Most effective discharge rate for SLA batteries.

[TheVictim]

I built a UPS from a wall charger, 12v 7.2AH SLA battery, and a small invertor. I'm getting 55 minutes of run time from this, which is far below the AH rating of the battery. I know that it is not going to deliver the full AH rating at more than the 20 hour rate (360mA). I don't want to build twenty batteries into the thing, but what is the point of optimal return as far as discharge rate is concerned? I already have a 2nd battery, will a ten hour discharge rate give me more than 2 hours of run time? I'll need to upgrade the charger in any case, even the one battery is pushing the poor wall wart to its limit.

 

[mneary]

Some manufacturers publish discharge curves. Then it's up to you to plot service time vs. cost.

Also, it's probably better to use one battery of the rating you need rather than trying to match several batteries in parallel.

 

[RadioRon]

It isn't clear what is going on here. My first concern is that the wall wart is not capable of charging the battery very fast so I'm wondering if you are simply not starting with a fully charged battery. I'd check the voltage of the battery when the wall wart is charging it to see if the voltage from the wall wart is high enough too.

What is the current or power draw of the load you are putting on this thing? Is the inverter a store-bought one or something you built? I just don't understand your claim that 55 minutes is far below the AH rating of the battery.

I think that you can expect the amount of charge you get out of the battery to approach its rating, even at relatively high draw, and anything less than 70% of rating would surprise me. If you want to learn more, why not have a look at an SLA battery manufacturers website for data on the effect of discharge rate vs battery capacity?

So, assuming you are getting, say, 80% of rated capacity at a discharge rate of C, then you should get about 69 watt-hours, meaning that your load is pulling about 531 mA at 120 V (nominally about 64 watts). This assumes an inverter efficiency of 85%.

 

[TheVictim]

Sounds like I should just stick with the setup I have now. 55 mins isn't TOO shabby.

 

[RadioRon]

Something to remember about lead acid batteries, including SLA types, is that heat is their enemy. If you operate them in a really warm environment, they won't last as long and won't wear for as many charge cycles as they are advertised to do. The manufacturer ratings are usually at 25 degrees C, and they derate them quite a lot for every 10 degrees C you go above 25 degrees C. So keep them cool or at room temperature if you can.

By the way, I had a look at capacity vs discharge for a Genesis NP7-12 which is nominally called a 7.0 AH battery and they say that capacity for 1 hour discharge is 4.2 AH which is a 40% derating, so my guess of 20% above was optimistic. I think that there isn't a specific point that gives best capacity, but I note that the capacity for 5 hour discharge is still 6 AH which is pretty good. The other thing that will determine what is "optimum" is that the life of the battery degrades a lot if you discharge it completely every time. This implies that you wouldn't want to exhaust your UPS every time it is needed. To me this means to shut down my PC as soon as is convenient if power is lost.

 

[TheVictim]

Thanks for the update. I think I'll shoot for four batteries. Time to find/make a larger charging circuit for it.

 

[RadioRon]

You can take some comfort in knowing that lead-acid is the easiest kind of battery to charge so is a very good choice for a UPS. All you have to do is apply 14.6 volts through a series resistor in the simplest case. Car battery chargers are, of course, a bit more sophisticated than that and happily, are readily available at any auto supply store at a reasonable price. So get yourself a car battery charger.

No problem putting a few batteries in parallel, should work ok if they are SLA type.

 

[TheVictim]

The decision to use multiple SLAs was due to the fact that I can get them new at a ridiculously low price. It was the motivation to build the UPS in the first place.