12v battery charges in 2 minutes with 2 amp charger

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Electric Rider   10 mW

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12v battery charges in 2 minutes with 2 amp charger

Post by Electric Rider » Dec 04 2010 8:21pm

I have this problem. I just added another 12v sla battery for a total of 4 ( for 48v) but only have the 36v charger - so I thought i'd charge up 3 of them as a pack, then drop one fully charged battery- Then add the remaining battery and charge that combo to 36 V - This should assure all batteries had a level 12v charge across the board to equal 36v right? ( the bateries are wired in series) - and thus I would have my 48 volts because all batteries were charged.

Well when I add that last battery to the other 2 fully charged batteries it only takes 2 minutes to charge to 36v. As a pack charging 3 at a time takes anywhere from 45 minutes to an hour and a half. I know it depends on how much i drain the pack but 2 minutes to top off the last battery to 12 volts? That seems really fast to me so i am thinking there is something wrong. I do not have a 12 meter to test the battery.

Any thoughts?

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MadRhino   100 GW

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Re: 12v battery charges in 2 minutes with 2 amp charger

Post by MadRhino » Dec 04 2010 9:24pm

I am far from being an expert in that matter, but here is my logic: When you add a 3rd battery that is discharged with the 2 other that are fully charged, it does not only draw power from your charger, but also a lot of it from the other 2. Then, the time involved depends also on the voltage of the discharged battery vs the charged ones, and the Ah that it can hold. The capacity of your batteries might be different, that is another factor. I can't see any other reason, except maybe inaccurate measure instrument.
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Electric Rider   10 mW

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Re: 12v battery charges in 2 minutes with 2 amp charger

Post by Electric Rider » Dec 04 2010 9:37pm

They are all the same AH. Interesting.. so in theory.. I could charge a battery connected in series with another battery and no charger?

This makes me think of electricity as fluid, like spilling over from one battery to the other until they equalize. IF then this is what is happening, then the charger tops off all 3 batteries till they reach 36v.

The charger is only 2 &1/2 months old ( since I bought it)
Last edited by Electric Rider on Dec 04 2010 9:39pm, edited 1 time in total.

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Re: 12v battery charges in 2 minutes with 2 amp charger

Post by Ricky_nz » Dec 04 2010 9:38pm

Electric Rider wrote:I have this problem. I just added another 12v sla battery for a total of 4 ( for 48v) but only have the 36v charger - so I thought i'd charge up 3 of them as a pack, then drop one fully charged battery- Then add the remaining battery and charge that combo to 36 V - This should assure all batteries had a level 12v charge across the board to equal 36v right? ( the bateries are wired in series) - and thus I would have my 48 volts because all batteries were charged.
Be careful one flat battery in series with 2 fully charged ones is overvoltaging the fully charged ones significantly during charging
That probably reaches a point where the charger detects full charge quickly. You may have had >17V over each fully charged battery with the charger on!
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Re: 12v battery charges in 2 minutes with 2 amp charger

Post by Electric Rider » Dec 04 2010 9:41pm

Ricky_nz wrote: Be careful one flat battery in series with 2 fully charged ones is overvoltaging the fully charged ones significantly during charging
That probably reaches a point where the charger detects full charge quickly. You may have had >17V over each fully charged battery with the charger on!
Er.. so my above summation is incorrect..? ( The post above yours - we posted about same time so you didn't see it) How can 2 batteries get that much overcharged by the charger that fast?

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Re: 12v battery charges in 2 minutes with 2 amp charger

Post by Ricky_nz » Dec 04 2010 9:56pm

Electric Rider wrote:
Ricky_nz wrote: Be careful one flat battery in series with 2 fully charged ones is overvoltaging the fully charged ones significantly during charging
That probably reaches a point where the charger detects full charge quickly. You may have had >17V over each fully charged battery with the charger on!
Er.. so my above summation is incorrect..? ( The post above yours - we posted about same time so you didn't see it) How can 2 batteries get that much overcharged by the charger that fast?
Heres a diagram.
You can assume a discarged battery has quite low impedance relative to the fully charged ones.
under charge the voltage accross the full ones rises which is bad for them.

Edit:
The fully charged ones can get quite warm.
This means your charger would stop because it sees a full voltage but the battery that started out flat would still be flat.
an easy way to check is measure the voltage sccorss each battery while charging.
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Re: 12v battery charges in 2 minutes with 2 amp charger

Post by Electric Rider » Dec 04 2010 10:12pm

Oh My. I would check each battery but I don't have a meter.. just one of those dinky 12v test lights. I don't trust that to be accurate.

Does it matter that the last battery added is not fully discharged?

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Re: 12v battery charges in 2 minutes with 2 amp charger

Post by Ricky_nz » Dec 04 2010 10:46pm

Electric Rider wrote:Oh My. I would check each battery but I don't have a meter.. just one of those dinky 12v test lights. I don't trust that to be accurate.

Does it matter that the last battery added is not fully discharged?
If it wasen't fully discharged the other 2 batteries would have got less voltage accross them but it could still be a bit high. They will fall back to their normal voltage when disconnected from the charger.

If you can afford even a really cheap digital multi-meter you would find it very handy for looking at this but also for diagnosing other issues you may come up with your ebike :).
A proper meter will allow you to read 0.01V accurately which is very handy sometimes, especially useful if you move over to LiPo or LiFePO4 in the future.
If it was only on for 2 minutes then your batteries are probably fine
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Re: 12v battery charges in 2 minutes with 2 amp charger

Post by Electric Rider » Dec 05 2010 1:07am

As soon as I see the light go Green I unplug the charger. I felt the batteries while charging in this manner last and they did not get hot to the touch. I am going to get a real meter sometime this week - and a 48v charger if I can afford a cheap one ( and find one) but I will have to charge them like this for the next few days. ( or just lose one battery which I really don't wanna do) I never discharge the pack below 50 %.

You say they will fall back to the normal voltage when disconnected from the charger - what happens to the electricity ? How does it get out if not discharged under load?

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Ricky_nz   10 kW

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Re: 12v battery charges in 2 minutes with 2 amp charger

Post by Ricky_nz » Dec 05 2010 1:27am

Electric Rider wrote: You say they will fall back to the normal voltage when disconnected from the charger - what happens to the electricity ? How does it get out if not discharged under load?
The excess energy goes into cooking the battery. In a wet lead acid it causes gassing (also known as boiling) converting the electrolyte to hydrogen which is not good. This is the reason wet cells need to be topped up ocasionally but impossible in a sealed cell.
At 16V gassing is rather vigorous in a open wet battery.

You might not have felt any heat in 2 minutes. I have some 18Ah SLA batteries that I was playing with and they do start to heat up if left at too high voltage and you can hear them bubbling internally (won't happen on gel based cells).

I'm not sure of the exact effect on gelled electrolyte cells but I believe they are damaged also.
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LI-ghtcycle   1.21 GW

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Re: 12v battery charges in 2 minutes with 2 amp charger

Post by LI-ghtcycle » Dec 05 2010 4:22am

You're playing with fire taking 2 fully charged batteries and then hooking them up to a 3rd battery with a significant lower state of charge, literally.

The voltage of the higher batteries to the lower battery if sever unequal, can literally cause an explosion.

I recommend getting a new 48V charger, but if you are careful and use even a cheap Harbor Freight meter, you can try to keep the batteries with-in say a few volts of each other, and you can always tie 2 batteries together in parallel (again, IF they are reasonably close in voltage, don't attach a 10V battery to a 14V hot off the charger, at the very least it will damage the battery and reduce it's useful life span) and allow them to equal out and then if they are close in voltage, then put them on the charger, and this way you can keep charging 3 at a time in series, but depending on how hard you are on them, they might only last 6 mons to a year before they start to sag pretty bad.

I think the reason your charger was shutting off so quickly is the lack of voltage in one causing the overcharging of the others and then it sensing the state of charge of the over-charged batteries telling it to stop, not that all the batteries are topped up, just that the total voltage is being reached for your "smart" charger to shut-off.

Just my $0.02
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Re: 12v battery charges in 2 minutes with 2 amp charger

Post by Electric Rider » Dec 05 2010 1:03pm

Thanks a bunch guys. Great answers. I know more about this now than I ever wanted to.

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Re: 12v battery charges in 2 minutes with 2 amp charger

Post by dogman dan » Dec 06 2010 8:21am

Refering back to original post. Don't do it that way! It's not quite so dangerous like paralelling two batteries of different voltages would be, but you are overcharging and damaging the two others that way, and it is possible they could burst on ya. Sounds like the charger is luckily just smart enough to keep the battery explosion from happening so far. So don't continue! The best outcome is you are ruining two batteries.

You need to get a 12v charger, and charge seperate. That means fully disconnected unless both chargers are isolated. Or you need to get a 48v charger. They are cheap on ebay, for 48v sla, so just get one of those.

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