# Battery: Rules of Thumb & Questions

mnittler
Solar Expert Posts:

**63**✭✭✭
I am looking for simple "Battery Rules of Thumb" (Inverter Related)

Battery Charger amps required to charge X Ahr battery bank in y hours?

Battery Ahr capacity = x Minimum recommended Charge current?

Battery Ahr capacity = y Maximum recommended Charge current?

If you have 6 each 8VDC 100 amp batteries in series for 48vdc is it considered 600 amp or 100 amps?

Generator size (watts) recommended to charge X Ahr battery bank in Y hours?

Battery Amps (Ahr) required for x watts inverter output for y hours?

(Assume x=500 watts 240vac split phase, y=6hrs)

Solar Panel total x watts required to charge Y amps battery bank in z hours?

Wire size for x Ahr battery bank of Y volts with Z length?

The batteries I am looking at (C&D) are rated in amps, How does this relate to others that are rated in Ahr?

Weight of battery * x = Amp capacity?

Battery normal % max charge voltage?

Battery normal % min discharge voltage for long life?

Throw any other Battery Rules of Thumb you can think of here also so they will all be in one place.

I know that these "Rules of Thumbs" will be just that, guides, not the holy grail.

Battery Charger amps required to charge X Ahr battery bank in y hours?

Battery Ahr capacity = x Minimum recommended Charge current?

Battery Ahr capacity = y Maximum recommended Charge current?

If you have 6 each 8VDC 100 amp batteries in series for 48vdc is it considered 600 amp or 100 amps?

Generator size (watts) recommended to charge X Ahr battery bank in Y hours?

Battery Amps (Ahr) required for x watts inverter output for y hours?

(Assume x=500 watts 240vac split phase, y=6hrs)

Solar Panel total x watts required to charge Y amps battery bank in z hours?

Wire size for x Ahr battery bank of Y volts with Z length?

The batteries I am looking at (C&D) are rated in amps, How does this relate to others that are rated in Ahr?

Weight of battery * x = Amp capacity?

Battery normal % max charge voltage?

Battery normal % min discharge voltage for long life?

Throw any other Battery Rules of Thumb you can think of here also so they will all be in one place.

I know that these "Rules of Thumbs" will be just that, guides, not the holy grail.

19.76kw Solar/GT Enphase IQ7+ MicroInverters

5.40kw Solar/GT ABB/Aurora 300 MicroInverters (AC coupled to Schneider/Xantrex XW6048 output)

6.00kw Solar/Hybrid Xantrex XW6048 Inverter w/2 strings Trojan L-16E-AC Batteries (48VDC)18kw Kohler Propane Generator

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## Comments

1,420✭✭✭✭Look throught the forum, there are a lot of references to specific gravity and battery voltage charts.

1,832✭✭✭✭Depends on the battery type and battery SOC. Let’s say you have a 500 Ah AGM {98% coulombic efficiency (Ah out/Ah in)} battery bank that 50% discharged. It’ll take a little over four hours to push the battery bank from 50% to 90% (bulk cycle), and anywhere from two hours to fours hours to finish off the last 10% (Absorb cycle).

Check your battery manufacturer. For example, Trojan recommends 10% to 13%. As a practical matter, ~5% is the lower limit, but that can result in really long recharge periods.

For VRLA (AGM or gel) batteries, the practical upper limit is ~30%, IMHO. Some battery manufacturers suggest a higher limit, but this is impractical, again IMHO. A bigger charger can reduce bulk charge time, but not absorb time.

When batteries are wired in series, the voltages add but the Ah specs remain constant. Accordingly, you’d have 6 x 8V = 48 V (nominal) x 100 Ah.

This takes us back to Q#1. Multiply charge Amps x charge voltage and divide by charger efficiency and power factor. For example, (50 Adc x 58.4 Vdc) / (85% x 0.98 ) = 3.5 kVA.

The ability to deliver 3.5 kVA continuously would probably require a generator rated 20% higher for its surge rating and “market power rating”, or ~4.2 kVA. These are sea-level- and ~60 F ambient temp ratings, so further deratings of ~3.5% per 1,000 ft elevation and ~1 degree per 10 F above 60 F would be required. Assuming a 48 V battery bank, a max discharge of 50%, and inverter efficiency of 90%:

3.3 kWh / 48 V nominal = 69 Ah

50% max discharge = 69Ah / 50% = 138 Ah

Depends primarily on the inverter’s max input current spec. Deratings may apply based on wire specs and conditions of use.

Please provide a URL to your preferred battery.

Useless concern, IMHO.

These are not a typical specs. % of what?

Battery technical references:

East Penn Deka / MK Battery

Concorde / Sun Xtender Battery

Jim / crewzer

63✭✭✭Thanks for all the helpful battery data.

http://www.cdstandbypower.com

Battery C&D: 4JC-100 can be found under batteries JC then look at 4JC-100.

I was thinking of putting together 2 strings of 6 of these batteries.

(8VDC * 6 batteries = 48VDC)

(100 amps * 2 strings = 200 amps) I guess they rate them differently.

The reason I am confused is the following from battery manufacture website:

1.78 FV/Cell

1 min = 119 amps

15 min = 87 amps

30 min = 67 amps

1 hr = 48 amps (Then is this one ahr?)

2 hr = 33 amps

3 hr = 25 amps

4 hr = 21 amps

5 hr = 18 amps

6 hr = 15 amps

8 hr = 12 amps

12 hr = 9 amps

Anyway what I wanted to know was how long will these batteries last at 500 watts output from inverter? I would just ratio this number for 1000 watts, 2000 watts, etc. From previous post then 200 amp battery bank would be 50% of amps which is 100 amps. 100 * 48 = 4800 watts for 1 hour (or 1200 watts for 4 hrs).

On the weight (this C&D battery weights 100 lbs) , I had figured that if one battery weight is 50 lbs and another of the same voltage is 100 lbs then the 100 lbs battery must be better (thicker plates or something).

18kw Kohler Propane Generator

32,212adminWe like to use C/20 (20 hour rate) matches pretty closely to how much current the average Off-Grid solar system draws from a battery bank.

Note the above chart is based on the ending cell voltage. 1.75 volts per cell is what we call "dead" (10.5 volts for 12 volt bank, 42 volts for a 48 volt bank).

As you can see, a C/20 or C/100 would have even higher AH ratings.

So, you are looking at a 48 volt bank and 1,000 watt inverter, and two parallel strings (assume 1/2 inverter current to each string):

- 1,000 watts * 1/48 volt bank * 1/0,80 inverter efficiency = 26 amps @ 48 volts for the inverter
- 26 amps / 2 strings = 13 amps per string

So, we can use your Battery rating for 15 amp steady draw or C/6 and 90 AH capacity (rounding up).Normally, we would tell you to only discharge the battery by 50% maximum for longer life:

- 90 AH * 1/13 amps * 0.50 max discharge = 3.46 hours = 3 hrs 28 min

So, for a 500 watt inverter, the battery will appear to have more Amp*Hrs of storage. And for a 2,000 watt inverter, the battery will appear to have less storage.If you want to get more in the details... Inverters are constant power devices. When the battery is first loaded, Vbatt is high. As the battery discharges, the battery voltage falls, and the inverter draws more power.

- 1,000 watts * 1/51.2 volt bank * 1/0,80 inverter efficiency = 24.4 amps @ "full" battery
- 1,000 watts * 1/42 volt bank * 1/0,80 inverter efficiency = 29.8 amps @ "dead" battery

Some UPS batteries I have seen specs. that also include "constant power draw" capacities too (instead of constant amperage draw like the above chart).I am guessing that they are partially populated with lead plates (inside same battery case). As the lower capacity batteries have fewer plates, they take more electrolyte to fill. Battery capacity appears to be roughly proportional to the number of lead plates in each cell.

It looks like purely a cost of manufacturing, rack design, and product mix. It is cheaper for them to put fewer lead plates in a battery than to redesign the battery case, mounts, bus bars, racks, etc. for a physically smaller battery. Batteries have same characteristics other than simply reduced AH rating (and other capacity related differences).

-Bill

5,697✭✭✭✭✭Only rule of thumb I'd like to mention, is since this is for a grid tie system, Deep cycle batteries like to be used some, so if you haven't had a need/lose of grid, every couple months I'd draw them down to 80% state of charge, just consider it exersize for the battery...

- Assorted other systems, pieces and to many panels in the closet to not do more projects.

132,212adminOK... Assuming you want to run the load for 12 hours and to 50% maximum discharge--The battery discharge rate would be C/24 -- Close enough to C/20 for our "standard" battery ratings (a few vendors do list C/24 rates instead).

48 Watts * 1/12 volts * 12 hours * 1/0.50 maximum discharge = 96 Amp*Hour @ 12 volt battery bank @ C/24 rate or roughly C/20 Rate

Note that battery capacity can vary by ~20% +/- ... And as a battery ages, its capacity does drop. Some vendors call a battery "dead" when it loses 20% of its capacity, others will call it dead when it loses 40% of its capacity.

For you, the battery is "dead" when it can no longer supply 50% of its capacity. Note that battery capacity does fall as batteries get cold, etc.

So, if you are in a a cold climate with an older battery, you may want to put some "fudge factors" in to ensure that your loads are reliably served for the expected life of the battery bank.

-Bill

63✭✭✭I call it Rule of Thumb for battery run time. Just an estimate using assumptions.

Inverter Run Time Calculation

Lets assume the following values just for the sake of an example. Substitute your own system parameters.

This Assumes the battery bank if fully charged and only a rough estimate.

Inverter Load = 400 watts

(This is your desired watt load) This just needs to be the average inverter load since in normal operations the loads will probably vary so most of the time it is just a good guess unless it is actually known.

Battery String Voltage = 48 vdc

(This is your system battery voltage normally it will be 6v, 12v, 24v, 48v, etc.)

In my case I have 8 each 6V batteries so the voltage is 48 vdc.

Battery String 20 Hr amp rating from the manufacture = 225 amp/hr

(This is your battery manufacture 20 Hr battery amp rating which can normally be found on the battery manufacture's web site) I have Trojan T-105 batteries so I use 225 amp hrs for the 20 hr amp rate. (20 Hr amp rating is not to be confused with CCA which is an entirely different battery rating. CCA means practically nothing for inverter run time calculations.)

Battery % of charge to be used up = 40% = .40

(If you want to use up 60% of the maximum charge then put .6 here which would leave 40% of full charge) The 40% of maximum charge that I used in this example should give you fairly good battery life. If you drain your batteries completely down to 0% of charge then your battery bank will be ruined and probably not recover. 0% of charge is a lot higher battery voltage than most people think but that is an entirely different subject.

Number of Battery Strings = 1

(Put the number of battery strings here) (This is not the number of batteries, it is the number of parallel battery strings)

Efficiency and Fudge factor = .83

(I just use this value as a combination of modern inverter Efficiency and a Fudge factor for lack of a better number.)

Another assumption is made here: The batteries are in fairly good condition, fully charged, and the manufacture 20 hr amperage rating is correct.

Formula:

Inverter Run Time Hrs = Battery string 20 hr amp rating * Battery percent charge to use up / Inverter load in watts * Battery string Voltage * Number of Battery strings * Eff & Fudge Factor

For the above assumptions here is the calculation:

Inverter Run Time hrs = 225 twenty hr amp rating * .4 percent of maximum charge to use up / 400 watts load * 48 volts * 1 strings * .83 Fudge Factor

= 8.964 run time hours (This is just an estimate for this example and will depend on battery charge, battery age, battery condition, and temperature.)

Remember this is only an estimate to get you started. Every installation will have it's own different situations and values.

18kw Kohler Propane Generator

54✭✭Have you done anything with ac coupling tje Xantrex to grid-tie inverters ? sara.j at embarqmail dot com

gridtie: sunny boy sma 2x 5k

gridtie: fronius ag5100 2x 5k

120 panels total on property

5 panels in series driving 90v 1hp pool pump