Batteries have an acceptance curve. You can see a plot of amps absorbed at a given discharge state and charge voltage that assume that the capacity of the charging source is infinite. The idea that the batteries absorb whatever the alternator can produce is a common one, even though it is wrong. For one example, my car has a 120 amp alternator and a 50 AH wet cell battery. The battery does not explode every time I start the car ☺ It is commonly stated that wet cells charge at 0.25C max and gel types are 0.5C max. This is largely due to the chemistry, the wet cell batteries *restrict themselves* to 0.25C. If you are charging the batteries too hard, there are a few ways to fix it. One is a temperature sensor on the battery. If excess charging heats the battery, the voltage will back off. Another is a sophisticated regulator that can control amp output as well as voltage. Back in the day we used this feature to save belts and drag on the engine more so than for battery issues. The third way is adjusting the voltage setpoints. In my experience too much charging is the last problem you’ll ever have on a sailboat, but say you did. Your 220 amp golf cart wet cell bank is my some miracle accepting 140 amps instead of the desired 55 amps and somehow the temp sensor is not backing it off or you don’t have a temp sensor. Just do a test with low batteries and reduce the set point on the regulator to see the desired 55 amp charge current. By far the bigger issue is set points too high for the float mode, so you leave the dock with full batteries and end up wrecking/boiling them with float voltage way too high. The slow speed charging rate is also very important to sailboats. Figure out your pulley ratios and look at the output curves of various alternators. You may find the 180 amp version puts out no more than the 120 amp version at low engine speeds and you may find out no alternator is going to get you much running at idle at anchor. I went through a few alternators to settle on a Balmar 60 amp unit on my boat. It puts out a fair amount more than the 120 amp 10si it replaced at low and medium RPMs, which is not a thing for powerboats but very much is for us. Those of us with Atomic 4s suffer from small engine pulleys that make it hard to get any decent output at low RPMs.
Joe Coquina * none of this applies to Lithium batteries. You can blow them up real good without a specialized battery management system From: CnC-List [mailto:cnc-list-boun...@cnc-list.com] On Behalf Of Josh Muckley via CnC-List Sent: Monday, October 16, 2017 9:32 PM To: C&C List <cnc-list@cnc-list.com<mailto:cnc-list@cnc-list.com>> Cc: Josh Muckley <muckl...@gmail.com<mailto:muckl...@gmail.com>> Subject: Re: Stus-List New Engine, now what size alternator Joe, the damage I was considering would be caused by exceeding the charge rate for the bank size. The conventional wisdom is that typical lead acid batteries should not be charged at a rate greater than 25% of their capacity. A 100 AHr battery would be limited to 25 amps. Isn't it possible to exceed the charge rate with a good regulator? Particularly applicable when the battery is more deeply discharged. All of the 3 and 4 stage regulators I've ever seen will ramp up to max amps and hold there until ~14.6 volts is reached (80% full - end of bulk charge) at which time the voltage will be held constant at ~14.6v as the amps are reduced. Once the amps lower to ~2amps the regulator shifts to float mode and lowers the voltage to ~13.3v and holds it indefinitely. As an example lets say that you are using a 100 amp alternator and regulator to charge a 100AHr battery bank that has been discharged to 50% capacity. Isn't it likely that the charge current will ramp to 100 amps? Or at least greater than 25 amps? What other regulator function would prevent this from happening? I suppose battery temperature could input to the alternator so as to reduce charge current. Though, I'm not sure that battery temperature responds quickly enough to prevent early over current damage, only overcurrent damage as a result of longer term charging current which has been applied long enough to raise temperature to the threshold. Josh
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