I'm building a remote weather station that will have a battery pack and be charged by a small solar panel. I'm hoping the unit will be self sustaining for as long as possible as I plan on mounting it in a difficult to access location (so inquisitive people don't steal it).
I've settled on 18650 batteries due to their capacity and size but some have a cycle life of 400 charges. If I get a small amount of sun each day would that count as one cycle or does it need to fully charge and discharge to count as a cycle? My concern is the batteries would have to be replaced every 400 days if that's the case and I'd be better off with something more suitable to the trickle feeding of charge that a solar panel would provide.
pretty sure 18650s vary like crazy allover the board... it just depends what brand/AH u buy.. I have had good results from some china cheap stuff.. I think japan might make the best 18650s..
why not just try some and go from there.. they vary way toooooo much to lay down any solid guidelines..
also-- cycles are considered as full deep recharges.. so minor/cloudy day charging really doesnt mean much
in that category..
Those are full charge/discharge cycles - and battery life estimates are a real crap shoot at best.
What really kills LiPo batteries is being stored at 100% charge, or over/undercharged. That's why the battery fails in laptops that are left plugged int 24/7 - LiPo is happiest stored at ~60% charge apparently.
You'll need to test it out with the 18650's you have in mind. There is HUGE variation in quality on 18650s. Some chinese manufacturers just put whatever number they want for the capacity - flat out lying about capacity.
yeah I was planning on spending a bit on the battery to be sure I wont have to access it once I'm happy with it's function. I'll pick up a couple of cheapies just because and probably the Panesonic NCR18650B 3400mAh batteries. Thanks for all the help! I just wanted to make sure I wasn't starting out in the wrong direction if there's an obvious alternative.
Keep in mind that 400 cycles doesn't mean the battery is useless. It means that battery has lost some of its capacity (usually 20-35%). The Panasonic NCR18650B 3400mAh chart shows >2500mAh are left after 300 cycles. Is that good enough to run your weather station for one day? If it is, then you're not performing a complete charge/discharge cycle each day and your battery life will be even longer.
I worked for a LiFePO4 company for a couple years, and these cycle life charts resulted from taking a sample of cells and quickly charging/discharging them while counting the coulombs each cycle. That makes it very easy to produce the capacity vs cycle graph in the datasheet, but it is very hard to relate to life in a product.
For example, hybrid cars go through hundreds of partial charge/ discharge cycles a day. If you sum up all of the charging and discharging time, it is a lot more than a few hundred full charge cycles yet the batteries last much longer. The average state-of-charge of the battery's life (previous poster was correct that life is extended if the battery averages 40-60% SOC), the battery temperature (particularly when it's being charged), and the load conditions all affect the battery's life.
The most drastic thing that affects battery life is mismanagement. You're using something to protect against overvoltage/undervoltage/imbalance?
EVERY Discharge and Recharge is a cycle, regardless of Depth of Discharge (D.O.D), even A.C. Ripple can affect cycle life. The cycle life needs to be qualified by D.O.D. 400 cycles without the D.O.D figure is useless.
I was planning on using protected 18650 batteries but unprotected seem a lot easier to source. If I end up going with unprotected I'll buy some prebuilt protection circuits for them as I don't really want to mess around with it.
Kiwi_Bloke:
EVERY Discharge and Recharge is a cycle, regardless of Depth of Discharge (D.O.D), even A.C. Ripple can affect cycle life. The cycle life needs to be qualified by D.O.D. 400 cycles without the D.O.D figure is useless.
In a datasheet, unless the chart says otherwise, the cycle life chart means CC-CV charge followed by 1C discharge at 25degC. If it's not a 100% discharge, how do you think they measure the capacity of the cell?
I agree that the 100% DoD cycle life data is difficult to extend to partial discharge applications.
acitta:
I was planning on using protected 18650 batteries but unprotected seem a lot easier to source. If I end up going with unprotected I'll buy some prebuilt protection circuits for them as I don't really want to mess around with it.
BigBobby:
If it's not a 100% discharge, how do you think they measure the capacity of the cell?
I can't really comment on Lithium's but for the SLA product there seems to be a fair amount of "guesstimate" or "lets just rate it slightly higher than the competition" and I really can't see the cheapo Lithiums being any better.
Kiwi_Bloke:
I can't really comment on Lithium's but for the SLA product there seems to be a fair amount of "guesstimate" or "lets just rate it slightly higher than the competition" and I really can't see the cheapo Lithiums being any better.
Lol...well for cheapo I see a lot worse than "guesstimate"...I see downright lying! Look at the 512GB USB flash drive reviews on amazon if you want to laugh at how bad cheapo can get.
The panasonic cell lists its test conditions on its datasheet, however, and they are typical for the industry. They charge CC-CV to their recommended charge voltage. They then 1C discharge at 25dec C to their recommend discharge voltage. Since it's a constant current, they just need to measure the discharge time to calculate the capacity (Q = I * T).
What that number means in someone's application, isn't much. But sheesh...can you think of a better way to estimate the life of a cell?
Is this station recording data 100% of the time? I mean you may be able to get away with a super cap. I made a very basic "weather station" (just temp and humidity) which sent a reading every 5 minutes via a 433Mhz module. It was happy with a small 6V Schottky + Zener regulated solar panel and 8F of 5.5V supercap.
It was an ATtiny85 driven using watchdog timer to reduce the consumption to around 9nA while "off" for the 4 minutes and 58 seconds out of the 5.
I did a quick google search and from the two sites I looked at, they say supercaps are much more stable lifetime wise. They are electrostatic based storage whereas batteries use electrochem...
The station will record data every 10 minutes and every half hour connect to the GSM network. If it has recieved an sms it'll respond with the data in sms form. It's the GSM module that'll use most of the power. I don't know too much about supercapacitors but I'll do a bit of research to see if they'll suit this application.
