I couldn't find my exact question when I searched so please forgive me if this is a) Obvious or b) well covered elsewhere. I have a Beneteau 323 with a pair of group 24 house batteries connected in parallel and an additional group 24 battery that can serve as an engine start which I generally don't use. The way Beneteau has wired the boat, the alternator only charges the battery(s) selected. The onboard charger charges both when I plug in. My solar panels are only connected to the house bank. I have hit 7 years of service on my Canadian Tire nautilus (Exide) agms and I am thinking of replacing the batteries for the coming season. I am contemplating swapping the group 24 house bank for group 27, just for the slightly higher amp hours but leaving the starting battery as a group 24. I'll keep them all as agms but with slightly different capacity. Will it matter? When the onboard charger is charging all of the batteries will that be a problem? Our power demands are modest (mostly the fridge and pumps), we have all LED lights and my solar panels mostly keep up with our use, even on cloudy days but we often spent 4 or 5 days at anchor and it would be nice to have a bit of a cushion on a string of cloudy/rainy days.
Battery sizing
- Thread starter Phil Mac
- Start date
That won't be a problem. The house bank was already larger than the single engine battery. Now it'll just be larger by a somewhat wider margin. Having the banks combined while on the shore charger won't hurt the house bank at all. It might hold the engine bank at an absorption voltage a little longer than ideal, but plenty of engine batteries live with that just fine.
If the charger has 2 outputs and one goes to the house bank, and one goes to the starter battery, I assume it treats each bank independently and won’t over charge the starter battery….isnt that right? Maybe it depends on how smart the charger is…
I think my charger is feeding back through the common post on my battery selector switch, so bot banks would charge essentially the same.
Greg
So long as you have room for the 2 Grp 27s, not an issue. You might even see if Group 31s will fit.
The Gap 27s have about 75-80 amp hour capacity, the 27s about 90ah, and the 31s about 100 ah. Depending on the battery, AGMs want to be charged at 20% of capacity or a little higher. At 180 ah, the batteries would be best charged at 30-40 amps. A 40 amp smart charger would be a slightly better choice as there is extra capacity to run house loads while charging and to provide a higher charge current to the start battery when both banks are charging. Remember, a 2 (or 3) bank charger can only provide the nominal charge current divided between the two banks, so a 20 amp charger can only provide 10 amps to each battery, or 15 to one and 5 to the other, and so on.
The Gap 27s have about 75-80 amp hour capacity, the 27s about 90ah, and the 31s about 100 ah. Depending on the battery, AGMs want to be charged at 20% of capacity or a little higher. At 180 ah, the batteries would be best charged at 30-40 amps. A 40 amp smart charger would be a slightly better choice as there is extra capacity to run house loads while charging and to provide a higher charge current to the start battery when both banks are charging. Remember, a 2 (or 3) bank charger can only provide the nominal charge current divided between the two banks, so a 20 amp charger can only provide 10 amps to each battery, or 15 to one and 5 to the other, and so on.
Thanks for the quick and helpful input. The group 27 batteries should fit but 31s would be a bit too tight if they are properly mounted. I will need to start looking for deals!
If you’re using standard plastic battery boxes 31’s fit in the same box as 27’s, so there’s no extra space needed at all. If you have some kind of custom box or mount then they will need a little more space.
In almost all cases smart chargers are just one power supply with the ability to isolate the batteries when not charging. It’s basically a single charger plus a built-in ACR. As long as one bank is still absorbing current the charger holds all banks at absorption voltage. See the section starting with Myth 10 at Making Sense of Automatic Charging Relays - Marine How To.
Agree with Dave & David; group 31s would be better choice if you’re anchoring 4-5 days. I have purchased Duracell AGM deep cycle batteries from Sam’s club at a reasonable cost. Not as good as lifeline; however, good enough for an occasional outing for a few days. My understanding is that Sam’s Duracells are manufactured by same manufacturer that makes West Marine batteries. I use a two output Xantrex 40 amp smart charger that was the oem charger supplied by Beneteau.
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WM batteries are made by East Penn as are batteries as all NAPA batteries and others.
Thanks. I'll need a trip down to the boat to measure my space. I had been thinking of swapping the boxes for a dual tray. Group 31s would be heavier but I leave my batteries on board for the winter.
Really appreciate the input.
Really appreciate the input.
BTW, AGMs do not need a sealed box. They do need to be secured in place so there is no more than 1" movement, but it is obviously better to have much less than 1" of movement.
FWIW… I went down this path a couple of years ago, and found that my original 20A charger could not keep up with the larger Group 31 batteries. When cruising, if the batteries were significantly discharged then a single overnight on shore power was not enough time to fully recharge the larger house batteries. I ended up buying and installing a larger 30A charger.