Jerry—
You've got to remember how batteries charge. All rechargeable lead-acid batteries, whether they're AGM, Wet Cell or Gel, will charge fairly quickly to about the 85% of charge capacity... once they hit 85%, the current acceptance rate drops very dramatically, going from bulk charging to absorption charging... once the batteries are fully charged, the intelligent chargers will go in to float mode.
Batteries tend to have problems if not charged fully on a regular basis or if overcharged constantly. If the batteries are left in a state of discharge for an extended period of time—the lead plates that make up the internal structure of the batteries start to sulfate...where the lead is converted to lead sulphate crystals, which generally do not convert back to lead. This reduces the batteries actual capacity. Do this often enough, the batteries die. If the batteries are overcharged, they can boil off electrolyte. In the case of wet cells, this means that they need to be monitored more often and have water added more often—in the case of AGM or Gel batteries, the electrolyte can be boiled dry, killing them.
When you're out cruising on a Telstar, if you are relying on a shore-power charger to keep your batteries topped off, you need to have a fairly decent sized charger, since you may be using the batteries fairly heavily while anchored out. Then when you get back to a marina slip, say for a single night's stay, you have to have the ability to put back most, if not all, of what was used while anchored out. The outboard motor has a very, very limited charging capacity, and if you've got your boat equipped with a refrigerator, navigation lights, cabin lights, stereo, etc... you'll be using more than the outboard can put back into the batteries, unless you're motoring constantly.
Also, as a general rule, lead-acid batteries last far longer if you do not discharge them too deeply. Ideally, you'd only discharge them to the 50% depth of discharge level. This ends up giving you the most battery power for the money spent as a general rule. If you discharge a battery deeply on a regular basis, say to 80% DOD, then you can reduce the number of charging cycles that the battery is capable of handling by a significant percentage.
Typically, you should calculate your battery bank size by doing an energy budget. I generally recommend you do one for when you're typically at anchor and one for your usage when underway, and then take the higher number of the two as the basis for your energy budget. You do this by taking the amperage a piece of equipment uses and multiplying by the number of hours you would typically be using it. Then you add up all the numbers to get the total number of amp-hours you use per day. I'd point out that the most accurate way to do this is to add a battery monitor, like the Victron BMV 600, which will keep track of your actual usage.
image courtesy of jamestown distributors.
Once you have a rough budget, you have to decide how many days you want to go between charging the batteries. That gives you a basis for sizing your battery bank. Say your daily amp-hour usage comes out to 50 amp-hours and you decide you want to be able to go two full days between battery recharging, since you will generally anchor out for a weekend... leaving Friday evening and getting back to the slip on Sunday evening. That means you need 100 amp-hours of battery capacity. However, since you only want to discharge your batteries to the 50% DOD level, that means you need 200 amp-hours of battery bank.
Now, Sunday evening comes around, and you're back at your slip. You plug the boat into the shorepower system. Wet cells can generally accept about 20-25% of their amp-hour capacity in amps during the bulk charging phase, AGMs can accept a lot more generally. I don't like gels, and won't discuss them, since I believe they've got the worst characteristics of the three types. You have to put back 100 amp-hours.
If you have a 1.25 amp trickle charger, it is going to take a really long time... over 80 hours, since lead-acid batteries self-discharge, with wet cells losing as much as .5% per day on a hot summer day. If you have a 30 amp charger, like the Iota 30 I mentioned... you'll go from 100 amp-hours
(remember the batteries are at 50% and you have a 200 amp-hour bank) to 85% charged pretty quickly... say in about 2.5 hours (30 amps x 2.5 = 75 amp-hours). To get from the 85% DOD level (170 amp-hours) up to 100%, may take another eight hours.
Now, if you're cruising for a week, and stop in the marina every other night, you'll probably be fine with a 30 amp charger. With a 15 amp charger, your batteries will slowly drop lower and lower, and you'll end up killing them prematurely, since the first stay at a marina, they'd get up to say 90%, then the next two days would drop them to the 40% level... and the next marina stay would only bring them to 80%...then the next two days would drop them to 30%...etc, until you're stuck someplace with dead batteries.
IMHO, a good battery monitor is one of the wisest and best investments you can make on a boat, if you plan on doing anything more than just weekending on it.
Jerry wrote:Dan,
30 Amp charger seems a bit of overkill for any size batteries when the charger is going to be plugged in overnight while in a slip. Can you give me a little more of your reasoning? I have read your solar wright up an may do that next year.
Thanks,
Jerry