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Thread: Charging 12 volt gel cells

  1. #1

    Default Charging 12 volt gel cells

    I have a seven amp/hour twelve volt sealed lead acid battery. After using it I put the charger on it. The voltmeter only read 12.70v in the morning.
    I checked the voltage coming out of the charger and it's only 12.10v.
    What happened? Is my trusty Radio Shack charger burnt out? It used to read 13.10v.

  2. #2
    Flashaholic* MrAl's Avatar
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    Default Re: Charging 12 volt gel cells

    Hello there,

    What is the voltage out of the charger when it's not connected to the battery?

    Take care,
    Al

  3. #3

    Default Re: Charging 12 volt gel cells

    The voltage is only 12.10v when not connected to the battery.
    I think the charger went bad.

  4. #4

    Default Re: Charging 12 volt gel cells

    Check the CURRENT out of the charger with the battery hooked up. Current should be flowing INTO the battery. If current is flowing INTO the charger, or you measure only a few mA, then it's busted.

  5. #5

    Default Re: Charging 12 volt gel cells

    seems to be busted to me, but before it broke, it charged your battery pretty well... [img]/ubbthreads/images/graemlins/tongue.gif[/img]

  6. #6

    Default Re: Charging 12 volt gel cells

    I went to Radio Shack and bought a new 12 volt 500ma charger.
    That solved the problem.
    The new charger puts out 14.4 volts, which I believe is the maximum voltage to charge a 12 volt lead acid with.
    I disconnected the charger when the battery reached 13.05v.
    Should I let the voltage go higher? I don't want to overcharge.

    I think Snovotill is right, the current was flowing from the battery to the old charger.


  7. #7
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    Default Re: Charging 12 volt gel cells

    14.4v is OK for cyclic charging. 13.9 is usually the manufacturer's recommendation for float charge. Actually, if the current is less than 1% of the capacity rating, (say 70ma for a 7.0ah battery) then it shouldn't matter what the voltage is. 13.05 isn't fully charged. See what the voltage is an hour or more later. I let mine go up to 13.9 or 14v if I don't have a meter to tell me the amperage. A fully charged, brand new gell cell should be up to over 13v after it sits for a day. 12.9 is OK for a used battery - give it at least an hour after you take the charger off for the surface charge to equalize.
    Russ

  8. #8
    Flashaholic* snakebite's Avatar
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    Default Re: Charging 12 volt gel cells

    a half wave unfiltered charger will be a bit low.
    hook it up and see if the battery voltage starts to rise.
    you also should let it go to about 14.5v in cyclic service.13.05 while charging is nowhwere near fully charged.

  9. #9

    Default Re: Charging 12 volt gel cells

    gee I forgot resting voltage for a gel cell was so high; 13.00?..I was thinking lead/ acid which are full at 12.7 [img]/ubbthreads/images/graemlins/tongue.gif[/img]


  10. #10

    Default Re: Charging 12 volt gel cells

    That old charger only went to 13.10, and sometimes I would let it sit overnight, thinking it would not overcharge. But at 500 ma it was more than 10% of the betteries ampacity, so maybe the batt did get overcharged.
    I used to have a CVC (Constant Voltage Charger) from a hobby shop that would start at 500ma but taper off to a 50 ma trickle charge at 14.45 volts. I could leave that charger plugged in all the time. Unfortunately I think my mother threw it out. Fortunately I have my own place to live now, so she can't touch my stuff anymore. The new CVC is larger and charges both 6v and 12v lead acids, and I held off on purchasing it because it's ten times the size of the old one.
    I believe there is a special chip or integrated circuit that regulates the voltage to a trickle at 14.45.
    Have any of you heard of such a charger?

    Anyway, I will use my voltmeter in the meantime to avoid overcharging. I had the batt down to 12.1 last night after a bike ride, then I charged it to 13.1 before unpluging it and going to bed. I don't want to overcharge while I'm sleeping.I will put the batt back on the charger to bring it up to 14v this afternoon while I have time to watch the voltmeter.
    Thank you all for your replies, no one at any bike forums has a clue how to run a 20watt halogen, all they talk about is their gears and tires.

  11. #11
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    Default Re: Charging 12 volt gel cells

    For a simple regulator like you are talking about, Take a look at the application notes for the LM317 (adjustable output voltage) and the lm7805 family (the 7812 (12v) and the 7815 (15v) are all in the same notes). You can use the 7812 or the 7815 with 1 or 2 diodes to get a good regulated charging voltage, 1 amp with a heatsink (and a wall wart transformer capable of one amp or more), for less than $2. Here's a starting point: http://www.fairchildsemi.com/pf/LM/LM7805.html

  12. #12
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    Default Re: Charging 12 volt gel cells

    Excellent info, Russ!

  13. #13
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    Default Re: Charging 12 volt gel cells

    I have seen a similar fault to this before.

    It sounds like one of the diodes in the bridge rectifier has gone open circuit.

    The theory bit:

    Many cheaper battery chargers for lead acids simply use a transformer and a rectifier bridge (often made up of four separate 1n4001 diodes). Sometimes they also have a smoothing capacitor. If you lose one half of the bridge, the capacitor may not reach the normal open circuit voltage, and a charge will appear to take about 10 times as long.

    Hope this is of help, you may find the fix consists of little more than a replacement diode costing a few pence / cents (insert currency of your choice!).

  14. #14

    Default Re: Charging 12 volt gel cells

    UK Owl:
    The old charger is a "wall wart" and can NOT be opened , unless a hacksaw is used.
    It's actually in my green recycling bin right now, with the empty cans and bottles.

  15. #15

    Default Re: Charging 12 volt gel cells

    The resting voltage this morning was 13.86v. It was up to 14.4v before I disconnected the charger..
    I let the surface charge equalize for about 8 hours , while I slept.
    How many more charges am I going to get out of this battery?

  16. #16
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    Default Re: Charging 12 volt gel cells

    The 14.4 volts is an overcharge on behalf of the charger. This is usually okay for a short time, e.g. at the end of a charge cycle, but if left long term at 14.4 volts trickle the battery will be damaged due to depletion of the electrolytes.

    A typical trickle charge would be 13.8 volts would be used as this is the normal 'float' voltage of 12 volt lead acit batteries.

    In a 12 volt battery you have 6 lead acid cells rated at a nominal 2.2 volts each, this gives you the 12 volts mentioned.

    However in reality this is actually 2.3 volts on a good cell (in the same way as 1.2 volt NiCd's give typically 1.35 volts when fully charged)giving you the 13.8 volts you are seeing. This 13.8 volts indicates the battery will take a good charge.

    To determine it's condition you could try 'drop' testing it, this is simply putting a large load on the battery for 15 seconds or so and seeing what voltage drop is measured at the battery terminals, but the value of the load (in some cases 1/2 ohm for large batteries) needed for the drop test will vary for different capacity batteries, and without manufactures datasheets for the performance of the battery under these conditions this could be inconclusive.

    You could try ensuring it is fully charged then testing for capacity. e.g. if rated at 8 amp hours, see how it performs giving 1 amp for 7 hours. Beware though that this way you could easilly take the charge too low, this will damage the battery.

    Your will also need to know if any conductive sludge has formed in the bottom of the battery. You can best prove this by first fully charging the battery, and then leave it on the shelf for a couple of weeks. When you test the battery after this time it should still have most of its charge. If not, this indicates you battery has been damaged.

    I hope this is of some help to you.

  17. #17
    Flashaholic* GJW's Avatar
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    Default Re: Charging 12 volt gel cells

    For gel cell charging here at work we've had a lot of luck with this:
    Gel cell charger

  18. #18
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    Default Re: Charging 12 volt gel cells

    That looks pretty good. The 1.5 amp current is a little low for anything over about 20ah, IMO. It is a multistage smart charger and I like the tricle mode at only 200ma & 13.6v.
    I'll stick with my 4 amp 13.9v power supply ($29), however. I have some larger batteries (a 48ah gel and a 105ah AGM) that need higher charge rates. I'm modifying one of these bench power supplies for adjustable voltage (using an LM317 chip) and up to 10amps (using 2 transistors, a better heat sink, and a fan). I may have to tone it down some to protect the transformer....

  19. #19
    Flashaholic* larryk's Avatar
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    Default Re: Charging 12 volt gel cells

    [ QUOTE ]
    You could try ensuring it is fully charged then testing for capacity. e.g. if rated at 8 amp hours, see how it performs giving 1 amp for 7 hours. Beware though that this way you could easilly take the charge too low, this will damage the battery.


    I never knew you could damage a gel cel battery by running it to low. I thought they would be similar to a lead acid battery. How low of a voltage can you drop a gel cell before damage ? Larry.




  20. #20
    Flashaholic* star882's Avatar
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    Default Re: Charging 12 volt gel cells

    "The 14.4 volts is an overcharge on behalf of the charger. This is usually okay for a short time, e.g. at the end of a charge cycle, but if left long term at 14.4 volts trickle the battery will be damaged due to depletion of the electrolytes.

    A typical trickle charge would be 13.8 volts would be used as this is the normal 'float' voltage of 12 volt lead acit batteries."
    Why not design a charger that charges to 14.4v, then go back to 13.8v once the battery reaches 14.4v?

  21. #21
    Flashaholic* mattheww50's Avatar
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    Default Re: Charging 12 volt gel cells

    I takes a lot of of overcharging to deplete the electrolyte. Getting rid of 18 grams of electrolyte in a cell will require about 60 amp hours.


  22. #22
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    Default Re: Charging 12 volt gel cells

    Your idea of a "smart" or two-stage charger is a good one. But check the prices on those. They are generally too expensive to include with the product - the manufacturer's use only the cheapest charger they can get away with.
    Note that volts & "trickle charge" don't necessarily go together. For a trickle charge the important factor is amps (or ma), the voltage doesn't matter. Obviously, for any particular battery, you would expect a certain charge at a certain voltage. But I can show you identical gell cell batteries (from the same lot) where the current is a factor of 2 different with both batteries connected in parallel (which, obviously, means they are at the same voltage). And that's with new batteries that have not had a chance to be altered by use in different environments.
    13.8v is regarded as the standard trickle charge voltage for a wet cell. Gell cells are usually specified for trickle charging at 13.6-13.9v, often with a current limit, too. Above that voltage, they normally specify a maximum allowable current. I sometimes use a charge rate up to 14.4v with a suitable current limit (often just a resistor) when necessary on gell cells. I have found that they last longer if you don't do that. Keeping them on float isn't ideal either, tho. An occasional use, running the battery down, seems to make them last longer. They don't last more than 8 years in any case - it appears that the plates dissolve if they don't sulfate or run out of electrolyte first. HTH,

  23. #23
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    Default Re: Charging 12 volt gel cells

    That sounds about right. So it (the 17ah gell cell)might last 3 years (at constant 14.4v float) before you deplete the electrolite enough to seriously damage the capacity. That's consistant with my experience. It's very difficult to find a properly regulated small charger when Chinese wall warts are selling at $2-3 each. And it may not be worth it for every application, particularly flashlights. Even so, I've found it worthwhile to add $2.00 regulators to several of my chargers for gell cells.

  24. #24
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    Default Re: Charging 12 volt gel cells

    Just a quick comment that probably means nothing overall ...

    In this apartment building the last time there was a major bulk changeout of the 6V gell cell batteries in the emergency lights scattered throughout the building was 1990. About 70 percent of those lights have had no battery service since then and appear to work fine ... the other 30 percent have had batteries replaced.

    These lights keep a constant trickle (float) across the cells as long as commercial power is available, and only draw power during monthly tests (Does it light up? OK.) or when there is a power failure. Once a year there is a 90 minute run test required.

    I have NO idea if this has any value to this thread, but it might ... [img]/ubbthreads/images/graemlins/blush.gif[/img]


  25. #25
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    Default Re: Charging 12 volt gel cells

    I think it brings up a good point, applicable to most rechargeable (and primary) batteries not just gell cells. Usually, the battery life is taken to be the point at which a certain percentage (usually 50%) will no longer hold a particular charge (50% is normal for gell cells, NiCads, NiMH, IIRC). I'm surprised that 70% of your gell cells are still working after 13 yrs - maybe being out in the cold helps. But it is obvious that they aren't doing much of a test. Those emergency lights may not run very long if and when they are really needed. I had an 11 year old gell cell that still worked - for about 5 minutes, and then it would dim fast. It was a 4AH 12v feeding a less than 1 amp bulb, on float in an emergency light. When I cut it open there was no sign of lead plate - the entire inside was a mushy black damp powder. You couldn't tell the positive plates from the negative because there weren't any. IIRC, the positive side uses a spongy lead (doesn't look much like lead) and the negative side is lead. I replaced all of the 12v gell cells in that emergency light several months ago - 3, 4ah gell cells in parallel power it when my power goes out. I have lots of flashlights using 4ah gell cells, although most of them are 6v.

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