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Firstly let me say that this is with a view to using a lead acid battery for backup purposes in case of power outage (mains power, a few lightbulbs but with daily outages lasting a few hours)

So let's provide a data sheet for the battery, deep cycle flooded lead acid type:

http://www.trojanbattery.com/pdf/datasheets/SPRE_12_225_DS.pdf

For the given battery, the SPRE 12 225, the open-circuit cell voltage is:

  • 100% charged 2.122V per cell
  • 50% charged 2.017V per cell
  • 20% charged 1.943V per cell (11.66V total)
  • 10% charged 1.918V per cell (11.51V total)

Meanwhile the capacity of the battery is defined based on discharging (at constant current) in 10, 20, 48, 72, or 100 hours until the cell voltage falls to 1.75V (10.5V total).

According to the manufacturer's data, the total lifetime capacity of the battery (number of cycles * depth of discharge) doesn't appear to be significantly different between 20% and 80% DoD.

number of battery cycles vs depth of discharge

So if we want to know how much real capacity the battery has, it seems we can use 12V * C * 80%. The C10 figure is 179Ah, C20 is 204Ah and C100 is 225 Ah. So at C10, 17.9A, that's 12 & 179 * 0.8 = 1718.4W.

However, is it possible that the voltage drop due to the current & internal resistance of the battery is distorting the figures? Or is this insignificant?

The manufacturer do not quote below a C10 figure (17.9A, equivalent to around 170W after inverter & distribution losses). Is this the maximum current draw? Or could you go to, say, C5?

I notice from the manufacturer's datasheet for their renewal energy storage range

http://www.trojanbattery.com/pdf/TrojanBattery_ProdSpecGuide_RE.pdf

that the ratio of, say C10 to C100 isn't completely consistent across battery chemistry, however they do quote C5 numbers for some of the AGM & gel batteries. Is there in fact a possibility to use a higher discharge current for gel/AGM vs lead acid? And as such perhaps a better battery chemistry for the deep cycle, fast discharge use case (let's say we we get a 1 hour powercut and want to discharge at, say C2, and have a generator for backup in the case that it goes over 1 hour)?

Secondly in terms of charging the battery, the SPRE 12 225 states 'a maximum charge current of 13% of C20 rate'. Does this mean 13% of the C20 capacity of 204Ah = 26.5 A charging current, and hence we can charge from 20% to 80% (122.4Ah), say, in just over 4.5 hours?

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    \$\begingroup\$ You made a few mistakes. If you multiply volts * amp-hours, your result will have the unit of Watt-hours, not Watts. So the capacity of the battery is 1718.4 Watt-hours. AKA Wh. Faster discharge will provide a lower Wh rating due to ESR voltage drop and also due to reduced capacity (in Amp hours) at rapid discharge rates. You can discharge faster than C/10, but the capacity will be diminished. In general, if you want rapid discharge, I would suggest a lithium chemistry (maybe LiFePO4). Also, if you need to store the battery at high temperatures, I would suggest LiFeP04. \$\endgroup\$ – mkeith May 20 '18 at 21:52
  • \$\begingroup\$ Lead acid batteries can be great for backup power, but if the temperature is going to be high (like 85F to 100F) the batteries will not last very long. A friend of mine does solar installations on Hawaii, and he said they won't use lead acid batteries anymore for that reason. You can also find this in the datasheets for some batteries. But LiFeP04 cost 4x what AGM costs. So that is something to consider. \$\endgroup\$ – mkeith May 20 '18 at 21:59
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doesn't appear to be significantly different between 20% and 80% DoD.

It appears to me if the DoD is 20% (80% SoC) you will get almost 5,000 cycles. If discharged to a depth of 80% the number of cycles will only be about 1,000, or 80% less life.

Is there in fact a possibility to use a higher discharge current for gel/AGM vs lead acid?

Gel and AGM are Lead Acid. You may mean Flooded Lead Acid. Discharge current is different for each type of Lead Acid. An AGM can be charged up to 5 times faster than a flooded or gel. Be aware that AGM does not like heat and a faster charge rate can increase the temperature.

For general info see: Battery University Learn Section: Battery Types
BU-201: How does the Lead Acid Battery Work?
BU-201a: Absorbent Glass Mat (AGM)
BU-201b: Gel Lead Acid Battery

in just over 4.5 hours?

No, typical flooded lead acid (e.g.SPRE 12 225) charge time is 12-16 hr.
See also Charging Lead Acid.
Your 20% to 80% is ambiguous. Do these percentages refer to SoC (capacity) or DoD (discharge depth)? If the battery is discharged (DoD) 20% (80% capacity remaining SoC=80%) then charge time could reasonably be 4.5 hr, but I did not see that information in the linked document.

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  • \$\begingroup\$ Here is a link to a datasheet for AGM batteries that says maximum charge current is unlimited, as long as voltages are respected. This suggests that at least some AGM batteries can be recharged MUCH faster than C/5. enersys-emea.com/reserve/pdf/EN-SBS-EON-AG-006_1213.pdf \$\endgroup\$ – mkeith May 21 '18 at 7:57
  • \$\begingroup\$ @mkeith Yes, an AGM can be charged up to 5 times faster than a flooded. The SPRE 12 225 referenced in the paragraph with the 4.5 hr. is a flooded battery not AGM. \$\endgroup\$ – Misunderstood May 22 '18 at 0:15
  • \$\begingroup\$ Yeah. A point which temporarily slipped my mind. Going to leave my comment anyway. Could help someone at some point in the future. \$\endgroup\$ – mkeith May 22 '18 at 3:00
  • \$\begingroup\$ "It appears to me if the DoD is 20% (80% SoC) you will get almost 5,000 cycles. If discharged to a depth of 80% the number of cycles will only be about 1,000, or 80% less life." However the total amount of power stored is identical in either case. \$\endgroup\$ – thelawnet May 22 '18 at 13:32
  • \$\begingroup\$ "Your 20% to 80% is ambiguous. Do these percentages refer to SoC (capacity) or DoD (discharge depth)? If the battery is discharged (DoD) 20% (80% capacity remaining SoC=80%) then charge time could reasonably be 4.5 hr, but I did not see that information in the linked document." I meant from 20% charged to 80% charged. As I understand it, the first stage of charge is fast, and then as the battery approaches fully charged it slows to a trickle. Hence while it might take 12 hours to charge from empty to 100%, the 20% to 80% is much faster. \$\endgroup\$ – thelawnet May 22 '18 at 13:34

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