Best practices for extending life of Li-Poly backup battery in product - Page 1
Forum section: electronics / general

author: tom66
author_rank: Super Contributor
timestamp: July 23, 2020, 09:14:51 am
content: Hi all,We're designing a product with a backup battery in it. The battery is a 2000mAh, 3.7V LiPo cell.The goal is that the cell have a lifespan of 5 years at minimum. The product will spend most of its time in standby and the cell will undergo very few cycles, pretty much only being used if there is a power cut.We don't want to keep the cell at 100% SoC all of the time which might "cook" it - conversely we do need decent capacity from the cell should it become necessary, at least 70% or so.With that in mind, the immediate thought is to try to keep the battery at 80% SoC most of the time, so I monitor the battery voltage and when it exceeds 4.08V, the charger is inhibited; when it drops below 4.05V, the charger is enabled again. (The measurement of voltage is based on a moving average filter so that we don't toggle at a higher frequency due to cell ESR or ADC noise.) This results in very mild cycling of the battery between 4.15V and 4.05V on a 60 second period. My concern is that this cycling causes more wear than keeping the cell at a higher voltage. The cell is charged at approx 750mA.Should the cycling period be slower? Should we find a way to reduce the charging current, or increase the cycling time, perhaps by allowing the cell to drop its terminal voltage further after charging is released?

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 23, 2020, 12:35:37 pm
content: Could you use a minimum on/off time instead of hysteresis? Or maybe even use a LDO with reverse current blocking as the charger to hold the voltage steady at the point you define. A resistor in series with the input can be used to limit the current.

author: tunk
author_rank: Super Contributor
timestamp: July 23, 2020, 12:56:11 pm
content: Would it be possible to use a standard charger module/chip with a Schottky diode in series with the cell?

author: ZigmundRat
author_rank: Regular Contributor
timestamp: July 23, 2020, 01:42:36 pm
content: Not a battery expert, but I can tell you that for longest life LiPo should be charged to ~3.85V. This also correlates with lowest self discharge. LiPo charged to ‘normal’ levels (4.2V) will experience higher self discharge rates. This is worst in the first 24 hours, but continues. I think you may be unintentionally exposing yourself to this effect. We also know that the more charge/discharge cycles, the shorter the life. So yes, it seems to me that your philosophy is sub-optimal as you charge to higher levels and keep repeating the charge cycle. I think you need to 1) charge to a lower level, 2) wait until SOC is lower before recharge, 3) consider a different capacity given 1 and 2.There are also other chemistries to consider. LiFe for example.A variety of questions come to mind. Is there over discharge protection? What level do you consider discharged? Are you using a ‘fuel gauge’ (or coulomb counter) IC or simple ADC? Is there a cutoff to isolate the cell when not being used for power?Also, while fact checking my opinions above, I ran into an informative article on Electronic Design:https://www.electronicdesign.com/technologies/test-measurement/article/21808344/measure-selfdischarge-using-ocv-on-lithiumion-cells] https://www.electronicdesign.com/technologies/test-measurement/article/21808344/measure-selfdischarge-using-ocv-on-lithiumion-cells - maybe it’s helpful. Any chance you could get access to a battery company FAE?

author: Siwastaja
author_rank: Super Contributor
timestamp: July 23, 2020, 06:37:58 pm
content: Reduce charging current, especially near top. Charging at 0.5C from 0% to 75%, then at 0.25C from 75% to 100% for average of 0.4375C is likely better for the lifetime than charging at 0.4375C from 0% to 100%. For the same reason, you can see EVs slowing down charging speed way before than the charging goes in CV mode (which obviously slows down charging).Consider derating even lower than 80%, if at all possible. 60% would be better if you can handle the larger cell required to offset for the unused energy.I've seen some data that on some cells, 80% does not provide any gain over 100%, but 60-70% starts doing the trick. This varies by chemistry.Indeed, avoid unnecessary cycling.Avoid high temperature. If your currents are small and you avoid unnecessary cycling, it's well likely calendar fading dominates the lifetime figure; basically the cell is fading use it or not! Lower temperature slows down the fading. The opposite is true if you do have to cycle, especially at higher charge currents. During charging, low temperature causes more damage. So an optimal system would magically have the cell sit at low temperatures, say in a fridge, then heat it up to maybe 35-40 degC for usage (especially charging). Obviously, this isn't possible in reality, and I'm not sure thermal cycling isn't a good idea either.

author: bdunham7
author_rank: Super Contributor
timestamp: July 23, 2020, 06:49:34 pm
content: Quote from: Siwastaja on July 23, 2020, 06:37:58 pmIs that true even if you reduce the charge current at lower temps? My electric car charges much more slowly (and a bit less full) if it is cold out. I was hoping that eliminated the cold-charge issues.