Energy required to recharge NiMH batteries?
- Thread starter bgiddins
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SilverFox
Flashaholic
Hello Bgiddins,
First of all, there is no such thing as an average NiMh AA battery...
If you go to the threads of interest at the top of this section of the forum and look up the Shoot out tread for NiMh cells, you will find graphs and tables that give you the watt hours discharged from the cells. This varies according to the charge rate.
To charge the cells back up, you will need at least this much, and will have to add to it some more to account for inefficiencies. For example, the charge efficiency when charging at 0.1C requires that you charge about 1.5 times the amount of the capacity of the cell. 1C charge rates usually end up with around 1.1 times the capacity of the cell.
When you move to Li-Ion chemistry, the charging efficiency is very close to 100%, so you just have to put back what you took out. Once again in the stickies you can find the Shoot out thread involving Li-Ion cells and go by the watt hours listed in the discharge graphs.
Tom
First of all, there is no such thing as an average NiMh AA battery...
If you go to the threads of interest at the top of this section of the forum and look up the Shoot out tread for NiMh cells, you will find graphs and tables that give you the watt hours discharged from the cells. This varies according to the charge rate.
To charge the cells back up, you will need at least this much, and will have to add to it some more to account for inefficiencies. For example, the charge efficiency when charging at 0.1C requires that you charge about 1.5 times the amount of the capacity of the cell. 1C charge rates usually end up with around 1.1 times the capacity of the cell.
When you move to Li-Ion chemistry, the charging efficiency is very close to 100%, so you just have to put back what you took out. Once again in the stickies you can find the Shoot out thread involving Li-Ion cells and go by the watt hours listed in the discharge graphs.
Tom
jerry i h
Enlightened
OK, this is the stupid-back-of-the-envelope-calcutation I use for NiMH and NiCd (but NOT Li-anything).
Say you have have Eneloop AA's rated at 2000mAH = 2.0 AH
You have a stupid, budget trickle charger that gives 150 mAH=0.15A
So:
hours to charge = capacity÷(applied current*efficiency factor)
hours = 2.0÷(0.150*0.8)
hours = 17 hours
I use 80% conversion efficiency (if you have a more accurate %, let me know?)
Say you have have Eneloop AA's rated at 2000mAH = 2.0 AH
You have a stupid, budget trickle charger that gives 150 mAH=0.15A
So:
hours to charge = capacity÷(applied current*efficiency factor)
hours = 2.0÷(0.150*0.8)
hours = 17 hours
I use 80% conversion efficiency (if you have a more accurate %, let me know?)
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Marduke
Flashaholic
Assuming you are wanting to know something along the lines of how much it costs:
Let's conservatively go with a high capacity cell, and a crappy inefficient charger.
2.7Ah cell, 1.2v nominal ~ 3.24 watt hours
50% efficiency, so double that to 6.48 watt hours of energy used to charge.
Let's assume you live in the worst possible location for electricity, Hawaii, at 31.56 cents per kilowatt hour.
So, you are paying about $0.002045 to recharge each cell.
Typically, chargers are much more efficient, and rates are MUCH lower for the typical person. The national average for electricity rates for the US is 10.68 cents per kilowatt hour, so you can quickly cut that estimate by 1/3 there. Let us also assume 80% efficiency. That drops your cost to $0.00043 per cell charge.
Let's conservatively go with a high capacity cell, and a crappy inefficient charger.
2.7Ah cell, 1.2v nominal ~ 3.24 watt hours
50% efficiency, so double that to 6.48 watt hours of energy used to charge.
Let's assume you live in the worst possible location for electricity, Hawaii, at 31.56 cents per kilowatt hour.
So, you are paying about $0.002045 to recharge each cell.
Typically, chargers are much more efficient, and rates are MUCH lower for the typical person. The national average for electricity rates for the US is 10.68 cents per kilowatt hour, so you can quickly cut that estimate by 1/3 there. Let us also assume 80% efficiency. That drops your cost to $0.00043 per cell charge.
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jtr1962
Flashaholic
Check your math. 6.48 watt-hours = 0.00648 kw-hr. That's 0.00648*$0.3156=$0.002045 or ~2/10ths of a cent to recharge the cell. I once figured that over 1000 recharges the cost of electricity roughly equals the initial cost of the cell. BTW, Hawaii may not have the highest electric rates for long. Right now electric here is about 26 cents per kw-hr but there's talk of it going up substantially.
Marduke
Flashaholic
Thanks, missed the 1x10^-2 conversion from cents per kilowatt hour to dollars per kilowatt hour. Just shifted the decimal to even cheaper numbers.
Light Sabre
Enlightened
2 or 3 years ago I did some tests mostly on primary batteries and a few NMH's (AA, AAA, D, and 9v). There are so many different types, alkalines, titanium, oxyride, lithium, duracell plus, etc. I wanted to know which one(s) were the biggest bang for the buck. I knew that the cost of the power in batteries was really high compared to the power from the wall outlet. If I remember right, D batteries cost ~40,000 times more and a button battery cost ~400,000 times more. I took the cents/kwh to $/kwh into account. It was an interesting experiment, that tooks several months to do. I have some DVM's that have an RS-232 port and software to collect all the data. Has a real time on screen graph, and saves the data as a text file that you can import to Excel. Did similar tests using CDS sensors and a home made integrating sphere with some of the flashlights I have. You can see a flashlights run time (same brand of alkalines on all) and whether it gradually dimished in brightness or just quit very abruptly just like someone turned off a light switch. IMO that's a big no no. :shakehead
jerry i h
Enlightened
Darn: KWH, not H. OK, so here is the rest of my calculation :shakehead:
Look at the bottom of my trickle charger, it takes 6 watts, so:
KWH = 17 hrs * 6 watts ÷ 1000
KWH = 0.10 KWH
utility bill says 12¢ per KWH, so
12¢ * 0.10 KWH
= 1.2¢
to charge an AA Eneloop
Look at the bottom of my trickle charger, it takes 6 watts, so:
KWH = 17 hrs * 6 watts ÷ 1000
KWH = 0.10 KWH
utility bill says 12¢ per KWH, so
12¢ * 0.10 KWH
= 1.2¢
to charge an AA Eneloop
probably less than that, 6 watts would be most likely with ALL the cells in it if it a 4 bay charger you are talking 0.3 cents/cell
at worst is takes double the power to recharge a cell.... 2.0Ahr * 1.2v is 2.4watts so 4.8watts could be overestimating while using a smart charger putting a half charge in could be 1 watt which would be 0.2 cents
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