the one hand, I'm always leery of hitting the water with only one battery onboard.
On the other, that AGM battery is truly awesome, and has never failed to hold on to a full charge even 6 months between chargings . And I already have it, so it's actually the cheaper option by far.

 

[John_S]

Re: Batterie(s); 2 flooded or 1 big AGM, help me decide which way to go

While I haven't used an AGM, how about just replacing your current deep-cycle with it? Buy a 2-bank charger that can handle the different types of batteries, and move some non-essentials, like the bait tank to the AGM.

BTW, Do you think the charger took out the deep-cycle? 3 batteries in 6 years is high given you are charging when you bring it home, and before you go.

 

[ajgraz]

Re: Batterie(s); 2 flooded or 1 big AGM, help me decide which way to go

BTW, Do you think the charger took out the deep-cycle? 3 batteries in 6 years is high given you are charging when you bring it home, and before you go.

I think it is possible the charger has been taking out the deep cycles (and maybe vice-versa). It is not a true 3-stage (only 2-stage, no float), and I've read that non-distributing multibank chargers can be troublesome if batteries are not somewhat equally discharged (i.e., if one requires far more charging than the other, as has likely always been the case for my deep cycles). Also, that crappy charger was discontinued after only a few years on the market, and the other short-lived Motorguide brand charger models all got recalled, if that means anything.

While I haven't used an AGM, how about just replacing your current deep-cycle with it? Buy a 2-bank charger that can handle the different types of batteries, and move some non-essentials, like the bait tank to the AGM.

I did consider that, but I have two issues (though neither is necessarily fatal):
1) Is there a charger that is known able to handle two different types? EVERY reasonably priced charger I have researched says you shouldn't mix types (flooded, gel, AGM) on the same multi-bank charger. On the other hand, the battery specs and web research says the AGM can take the same charge profile as flooded lead acid?!? I'm confused about this, and was hoping to avoid the issue
2) The AGM weighs 100lbs alone. That's lighter than both flooded batteries together, and I'm hoping to shed some weight if possible, not add a lot of it.

If someone can point me to a 2-bank charger that says you can mix battery types, I will consider this a third option.

 

[John_S]

Re: Batterie(s); 2 flooded or 1 big AGM, help me decide which way to go

Yes, it is heavier, but you already own it. You could upgrade that TM to 55lb. How much heavier is it from a good group 31? An extra 25lbs with plenty of reserve VS spending over $100+ for a good lead-acid, I'd pick the former.

You would have to call and talk to tech support about mixing battery types. I didn't see any warnings about doing that when I looked at the specs on a couple of Guest chargers. You could always use whatever charger you are using on the AGM now, probably have to give up the convience of leaving it attached.

It sounds like you have had a number of issues with "value" chargers. Consider a more expensive one. Nothing says you have to sell it with the boat.

Otherwise, I'd sellect option 1. I don't like having a single "do all" battery, in your option 2.

 

[ajgraz]

Re: Batterie(s); 2 flooded or 1 big AGM, help me decide which way to go

I agree with the notion of not relying on one single battery, at least not if I'm going farther from land than I'd be willing to paddle .

Turns out at least one of the banks on my 2-bank charger is good; the other may still be good, may be just that the green LED is burnt out. Testing it on a partly-discharged battery tonight. So for now at least, I may be OK on charger.

Now here's a question; given the piddly output of the charging circuit on my outboard, is it worth considering a VSR (voltage sensing relay), like maybe this one from yandina?:
http://www.yandina.com/acrobats/C100Data.pdf

If I understand this thing correctly, it will allow me to charge both my flooded starter battery and my AGM house battery from the outboard, and not have to install a regulator at the engine (thanks to the built-in regulation for the house battery). Also, it will allow me to hook up my onboard charger (one or both banks, whichever are working)...or even my garage charger...to just the house battery, and that will charge both. Do I have that right?

In any case, VSR or not, after some more figuring I'm now also considering putting everything besides the outboard starter on my massive AGM house battery (VHF, FF, lights, bait pump, bilge pump, trolling motor). I did some experimentation today and determined that the trolling motor does NOT appear to cause any wierdness with the fishfinder when they are both on the same battery, so that's one potential issue out of the way. Plus this way I can see the voltage on both batteries (starting battery though the meter on the dash sensing through the control box, house battery through the fishfinder). And I won't really have to rewire at all, just swap the mains (to the fuse panel) from the starter battery to the AGM.

Does this seem like a sound plan? Any specific reason I should keep any of my non-starting circuits on the starter battery?

 

[John_S]

Re: Batterie(s); 2 flooded or 1 big AGM, help me decide which way to go

I haven't used a VSR and don't think I'd want the unregulated output charging an expensive AGM. I'd rather just charge it with a smart charger when I got off the water. The outboard will not be able to keep up with most electrical use anyway.

I'd leave some loads on the starter battery, or at least be able to switch over. When the starter battery if fully charged and you leave the dock, the unregulated outboard output can cause an over-voltage condition. It is nice to be able to turn on some lights or other items to keep it in check.