Originally Posted by btrayfors
An excellent question!
First impression is that an 8D AGM
dedicated to starting a small-to-medium size diesel
is WAY overkill. A group 31 flooded or AGM
would be more than enough (like the Lifeline GPL-3100T). That third 8D could be used for the house bank, increasing its capacity to 675AH.
If I understand correctly, you have two 50A alternators and one 50A battery charger
. So, at dockside or with a generator
you have a maximum of 50A charging
capacity. And, with the engine
running you have a maximum of 100A charging
Those AGMs when 50% discharged will take up to their AH rating in charging amps, e.g., the two 8Ds now in your house bank can take up to 450 charging amps rather easily. Three of them could take 675 charging amps.
While it's highly unlikely that you could ever have enough charging capacity to reach those numbers, it is a good idea to have charging capacity of at least 25-30% of AGM's rated capacity.....so, say, a minimum of 100-125A charging capacity.
From that perspective, it looks like you're way under-equipped in terms of charging ability, particularly for short dockside overnites or for generator-supplied charging.
Also, it's unlikely that the two alternators could put out their full 50A rating each for very long, without overheating
. And, through the natural process of heating
up to operating temperature, alternator
output drops. So, let's assume that those two alternators -- working to their normal operating temperature capacity -- could together put out about 80-85 amps.
AGMs have a relatively low internal resistance, so can take lots of amps even in the acceptance phase.
Whether or not it's worth it to you to upgrade your charging capacity depends on how you use the boat, and whether or not you're comfortable with charging those AGMs long enough for them to reach a full charge
at least once every couple of weeks.
Yes, I know about the oversized/underutilized eng start batt, but only because you & others told me so awhile back in a different thread! The boat came to me from the PO with the 3rd 8D for starting only(??), and neither I nor apparently the tech who I had replace all the batts & charger
two years ago knew any better at the time.
Your advice is particularly apt since I have crossover relays that allow me to draw on the two house batts if need be when starting either the eng or genset! (The gen has it's own, much smaller start batt which is charged by a BEP VSR echo-type charger off the eng batt -- maybe this could also be used for an eng start). After two years in he current
configuration, however, the consensus seems to be to now wait until I need to replace all the batts. to reconfigure. Agreed?
Yes, dockside or via my genset I have a max of 50A charging capacity with my Xantrex. When motoring, I have one 50A eng alt. that charges only the one 8D start batt., and a 2nd 50A eng alt. that charges only the 2 house batts that comprise the house bank. So the two alts are not combined -- they charge the two banks separately.
So if/when I reconfigure to put the 3rd 8D into service
as a house batt., the wiring
for the input/charging side of the system -- via the Xantrex or the eng alts -- can theoretically remain the same, albeit with enhanced charging capacity as Bill suggests. The existing Xantrex, for e.g., could be dedicated to a single
8D, and a second, 100A charger for the remaining two 8D's. (Xantrex doesn't seem to make the XC series anymoe so I'm not sure I could combine them). Likewise, I could leave one of the 50A alts that already charges only one 8D in place, and then upgrade the 2nd one to 100+ amps for the other two 8D's. Maybe??
On the output side, I have a 1-2-ALL-OFF switch for the two house 8D's, and a separate On-Off switch for the eng start 8D. Not exactly sure what to do about this yet but hopefully there's a reasonably simple solution w/o having to invest in more components. Could I combine all 3 house batts. onto the one switch? Does that mean I need a 1-2-3
-ALL-OFF switch or does it not matter if all 3 are wired in parallel as one house bank? Or maybe re-wire the eng switch to use for the new #3 house?
I'd like to think I could avoid bringing more complexity into this already far too complicated system but we'll see. I'd also like to think I could accomplish much of this myself this time, but we'll definitely have to wait & see on that one! If not, at least this time I'm hoping -- thanks mostly to this forum -- I can be more on top of what is going on. Thanks for all the great advice & apologies for getting off-topic here.