Best way to charge 12v boombox?

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Robban98422

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Joined 2014
#1
Hi there, i have made two "portable boomboxes" works like a charm and good looking 🙂

But i want a better way to charge my battery, Now i just open the suitcase and put a car-charger on the battery, pretty simple.

but i want a dc-power connection, that i just plug in the wallsocket? how do i do this the most simple way?

The battery i use is a mc-battery 12v 7ah SMF











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#3
Robban98422 said:
But i want a better way to charge my battery, Now i just open the suitcase and put a car-charger on the battery, pretty simple. but i want a dc-power connection, that i just plug in the wallsocket? how do i do this the most simple way? The battery i use is a mc-battery 12v 7ah SMF
Click to expand...

Whatever connector you add to connect the external charger, wire a 10A or so diode in series with it to the battery, so you can't reverse the polarity.
 
#4
hmm i meybe was unclear.

to charge now, i need to open the suitcase and put a 12v car charger on the battery.

i want to connect the battery to a "dc-power female connector" that is built in the case.
and then use a laptop charger or whatever?

is this possible?
 
#5
Robban98422 said:
hmm i meybe was unclear.

to charge now, i need to open the suitcase and put a 12v car charger on the battery.

i want to connect the battery to a "dc-power female connector" that is built in the case.
and then use a laptop charger or whatever?

is this possible?
Click to expand...

Sure, if you can find the proper mating male connector, which is often difficult. You may have to replace it to get a matching pair.
I'd still use a series diode unless it's impossible to reverse the leads at either end. You don't want your boom box to actually do that.
 
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#6
Be careful with the amount of charge current that you allow to get to the battery. 300 mills max. or you'll dry it out. Don't allow the charge voltage to go above 13.2-13.6 volts..

Find an A.C. wall wart that puts out a regulated 12 v.d.c. at ~500 mills. They usually regulate voltage at ~13.4-13.8 volts. Put a 100 ohm 5 watt wire wound resister in series with the wall wart and the battery. This will buffer the current and allow the voltage to rise as the battery charges. You don't want to fast charge a SLA type battery..

Don't use that car charger on your battery! They are ruthless and apply full current! OK for a flooded cell but will ruin an SLA.. (Sealed Lead Acid)
 
#7
 
#8
SMF.. Sealed Maintenance Free / SLA.. Sealed Lead Acid. Same thing. Sealed lead acid battery with no provision to check the electrolyte. Requires a more controlled charge than a flooded cell..

The power jack and wall wart that you found would work fine. 2 amps is a bit overkill. Monitor the voltage carefully! Check the output voltage of the adaptor at open circuit to be sure that it regulates at or below 13.8 volts. (It's a switching supply and assume that it's regulated). Add one of these between the battery and the power jack to buffer the current a little..

10pcs 100 Ohm 100R?J 5 Watt Axial Ceramic Cement Power Resistor 5W 1470 | eBay

Connect all of your loads directly to the battery, not at the power jack. If you want to add a stiffening cap, parallel it with the battery, not at the power jack..

Install a fuse between the battery and the power jack for safety reasons. Install a fuse between all of your loads and the battery, just in case..
 
#10
You would need to check the voltage at the battery and then at the external power input and notice the difference. The voltage will read a little higher at the power input because of the resistor during a charge..

When the battery reaches 13.6 - 13.8 volts, (checking it right at the battery) it should be pretty much charged. Invest in one of these and mount it in your case somewhere so you can monitor the condition of the battery full time. (No need to open the case to get to the battery and monitor the charge)

Mini DC 0 1 30V LED Panel Voltage Meter 3 Digital Display Voltmeter Motorcycle | eBay

Don't allow the battery to discharge any lower than 11 volts or you will damage it! The amp will work fine to a lower voltage but over time, you will destroy the battery..
 
#12
I own a few of those Lepai amps myself. I usually figure the run time at ~2 aHr./Hr. at moderate (Loud) volume..

You shouldn't discharge a battery any lower than ~25% of its total capacity. In your case, you have 7 aHr. of reserve. Divide that by 4.. 1.75 aHr. On a fully charged battery, you can safely run the amp for a little less than an hour at high volume and not discharge the battery to a level of possible damage. At a lower volume, you could operate for several hours without concern. The higher the volume, the shorter "Safe" run time. Just monitor the voltage of the battery. If it gets anywhere near 11 volts, you had better recharge..

You can increase your run time with a bigger battery. (Higher aHr. rating) Using the same charge scenario but you would need to increase the wattage rating of the 100 ohm resistor to 10/15 watt.
 
#14
From the instructions of the Harbor Freight Product..

Intended Applications [FONT=Arial,Arial][FONT=Arial,Arial]Use this product only to maintain the charge of a flooded lead-acid battery. [/FONT][/FONT]

TO PREVENT BATTERY DAMAGE: [FONT=Arial,Arial][FONT=Arial,Arial]Do not use on *gel lead-acid* or AGM (Absorbed Glass Mat) type batteries. [/FONT][/FONT]
[FONT=Arial,Arial][FONT=Arial,Arial][/FONT][/FONT]
[FONT=Arial,Arial][FONT=Arial,Arial]*gel lead-acid* = SLA type battery[/FONT][/FONT]
[FONT=Arial,Arial][FONT=Arial,Arial][/FONT][/FONT]
[FONT=Arial,Arial][FONT=Arial,Arial]To clarify..[/FONT][/FONT]
[FONT=Arial,Arial][FONT=Arial,Arial][/FONT][/FONT]
[FONT=Arial,Arial][FONT=Arial,Arial]The above product would work but they give the intent that one could just plug it in and forget it. Not true with this type of battery. It's basically the same as my design but the voltage of the battery still needs to be monitored closely. Once the charge voltage is achieved, the current needs to be disconnected totally or it will dry out the battery..[/FONT][/FONT]
[FONT=Arial,Arial][FONT=Arial,Arial][/FONT][/FONT]
[FONT=Arial,Arial][FONT=Arial,Arial]SLA type batteries are used in items that have a very complex charge system. (UPS for example) They monitor the voltage carefully and apply small bursts of low current from time to time to maintain the float voltage. A standard charger isn't that complex and applies a small amount of constant current to maintain the float voltage. A flooded type cell (Motorcycle, Lawn Tractor) is much more forgiving and can afford to lose a little water. A gel cell can't, being there isn't much to lose in the first place..
[/FONT][/FONT]
 
#16
Robban98422 said:
With a little modifikation that looks great 🙂 And veeeeery cheap.

will try to find something similiar here in Sweden, or else i just try Ebay
Click to expand...

There's also:
1.5 Amp Three Stage Onboard Battery Charger/Maintainer

Amazon.com: Battery Tender 021-0123 Battery Tender Junior 12V Battery Charger: Automotive

or DIY (at own risk):
Simple auto cut off 12V battery charger | Eleccircuit.com

Automatic 12V Lead Acid Battery Charger

or search for automatic battery charger circuit
 
#17
rayma said:
Whatever connector you add to connect the external charger, wire a 10A or so diode in series with it to the battery, so you can't reverse the polarity.
Click to expand...

The diode will confuse the charger, it can't read out the battery voltage. Also, you can't charge the battery fully due to the forward voltage drop.

OffGridKindaGuy said:
I own a few of those Lepai amps myself. I usually figure the run time at ~2 aHr./Hr. at moderate (Loud) volume.
Click to expand...

I suppose you're talking about the LP2020A+? Those won't use any more than 0.4A with music due to the crest factor.


And why DIY something together with a PSU, there are tons of good SLA chargers on the market. Maximum charging current should be 1/3C, preferably 1/10C
 
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