I am trying to understand the relationship between AH and WHour for lithium batteries. I know the Watthour=AH x voltage formula. What I am trying to understand is for a 12 V lithium battery how a smart shunt or BMS measures the AH in relation to WH. Does it use 12.8V or 13.3V or something different? When a shunt gives a AH value is it multiplied by nominal lithium battery voltage or 12.8 which is lead acid battery voltage? My shunt seems to use 13.3V approximately but it seems to change. it shows both AH and WH values. When I purchase a LFP battery does the seller advertised AH base on 12.8 V therefore with smart shunt should expect less AH?
What is the relation between AH and WHour for a 12 lifepo4 battery?
- Thread starter adoyuran
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sunshine_eggo
Happy Breffast!
Ah needs voltage to be meaningful.
Wh is independent of voltage (because it uses voltage to compute it).
A 100Ah 12.8V LFP battery has 1280Wh
A 100Ah 51.2V LFP battery has 5120Wh
The 100Ah requires a voltage to meaningfully compare to other things.
The Wh value is directly comparable independent of voltage.
Shunts work by counting Amps. It take V and I samples every t seconds/minutes and computes Ah and Wh based on measured Amps and voltages averaged over time t.
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Thank you
So for the same battery, if the watt hour capacity is fixed, when different loads applied since voltage and current maybe different, does the AH capacity change when lets says load is small vs high? Or is AH also fixed in relation to WH capacity. If so what is the conversion voltage value?
sunshine_eggo
Happy Breffast!
Voltage for "the same battery" can't be different. You can't put a 6V or 24V load on a 12V battery.
When dealing with a given voltage exclusively, Ah can be used exclusively.
Unlike lead acid, with lithium batteries, you pretty much get 100Ah out of a battery whether it's 1A load or a 50A load.
Looping back around to this:
I want to make sure I explained that.
Let's say you have a load. Every second (or other consistent time frame), it measures voltage and current. It then computes Ah and Wh via:
Current * 1 second/(3600 seconds/ 1h) * voltage and adds this to a running total.
Example:
Second 1: 10A / 3600 = .0028Ah; then it multiples that by the voltage, 13.40 = 0.0372Wh
It then records these values.
Second 2: 9.8A / 3600 = .0027Ah; then it multiplies it by the voltage, 13.39 = 0.0365Wh
It then adds it to the previous value and records the new total.
Rinse and repeat.
Ok perfect
This is a good explanation
If this is true though, since overall lithium voltage is higher than lead acid, AH value for the same WH capacity would be lower since Voltage is higher than 12.8. When they sell batteries do they use 12.8 V as a standard capacity for AH ? For example a 100 AH battery does it have 100x12.8=1280 WH or does it have 100x13.3=1330WH?
sunshine_eggo
Happy Breffast!
12.8V is the nominal voltage of 4S LFP. When calculating rated Wh from rated Ah, you use the nominal voltage.
When MEASURING Wh, you use the method mentioned above.
Measured Wh will typically be a little higher than calculated as the AVERAGE (not nominal) voltage during a discharge will frequently come out a little higher than 12.8V - maybe 13.0V.
The quality of the installation can also affect this. Any wiring losses or poor connections may drive battery voltage lower for a given current. In this case, you might measure less than rated Wh. the missing Wh will have been dissipated as heat in the wiring or poor connections.
Daddy Tanuki
Solar Wizard
?. @sunshine_eggo is rarely wrong, this is the best explanation I have seen in a while. But to be honest it does depend upon your BMS’s program. Some are better some are worse.
Ok thank you for that clarification
If I bought a 100AH battery which has 1280WH energy storage, when I run it under the load and it discharges at average 13V, the measured AH should then be 100/13x12.8=98.46AH. Is that correct? So it is ok to measure a little less AH than the sellers advertised AH, correct?
sunshine_eggo
Happy Breffast!
No. No. The only time 12.8 is used is for calculations when test data are not available.
A 100Ah battery will likely test a little over 100Ah because they de-rate them a little to ensure they meet specification. It's common for Battleborn to test in the 107-110Ah range. Will has done many tests on various brands where batteries test a bit over spec.
If the test only measured Ah, you could approximate Wh by multiplying by 12.8V. If the test also measured Wh, you have measured Ah and Wh, and no calculations are needed. Using 12.8V when Ah and Wh are available is inaccurate and pointless.
RCinFLA
Solar Wizard
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A battery monitor takes small time samples for current readings and voltage readings.
So a monitor is summing amp-seconds and watt-seconds. It may not be precisely seconds in increment time periods, but it is in that ballpark and is consistent small time increments.
Just keep a running tally sum of increments and convert time reference to hours, either AH's or wH's.
Mathematically, it is integration by small increment steps.
To account for battery charging efficiency or magnitude of current discharge efficiency, a monitor may apply a fudge factor derating on each sample before it is summed into running tally count. There is user provided setup parameters (charge efficiency and Peukert factor) that determine these fudge factors.
There will be some errors in current reading that will accumulate in the tally sum. A monitor clears the tally count when battery is fully charged. It is necessary to do this periodically to clear out accumulated errors and re-start tally count fresh from a known reference point (full state of charge). Failure to do this (fully charge battery) for a long period of time will result in poor AH capacity available readout accuracy.
It is like dead-reckoning navigation, you need a known reference point once in a while to clear out cumulative errors.
So a monitor is summing amp-seconds and watt-seconds. It may not be precisely seconds in increment time periods, but it is in that ballpark and is consistent small time increments.
Just keep a running tally sum of increments and convert time reference to hours, either AH's or wH's.
Mathematically, it is integration by small increment steps.
To account for battery charging efficiency or magnitude of current discharge efficiency, a monitor may apply a fudge factor derating on each sample before it is summed into running tally count. There is user provided setup parameters (charge efficiency and Peukert factor) that determine these fudge factors.
There will be some errors in current reading that will accumulate in the tally sum. A monitor clears the tally count when battery is fully charged. It is necessary to do this periodically to clear out accumulated errors and re-start tally count fresh from a known reference point (full state of charge). Failure to do this (fully charge battery) for a long period of time will result in poor AH capacity available readout accuracy.
It is like dead-reckoning navigation, you need a known reference point once in a while to clear out cumulative errors.
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Minimoose
New Member
This screws with my head too. If I have 4 batteries in a series for 48 volts does that mean it will run 4 times longer that 4 batteries in parallel at 12 volts? Amps are amps no matter what voltage right? I just figured the amps would be used slightly more efficiently at 48 volts.
Ok.thank you very much
I appreciate a through explanation. I think this explanation will be very valuable for many people.
Thank you
sunshine_eggo
Happy Breffast!
No. No.
A 1200W load at 12V consumes 100A. 100Ah will last 1 hour, 400Ah will last 4 hour.
A 1200W load at 48V consumes 25A. 100Ah will last 4 hour
That's why Wh helps simplify things.
12V * 4 * 100Ah = 4800Wh
48V * 1 * 100Ah = 4800Wh
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Minimoose
New Member
So I could basically swap my 4 12v 100ah batteries to 24v or48v and double or quadruple my run time? That would be cheaper than buying more batteries.
sunshine_eggo
Happy Breffast!
No. I literally demonstrated the opposite. Let's try again:
Parallel: 4 * 12V * 100Ah = 4800Wh (12V @ 400Ah)
Series: 1 * 48V * 100Ah = 4800Wh (48V @ 100Ah)
Both arrangements contain the exact same amount of energy.
On a 12V system (4P = 400Ah), a 1200W load will consume 100A. 400Ah will last 4h.
On a 48V system (4S = 100Ah), a 1200W load will consume 25A. 100Ah will last 4h.
Ampster
Renewable Energy Hobbyist
Only if you reduced the Wattage to one quarter of the the initial Wattage. Look again at the forumula that you quoted from @sunshine_eggo and particularly his statement that, Watts simplifies things. Or read is response just above.
Minimoose
New Member
Okay...makes sense now! Thank you!
Minimoose
New Member
Yes. I was looking at it the wrong way. Only way to increase run time is more storage for the same load. Thanks!
Well… depending on a LOT of factors, arranging the batteries to 48V will give you a little more runtime, because the same 1200W load at 12V will be maxing the loads on he battery, and heating everything up more because of high amp waste heat.
At 48V, he 1200W load may run a bit longer than 4 hours since the drain is smoother on the wiring at 25A vs 100A.
At 48V, he 1200W load may run a bit longer than 4 hours since the drain is smoother on the wiring at 25A vs 100A.
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