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Discussion Starter · #1 ·
OK, small boat, previously used only as day sailor. I hope to camp with it. It has an odd electrical system. Two group 27 batteries (not matched, different manufacturers, different ages) in series to a 24v trolling motor, that part is normal. But the 12v for the house is just tapped off of one of the batteries, that seems odd. And they are charged by a single unregulated solar panel that gets physically moved from battery to battery, and that seems really odd.

I'm on day seven of the boat in the water and this system seems to be working. I've been checking the batteries and recording the voltage every day when I get on. They've both been reading in the 12.7 to 12.8 range.

Last night I got carried away with the sailing and ended up putting it away in the dark. I my carelessness I left the trolling motor in idle instead of off. When I got there today the batteries measured:
A 12.20
B 12.45

I used the motor a modest amount today. From the mooring to the dock and back. To the dock and back to pick up a friend. To the dock and back to drop him off. Back to the mooring. I messed around a bit putting sails away and having a snack, then I measured the batteries again:
A 12.35
B 12.51

Whaaaa?!! How can they have more charge than before I used them?!? Would moderate use heat them up and would that cause the voltage to go up?
 

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Discussion Starter · #3 ·
You are measuring Voltage, not charge. These are two different things!
That makes sense, but... If a battery rests a while, should voltage = charge?

I came to a fully rested battery. It read 12.20. I used it for a bit, then let it rested for a little while, maybe 30 minutes, and it read 12.35. That just seems weird!

I'll go back out tomorrow and check again and see what I get. I left the solar panel connected to the one with the lowest voltage.

(Seriously, as a kid I had a Radio Shack kit and I thought I knew all about electricity. Now I realize that electricity might be magic.)
 

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Discussion Starter · #4 ·
Oh, I get it! I am slow some days.

I left the trolling motor in idle so the batteries had been under a low load for around 20 hours. I turned that off, but it was probably only five or ten minutes later I tested the battery and found the low voltage readings. The voltage bounced back after the load was removed! Duh! And even though I trolled around a little more, by the end of the day the batteries ended up higher than when I first tested them.

What did Gilda Radner say? "Never mind!"
 

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Discussion Starter · #6 ·
Seems like you diagnosed the problem. I was going to question the quality of the multi-meter as well or its connection to the bank.
I have no idea if it's accurate, but it is consistent. I've taken multiple readings and it stays +- 0.01 vdc.

I'm more interested in how one taps 12v off a 24v two batt series??
This seems odd to me too, but it works. Here's the setup:


As far as I can tell there's no fuse between the batteries an the troller. I assume that's a no-no?

And they get charged by just unclamping the panel from the terminals of one battery and moving it to the other battery. That doesn't seem dangerous, it's just kinda hack.
 

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Discussion Starter · #9 ·
It's definitely a 12v panel and it's definitely a 24v motor. I believe by the time they get to 74-pound-thrust they're all 24v.

When I test at the motor socket I get 25+ volts so the batteries are in series.

When I test the solar panel I get around 18v, so it's definitely a 12v solar panel.

I'm getting the 12v battery reading by putting the multi-meter right on the terminals of the battery. It makes sense that I'm getting 12v readings there, because without the trolling motor being on it's an open circuit. I think.

I should test the auxiliary power outlets. I imagine I'll get a 12v reading. The lights work and don't burn crazy bright, so they're probably getting 12v.



Yes, I should get a 24v panel so I could charge them properly. Then project creep sets it… I'll need to mount it, it shouldn't just sit in the cockpit like the little panel does. I'll need a proper controller. A DC to DC converter to get my 12v. Oh, heck with it, while I'm at it I might as well get an inverter too so I can bring power tools out to the boat. If I'm doing all that I should really re-do the house wiring too. Probably a new house panel with breakers instead of the fuses in the current one. Cha-ching $$$$ :)
 

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Discussion Starter · #15 ·
You need to define "works"... Electrically it can create problems for the batteries even though it appears to work..

That type of wiring can lead to short battery life. You create out of balance cells/batteries for the 24V bank. Any series batteries really need to be parallel balanced before being placed into service then regularly equalized and charged at proper gassing voltages to ensure they both get "full". Otherwise or they can become out of balance...
It's far from ideal, that's for sure! I have printed out your article Installing A Small Marine Solar System. Having two batteries in series complicates things.

Frankly I'm surprised at how well the batteries hold a charge. They're different brands and different ages, one four years old and the other older. But they both charge up to about 12.8 and seem to be working well. I've been checking them every time I get to the boat.

I think the way I would like to set it up is with the primary system being 24v, since the primary load is the 24v trolling motor. So I'd get a 24v solar panel and a good solar charge controller, a DC to DC converter to supply 12v for the relatively modest demands of the lights and iThing charging (and possible tiller pilot), and maybe an inverter so I could run power tools and my wife's CPAP on our rare overnights.

I realize that's three different voltage systems on a 22' boat.... I might be overdoing this...
 
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