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So in moving the boat from the dock to the marina about 12 miles away the alternator died. This meant that the boat has been running on batteries for the trip and the last couple days as the haul out marina didn't have 30 amp power available to match the receptacles on the boat. So from wednesday noon to Friday noon the batteries carried the whole load.. cranking, instruments, and most evilly of all the fridge.
last fall I put in 4 group 31 AGM batteries for a battery bank of 420 amp hours at 12 volts.
I didn't intend to leave the batteries running the fridge all that time. I had intended to turn it off. 🙂 I forgot that the prior owner wired directly to the batteries for the fridge and it has a separate switch to turn it on or off. So on wednesday when we left I turned the master power off and didn't think about it. The next day I looked at the fridge to see how the stuff in it was holding up and there was still ice in it. Actually it looked as if it was still very cold. 🙂 you would think I would have gotten a clue at that point. It was only later driving home that I stopped to think... duh the fridge is actually runnning. The battery monitor wasn't showing any current draw when I looked at it because the compressor wasn't runnning.
So I stopped and got an adapter yesterday to plug into a standard 110 volt outlet and then be able to plug the boat into that to run the battery charger. Before doing that the battery monitor showed that I had used 74 amp hours out of the battery bank and that the batterys were at 75% or so state of charge.. 🙂 I have to say that I was pretty happy to see that. 3 or 4 times starting the engine, 2 solid days of fridge draw plus instrements and stuff for 5 hours or more on the trip up the ICW. I think that once I get the solar panels installed and the solar charge controller, we will have plenty of power no matter where we are.
When charging from that deep of a discharge (this is the first time I have drawn the bank down) the battery charger said that it was putting out 52 amps but the battery monitor only showed about 42 amps going into the battery bank. I found this to be a little weird. Not sure how to test it.. the voltage that the battery charger was charging at was the same voltage that the battery monitor was showing so I'm not getting a voltage drop between the two. Strange..
anyone have any thoughts on what could be happening here?
scott
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Hey Scott,
Stop me if you know all this, but it sounds like you've got the charge efficiency setting on your battery monitor set to 80%
If your battery monitor is anything like mine ( BEP 600-DCM ), then it uses a charge efficiency setting and an exponent setting for Peukert's equation to try and guesstimate how much power you have remaining in your battery bank. The monitor tries to figure out how much power is in you batteries based on how many amps go across the shunt. Unfortunately your batteries don't absorb all the amps you throw at them, hence the charge efficiency setting. They also discharge at different rates depending on your draw. A 10 amp load for 10 hours actually discharges your batteries more than a 5 amp load for 20 hours. Peukert's equation is used to .calculate the additional discharge. You also have to have your amp draw properly zeroed. Once you get all these settings dialed in, the battery monitor is a very useful tool, but the remaining capacity is still just a best guess and not an accurate number. The more telling number would have been the voltage remaining when you got back to your boat. If it was still up in the 12.3-12.4 range then you probably still had 75% charge remaining, but if it was much lower than that, I'd say that your monitor needs some adjusting.
Jason
DE32 Bodhran
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Jason,
lol, actually I do know all that but hadn't stopped to think about that in relation to the battery monitor. I vaguely remember setting a peukert's factor for the batteries in it. I would never have thought of it affecting the display of amps in though 🙂 thanks for the heads up.. I will have to re-read my manual and double check the the peukert's setting I used against the manufactures recommended number.. not sure if I went with a default or with the battery specific number.
thanks for the heads up on it.
I think the voltage was 12.48 so it looks as if the monitor is tracking pretty good.
scott
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This is the first time I have actually drawn the battery bank down since installing it. With the issues with the engine and the stuffing box the boat has been at the dock a long time. So the batteries have just been floating on the battery charger and all house current has been provided by the charger. It's good to see realistic numbers (amps out) that support the size of the bank. The charger did a good job of starting to charge the bank back up yesterday before I left the boat. It's a four/three stage charger and put out 52 amps at bulk for a couple hours and then moved to an absorbtion stage.. was doing that when I left. Should be floating by the time I get back to the boat today. So far the charger a Zantrex 60amp, 3 bank, multi chemistry charger has been doing a great job.. Once I get the solar stuff installed though I doubt that the shore charger will get used much.
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