Simple 12v SLA circuit

[philbx1]

Hey all,

Just trying to finish off a 'portable' microcontroller rig, and I'd like to go super simple with the
power supply on this, and more importantly get the components in order.

So, I have a 12v 7.2 aH SLA battery, SLA charger and various boards I need to power (including
a Futurlec Atmega128 board, which BTW has a weird negative center DC connector, but standard
7805 reg. The other power is for a UBEC reg (for RC model servos).

What I'm asking in a roundabout way is what the minimum requirements are for a circuit, and in which order components are placed.
This circuit should have  fusing and polarity protection and maybe also DC voltage smoothing.

So, first off I thought I'd remove the DC connector from the Futurlec board as I don't have any male connectors to fit this (the inner pin is too big for a start, let alone negative voltage).

Having the MCU board sorted I then need to handle the battery and charging inputs.
The SLA charger I bought from Altronics has female spade terminals. No biggy there though...

Anyway, just playing around with Eagle I thought the following  might be the order of things,
(maybe attached) but as usual, I'm probably wrong.

Any thoughts on this guys?

[ciccio]

There are many different types of DC power jacks, and the one with large (2.3 mm) center pin is usually wired with positive at center.  It seems very strange that you can't find one...
The smaller one, with 2.1 mm center pin, is usually wired with negative center.
The so called "universal" power adapters have interchangeable plugs that can be rotated for changing polarity.

Regarding your circuit, I'm not sure that I've fully understood your needs, but it seems correct.

EDIT: The 1 A fuse will blow if you try to recharge an "empty" battery.
SLA batteries must be charged with a constant voltage charger,  not the constant current type used for Ni-Cad or Ni-MH. The max current is limited by the charger, so it must be less than 1 A

Regards

[philbx1]

Hey Ciccio,

Many thanks for your reply and it's much appreciated to get feedback.

I've decided to go for a more 'modular' approach and pretty much made everything a screw terminal.

I have desoldered the Futurlec negative centre pin DC input and will wire that in my own way.
I've also added switch inputs that although might not be 'safe', will allow better selection of power input.

I realised that any diode will drop power, so I've also removed these from the solar/charge inputs.

I'll alo find a higher-rated fuse as per your suggestion.

Anyway, thanks again for your comments. Much appreciated.

Hopefully the new revision images are attached to this posting...

There are many different types of DC power jacks, and the one with large (2.3 mm) center pin is usually wired with positive at center.  It seems very strange that you can't find one...
The smaller one, with 2.1 mm center pin, is usually wired with negative center.
The so called "universal" power adapters have interchangeable plugs that can be rotated for changing polarity.

Still not sure about the big capacitor that may use power. More research there by me...

Regarding your circuit, I'm not sure that I've fully understood your needs, but it seems correct.

EDIT: The 1 A fuse will blow if you try to recharge an "empty" battery.
SLA batteries must be charged with a constant voltage charger,  not the constant current type used for Ni-Cad or Ni-MH. The max current is limited by the charger, so it must be less than 1 A

Regards

[IanB]

I realised that any diode will drop power, so I've also removed these from the solar/charge inputs.

Beware that solar panels usually need to have blocking diodes somewhere in the circuit to prevent current reversal in the event the solar panel is shaded. Otherwise the panel could drain the battery rather than charge it.

[Simon]

depending on the size of a solar panel you need to have a proper charge controller circuit. I once put a 12V 44Ah battery on a 40W panel and it shot right up to 16V and the electrolyte boiled, not a very good idea

[Zero999]

depending on the size of a solar panel you need to have a proper charge controller circuit. I once put a 12V 44Ah battery on a 40W panel and it shot right up to 16V and the electrolyte boiled, not a very good idea

That doesn't surprise me. I had the same experience with two 12V 14Ah gel cell batteries and two much smaller 2.4W panels connected in series. Fortunately I disconnected the batteries before too much damage was done and the batteries are still fine today.

Sealed lead acid batteries don't like trickle charging, especially the gel type. If the charge current exceeds the natural battery leakage current my much it will start to boil the electrolyte.

Fortunately it's easy to build a charge controller. The battery needs to be charged with a limited current until the voltage reaches 14.4V, then float charged at 13.8V, by which time it's fully charged. You can do this by connecting the battery to a 13.8V linear regulator which is bypassed by a switch (e.g.e a relay or transistor) until the voltage reaches 14.4V, then the switch can be opened until the voltage falls below something like 12.5V. Don't worry about current limiting, the solar cell will already do that. This can easily be done with a comparator - there's no need for a fancy MCU unless you want to do something more sophisticated.

[philbx1]

Hey Simon,

Thanks for the info. The solar panel I currently have is pretty much a 'toy' (el-cheapo car battery top-up @ 100mA). This is probably overkill, but it's currently hooked up to
one of these http://secure.oatleyelectronics.com//product_info.php?products_id=690&osCsid=bbb95ad9b066b799578c04c7baefa857

I also have a smaller panel (shed LED panel) with one of these which would also hopefully not do too much in regards to boiling the 7.2aH battery
http://talkingelectronics.com/projects/SolarCharger/SolarCharger.html

Another one I have is http://www.futurlec.com.au/Solar_Charger.jsp
which I'd rather use on another project using a bigger panel and my 12aH 12v SLA battery.

An interesting point you bought up Simon. I have (as per the circuit) a 2200uF 50v) electro
which I thought might be good idea as a 'smoothing' cap, and I'd assume it would handle
smaller panels OK (unlike your 40w monster), but I'd like to find out if it would actually 'rob' power from the circuit at all.

Thanks much again for the posting.

depending on the size of a solar panel you need to have a proper charge controller circuit. I once put a 12V 44Ah battery on a 40W panel and it shot right up to 16V and the electrolyte boiled, not a very good idea

[philbx1]

Hi Hero999,

Thanks for your reply also. Being new to solar charging, your input helps me (and others)
a lot.
If you have time, please do read my reply to Simon and check out some of the links
as I may still be doing things wrong.

That doesn't surprise me. I had the same experience with two 12V 14Ah gel cell batteries and two much smaller 2.4W panels connected in series. Fortunately I disconnected the batteries before too much damage was done and the batteries are still fine today.

[Simon]

a large cap won't hurt, it will only "rob" the battery of it's leakage current but then it does not help that much either

[Lightages]

Sealed Lead Acid batteries are not all the same. GEL and AGM types are slightly different and have different charge and discharge characteristics. Batteries of the same type can have different charging requirements based on the manufacture's specs.

I have a fairly big solar system and live completely off the grid. I have 16 x 205 watt panels, a 4500 watt inverter and 12 x 250AH AGM batteries to make a 48V battery system at 750AH. Although this is orders of magnitude bigger than what you want to do, the principals are the same.

When the batteries are partially discharged, the first stage of charging is called "bulk" charging and the charging system acts as a current source a lets the voltage go where it will. That is up until a threshold voltage (in my case 58.8volts) at which time the charger then changes to "absorption" mode where it switches to a constant voltage source (in my case it holds the voltage at 58.8 volts). The absorption mode is maintained until the batteries draw less than 2% of their rated AHs, or 2 hours, whichever comes first. At this point the charge controller switches to "float" mode. Float mode is the voltage recommended by the manufacturer as "float" or "maintenance" voltage (in my case 54.4 volts).

So, every lead acid type battery should be charged in this manner, but to the manufacturer's specs for voltages and no more than the maximum bulk rate that they list for the battery. These numbers are specified at a standard temperature, usually around 25°C. If the temperature is above or below this then a temperature coefficient of around -80mv/°C is applied, again the exact number is from the manufacturer. Temperature coefficients can be ignored for the most part but charging without this accounted for in extreme temperatures will result in voltages that might damage the battery or shorten its life. There is also the consideration of the minimum charge rate. It is usually recommended that for GEL and AGM type batteries to charge during bulk rate at no less than 5% of the battery's rated capacity.

Another rule of thumb is to never discharge a lead acid type battery below 50% of full charge. This will ensure the longest life of the battery. It is better to keep the battery above 70% of full charge. Taking a lead acid type batery to 0% charge is a sure way to kill it immediately.