Leave an unplugged battery charger connected to lead-acid battery?

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I posted a question in the newsgroup "rec.boats" about recharging a lead-acid battery for an electric boat motor after returning from a boating/fishing trip. The original question was about whether it's okay to leave the battery charger on for a few days to a week. I found out that, unless I have the more modern automatic type of battery charger, that is a bad idea -- due to overcharging the battery.
Then someone suggested just plugging the battery charger into a timer and setting the timer to turn the power off to the battery charger after say 12 hours.
My question now is,
"If I do the timer idea, could the fact that the battery charger will still be set to "ON", and will still be connected to the battery after the timer cuts power to the battery charger, cause the battery to discharge and drain back through the battery charger?"
I tried a Google search but didn't find too much info that I could use.
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The battery chargers I've used, the DC output goes through diodes. So, discharge isn't an issue.
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From what I recall from long ago, all diodes have a leakage current rating. Whether it's significant in this case I don't know.
FAIK, the old Si/Ge diodes may be obsolete in today's electronics.
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On 5/15/2009 3:19 PM Red Green spake thus:

They all have a leakage current rating; the leakage is very, very small.

Si/Ge? You mean one or the other; modern diodes are all silicon, except for a few small-signal ones (1N34, etc.). Not used in battery chargers, that's for sure.
And of course before that there were selenium and copper-oxide rectifiers, both long obsolete.
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The reverse leakage should be so small as to be insignificant compared to the battery self-discharge.
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hr(bob) snipped-for-privacy@att.net wrote:

Of course battery internal resistance is a durrent path for discharge.
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wrote:

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snipped-for-privacy@snyder.on.ca wrote:

Not series resistance, the internal resistance of the battery. The lower the internal resistance, the higher the self discharge (but the higher the maximum current as well). Short out an Alkaline AA cell and nothing much happens because of the high internal resistance. Short out a NiCad or NiMH AA cell, or a car battery, and the result is much different.
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SMS wrote:

Don't alkalines have lower internal resistance and lower self-discharge than carbon-zinc cells?
In my experience, a new nicad may still have a good charge after a month on the shelf. There may come a point when it will discharge itself in a week. I don't think its internal resistance is any lower at that point.
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wrote:

Self discharge and internal resistance are TOTALLY unrelated. A battery is a series of cells. Each cell has an internal resistance - which is between + and - terminals. This is "series" resistance. If there is no load on the battery there is no current flow, so no effect. IF both internal resistance and self discharge are lower on a given battery than another, the two items are co-incidence, not causual.
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On Fri, 15 May 2009 15:36:21 -0700, David Nebenzahl

Shottky are becoming more common due to their lower voltage drop (forward). Half a standard silicon diode drop. (like germanium)
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Red Green wrote:

Here's the rectifiers I like:
"http://www.kpsurplus.com/products/view/16558 "
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BetaB4 wrote:

automatic drained 7ma. It would take a week to draw off 1 amp-hour.
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Batteries need to be monitored by voltage, not guessing. Your battery charger may or may not work or even charge properly to 100%. www.batteryuniversity.com has all the info you will ever need to maintain them properly, but basicly keep the voltage to low and its ruined by sulfation, keep it to high and the plates deteriorate. Battery maintainers take care of these issues and they are cheap. Knowing proper voltage is something you will always need to know and check to be sure your chargers actualy are working as intended.
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On Fri, 15 May 2009 17:47:00 -0700 (PDT), ransley

A good 3 stage marine charger (particularly the "mountable" type) should be safe to leave connected, either plugged in or not. Some (mainly older units) have "relay" isolation - physically disconnected when not charging. (or not powered on)
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On Fri, 15 May 2009 17:47:00 -0700 (PDT), ransley

When I charge any battery, I sit next to the charger all day if required and watch the meter. If I get tired, I put the battery and charger in my bed and set my alarm to wake me every 10 minutes to check the meter. Because I have many batteries to charge, I had to quit my job so I could keep up with monitoring them. That worked fine until I ran out of money and they utility company shut off my electricity. Now I have a solar charger, which often takes days to charge a battery. I now spend all my time watching them charge and can no longer do anything else. I have lost 72lbs from not eating for days, because I can not leave the battery long enough to use my food stamps to grocery shop or to cook. However, my batteries are always accurately charged and last a long time.
Please send me money $$$$$$$
Padro
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I'm sure (well, pretty sure), as others have pointed out, that any draw from leaving it connected to your battery would be insignificant. But since I'm too lazy to actually check it I disconnect the charger from the battery when done. Hell, I might foget about it for weeks or more.
As ransley stated you really need to know your charge voltage and charge rate and duration to properly care for your batteries. This info is usually available from the manufacturer. Some "Smart Chargers" are probably pretty safe to use in general but some other "Automatic" chargers may ruin your batteries in a fairly short time.
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Years ago I got a "great" charger, only to find out many years later it was never calibrated to charge 100% or went out of calibration, My batteries never lasted or turned over the car when -20f. One day after learning about what 100% charge is I luckily inside found a screw that adjusted the voltage up. I think most new units are computer chip and probably unadjustable. Not having a battery maintainer has ruined many batteries for me.
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ransley wrote:

I think the screw was for temperature compensation. At -20F, a typical battery may require 16V. At 30F it may require 15V and at 110F, 13V. Chargers and regulators used to be set manually, taking the estimated temperature into account.
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E Z Peaces wrote:

Use a hydrometer to check the specific gravity of the electrolyte in a lead acid battery if it's one that you can open. Some batteries actually have a little indicator that turns green when the specific gravity is within the proper range.
TDD
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