Low curent pulse charging a SLA battery - Page 1
Forum section: electronics / projects

author: Copernicus
author_rank: Newbie
timestamp: October 01, 2018, 10:52:40 pm
content: I'm working on charging a 12V 2.5Ah sealed lead acid battery from an energy harvesting device (a turbine). Right now, it dosen't produce enough power to charge at 0.1C continuously.It produces about 10W second (10 Joules) of energy every minute, and the energy is stored in a supercap. Once the supercap is full, it will then turn on a CC/CV charger to charge the battery. And then repeat.So about every min, the Capacitor has 10Ws of power andat 0.1C the charge current would be 250mA, at a nominal 12V this is 3W = ~3.6 charge of time / minat 0.05C the charge current would be 125mA, at a nominal 12V this is 1.5W = ~6.6s charge time / minat 0.01C the charge current would be 25mA, at a nominal 12V this is 0.3W = ~36s charge time / minat 0.005C the charge current would be 12.5mA, at a nominal 12V this is 0.15W = continuously chargeThe battery is 6 of these cells:https://www.digikey.ca/product-detail/en/enersys/0810-0004/842-1000-ND/2003832So obviously, It will take a long time to charge (about a week), which is okay for my application (backup battery).Does anyone have any experience with this type of application? is it better to pulse at 0.1C for 4s every minute, or have it continuously charge at 0.005C? Or somewhere in the middle?

author: Siwastaja
author_rank: Super Contributor
timestamp: October 02, 2018, 12:17:40 pm
content: I would seriously consider li-ion in your application, since li-ion has very straightforward reaction kinetics compared to LA.Leakage is almost negligible (especially at lower SoC and/or temperature it can be less than 1%/year), coulombic efficiency is around 99.97%, and the cell doesn't require floating at 100% SoC for good lifetime - quite the opposite, so there's no harm done discharging your energy storage almost completely, even if charging is delayed or is slow.For extremely low leakage and best lifetime, you could cycle it between about 50% SoC and 10% SoC. Even capacity-derated to 40%, it would be lighter and smaller than an SLA, and expected lifetime would be at least 10, potentially 20 years.With li-ion, AFAIK, there would be absolutely no magic regarding charge rate, and the efficiency just goes higher the slower you charge, so at your 0.005C "continuous charge" I guess you would be around 99.9% efficient cell-wise. You could get rid of the super cap completely, simplifying the design, and reducing the DC/DC conversion losses by reducing the number of converters from 2 to 1.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 02, 2018, 05:09:02 pm
content: I'm not aware of any problems trickle-charging at tiny rates.It just takes forever.Obviously, make sure you have enough power to overcome self-discharge.Just stop charging when it reaches Vfloat. Mind considerations for temperature (voltage tempco), and what exact voltage is best when floating for extended periods of time versus an intermittent float.Tim

author: viperidae
author_rank: Frequent Contributor
timestamp: October 04, 2018, 05:14:36 am
content: What temperature are they going to be when out in the field? Self discharge increases significantly in lead acid batteries as temperature rises.50c can discharge a lead acid battery to the point of failure in a few months. 25c takes about a year to get to damaging levels of charge.

author: mzzj
author_rank: Super Contributor
timestamp: October 04, 2018, 06:25:21 am
content: Quote from: T3sl4co1l on October 02, 2018, 05:09:02 pmI can't remember what was the situation with lead acid but Nicd/Nimh charging efficiency is really poor if you attempt to charge at low currents.Like "Siwastaja" said the li-ion could be the most straightforward solution for low charge rates.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 04, 2018, 07:17:21 am
content: FYI, some examples of my measurements on li-ion self-discharge:Samsung 29E at4.20Vat23degC: leak 3.6%/year, C fade 3.0%/yearSamsung 29E at4.00Vat23degC: leak 2.2%/year, C fade 2.3%/yearSamsung 29E at3.60Vat23degC: leak 0.6%/year, C fade 0.0%/yearSamsung 29E at3.45Vat23degC: leak 0.1%/year, C fade -0.2%/yearSamsung 29E at4.20Vat approx35degC: leak 6.2%/year, C fade 6.1%/yearSamsung 29E at3.60Vat approx35degC: leak 1.3%/year, C fade 0.9%/yearSamsung 29E at4.20Vat approx4degC: leak 1.7%/year, C fade 1.7%/yearSony VTC5 at4.20Vat23degC: leak 1.8%/year, C fade 1.3%/yearSony VTC5 at3.60Vat23degC: leak 0.4%/year, C fade 0.9%/yearSony VTC5 at4.20Vat approx35degC: leak 4.3%/year, C fade 2.7%/yearExpect about +/-0.5% of inaccuracy in the measurement methodology. The giveaway is, if you want to store miniscule amounts of energy without wasting it in self-discharge (or non-wanted chemical reactions such as sulfation in lead-acid), especially if the amount of harvested energy is uncertain, definitely use li-ion; keep it as cool as possible; and don't charge fully, keep at around 3.6-3.7V (50% SoC). It's OK to discharge to almost 0% so you have half of the capacity usable! As you can see from my numbers, when used like this, you can approximate the self-discharge to 0, and lifetime infinite, and it'll be close enough in most practical applications. There may be unexpected reactions happening when the test time is much longer than my 1.5 years, but the cells stored at about 50% SoC at room temp didn't showanysigns of any kind of degradation in 1.5 years.

author: ogden
author_rank: Super Contributor
timestamp: October 04, 2018, 07:17:50 am
content: Quote from: Siwastaja on October 02, 2018, 12:17:40 pmRight. It's worth to mention that your "battery out of six individual cells" approach makes most expensive 2.5AH 12V LA battery in the world. If you are not changing to Li-Ion, at least pick this one:https://www.digikey.ca/product-detail/en/b-b-battery/BP2.3-12-T1/522-1008-ND/653329