SLA battery charging advise, help!

[torclausen]

Hello,

I am working on a project where kids build boomboxes. I'm trying to work the bugs out of the basic design.

Lepai 2020
SLA 4.5 Amp/hr battery
Cheapo car trickle charger 500 mA

I've been messing around with these sort of setups for years. Here is my question ~

How would you handle the charging? Currently, I alligator clip the charger on the battery when charging is desired.

Main problem is when the amplifier is left on, the battery drops so low that the little charger get overloaded when it tries to charge it and clicks off. I put a larger charger on to get the voltage high enough for the little charger to take over, but thats too confusing.

Simple and affordable is key. Any way to install a low voltage kill switch to protect the battery?

Help!

Many thanks, love this forum.

 

[terrences]

There are two ways to do this, one is a new charger rebuild with new parts. Lots of links to do this online and on this forum. If you want to go cheap, you can take apart the trickle charger and replace the transformer in the circuit with an 80VA one that has the same VAC secondaries. Just make sure the parts inside can handle the increased heat. If they can't, replace them with higher spec parts or mount the ones you have on a heatsink. The milliamp output of your circuit seems fine and it has shut off protection so it just needs more capacity.

 

[torclausen]

Thanks Terrences.. I'm attempting to make a project I can do in a day or two with kids. So I'm looking for a more off the shelf fix. Is it possible to kill the amp before the voltage gets too low? and inline low voltage kill switch?

Is that a good way to deal with this?

 

[torclausen]

this is too much money, but it looks about right...

http://www.amazon.com/Seco-Larm-Vol...1413066414&sr=8-3&keywords=low+voltage+switch

Then they could hook up the charger when the boombox quits, and it won't be so damn hard to bring back up.

 

[Richidoo]

Battery Tender can handle charging a low battery. It will charge @ 1.25A until 80%, then slow down until battery is full, then trickle, all automatic. It is a smart charger with microprocessor. It is very well built. When I was using a battery powered system I left this charger connected to the battery all the time. I just flipped the wall switch when I wanted to turn on the charger. Even when the charger was charging while playing music I could not hear any effect on the sound of the amp. They also have a 5A version for faster charging.

A nice charging jack is Switchcraft 712A which is good for 5A, and matching plug is part 760.

 

[ttan98]

Hi,

I use battery charger to charge my SLA battery, I think your battery is too small in capacity, I use 4*12V battery series & parallel to make 24V battery. When I play music the charger is disconnected from the main amplifier and I play music for 3-4 hours before the voltage drops to about 22V. After I finish playing music I switch to charge mode, the charger takes over. I use a single switch up for playing and down for charging. You should not let the battery to run too low.
I suggest you add another battery in parallel to your existing one. Charge the 2 batteries to fully charged and then play your music after playing music(do not let the battery to run too far low voltage)then recharge your batteries. I have not problem that way. My charger is very simple, Transformer via full bridge output is around 26 Vdc to charge my 24V batteries.

 

[AndrewT]

You could add a current limiter between the charger and the battery.
An lm317 wired as a CCS is very easy to build and use.

 

[terrences]

Hi,

I use battery charger to charge my SLA battery, I think your battery is too small in capacity, I use 4*12V battery series & parallel to make 24V battery. When I play music the charger is disconnected from the main amplifier and I play music for 3-4 hours before the voltage drops to about 22V. After I finish playing music I switch to charge mode, the charger takes over. I use a single switch up for playing and down for charging. You should not let the battery to run too low.
I suggest you add another battery in parallel to your existing one. Charge the 2 batteries to fully charged and then play your music after playing music(do not let the battery to run too far low voltage)then recharge your batteries. I have not problem that way. My charger is very simple, Transformer via full bridge output is around 26 Vdc to charge my 24V batteries.

He gets music before the charging circuit cuts out so that's why the battery drains. If the charging circuit stayed up, the battery would be fine.

this is too much money, but it looks about right...

Amazon.com : Seco-Larm Low Voltage Battery Cut-Off 12/24V Adjustable Restore Point Master On/Off Switch : Home Security Systems : Camera & Photo

Then they could hook up the charger when the boombox quits, and it won't be so damn hard to bring back up.

This is not what you need. Your battery will still drain and cut out. I don't think the charging circuit is your problem here, assuming the battery is in good condition and the circuit parts haven't been damaged (ie: the charger is working properly) the solution is in changing the over stressed piddly transformer supplying the current. The amp current draw is too demanding for the transformer so it heats over spec triggering the circuit protection, is what I believe is happening. Change your transformer to a beefier one and problem solved. Even if you take the advice of AndrewT, you'll need a new transformer. If none of this makes sense to you, take Richidoo's advice and buy the battery tender.

 

[Mooly]

You need a simple low voltage cut off circuit such as this that will trip at 10.8 volts and prevent the battery discharging further.

Over discharge of SLA's will kill them as will charging with an inappropriate charger.

 

[JMFahey]

Lepai 2020 <--- check
SLA 4.5 Amp/hr battery <--- check
Cheapo car trickle charger 500 mA <--- ouch !!! 😱

How would you handle the charging? Currently, I alligator clip the charger on the battery when charging is desired.

Main problem is when the amplifier is left on, the battery drops so low that the little charger get overloaded when it tries to charge it and clicks off. I put a larger charger on to get the voltage high enough for the little charger to take over, but thats too confusing.

Obviously your little charger is not enough, period.
16/20W (I bet the kids run their boomboxes into clipping) times 12.6V times , say, 90% efficiency means your battery supplies almost 2A on a fairly regular way.

If you pull 2 liters water per hour out of a 4.5 liters bucket and your tap supplies only 1/2 liter per hour it's clear that soon you will end up with an empty bucket.

2 answers:
a) you charge your batteries overnight (12 hours) and next day you have , say, 3 hours autonomy (how long does your class last?)
or
b) you get a 2A charger .

I'm painfully aware about battery charging and actual use, I commercially make battery powered amps for street musicians.

FWIW Here's my biamped "Callejero 30":

20+10W, 12V 7AH battery, built in "intelligent" charger, recharges in 8/10 hours and does not damage the battery if left on forever.

 

[spwalek]

Try this it uses a current limited approach. It will increase the build time but works well and can be customized to the rating of the SLA. I use this circuit with cast off laptop power bricks instead of the transformer and bridge.

Battery charger

 

[torclausen]

Thank you everyone! Played at full volume, my boxes are playing over 8 hours. I actually play them outside on full with heavy blankets over them.

I'm leaning toward advising the battery tender. For $15, I get an off the shelf solution. $25 bucks instead of the $9 dollar bottom end trickle chargers. I think its worth it. I think I'm leaning toward the plugs listed above too, as the alligator clips can be put on wrong.

I do like the charging limiter as well. Hadn't thought of that. There must be one of those on all these batteries that charge via USB. I've seen large batteries charged at 5 volts at 1 amp.

T