[sts9fan]

I am currently working on a "boombox" for S&Gs and was wondering if anyone could enlighten me on battery supplies.  I am going to be using a Amp6basic kit from 41hz.com and two 12volt SLAs.  I need to figure out how to charge these guys.  I have been looking around and have a few ideas but any pointer or links would be great.

[Daryl]

SLA's are very sensitive to overcharging, deep discharges and being stored partially/fully discharged.

The plates of SLA's corrode if they are not fully charged so you want to charge them immediately after use and when not being used they must be periodicly topped off or connected to a voltage source.

The individual cells of SLA's will not have the exact same capacity and so if you discharge an SLA completly the first cell to discharge will be to be charged in reverse (polarity reversal) by the other cells in the battery which are still discharging ruining the battery usually by the second time you completly drain it.

I recomend a regulated constant voltage supply to charge SLA's.

About 13.4 volts should do it.

[Gordy]

Here's a great charger for what you've planned... http://www.zbattery.com/12bc1000d-1.html   I use one to charge an SLA supply for my dac and RWA uses them as well, or did anyway.  Also, I like to use a dpdt on-0ff-on toggle switch...    http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=355227&Row=170221&Site=US  to connect the charger to the system.  Using a toggle switch allows you to keep everything connected permanently and is far less hassle "switching" from power to charge mode so you're (I'm) more apt to keep the batteries fully charged or charging between runs.  HTH's 

edit: Never allow the batteries to go below 12v!!! 

[sts9fan]

great tips!  So how do I make sure it never goes below 12 volts?  Use a gauge?  Or just be cautious?  What will happen?

[Gordy]

If SLA's run down below 12v they quickly loose their ability to take a full charge and run times become shorter and shorter between charges.  The battery will become useless in no time.  What I did was to monitor the voltage with a VOM and time the first couple of run cycles, until ~12.2v was reached.  That gave me the max run time I was allowed but, I like to stay at least a few hours away from that... say 8 hours if my max is 12. 

Actually, my first experience with batteries gave me just about exactly that example and one premature nod off     cooked the battery, so I've since always aimed for a 24 hour max time.  This'll be far more important with an amp than a dac I think, since you might be using an amp for much longer runs with a tuner or streaming music for back ground music. 

edit... again... Using batteries this way I've had no noticable lose of run time over a two year period and those particular batteries were bought from a surplus house and were from dated 2001! 

[jrebman]

You really should consider using an automatic or float SLA battery charger.  It monitors the voltage and shuts off when the appropriate level is reached so you don't overcharge them.

[Reverie]

My amp4 battery power diagram. Still not entirely complete. 

Needs tweaking for an amp6 - 12v instead of 24, and I'm not sure if you have a 5v section either.

Good luck! 

[sts9fan]

Thanks!!

[Occam]

....
Needs tweaking for an amp6 - 12v instead of 24, and I'm not sure if you have a 5v section either.

Sts9fan - Its allway helpful to post in your original question as much information as you can so that the responders stay on point. 8 posts in, Reverie lets us know that your Amp6 runs off a nominal 12v dc powersupply (not a 24vdc supply), assuming you've the Amp6-Basic, rather than the Amp6.
http://www.41hz.com/
If its the non Basic version, you need an AC supply (transformer) to power the unit.

At its simplest, you need a 12v SLA battery charger, that automatically senses and switches between normal charge and trickle charge when the SLA reaches a specific voltage. These are identifiable generally by the inclusion of a red and green led. You also need 2 dpdt, center off, switches. One switches between charge Batt1, charge nothing, and charge Batt2. The second switches between power from Batt1, off, and power from Batt2.
Reverie's suggestion of stiffening caps across the battery is an excellent one. You only need one, across the output of the switch that determines which battery is powering the amp. Batteries are not noiseless, they generate noise via their electrochemical action, which the cap helps to ameliorate, It will also minimize the noise from the charger if you happen to be 'listening' to a battery which you are simultaneously charging.

[Audiovista]

Try to find "deep cycle" batteries which will tolerate being discharged to 11V (1.85V per cell) without huge penalty on their life.

The deep cycle SLA batteries are used in electric wheelchairs (and other motive applications) as traction batteries.

[sts9fan]

Occam,

It is an Amp6 Basic.  Sorry I did not supply enough info but I was fishing for general ideas to help this and future projects.  Thank you for the two switch idea!  Is it possible to attach two batteries so to only get 12 volts?  This should give longer use time between charges right? Then I would only need one switch with on-off-charge.  What size cap would I want to use?  What factor make me choose that size?

Thank for the help!
Kris

[Occam]

Kris,

Yes, you can parallel 2 batteries, but that is potentially problematic. Ideally, if the batteries are the same make and model, and purchased at the same time, they will forever stay in sync. But potentially, due to manufacturing vagaries, one could need charging at at full voltage, while the other has reached its voltage and should be (automatically) switched to the appropriate trickle charge voltage. Dunno exactly how problematic this could be. Perhaps someone with more empirical experience could chime in.

If you want more playing time, my solution would be to buy larger batteries. You can look at the costs -
http://www.gruberpower.com/gruberpower/advertising/Batteries/Alarmbatteries/
and see that amp-hours vs. cost is reasonably linear. (at the bottom of the page you can also link to well priced trickle chargers).

You could also use a non center off dpdt switch to control which battery is charging, as this would ensure that one of them is allways charging, so that when you see the green led on the charger lit, you switch to that battery, charging the other. This would ensure that you can allways power from a fully charged battery, minimizing the potential for running the other battery down.

As to what caps to use across the battery, that would depend on its purpose. If its to provide instantaneous power to compensate for any current limiting temporary sulfation, use a 10k uf Panasonic FM (or Nichicon HE) 20v electrolytic, available from Digikey. If its purpose is to eliminate the 'popcorn' noise of sulfation itself, use a bunch of film caps in parallel with very low inductance. 1uf + .1uf + .01uf, with the bypassed caps extending the spectral efficacy. Very low inductance stacked film caps from Panasonic (V series polyester) and Epcos (BC3252 for polyester and BC3262 polypropylene) are very inexpensive and available from Digikey. Or use electrolytic and film caps together; they're cheap.
And obviously, you can use caps that are as bespoke as you wish.
Will these caps provide discernable improvements? From my limited experience, I believe so. But if not, I'm not going to loose any sleep over your potentially wasting $3 on caps. There is a reason the Good Lord has, in Her infinite wisdom, given use dpdt switches, so that we can put components in and out of circuits while under power and make our own judgements, free will and all that....

FWIW,
Paul

[sts9fan]

Thanks for taking the time for such a good response.

Kris

[art]

An old telecom industry approach.......

Put a diode in series with each battery. The one with the highest voltage will conduct.....the other will set there until its time to conduct.

Of course, the diodes need to be large enough to handle the peak current, and you will lose 0.7 v or so.

If you are running a trickle charger (13.2-13.4 V, depending on who's data you trust), the diode drop won't kill you.

Adding caps to lower the intrinsic noise (batteries aren't as quiet as most think) and finding deep-discharge ones as also good ideas. The automotive world calls them "marine batteries", although all batteries sold as such are not always deep-discharge. Go figure.

Pat

[sts9fan]

So what is the best way to determine peak current?