# 12 SLA charging circuit for amp

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#### Caleb Chia

Hi, I'm building a battery powered amp, and was am deciding what charging circuit to use.

One option- buy a plugpack sealed lead acid battery charger. The only annoyance is that you cant run the amp when the circuit is charging as the charger makes noise/hum as it switches.
I guess I could put heaps of filters and stuff between the battery and amp but I suspect the charger pulses the voltage on and off, which would be hard to filter.

Or I was thinking of just whacking a transformer in and put a regulator set to 13.8V. However I saw in another thread about it needing current limiting when charging a discharged SLA. What sort of circuit would be good for current limiting in this case?

Cheers,
Caleb

#### whizgeek

just use a small low current transformer. There is no real need for current limiting

#### FoMoCo

just use a small low current transformer. There is no real need for current limiting
There is a need for voltage limiting.
Also, the small transformer may overheat. Although it can't damage the battery, unless designed for it, transformers do not like sustained overloads.

#### Caleb Chia

After looking at available transformers, I think I might go for a laptop power supply (16V) and put a regulator circuit, with a current limiting resistor before the regulator, maybe a few ohms. Would this slow down the charging too much? 16V -12V= 4V drop, with a 4ohm resistor to limit it at 1A charging current, which is within the rating of the PSU and the regulator. Is this calculation correct? Of course the current will be less due to internal resistance of the battery.

#### Redshift187

Just use a constant voltage charger made with a simple LM317 regulator set to 13.1-13.5V. The IC will current limit to a safe level, and you won't overcharge no matter how long you charge if you stay between those voltages. In addition, this supply should be quite enough to listen while you charge, though the regulator might not give enough current for the amp (we don't know anything about your amp).

Internal resistance of an SLA battery is like 0.003 ohms... that's not going to affect the currents you're dealing with at all.

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#### Arty

use a relay to detach the charger from the battery while the amp is on.
preferably a delayed relay setup would work as:
1. detach the charger
2. attach power to battery.

then when the amp is off, the charger should be re-attached to the battery.

#### spwalek

I use this circuit for exactly what you want to do and it works well.

Battery charger

you can feed it with a laptop brick, or a transformer and bridge. You do need to heatsink the lm317 though.

#### Redshift187

The transistor in the circuit is for current limiting, according to the website limiting to 1A. The LM317 will limit to 1.5A on its own, I don't think you need the added complexity.

#### spwalek

Tyler,

Yes but the SLA battery's charge requirements vary by size/amp hour etc. The added complexity just extends the life of the battery by limiting charge current to your specific SLA requirements. R = 0.6V / max current

Steve

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#### paulb

Here is what I have used. I can't remember offhand whether it is for a 6V or 12V battery, probably 12V.

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• UC3906 charger.jpg
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#### MarianB

A well discharged LA battery draws verry high current when charging, the first step in charging it would have to be with a constant current source, limmited at about 10% of the battery capacity, the voltage should be at about 14-14,4V on the charging unit. That is why you need current limmiting, either with SMPS or 50/60Hz power transformer. There are loads of ways to go, you just have to be sure what you want for it.

#### Redshift187

Using a voltage regulator (like with LM317) with current limiting (however it's implemented) acts as a constant current charger until the current demands drop as the battery charges, at which point it becomes a constant voltage charger.

#### Caleb Chia

Thanks a lot for the ideas, that regulator circuit looks nice and simple.
I didnt realize the voltage regulator ICs had their own current limiting.
So it wont damage the IC to run it at its max amperege for extended periods of time?

1.5 amps charging current should be fine for the battery, I'm using a 7.2Ah SLA.

#### Redshift187

It's fine for these ICs to run at the current limit, so long as they are heatsinked sufficiently. 1.5A is a little high for a 7.2Ah battery, what I've read is C/10, or 0.72A. But you might not hit 1.5A if it doesn't run down too far between charges. Even so, what is the cost of that SLA? If it's not expensive, I'd just go for it and see how it lasts.

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