How long can I leave a cheap 13.2v battery charger connected? - SailNet Community
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post #1 of 21 Old 01-01-2015 Thread Starter
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How long can I leave a cheap 13.2v battery charger connected?

I just bought a very inexpensive battery maintainer from Harbor Freight to charge an extra wet-cell battery that I have. One of my intentions is to test the battery capacity this winter.



The battery is a group 27, combination starter/deep cycle battery. It is in the garage, where it will be between 50 and 20 degrees this winter. Before applying the charge, I checked the specific gravity of one of the cells and it was well into the red on the gauge. The battery needed to be charged.

The charger is spec'd for 1/2 amp and to maintain the battery at 13.2 volts. I've had it connected to the battery for a week now and the voltage is consistently maintained at 13.2 volts.

First question: At 13.2 volts, is there a limit to how long I can keep it on the charger?

After the week of charging, when I take the battery off the charger momentarily, it shows 12.5 volts. (Haven't re-tested the specific gravity yet.)

I'm planning to do a load test on the battery with a 12v, 25 watt element that I used for the wind generator's dump load. (Meaning about a 2 amp load.) I figure I'll take 25 amp-hours out, let it sit for a day and check the resulting battery voltage and specific gravity. In this way I'll know how many amp-hours the battery is still good for.

Second question: Anyone see any issues with my battery testing plans?

Regards,
Brad

(I have a fantastic, Victron battery monitor on the boat, but I'm doing all this at home. I'll just keep an eye on the voltage across the 25 watt element and use the V=IR equation to track the current usage and get to a fairly exact 25 amp-hour draw-down.)

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Re: How long can I leave a cheap 13.2v battery charger connected?

A few thoughts:

1. You could leave the battery connected as long as you want @ 13.2VDC.

2. Problem is, even at 13.2VDC the wet-cell battery will sulfate, though slowly.

3. Your reading of 12.5VDC after momentarily disconnecting the battery charger is pretty low. It should be at least 12.6-12.8VDC.

4. This leads me to suspect that your battery has lost significant capacity and is in need of equalizing. This requires a 15.5 to 16VDC charge for several hours, maybe repeated a couple of times.

5. Combination "starting/deep cycle batteries" and "no maintenance" flooded batteries are not a good choice for a boat.

6. Your plan for a timed discharge is unclear to me. Batteries are generally rated at the 20-hour rate. The way you test them is to apply a resistive load roughly C/20 until the voltage is drawn down to 10.5VDC. It should take ~ 20 hours if the battery is near 100% capacity. By measuring the total AH drawn before the battery reaches 10.5VDC you'll know its real capacity.

7. In any case, you'll learn a lot in the process. See if you can borrow or acquire a charger capable of doing an equalization charge. You may be able to rejeuvenate the battery a bit.

8. Battery monitors like the Victron on your boat can be great, but they can also deceive unless calibrated and tested regularly.

Good luck,

Bill

Last edited by btrayfors; 01-01-2015 at 09:20 PM.
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post #3 of 21 Old 01-01-2015
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Re: How long can I leave a cheap 13.2v battery charger connected?

We used a lot of group-27 *marine deep cycle* batteries in powered wheelchairs; always tested them with a Load Tester. The combination batteries did not hold up well at all (some of our customers tried pinching pennies -- didn't work out for them).

Sounds like you need to give it a real charge before you put it on maintenance charge. You have to test specific gravity of *all* cells -- they should be about equal -- if not, you could have problems with plates or separators.

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I don't know about freezing batteries (I'm in Florida) , but it sounds like a bad idea; maybe the trickle charge will keep it warm.

Is the battery in a box, in case of an overflow? Isolated from the floor? .....a concrete floor can drain a battery pretty quickly. What kind of ventilation do you have in the charging area? A little hydrogen, a little oxygen, and a spark can do an amazing amount of damage, like 'take the roof off your house' bad -- no wonder they used it to fuel the Saturn V to launch Apollo to the Moon.

Lots of info, including temperature correction charts, here.

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post #4 of 21 Old 01-01-2015
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Re: How long can I leave a cheap 13.2v battery charger connected?

I have had that same charger on my GRP 27 for a year without incident still has same ah it had then recently tested with a tester similar to the one pictured I use it as a backup to my 8d on my lil islander

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Re: How long can I leave a cheap 13.2v battery charger connected?

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Originally Posted by manatee View Post
I don't know about freezing batteries (I'm in Florida) , but it sounds like a bad idea; maybe the trickle charge will keep it warm.

Isolated from the floor? .....a concrete floor can drain a battery pretty quickly.
Battery cases are plastic now and leaving them on a concrete floor will not have any ill effects. Before the plastic cases it was an issue - many decades ago.

As long as the battery is fully charged it will not freeze - at least not until it is really, really cold.

From Trojan:

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post #6 of 21 Old 01-02-2015
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Re: How long can I leave a cheap 13.2v battery charger connected?

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Battery cases are plastic now and leaving them on a concrete floor will not have any ill effects. Before the plastic cases it was an issue - many decades ago.

As long as the battery is fully charged it will not freeze - at least not until it is really, really cold.

From Trojan:

I have had a battery in my car freeze in Wyoming at 30 below and I know it was a good battery so from my personal experience I would assume that the trojan data is slightly biased in their favor IMHO

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Re: How long can I leave a cheap 13.2v battery charger connected?

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I have had a battery in my car freeze in Wyoming at 30 below and I know it was a good battery so from my personal experience I would assume that the trojan data is slightly biased in their favor IMHO
This link Can a Car Battery Freeze? - KickingTires states -76F for a fully charged battery, as do other links. It all depends on the specific gravity of the electrolyte. Your battery was probably not fully charged.

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Re: How long can I leave a cheap 13.2v battery charger connected?

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Originally Posted by btrayfors View Post
A few thoughts:

1. You could leave the battery connected as long as you want @ 13.2VDC.

2. Problem is, even at 13.2VDC the wet-cell battery will sulfate, though slowly.

3. Your reading of 12.5VDC after momentarily disconnecting the battery charger is pretty low. It should be at least 12.6-12.8VDC.

4. This leads me to suspect that your battery has lost significant capacity and is in need of equalizing. This requires a 15.5 to 16VDC charge for several hours, maybe repeated a couple of times.

5. Combination "starting/deep cycle batteries" and "no maintenance" flooded batteries are not a good choice for a boat.

6. Your plan for a timed discharge is unclear to me. Batteries are generally rated at the 20-hour rate. The way you test them is to apply a resistive load roughly C/20 until the voltage is drawn down to 10.5VDC. It should take ~ 20 hours if the battery is near 100% capacity. By measuring the total AH drawn before the battery reaches 10.5VDC you'll know its real capacity.

7. In any case, you'll learn a lot in the process. See if you can borrow or acquire a charger capable of doing an equalization charge. You may be able to rejeuvenate the battery a bit.

8. Battery monitors like the Victron on your boat can be great, but they can also deceive unless calibrated and tested regularly.

Good luck,

Bill
Why would a combination deep cycle / starting battery be a bad choice for the starting battery on a boat?

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Re: How long can I leave a cheap 13.2v battery charger connected?

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This link Can a Car Battery Freeze? - KickingTires states -76F for a fully charged battery, as do other links. It all depends on the specific gravity of the electrolyte. Your battery was probably not fully charged.
Ithe battery was a 6 month old interstate car battery in a Subaru I had just driven from bremerton was to my sisters in burns wy it was so cold taters in the kitchen froze where they touched the north wall it was 70 plus in the mobile home at that time didn't check the specific gravity just started the car and the battery thawed out while I drove back here to the base after Christmas in 1990 just my experience and I don't ever want to experience it that cold again

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Re: How long can I leave a cheap 13.2v battery charger connected?

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Originally Posted by newhaul View Post
Ithe battery was a 6 month old interstate car battery in a Subaru I had just driven from bremerton was to my sisters in burns wy it was so cold taters in the kitchen froze where they touched the north wall it was 70 plus in the mobile home at that time didn't check the specific gravity just started the car and the battery thawed out while I drove back here to the base after Christmas in 1990 just my experience and I don't ever want to experience it that cold again
Your battery never likely froze it just lost the ability to crank your engine at that temp.. In cold temps the battery looses deliverable cranking ability as well as usable Ah capacity. It comes back when the battery warms... If batteries froze at -30 it would be real tough to live in Fairbanks, where I was born....
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