SLA battery exercise  - Page 1
Forum section: electronics / general

author: BradC
author_rank: Super Contributor
timestamp: May 25, 2025, 05:11:05 am
content: This is an interesting one I thought worth posting.I have a battery (specifically a DryPower 12SB35WHR) I use as the start battery for my little petrol generator. I bought this one in late 2021 and it's date coded April 21.It has been used a few times on an as-needed basis since installation, and maybe once every 6 months I'll plug a charger in and run a 3 state charge cycle (14.4->13.8->13.2v) overnight.I wondered how it was doing, so I pulled it out and ran a discharge cycle on my BD200. I used the 10 hour specs (~570mA to 10.5V). Due to me doing something dumb the first cycle cut off early (10.67v). Blue curve. Note the marked drop in voltage at the start of the cycle and clear offset of the curve.It didn't look too bad, so I charged it using the 3 state cycle, let it rest overnight and did another discharge (same specs). Purple curve. Voltage held quite a bit better.I then had a look around various manufacturers "manuals" for SLA battery care and came across an interesting one from Kung Long. It specified to do a standard discharge, followed by a second discharge at C/40 to 10.5v, then a specific charge cycle (C/3 to 14.4V then C/20 constant current until the voltage has maxed out for 3 hours). The second discharge pulled over an extra 2Ah out of the battery and took ages.I set up the charger with the specified algorithm. For the C/20 charge I set the max voltage at 16.5V on the PSU, and it charged up in a pretty linear fashion until it hit 16V, then jumped up and sat against the 16.5V limit for 3 hours while the current slowly dropped.I let it rest overnight and ran another discharge cycle. Now the BD200 has some quirks with its constant current discharge at low currents, so while I asked for 570mA, I got anywhere between 540mA and 615mA across the cycles. If you reverse calculate the values on the graph you can see the difference, however the trend in capacity improvement is pretty clear.Due to my crappy gnuplot code the numbering on the voltage curves is not in sequence. It's Blue, Purple, Orange in order of cycles. Pretty clear to see the improvement as the battery gets a bit of exercise, but the jump between the standard charge/discharge and the Kung Long cycle is fairly marked. That last cycle was the C/20 capacity at a C/10 rate.Now I tried a similar set of cycles on a pair of 6V batteries that have obvious sulfation and saw zero improvement, so I'm not suggesting this is in an way rejuvenating the battery. Just something I'd never seen before. Reckon I might get another couple of years out of this one.

author: thm_w
author_rank: Super Contributor
timestamp: May 26, 2025, 09:54:06 pm
content: So you did an equalization charge then?https://chargetek.com/assets/equal.pdfhttps://northeastbattery.com/equalizing-charges/Once it does die you can replace it with lifepo4 for ~$25

author: BradC
author_rank: Super Contributor
timestamp: May 27, 2025, 12:19:35 am
content: Quote from: thm_w on May 26, 2025, 09:54:06 pmIt was effectively an equalization charge, but I've tried those before, even running as high as 16.5v at low current overnight. It appeared to be the deep slow discharge prior to that which really woke it back up.How will a lifepo4 do inside a generator at high temperatures and managing 90A to spin the starter?

author: thm_w
author_rank: Super Contributor
timestamp: May 27, 2025, 09:46:12 pm
content: You changed two variables so you can't say which had an effect but logically the 16V vs 14V charge would seem to me to be more likely to change the capacity. I can't see a slow discharge doing anything.That battery is only rated to 40-50C ambient, high ambient will reduce cycle life significantly, though in this case its not being cycled.The 90A discharge would require a starter lifepo4 which is a bit more expensive, maybe $50-60. Or hopefully sodium ion soon enough.

author: BradC
author_rank: Super Contributor
timestamp: May 28, 2025, 09:06:46 am
content: Yeah, fair call. Previous charges to 16V have been current limited significantly, so that probably helped.I can't find a lifepo4 battery that'll fit in this location, so that rules it out anyway. I've got 4 years out of this one. When it pegs it, another $35 isn't going to break the bank.The original battery the generator came with lasted about 2 years before giving up and they're > $60 to replace, so I'm ahead anyway.