TA2020 Amp and 12V 7Ah SLA Battery?? - diyAudio
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Old 6th June 2012, 10:12 PM   #1
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Default TA2020 Amp and 12V 7Ah SLA Battery??

Hello everyone.

I have a question about this power supply.

I know it will work well for the Amp itself. I'm wondering if I could wire a switch or somehow have it charge my 12V 7Ah SLA battery when the Amp isn't being used.

Is there a better option out there? I'm building a smaller, external PSU TA2020 based portable unit. Just wondering how I can have a really slick, dual powered setup.

Thanks in advance.
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Old 8th June 2012, 04:32 PM   #2
jitter is offline jitter  Netherlands
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To charge a SLA, you cannot simply hook up a power supply and leave it be without the risk of overcharging. But with your PSU, you wouldn't be able to charge a 12 V SLA anyway. Even though the nominal voltage of the battery is 12 V, charging voltage must be higher.
E.g. the Panasonic LC-R127R2PG needs a charging voltage of 14.5-14.9 V (cycle use) or 13.6-13.8 V (standby use). And don't forget that the charge current must not be higher than what is specified for that particular battery (so called "initial charge current", for he LC-R127R2PG it's 2.88 A).

Edit: there's a nice little book on SLA's attached to this post.

Last edited by jitter; 8th June 2012 at 04:50 PM.
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Old 8th June 2012, 07:36 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

Click the image to open in full size.

uses this :

12 VDC 2000mA 2A Switching Power Supply US and EU 2.5mm plug 120-052

see :
Parts Express DIY Project

All very confusing as I know lead acid batteries are 13.8V DC.
I can't see the point of the diode at all - it will simply reduce
charging voltage by 0.6V - any ideas what its purpose is ?
(Reverse polarity of the input I suppose, get rid of it.)

The supply will charge a discharged battery up to a point for sure.

It seems fine for constant trickle charge using the schematic above,
(diode or not). What is won't do is charge it quickly, and presumably (?)
not to full capacity, the question is how near full capacity it gets ?

How well the above works is the question, not will it work.

rgds, sreten.
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Last edited by sreten; 8th June 2012 at 07:52 PM.
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Old 9th June 2012, 05:06 AM   #4
jitter is offline jitter  Netherlands
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Sreten, given the desciption "automotive LED", my guess is the schematic you linked to is originally meant for use in a car (otherwise I'd expect they just specified a LED and value of resistor).
The power grid of a car is about 14-14.5 V with the engine running, that would make fully charging of the SLA possible. To prevent charging a flat SLA with too much current it must be limited, and a current limiting resistor is indeed also there, another hint pointing towards car usage.
This schematic might be usable on a 15 VDC adapter, though, but make sure not to keep it connected endlessly.

IMHO, the designer didn't really know the basics of lead acid batteries and their charging needs and assumed a 12 V SLA can be charged at 12 V. Charging the SLA from a 12 V adapter with a series diode will leave it very, very undercharged. Even in standby use (charger always connected) the voltage must be 13.6-13.8 V just to keep it charged to a usable capacity. In cycle use you can get away with higher voltage charging (limit the initial current!) of 14.5-14.9 V for quicker charging.
It's very likely on a 12 VDC adapter, the SLA will reach end-of-life very quickly because of continuous undercharge (SLAs definitely don't like that, I lost two SLAs just leaving them on the shelf for over a year without recharging, even though I fully charged them before storage).

Good SLA chargers charge with the higher voltage and then switch to maintenance charge at the lower voltage. Make sure to buy a charger that is suitable for the capacity of the SLA, otherwise initial charging current might be exceeded (esp. with small SLAs).

Last edited by jitter; 9th June 2012 at 05:22 AM.
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Old 9th June 2012, 06:48 AM   #5
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The diode prevents the battery from bleeding back through the LED and charge circuit when the charge power is turned off.
Quote:
This schematic might be usable on a 15 VDC adapter, though, but make sure not to keep it connected endlessly.
With a 15VDC supply, using 2 series diodes instead of one can get the charge voltage into the 13.6-13.8 range. And it can be connected for an extended time more safely. If the OP is looking for simple yet effective, it's a pretty good candidate.
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Old 9th June 2012, 10:35 AM   #6
sreten is offline sreten  United Kingdom
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Hi,

I agree a charging from inside a car should work much better.
And you'd then need the diode to safely use the 12V supply.

Click the image to open in full size.

Looks like another case of poor design in PE's "showcase" of such.

rgds, sreten.
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Old 10th June 2012, 07:30 PM   #7
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With the 20 ohm resistor plus silicon diode in series you're only looking at 20-something mA trickle from a 15V supply. It would seem better to have one series diode and one resistor, rather than two diodes as two diodes does nothing to limit the current upon initial charging and if the current is too high the switching PSU will shut off due to being too far under voltage... assuming a reasonably supply design, otherwise it might damage itself.

15V supply also seems better for the amp, more power output then vs 12V.
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Old 10th June 2012, 10:35 PM   #8
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I didn't take into consideration the voltage drop across the 20-ohm resistor. But I didn't say anything about removing the 20-ohm resistor either; so I remain doubtful that current-limiting is much of a concern. Perhaps 2 diodes and a smaller resistor? At this point it's up to the OP to determine the best combination for his particular SLA and charge requirements.
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Old 10th June 2012, 11:04 PM   #9
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^ That would work too. How much current limiting is a concern would depend on how low the battery is and how much current the PSU is capable of since we are suggesting a (different) higher voltage PSU.
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Old 11th June 2012, 08:58 PM   #10
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Awesome info everyone. Some new info and developments:

I want a single power supply to both, charge my Battery in my speaker dock, and also run the Amp when plugged in. I could have a 3-Way switch so the batteries and PSU aren't powering the AMP, but I'm just wondering how to safely charge a SLA battery AND power a 12V Amp with the same unit.

Thanks again for all the input.. I haven't bought anything for this project yet, so I'm open for ideas.
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