Battery Discharging – Less can be More

Re: Solar Energy, A Complement To FTA by pranil(m): 2:06pm On Jun 20

Reprobate:
so on luminous india site, they stated that the 220ah 12v flooded batt will last about 4hrs on 500w load , and 29hrs on 100watt load.
am a lil confused, assume I connect the following, which will last longer?

A. 2x 12v 220ah batt to a 1.5kva 24v inverter to power a 500w load

B. 1 X 12v 220ah batt to a 1.5kva 24v inverter to power a 500w load.

Can you correct B above ( 1 x12 V ) cannot be connected to 24 V inverter .

In general any Lead Acid battery ( wet , AGM, GEl) likes to be used on C10 or C5 rate based on design . Any higher current drawn above C10 will result in reducing teh overall AH ( energy delivery) of the battery .

So a 220 Ah battery will be able to deliver 220 AH ( 220*12V-2640 watt) if the load is 22 Amps ( 12 V x 22 amps- 264 Watts) for 10 Hours
Same battery at Higher load will likely drop to 170 AH

Also Keep in Mind the battery should be used only upto 50 % DOD in Home solar applications which means a 12 V 220Ah battery system should be considered In design to support a load of 264 watts only for 5 hours

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I think you meant C20. Most flooded acid batteries in the market especially the Tubular Battery form factor commonly sold are rated at C20. Although both of Sukam and Luminous have started to offer C10 batteries  unfortunately none if them is sold in Nigeria (With exception of their AGM batteries which are rated C10).  The example on the luminous site is just their attempt to explain in layman terms the peukert law which generally states the the relationship between battery discharge rate and current availability is exponential and not linear. If you take 11Amp / hour from a 220A  battery rated at C20.  The battery would be fully discharged after 20 hours. (20x 11= 220).
However if you discharge same battery at 20A per hour.. Rather than the battery lasting for 11 hours (which would be the case if discharge rate was linear) the same battery might only last 8 or 7 hours before becoming fully discharged.

On a flip side if you discharge a battery at a rate lower than its rated discharge rate. That is a 220AH battery being discharged  at C40 or C30 which would be 5.5A or 7.3A respectfully. Then you could technically get more current from the battery than the rated 220A you might get close to 230 or 240A before the battery becomes fully discharged.

Ways to use the peukert effect to your advantage includes.

  • Ensuring you use energy efficient devices which would allow you do more with less. 
  • Utilise opportunity loads i.e use most of your heavy loads in afternoons when you are at pick generation. This would put less pressure on your battery bank. Might also mean you solar array is a bit more over sized for the battery. 
  • Reduce night loads to minimum. 

My offgrid setup is designed to take advantage of the peukert law. The combination of running heavy loads like freezers, fridge, Microwave Oven, Washing Machine etc during the day where 90% – 100% of the required energy comes directly from the solar panels and running a small and energy efficient load at night. I have ensured that my 220A  battery is cycled daily at between 10 to 30% depth of charge. The shallow cycling of the batteries would ensure a longer shelve life.