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#71381 - 10/28/06 10:28 PM Battery help  
BigB  Offline
Joined: Mar 2004
Posts: 721
Tucson, AZ USA
Any of the DC experts out there that can help me out, on a 12V sealed lead acid battery what should the voltage be on a healthy fully charged battery? These are the batteries commonly used on emergency lighting, gate openers and alarm systems.

Also, what is the optimum charging voltage?

I am troubleshooting one of my toys that has a low battery shutoff, but I don't know what it shuts off at. The battery measures about 13.5 volts. As soon as the load is applied it drops to around 12 volts and the automatic shutoff kicks in.


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#71382 - 10/28/06 11:45 PM Re: Battery help  
JoeTestingEngr  Offline
Joined: Nov 2005
Posts: 791
Chicago, Il.
Hey Brian, it's funny you should be asking this now. Just yesterday I was crawling around inside an inverter that has 37, 12V, 17AH batteries in the bottom of it. Going in there with my Fluke is of little value unless I see signs of a dead (low V) or open cell (hi V). You need to put a load close to your normal load to get a hint of how it will work under load. I chose to screw a bracket mounted 5 ohm power resistor onto a block of wood. I took a small piece of copper-clad and took the Dremel to it. I mounted a 9.6V zener, 10 ohm resistor and red LED in series and screwed the board down on the wood wired parallel to the 5 ohm power resistor. I put on red and black test leads long enough to get to all the batteries. After bypassing so that the only dangerous voltage was the battery string itself, I checked the whole string. A healthy battery will supply about 2.5 amps to the resistor and not drop the battery terminal voltage much. The LED will only glow if the battery is 11.4 volts or higher and glows at normal brightness on good batteries. I lucked out yesterday, with all of the cells looking pretty good.

It sounds likes like your VRLA or SLA battery is just going south. Their life span is determined by many things but how often and how deeply they are discharged are big factors. The datasheet will usually show float charging at about 13.7 volts and cycle charging at about 14.4 volts. When I check a battery on the bench, I set the supply to cycle voltage and look at my current. If I see a high current, I'll current limit down to the charging current that I want and figure that the battery is probably fine. If it takes little current, it might be ruined by deep discharge and need replaced. I'll usually set the CL down and walk away for a few hours because sometimes batteries tend to "wake up" after being on charge for awhile. If it does, I'll go ahead and charge it and test it under load later. I don't think of them as the same AH rating after that but hang onto them for lab work.

How would my charger work? I would probably set my voltage output at 14.4 volts for a cycle charge. I would pick my current limit based on the mfg. datasheet for the given amp hour rating. The charger would provide constant current to the battery until it stabilized at 14.4 volts. Then when the current dropped to a given value, perhaps 100mA, the output voltage would drop to 13.7V or float value. That way you don't cook the battery.
Hope this helps,

#71383 - 10/29/06 10:57 AM Re: Battery help  
BigB  Offline
Joined: Mar 2004
Posts: 721
Tucson, AZ USA
Thanks Joe, I knew there would be some good battery knowledge on this forum. I think I may have figured it out. We are only talking about a single 4.5AH battery here. The charger output is 13.5V @ 1800ma. The unit only gets occasional use, so I had the charger connected to a timed circuit so it would charge once a day for 1 hour. Apparantly that either was not enough to keep it fully charged, or the battery is defective. I charged it for 14 hours last night, now the unit works fine. The battery shows 12.67 and dips to around 12.4 when the load is applied, it does not trigger the low voltage shut off. I'll see how it holds up.

I know it would be cheap and easy to just get another battery, but what fun would that be? We are talking some serious backyard workshop time here.

As far as storage, what would you recommend for charging lengths and intervals?

Thanks for the help... Brian

#71384 - 10/29/06 11:40 AM Re: Battery help  
LarryC  Offline
Joined: Jul 2004
Posts: 783
Winchester, NH, US
Make sure the top of the battery is clean. The battery may be self discharging across the top of the battery. Did you check the current draw when the device is not turned on? Do an equalizing charge of about 15 Volts at about 450 mA overnight maybe every 3 months or so.

Larry C

#71385 - 10/29/06 12:00 PM Re: Battery help  
JoeTestingEngr  Offline
Joined: Nov 2005
Posts: 791
Chicago, Il.
Brian, you could probably leave that charger hooked up indefinately. If it is putting out a measured 13.5 volts, you are probably only trickle charging the battery. The charger may be rated 1800mA, but that doesn't mean it will be delivering it at 13.5 volts output. If you put heavy use on the battery when you do use it, the charger might actually put out that much current for a while. Then the current will trail off as the battery charges. Do your instructions give you a maximum time? I would think that your one hour a day maintenance charge would more than make up for the battery self-discharge. Anytime you actually use the device, go ahead and put a full charge on it again. If you have the luxury of being around it, feel the case every couple hours to see if you notice it being any warmer than the previous time. Thats when I would stop the charging.
Here is the link for a Panasonic SLA of a rating close to yours. Please note that they list 13.6 volts as the lowest control voltage for trickle use.

[This message has been edited by JoeTestingEngr (edited 10-29-2006).]

#71386 - 10/29/06 08:36 PM Re: Battery help  
BigB  Offline
Joined: Mar 2004
Posts: 721
Tucson, AZ USA
Larry, the battery is nearly new and spotless. There is zero current draw when shut off.

Joe, yes the instructions say you can leave the charger hooked up all the time. It states that the battery cannot be overcharged. It looks like that is what I should do.

#71387 - 10/29/06 11:24 PM Re: Battery help  
Active 1  Offline
Joined: Feb 2003
Posts: 687
Grayslake IL, USA
The answer if it's a car battery:
6 cells at 2.1v each.

Slightly over like 12.8 is fine.
12.5 is not concidered fully charged (but would still probibly work).
13 is overcharged.

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