Charging 12V SLA from car

[Timvp]

So, here I am trying to read up on my stuff - not understanding most...

I would like to make a simple mono boombox in one of those small metal carrying suitcases to make the mp3 device heard!

What I have:
- 7Ah 12V battery
- Class T amp
- decent & sensitive speaker
- I'll make the signal mono using 4.7k resitors

Now, I'd like to charge the thing from a car cigarette lighter. Simple?

All I need, I think, is over charge protection, no? Or?

Dear forum, any ideas are appreciated

[DigitalJunkie]

It should work fine just connected in parallel with the cig. lighter plug. The charging parameters of a 12V flooded car battery,and a 12V SLA are more-or-less the same.

It might be good to put a diode in series with the cig. lighter cord (I'd suggest a schottky type) to prevent the SLA from discharging into the car's electrical system (like,when you go to crank the engine).

Other than that,I'm not sure you 'need' anything else. (ohh a fuse.Don't forget a fuse!)

[Timvp]

Perfect, thanks a lot Digital Junkie.

[palstanturhin]

The car cigarette lighter is fused - right?
With what fuse? 5A? 10A?
Not to blow the fuse when connecting your 7Ah battery to it, I could recommend (with the diode) also a series resistor. Lets say 2.2ohm 100W + heat sink (a big one - just in case you were also booming your box when charging)...

[Timvp]

Okay, I believe, oddly, I have one of those lying around as well. Is there a particular reason why you'd say 2.2 Ohm? And what is the function of the resistor (say that I will be booming the box).

Thanks for the tip anyway.

I am putting this thing together from old materials I have lying around. The lead acid battery has not been used for a while, so it may be dead. I have it on the charger now to see if the battery can still hold a charge.

If it's dead, should I invest in a lipo 12V 9800mAh pack (the blue ones on ebay) and make/get/build in a charger for it? Or just a new SLA, any experiences here?

Ah, the amp I will be using will draw a max. of 25W.

[jitter]

Originally Posted by DigitalJunkie

It should work fine just connected in parallel with the cig. lighter plug. The charging parameters of a 12V flooded car battery,and a 12V SLA are more-or-less the same.

Not so fast.

Depending on the type of battery, the initial charging current may need to be limited. This is especially the case with sealed lead acid batteries (SLAs).

Without current limiting, a flat battery will try to draw quite a lot of current. A car battery + running alternator will supply ample. The initial charging current of a 12 V / 7 Ah SLA must be limited to approx. 2.5 A. Depending on the make it can vary, but it should be printed on the case (e.g. the Panasonic LC-R127R2PG must be charged with 2.88 A or less).

If you ignore this things can go bad, like overheating it and possibly blowing it up.

[gmarsh]

If you're using SLA, the 'big resistor' method is probably the way to go - and by far the cheapest.

Given 2A maximum charge current and worst case voltage difference of 14.4V at the car and 12V at the battery (2.4V drop) that's a 1.2 ohm resistor, which will dissipate 2.4^2/1.2 = 4.8 watts. I'd use a 10 watt ceramic resistor. Going with the 2.2 ohm resistor, it's 2.6 watts and a 5 watt resistor will suffice.

If you're using LiPo, don't use the resistor method or you might find yourself with a car fire... I'd highly recommend using a proper LiPo charger instead, running off a cheap 12V->120V inverter. Buying the bits to run that would probably be cheaper than trying to build anything.

I designed a charger a few years back which does exactly what you've got in mind, charging a SLA from automotive +12V, using a Linear LT1513 buck/boost. But buying one of those chips, getting appropriate magnetic material, winding coupled inductors, getting a PCB made, etc... would probably get expensive really fast.

[gmarsh]

Lastly, whatever you do... MAKE SURE YOU USE A FUSE ON THE SLA BATTERY.