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post #1 of 15 Old 06-13-2014 Thread Starter
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Battery voltage rises after use?!

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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post #2 of 15 Old 06-13-2014
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Re: Battery voltage rises after use?!

You are measuring Voltage, not charge. These are two different things!
(if anything, charge is more related to current, Amps, than voltage. Combined voltage and Amps form the used power, which integrated over time will give consumed energy - which many think is the same as charge)

It is not uncommon that batteries regain some voltage when not in use; quite usual in fact. This is as when in use the load will decrease the peak voltage, all depending in resistanses in battery (-ies) and load resitanses. Unloaded, the battery will go back to peak voltage, a process that takes some time.

- this can also be explained from a battery intrisic point of view, with chemical balances.


To some of the other aspects
- a "small" solar panel can be directly connected to a "large" battery, that is no problem. Again, Solar panel peak voltage will decrease due to the low battery resistance. Regulators are used to a) see to that peak voltage is below boiling point and b) convert peak voltage to useable energy at lower voltage.
- to tap 12V for house use from one of the batteries seems a simple solution. Probably the best battery should be used for this. And solar panel mainly connected to this one - depending on your consumption.


/J
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Re: Battery voltage rises after use?!

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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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Re: Battery voltage rises after use?!

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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Re: Battery voltage rises after use?!

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'm more interested in how one taps 12v off a 24v two batt series??


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Re: Battery voltage rises after use?!

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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.


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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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Re: Battery voltage rises after use?!

"Fully rested" would mean 8-24 hours.

Meters can have a float of +-2 or 3 LSD, in addition to any percentage error, so measuring "12.35" can show "12.33" or "12.37" and it will all be the same, it is just the rightmost digit floating from one use to the next. Always be skeptical of meter errors.

Sounds like the SS Kludge Job has a mind of it's own.(G) What would a "24V" solar panel cost you, so you could at least charge both batteries at once?
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Re: Battery voltage rises after use?!

I swear that diagram would suggest the panel was 24v as well. While not advised, some do take the series voltage entirely off the last batt in the series.

It's just better charging and use balance, if you take neg from one end and pos from the other, like your trolling motor does.

Actually, how did you get your meter to show 12v? Did you disconnect the series first? Are you sure the jumper doesn't go pos to pos and you really have a 12v trolling motor?


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Re: Battery voltage rises after use?!

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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Re: Battery voltage rises after use?!

Quote:
Originally Posted by Minnesail View Post

When I test the solar panel I get around 18v, so it's definitely a 12v solar panel.
Why would a 12v panel put out 18v?

Quote:
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.
Not to me. Once you connected them in series, it should be 24v. We need an expert here, I get a headache.

But there is clearly something more to it, if you test the panel and it shows 12v (or more likely something in the 13s). But I don't see how, from that diagram.


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