How much current can a 12V 5Ah SLA battery supply?

[Simon99]

I am trying to diagnose whether I have damaged my new battery by discharging it. Here is a photo of the battery with the specifications and a photo of the current while discharging it.P1110362.JPGP1110365.JPG

To make a long story short, I bought it, used it to charge a phone at 280mA over 5 hours, then charged a few other things, then set it up outside with a solar panel project that failed (That's another story) but I think it became deeply discharged. I am not entirely sure but the standing voltage was at 11.7V.

Anyway, I put it on a charger yesterday and set the charger to 1.5A because that is what I read on the battery as the "Cyclic Usage" "Initial Current". I later read elsewhere that SLAs are limited to 10% of their amp hours. That would be 500mA. Did I make a mistake? The charger only supplied 1.4 amps to it for about 90 minutes, after which the current slowly decreased. A couple of hours later, it got down to 100mA then the charger shut off.

Now I am testing it for discharge. I set the unit at 1A but it will only supply 400mA discharging. Later it went up to 500mA but now it is back at 410mA.

What it the max. current the battery should supply? How can I tell if it is damaged? What should the max charge rate be?

[Sunking]

No it is a UPS and Robotic AGM battery. You can charge it at 20 amps if you monitor temperature and voltage. Being a UPS and Robotic battery also means you can discharge it at very high rates up to 4C or 20 amps. If you put it on a 1.5 amp charger for 5 hours and the resting voltage is 11.7 volts, toss it in the trash. FWIW this is not the right type of battery for cycling operations. It is made to discharge at very high rates and only has a few hundred cycle in it, as opposed to thousands of cycles.

If I were to put this into a solar charger I would be using something like a 50 to 100 watt panel. The battery is not made to be slow charged.

[PNjunction]

To make a long story short, I bought it, used it to charge a phone at 280mA over 5 hours, then charged a few other things, then set it up outside with a solar panel project that failed (That's another story) but I think it became deeply discharged. I am not entirely sure but the standing voltage was at 11.7V.

Remember that for most cycling purposes, you don't want to take the battery below 50% capacity for the longest cycle life. Ideally one would want to fall no less than say 20-25%. So in reality, if you want more than a handful of cycles out of it, you only have 2.5ah (50%) to play with.

You can get a rough idea by measuring the SOC voltage after a 4 hour rest of no-charge / no discharge:

12.8+ 100% charged
12.5 75% charged
12.2 50% charged
11.95 25% charged
11.95- don't go this low.

Anyway, I put it on a charger yesterday and set the charger to 1.5A because that is what I read on the battery as the "Cyclic Usage" "Initial Current". I later read elsewhere that SLAs are limited to 10% of their amp hours. That would be 500mA. Did I make a mistake? The charger only supplied 1.4 amps to it for about 90 minutes, after which the current slowly decreased. A couple of hours later, it got down to 100mA then the charger shut off.

Sounds normal. Mostly you'll read that these ups style agms can only handle about C/4 max input current. So the limit here of 1.25a makes sense. (5/4). Off hand, this rate is also about the maximum you want to discharge at - otherwise the voltage will crash very fast usually well below the voltage your gear can operate at. But you CAN dump a lot of current fast if you really want to - but look at the Discharge Charecteristic charts (link below) and look how fast pulling 10amps out of that thing will last - about 8 minutes, and thats at 8 volts! Not nice. At the other side of the chart is your cellphone at .25ah. That can go a LOT longer, like 20 hours. (real world usually less)

BUT, charts like these usually show times when totally discharging the battery. You don't want to go there. Cut the times listed in half for duration.

This similar battery specification may help. Look for the Powersonic PS-1250 in the chart and download the pdf documentation.

http://www.power-sonic.com/ps_psg_series.php

Update: that battery is a UPGI UB1250 - the pdf docs for the model D5741 can be found here:

http://upgi.com/products/batterysearch.aspx

Without any test gear, what I might do for a quickie test is just use that cellphone charger if it truly does draw 280 ma for about 9 hours from a fully charged agm. You may have to use the phone a few times to recharge for 9 hours total. 280 * 9 = 2.520a, about 50% of that agm. Let it rest for 4 hours and measure the voltage. If you are nowhere near 12.2 volts, then either the battery is bad to begin with, or you didn't start out with a fully charged agm. Or the load isn't really 280ma constant.

Obviously none of this may be accurate without quality measurements, but I think you get the idea.

[PNjunction]

If I were to put this into a solar charger I would be using something like a 50 to 100 watt panel. The battery is not made to be slow charged.

Funny you should mention that - I'm now running 100 watts of panel through a Morningstar charge controller into a 12v, 5ah agm - but it is a homebrew 6-pack of Enersys-Hawker Cyclon agm batteries, (two 6v monoblocs in series) where the cost of admission is a minimum current input of C * 0.4

This will have to do until Trojan makes a 5ah battery.

[Simon99]

this is not the right type of battery for cycling operations. It is made to discharge at very high rates and only has a few hundred cycle in it, as opposed to thousands of cycles.

Thanks for this information. How can I determine that a given battery is designed for cycling?

I have this one working out in a paddock on a farm right now with a 10W panel: SLA-12V7-F2

http://www.batterymart.com/p-12v-7ah-sealed-lead-acid-battery-f2.html

It has been running for 3 months. Here is a picture:

2012-07-16-594.jpg2012-07-16-592.jpg

Am I going to have the same trouble? Low cycles?

[Simon99]

You can get a rough idea by measuring the SOC voltage after a 4 hour rest of no-charge / no discharge:

12.8+ 100% charged
12.5 75% charged
12.2 50% charged
11.95 25% charged
11.95- don't go this low.

...

Without any test gear, what I might do for a quickie test is just use that cellphone charger if it truly does draw 280 ma for about 9 hours from a fully charged agm. You may have to use the phone a few times to recharge for 9 hours total. 280 * 9 = 2.520a, about 50% of that agm. Let it rest for 4 hours and measure the voltage. If you are nowhere near 12.2 volts, then either the battery is bad to begin with, or you didn't start out with a fully charged agm. Or the load isn't really 280ma constant.

I fully charged the battery yesterday, connected it to the panel and phone charger in full sun for a few hours (Where the net charge to the battery was marginal) and then left it on the phone charger overnight. The current did taper down from 280mA to 15mA this morning. I have disconnected the battery, will let it sit for 4 hours then measure the voltage and reply on this thread.

[Simon99]

I'm now running 100 watts of panel into a 12v, 5ah agm - but it is a homebrew 6-pack of Enersys-Hawker Cyclon agm batteries, (two 6v monoblocs in series) where the cost of admission is a minimum current input of C * 0.4. This will have to do until Trojan makes a 5ah battery.

My knowledge of battery characteristics and quality is thin, but why would you assemble your own battery parts when you can buy a 12V 5Ah battery online from Battery Mart as I did for $23 incl. shipping? Is it that batteries that are suited for cycling are more expensive, that the quality of what I bought was poor, that you just like building your own stuff of happened to have the parts handy?

[Simon99]

Okay the battery has been standing at room temperature for almost 4 hours. I took a reading and it measures 12.45V. So interpolating the scale given earlier, that represents 70% charge. I have now put it back on the charger at 1.5 amps initial current. We'll see how that goes.

[PNjunction]

Your resting SOC test sounds reasonable.

Be careful with the charger if it is a manual automotive type. You can do it if you are extremely careful, but most would recommend a 2 or 3 stage smart-charger designed to handle agm properly. With the panel and charge controller, it is just a simple CV limit - the battery bulks up to the CV voltage setting, and then because of that limitation, the battery will naturally drop current levels because the battery is not allowed to rise above the set CV voltage. What you are seeing is normal. The key to this recharging via solar is to be able to do it in the solar-insolation timeframe of your area. Othwerise, the batteries will always be in a state of under-charge (defecit charge), sulfate, and eventually fail to provide any meaningful capacity.

The reason I built my own pack with the Enersys Cyclon's is that they will take massive amounts of current so charge times can be really fast, which are important to me when I have limited amounts of solar-insolation available. I'm not limited to the C/4 maximum rate of the typical ups-style agm's, but START at C/4 with these. This ability to recharge very very quickly may not be needed in your case for something that only gets use once in awhile. I'm cycling stuff down to 50% daily more or less. I can get away with it on a small scale like this, but having the necessary amount of panel current goes impractically higher when you get to the larger batteries unless there is a real need for this fast-charge capability. Also note that the Cyclon's are very expensive compared to the typical Powersonic or UPGI agm's. I had the need for super-fast recharging - but it costs me in more supportive infrastructure.

In your case, the Cyclon's may not be justified.

[Simon99]

It is a decent multi stage electronics charger. I bought it at Fry's last week. $80.

P1110397.JPG

I asked it to start at 1.5 amps. It was putting out 1.41 initially. An hour has passed and it is down to 650mA. I expect it will tail off to 100mA over the next few hours.

I buy my project parts here in California and take them to Australia. Parts are a lot cheaper here and the variety is a lot better. I go there often enough that I take them with me. A 10W panel I can fit in my checked luggage. Not sure about 20W. But big heavy things are expensive via unaccompanied air freight. It was going to cost $300 to send my 12lb assembly. Almost as much as it cost me to build it. It might be most cost effective to just continue with the small 10W panel and replace the $25 battery every 12 months.

One of the most interesting lessons I have learned in the last 24 hours is that small SLA batteries are not all the same and that maybe I should be looking for a different type. I didn't really think about the deep cycle part of it. Obviously with solar, that is an issue.

[PNjunction]

It might be most cost effective to just continue with the small 10W panel and replace the $25 battery every 12 months.

You can still play with the numbers. 10w panel = .5a best condition. You use up 2.5a of the 5ah battery, 50% DOD. It will take at a minimum 2.5 / .5 * 1.76 = 8.8 hours to recharge from half full. Real world, about 10 hours. If you have 4 hours of solar-insolation per day, it would take a little more than 2 days to fully recover from that discharge depth from the 5ah battery with that panel. If that works for your application, great. If you only discharge to 25% DOD, 1.25 / .5 * 1.76 = 4.4 hours, or a little more than 1 solar day. These are typically best conditions. Throw in clouds, any little shadow crossing your panel, etc, and times increase.

With that programmable charger, you can simulate various panel output currents vs time for solar-insolation and see how well charged you are after a resting 4 hour period. Then discharge with your phone like a normal operation, and compare over a few cycles.

One of the most interesting lessons I have learned in the last 24 hours is that small SLA batteries are not all the same and that maybe I should be looking for a different type. I didn't really think about the deep cycle part of it. Obviously with solar, that is an issue.

Don't worry - destroying your first battery or two is part of the solar initiation.

[Simon99]

Yes, I have decided to go with a 10W panel as it has been working fine on the existing prototype, not having missed a beat in 3 months. The 5W panel just didn't cut it.

One thing that has been really difficult with this project is to work out how much power the phone consumes. I know that when it is dead flat and I hook it up to a charger, it draws 280mA for 4 hours, then slowly goes down to 10mA over the next 2 hours. That is easy to watch with a multimeter. The tricky bit is once it is fully charged, how much it draws over a 24 hour period. Mobile phones were not designed specifically to be plugged into chargers 24x7, although I have found they can handle it - mine has been plugged in for 3 months. Sometimes when charged, they will draw no current at all. Other times later when you look at the multimeter they are drawing 100mA. This you would expect because eventually the Li-Ion battery depletes. But it has been hard to determine, which was key to my calculations of what panel size I needed. I thought 5W would be fine to keep the SLA battery topped up. It turns out that it was not enough for the charge controller.

[PNjunction]

If you really want to get into it, a small inline dc ammeter like a "Watts up" unit could be handy. Powerwerx and various RC places have them. It keeps a running total of power along with other useful data.

For the time being, you could just drain the phone in a worst case condition. Starting from a fully-charged agm, recharge the phone. Measure the agm voltage after a 4 hour rest as a worst-case discharge point. Using the programmable supply, emulate a 10w panel by charging the agm at 500ma. If the worst case scenario in Australia is say 3 hours solar-insolation, just let it charge for 3 hours and pull the charger off. After a 4 hour rest, measure the agm terminal voltage to get an idea of how well your panel would charge the agm in only one day referring to the typical SOC voltage you used earlier.

That's the other question - do you use this thing daily? Are you willing to go more than one day to recharge the agm? In either case, it sounds like you are in the ballpark already, but you've got the tools to fine-tune it if you want to. Looks like a nicely built project - I wish mine looked as good.

[Simon99]

Yeah that Watts Up unit is what I need. $60. Hmmm. I already have a multimeter. Pity there isn't a cheaper single function version.

I have started on the tests you suggested. I actually have a 10W panel on order, so I will redo the tests with the panel in full sun to see how it compares with the charger at 500mA.

Do you use this thing daily? Are you willing to go more than one day to recharge the agm?

I sure do. The first prototype has been running out in the field for over 3 months. The one I have been building is the second prototype. I went smaller on the panel and battery but obviously the reduction in panel size was a mistake. My calculations showed it should work but I did not factor in some things about the charge controller.

My third prototype will involve a custom pan tilt zoom phone using servos and an Arduino board that I will be able to remotely position for photos / video to monitor the property. It will be on top of the highest hill. I have already put a telegraph pole into the ground. Waiting on the hardware.

[Simon99]

The results are in. 4 hours after I charged the battery, the voltage read 13.14. I fully discharged the phone, which then caused problems with the phone when I put it on the charger, plus the charge controller was flashing red saying the battery was bad. I think this new charge controller is sick. Anyway, after lots of stuffing around, I managed to 100% charge the phone from the battery. After leaving it overnight disconnected, this morning the battery reads 12.54V. According to the guide above, this means a full charge of the phone used 25% of the battery. Sounds like not too much drain.

A new charge controller is on the way, courtesy of the sales guy, and I have ordered a 10W panel. Looking forward to mounting the thing outside for a few weeks.