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# How can I rig a 12v sla battery to power an amp and be charged at same time.? Answered

I have a 12v sla battery (the kind from a kids play atv) that I want to use to make a portable mp3 player . I want to be able to use the battery and charge it at the same time if I want. The battery has only a positive wire and a negative wire coming from it. Help!

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well, you can hook the charger to the battery pack, but you'll be pushing an elevated voltage ~17V to the load...make sure the load can handle that high of an input before doing so. if the MP3 player's maximum supply tolerance is lower than the charging voltage of ~17V, you'd have to temporarily bypass its normal connection and route it thru an auxiliary 12V regulator circuit to avoid an overvoltage condition @ the MP3 player. that is, you'd be charging the battery *and powering the regulator circuit from the charger. The regulator in turn would power the MP3 player.

would a trickle charger work?

It might.

1. Before anything else....do you have a power adapter for using the mp3 player off the cigarette lighter in a car? If so, check the voltage at the working end of the adapter. If it reads ~17 V while driving, (not idling) then you're in business. If it's 12V, then things are more complicated.

2. if not #1, then check the manual or the side of the player for Voltage and power requirements. (I = P/V) P=power

if the load current is <<100mA, then check the trickle charge voltage with the battery connected to the charger, both with the battery fully charged and with it somewhat drained, to see the variation in charging voltage that you'll encounter. some chargers use a fixed voltage, some don't.

anyway, if the trickle voltage is within about 1-1/2 V of 12V.....so up to about 13.5 V, then I'd say go ahead, but be careful to pull it off the charger quickly if it gets warmer than normal...

best wishes