Properly charging an SLA battery

[AbyssUnderground]

Ive been wondering for a while how I should "properly" charge my SLA battery. Do I have to give an absorbsion period, such as a set voltage like 13.8v-14.2v for a certain period or should I just let it charge till the charge controller cuts off once it hits its cut-off voltage of 14.2v?

If I have to give this period can anyone recommend a cheap-ish solar charge controller that would do this automatically? Im planning on lots more solar panels in the future so it needs room for upgrading (up to 200w should be fine, I currently have 6w(!!!) but I plan an extra 40w in the next few months when I have money).

Thanks,

Andy.

[Bruce S]

Andy;

 SLA,s are pretty much the same as Lead-acid batts. So charging them with the same care & Feeding you would any good batt on your system will be just fine.

I would also go to say that they also have the problems of making sure they aren't discharged too far down, plus the good ole desulphating routine will help if any are old units coming out of UPS systems.

Hope this helps

Bruce S

 

[AbyssUnderground]

The thing is this IS the only battery on my system, and I was simply wondering if it needed this absorbsion period and how best to go about it.

[Bruce S]

Well, knowing that and taking into account that if your current solar charger is outputting say 31.8 - 14.2 and a good rating of 6w that this can be considered a slightly better than trickle charge. ( A trickle charge is considered at .1C and fast @ 1C) you should have no problems using the one you are currently using.

Since it is a 6 watt charger you also should have no problem with it over charging , but it will also take several days to fully charge this battery, depending on the Amp hour rating.

Once you begin using the 40 watt charger you should look into a charging regulator.

Hope this helps :--) a little

Bruce S

 

[Flux]

Yes you need the absorbtion phase, once it is significantly charged then the bulk charge rate will raise the voltage to gassing at quite modest currents.

You hold it at the absorbtion voltage and the current will gradually taper off and complete the charge. Once it is fully charged you may want to reduce the voltage to a lower float level but much depends on the speed with which you charge it and the time it spends at full charge.

If it floats much of the time you can use a compromise voltage of about 13.8 that will be more of a float value. It will eventually charge fairly completely but rather slowly. If you discharge it too often you may need to raise the voltage for a period ( much like an equalising charge for flooded cells but you must not take the volts high enough to drive off the water)

If it gets discharged frequently then you may wish to raise the voltage higher to a maximum of about 14.2v. If it is likely to remain in this state for days at a time you would be advised to drop it back to a lower float voltage as the high volts cause positive plate corrosion.

If have had it up to 14v or more for an hour or so then as long as there is no discharge you can cut the charge completely as long as you don't let it stand for weeks.

There is no perfect scheme or perfect controller but remember the biggest enemy is large % discharge and leaving at low volts for significant periods.

Some controllers stop the charge and others divert excess volts and effectively float.

As far as I can see all the charge cycles are based round a regular discharge and re-charge,

When you are charging and discharging at the same time much depends on the periods of each and controllers are not sure to what to do and neither do I.

[ghurd]

I use quite a few small ones. 7AH. With small loads (ie: 100ma LED lighting) and small charging current (ie: VW PVs), very frequently (every day) charged and used.

I get best results with both less than .1C and 13.8V maximum.

What I have seen...

Fast charge kills them quick. High drains kill them quick.

Over-voltage kills them soon. Too long at under-voltage kills them sooner or later.

Not much different than what is printed on many batteries like that.

You really should check out the PB137ACV chip, about US$1.

Add a heat sink and 2 caps, maybe from a TV or monitor.

Perfect for 6W and a small SLA.

G-

[AbyssUnderground]

If you take a look at my site in my profile you can find out most of the info you need to advise me, that is if you have the time or patience to do so.

Its a 17.2Ah lead acid with 6w of solar and a cheap 5A charge controller which cuts off at 14.2v but doesnt give chance for absorbtion.

In the future I hope to have about 40w of solar and a small wind generator with about 40-80Ah of battery storage.

All I want to know is if I need to absorb and how should I do it.

So should I make it absorb at about 14.2v? I have a mains charger that is 13.8v regulated and the charger for the powerpack is unregulated and gets the voltage up to around 14.5-15.5v if left long enough at 500mA max.

[ghurd]

I DO know what you have.

Do you?

I told you the exact peak power you were likely to see...

 and that is exactly what you saw... remember?

I never "actually got bored reading" (your earlier post) about what I wanted to know.

"Mine cuts off at 14.2v and turns back on at 13.1v" (another of your earlier posts).

"Cheap" also comes to mind in regard to the controller.

Most of the bulk or absorbtion charge is cut off due to hysteresis (what I said in an earlier post, "if you have the time or patience to" read it)

Most of the solar power is lost as the battery gets near full.

You won't do much absorb charging with what controller you have.

Very highly regarded and knowledgeable people tell you the answer, flat out, but you ignore or insult them.

And I do not count myself as highly regarded or knowledgeable.

Spend and waste your father's money as you wish.

[AbyssUnderground]

Excuse me? In my readings on this board you have insulted many more people than I have and you have the cheek to insult me? I ask for answers and make sure that what I am assuming is correct. I appriciate EVERY bit of knowledge and EVERY answer I get on this board, but insulting me because Im trying to do my best is not the way to go about life. You should think about the things you say before you post them.

Back on topic:

The peak power has nothing to do with this topic. Yes, the charge controller is cheap, but being a student, money is not abundant and I have to wait until I can get these things. Even £50 to me is a lot of money since I don't currently have a job and Im in full time college. Because of this Im trying to do things cheaply and as best I can with the money I have available.

So to finish this topic off, Id like a simple one word answer to this question:

Is absorbtion charging, charging at about 14.2v for a few hours?

Thanks for everyone who has at least tried to give some usable information unlike ghurd...

[altosack]

On the flame war front: sorry, but you started it, not ghurd (this time ;-).

On topic:

You want a one-word answer to your question, but I can't do it, except to say "no", but it's not just no, it's partly yes, but...

The absorption phase is voltage regulation while the current decreases, then, ideally after the current decreases to a percentage of the battery size (but usually after a fixed time period since cheap charge controllers don't allow you to input your battery size), it decreases to float voltage. This is the "ideal" 3-stage charging.

However, cheaper charge controllers (included those on car alternators) only have two-stages, bulk and absorption/float, i.e., it never reduces the voltage to float. They compromise by having a higher voltage than normal float, but lower than normal absorption. This has the consequence of not fully charging the battery very well; however, it's not too bad in a car because usually the battery is not discharged much.

Also, in a SLA battery instead of a FLA one, the absorption stage is set a bit lower (the 14.1V of a MorningStar SunGuard or equivalent is for a FLA; it's a higher float than I would want for a SLA), and if there is an equalization stage (usually not), it's at a much lower voltage.

The only way you're going to get a charge controller that does proper 3-stage charging for a SLA (there are jumper settings for different battery types) is to pay for it. The cheapest I know of are the MorningStar SunSaver series, about $60 US or so for 10A.

Well, you can also buy yourself a cheap microcontroller (e.g., ATmega48V for $1.90 from futurlec), a Power MOSFET, a gate driver (not really necessary if you set the PWM slow at 3-5kHz, but you'll be able to hear it then) and a few other bits and you can make your own 20A/12/24/48V for about $10 that will do whatever charge algorithm you're willing to code. A great learning experience for someone in college (or even for us older farts ;-).

Dave

[Flux]

I don't normally bite but I don't have time to read other peoples websites to answer questions. I answer on what is asked here.

I thought I covered most of the issues when I said that normal charge algorythms are based on charging a battery from a constantly available supply with no discharge present.

I am not quite sure you realise that these charge modes of bulk, absorbtion and float are a bit arbitrary.

If your charge current is very limited then you are effectively doing the absorbtion bit with the bulk and the battery accepts the lot.

At higher current the battery will accept all the charge at first but as it comes up it will not accept the high current and it will go up to gassing voltage.

You limit the voltage and the battery will then accept what charge it can and the current will fall. Eventually it will fall to a very low value and the battery is fully charged.

If you have unlimited current available and you want to charge quickly you stop at the point when charge has fallen to this low value and you have to detect it.

More often the voltage is set a bit lower and charge is limited in the bulk phase so that it reaches the set voltage near the end of the charge cycle. Rather than sense current most things set a timer when it reaches fixed voltage and assume that it will be charged in this time. This may be what you are worrying about.

With solar or wind or any variable charge the point at which you enter voltage limit will depend on the charging current. On a poor day you may be fully charged from low current. On a good day you may reach voltage limit at possibly 90% charge and ideally you need to stay at absorbtion voltage for a time to complete the charge.

If you stop when you reach the fixed voltage you will be a bit undercharged but if you drop to a lower float voltage it will mostly creep up to full charge eventually but as I said previously you may have to correct at intervals.

Normally most controllers compromise and take volts to a little over 14v then cycle on and off between about 13.6 and this value.

Sadly batteries die whatever you do and user observation and intervention will help to extend life to some extent. After 150 years the lead acid battery is still crap and I don't see that for all modern technology they last any longer than the Plante cells of the 1920's. They are lighter cleaner , relatively more efficient and certainly cheaper but don't last any longer.

flux

[AbyssUnderground]

Yes I may have started the flame war, I dont deny it. But never the less...

It seems the answer is clearer now thanks to altosack and Flux. Ill see what I can find in terms of charge contollers to do the job automatically. For now though Ill have to manually do the absorbsion or leave it to "almost" fully charge without it.