Need help hooking up SLA batteries

[Saurav]

Hi,

I have 3 12V SLAs arriving in a few days (my phono stage needs 36V - 45V). I also ordered one charger, because it said it could charge 4 batteries in parallel. So I need the batteries hooked up in series to drive the phono stage, and in parallel to be charged. This is where things get complicated, and I need help. I figured I could do it with a 6PDT switch. Hook up the batteries to the 6 contacts in the middle. Put the charger on one side and wire it up so the batteries are in parallel, put the phono stage on the other side and wire that up so they're in series. That should be doable, right? The problem is, I can't find any 6PDT switches. The phono stage has about 120,000uF of PS capacitance, so I want a switch that's rated for a reasonable amount of current just to be on the safe side.

So... does such a switch exist? If not, is there any other way of doing it? A separate charger for each battery and 3 DPDT switches? A 36V charger - does that exist? Will the batteries be OK if they're charged in series?

I'm sure other people here have hooked up SLAs in series to get higher voltages. How do you charge the batteries?

Thanks,
Saurav

 

[TriodeGuy]

I'm pretty much in the same boat as you but my case is just the reverse of yours... I need hook 4 12V SLA in series for charging break them into parallel for driving my stuff. I found an 8PDT 2A switch...would this be enough current for your needs??? It has an extra two circuits but it should work privided it meets your current requirements.

scroll down to the bottom its one of the last ones listed.

switch

 

[Saurav]

That could work, I think. How do the contacts work? 8PDT would need... 24 contacts, right? 8x3? So is that 12 contacts per wafer? I'm trying to visualize how that hooks up. Are every 3 adjacent contacts one 'set'? So the battery goes in the middle, the wires to the charger go on one side and the wires to the circuit go on the other?

2A should be enough, I think.

So you have a 48V charger? Is it safe/OK to charge SLAs in series? If I can do that, I could just buy a 36V charger and simplify everything. The batteries should drain equally... I don't know if that matters while charging or not. I'd think that charging a series string of batteries which are drained to different degrees might not be a good idea?

 

[cowanrg]

i did this awhile back with relays for a pre-amp that never happened...

basically, there was a power switch (DPDT) that directed power either to a bank of relays, OR to a charger. when the relays were NOT powered, the batteries were wired as 5 12v batteries or whatever. and the charger was powered. when the switch changed the power to the relays, the charger went off, and the batteries were wired in series for 60v. it was a pretty easy solution. when the unit was on, the relays triggered and you had power. when it was off, your charger started and you were disconnected from your pre-amp circuit of whatever you have it connected to.

and since the power supply for the charger was ALSO the power supply for the relays, it cut down on complexity. it worked like a charm and it may be used in a scaled down version for my active crossover when i get around to it.

 

[Saurav]

Hmm... that's a possibility, using 6 sets of relays instead of a 6-pole switch. I'd still prefer a switch though, because there's no power supply to worry about. Thanks for the suggestion. I've seen 6P relays on a few websites, but never considered that I could use those instead of a 6PDT switch.

 

[paulb]

I think this circuit will work for 3 batteries, 1 charger, and a 4PDT switch. We ended up switching to using 2 batteries, so I never used it. So please check the circuit carefully before you try it.
The switches are all part of a 4PDT switch, which is shown in the "play" position. "Charge" is the other position.

 

[Stocker]

If you insist on using a switch, NKK is likely to have what you need.

http://www.nkkswitches.com/

They have a great paper catalog too.

 

[cowanrg]

Hmm... that's a possibility, using 6 sets of relays instead of a 6-pole switch. I'd still prefer a switch though, because there's no power supply to worry about. Thanks for the suggestion. I've seen 6P relays on a few websites, but never considered that I could use those instead of a 6PDT switch.

you dont have to worry about a power supply... the charger has to get power somehow. you just route the power through your on/off switch is all. you are basically turning on the charger when you turn off the supply and vice versa.

you can scale the design for as many batteries as you need for your voltage by adding relays (or using the relays you suggest). in addition to the charger and batteries, for 5 batteries, the setup fit on a 4" x 2" circuit board and included terminal blocks for connecting all the wires.

i ended up going this route for the simplicity and cost of parts. the switches were hard to find, and expensive. 12v relays are very cheap surplus (i got some for $1).

 

[TriodeGuy]

That could work, I think. How do the contacts work? 8PDT would need... 24 contacts, right? 8x3? So is that 12 contacts per wafer? I'm trying to visualize how that hooks up. Are every 3 adjacent contacts one 'set'? So the battery goes in the middle, the wires to the charger go on one side and the wires to the circuit go on the other?

2A should be enough, I think.

So you have a 48V charger? Is it safe/OK to charge SLAs in series? If I can do that, I could just buy a 36V charger and simplify everything. The batteries should drain equally... I don't know if that matters while charging or not. I'd think that charging a series string of batteries which are drained to different degrees might not be a good idea?

I'm not exactly sure how the contacts work. The site doesn't list the manufacturers name or give a data sheet. I went ahead and ordered one since it's just 8 bucks I suppose I'll figure it out when it arrives.

Yeah it's a 48V charger. It was sold originally as some type of backup UPS. I think it was maybe used for networking equipment? Bought it from a web site that liquidates excess parts for industrial manufacturers. It contains 4 7ah SLA's wired it series. Whenever the output voltage drops below a certain level the batteries are removed from the load and hooked up to the charger still in series. I can't really say much about whether it's a good way to charge the batteries, but it was definitely designed to work that way.

It's a really solid and well built unit. Right now I'm planning on housing a modded Sonic Impact board and maybe a DAC in it.

The thing I like about using a single switch over relays is that since you have a "hard" mechanical connection with the switch's shaft, all the contacts are pretty much guaranteed to be made/broken at the same time. Relays have a bad habit of sticking after being in use for awhile.. I think if I were going to design a relay board I'd want to set all the relays to an intermediate "off" position before changing states. Maybe even have some type of "AND" logic in there to determine when all relays were off before changing...but then of course that would be making things a bit more complicated.

 

[Saurav]

That 4PDT schematic is pretty clever Looks like that should work.

Thanks for all the other suggestions. I'll see if I can find a 36V charger. If not, I think I'll stick with a switch. How much did you pay for that charger, if you don't mind my asking? Also, when is your switch arriving? Once you figure out how that works, I would appreciate it if you could let me know.

 

[TriodeGuy]

The liquidation price had been set to $25.00. Damn cheap for the quality unit that it is. The only catch was the shipping charges. I live on the east coast and this thing was located in CA. With the 4 batteries it weighs a ton so the weight brought the total cost to me to just over $100. But still if you think about all I got out of it...well built chassis, four SLA's and a charger. It's still well worth the money.

I just ordered the switch a couple of days ago so hopefully I'll get it by Mon or Tue of next week. I'll let you know how it goes.

 

[Saurav]

Sounds good. My batteries are showing up today, I think, and I can move wires around manually for a few days.