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I am interested in knowing what is the max amps that I can use to safely (ensuring the battery doesn't get too hot, and maximizing battery life). The battery is Panasonic LC-R127R2PG (12V,7.2Ah/20HR) Valve Regulated Lead Acid. I googled and found the spec sheet http://www.farnell.com/datasheets/1674772.pdf and it's listed as: "Charging Method - (Cycle use) Control voltage: 14.5 - 14.9V; Initial current: 2.88A or smaller. And Trickle use Control voltage: 13.6 - 13.8V; Initial current: 1.08A or smaller."

That seems to be super small and will take a while to charge at only 2.88 amps.

Also I am looking at solar to recharge the batteries as back up/emergency. Does anyone have a particular solar panel that he/she would recommend for this application.

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  • \$\begingroup\$ Not sure what you are asking, you say you are interested in knowing the charge rate, and then you tell us the rate listed in the datasheet, so what is it that you want to know exactly? \$\endgroup\$ – PlasmaHH Oct 14 '16 at 21:05
  • \$\begingroup\$ You can't charge a battery "faster" if your staying in the recommended voltage charging limits. If you set your charger to 14.9V then the internal resistance of the battery will determine your current. You can go to a higher voltage and higher current, but you'll probably blow up or damage your battery. If you need a faster charging rate, you'll have to switch to a battery with a different chemistry. \$\endgroup\$ – Voltage Spike Oct 14 '16 at 21:17
  • \$\begingroup\$ "the max amps I can use to safely" ... safely what? \$\endgroup\$ – Brian Drummond Oct 14 '16 at 21:27
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Assuming the question "what is the max amps that I can use" means real use (which is "battery discharge"), the provided specification has a clear answer: safe discharge current is 21.6A.

The other table (page 56) shows 3min discharge at 38.9A

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One can abuse a battery and shorten it's lifespan of charge cycles or one can follow the recommended C rate with some margin to extend the life.

Unlike LiPo , SLA batteries cannot be C/20 (1 hour) due to higher inherent ESR, which limits the charge rate and self heating temperature rise.

If the 1C (20h) discharge rate is 7.2Ah in 20h or "1C" and the charge rate is 2.88A MAX then the charge ratio = 7.2Ah/(2.88A*20h)C= C/8 is the fastest recommended, which also reduces the Ah capacity somewhat and due to absorption losses, you won't get all the Ah out for the Ah that you put in. The curves from Panasonic will show this in better detail.

So if your peak Solar day on average is only a few (say 2) hours (hypothetically) on average or 2/20= C/10 then a C/8 battery will not have the charge rate capacity you need, so more batteries are required in parallel-series array as required if you can supply more power than can be stored.

You can use Wh instead of Ah to make more sense of this in correct sizing Solar power to Battery Wh charge rate when using high-e DC-DC converters.

What may be more important is your storage capacity for number of days with little solar power from dark clouds. Then with more capacity, your charge rate capacity also increases. Battery storage costs can significantly be more than your PV array costs when you need more,storage capacity, but in the long run, it may lay off.

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