Battery IR 12V - Page 1
Forum section: electronics / general

author: itvend
author_rank: Contributor
timestamp: September 16, 2015, 03:19:47 pm
content: What is the internal resistance?

author: Augustus
author_rank: Regular Contributor
timestamp: September 16, 2015, 04:26:21 pm
content: Very low... Fully charged in the order of 10...20 Milliohms would be my guess...

author: Delta
author_rank: Super Contributor
timestamp: September 16, 2015, 10:26:31 pm
content: Erm, measure it!

author: bktemp
author_rank: Super Contributor
timestamp: September 17, 2015, 06:02:40 am
content: Quote from: Augustus on September 16, 2015, 04:26:21 pmMy measurements are about the same value. UPS grade batteries had 10-20 mohm, cheap ones 20-30.After 5 years of use in an UPS it was at 70 miliohms and the self test failed.

author: Chris C
author_rank: Frequent Contributor
timestamp: September 17, 2015, 06:17:08 am
content: Under ideal conditions pretty darn low. But there are some caveats.These use a gelled electrolyte, which charge moves through sluggishly. So the faster you draw power from them, the faster the electrolyte immediately surrounding the electrodes becomes depleted. Drawing high current therefore causes the voltage to drop early, as if the battery had a fraction of its true capacity, and I guess you could say the effective resistance appears to increase too. But stop the high current drain, the battery rebounds, and the remainder of the charge becomes accessible again; as the charges have time to move around and equalize.Any kind of lead-acid battery also doesn't like to be discharged too deeply, or for too long. Doing that causes the formation of non-conductive lead sulfate crystals on the electrodes, which permanently raise the internal resistance. This is a non-reversible chemical process. With a liquid electrolyte you can at least attach a high voltage pulsed or AC source briefly, causing vibrations which crack the crystals and cause them to fall off. But with a gelled electrolyte, this doesn't help much; as the crystals are pressed against the electrodes, and cannot fall away.Unfortunately a UPS does all the things this type of battery doesn't like. Especially if allowed to run for the maximum possible time, the UPS manufacturers drain the battery far too much in order to get competitive run-time figures. The battery may not last more than two blackouts used in this way. You can extend the battery life by hooking up the USB cable, and setting the computer and UPS to turn off as quickly as possible.

author: Balaur
author_rank: Unknown
timestamp: September 17, 2015, 10:19:03 am
content: Quote from: Chris C on September 17, 2015, 06:17:08 amTotally agree on this. In addition, let's not mention the multiple-batteries packs, charged in series, without any balancing whatsoever.An UPS will work perfectly until you really need it.

author: bktemp
author_rank: Super Contributor
timestamp: September 17, 2015, 10:24:17 am
content: Quote from: Balaur on September 17, 2015, 10:19:03 amEven a single 12V battery has many cells connected in series without any balancing.You can get 2.1V single SLA cells, but they are not really practical and therefore unusual.

author: Balaur
author_rank: Unknown
timestamp: September 17, 2015, 10:34:08 am
content: Quote from: bktemp on September 17, 2015, 10:24:17 amYes, I know. But it's a difference between the 6 cells in a battery and the 48 cells in the 8-battery pack that the work UPS uses.

author: Chris C
author_rank: Frequent Contributor
timestamp: September 17, 2015, 10:39:19 am
content: Quote from: Balaur on September 17, 2015, 10:34:08 amThat sounds awful. Glad I have not encountered any of those. Work used to use some tools with 36V ni-cad packs, no balancing whatsoever, a month didn't go by without having to rebuild one.

author: macboy
author_rank: Super Contributor
timestamp: September 17, 2015, 06:09:08 pm
content: Quote from: Chris C on September 17, 2015, 10:39:19 amThat's due to abuse in the form of over-discharge, not a balancing problem per se. The weakest (lowest capacity) cells will be charged backwards by the remaining good cells that are still happily pushing current. This is compounded by people who want to drive those last few screws or whatever before charging. That's the worst thing you can do. When you sense the pack is getting weak, stop and recharge. Those lowest capacity cells don't just die; they are murdered.Ni-cads are actually very robust and don't need balancing because you can simply over-charge them (slowly, at <= 1/10C) and all of the cells will happily become fully charged and ready. Not so simple with Pb or Li, neither of which can be safely over-charged.

author: Chris C
author_rank: Frequent Contributor
timestamp: September 17, 2015, 10:37:49 pm
content: Quote from: macboy on September 17, 2015, 06:09:08 pmI agree, though I think this specific case was a bit of an exception. It wasn't a motorized tool, but a $30,000 portable computer-based X-ray spectrometer. So no feedback based on motor speed, instead it had an automatic shut-off that was set at a fairly sane voltage. But with 30 cells in series and no individual cell voltage sensing, there was really no shut-off voltage that could be set that would both allow a reasonable run-time, and guarantee shut-off before a weak cell could become reverse charged. To make things worse...Quote from: macboy on September 17, 2015, 06:09:08 pm...This was used out in the field, in 8-12 hour shifts. Batteries were heavy, and ran for about 60-90 minutes. The charger was $5,000, and could be switched between a fast (1C) and a slow (1/10C) charge mode. You can guess how many batteries they traveled with, how many chargers were purchased, and which mode was used even when not necessary.My old personal ni-cad hand tools and lawn equipment didn't fare too well either. I tried to treat the batteries gently, but something always killed them regardless. Sporadic use and winter seasons, combined with high self-discharge. "Constant power" circuits maintaining motor speed regardless of battery voltage, up to the point where they were dangerously low. Dumb chargers. I'm actually glad Li is more complex, as it tends to enforce better designs. Manufacturers tend to skimp when the outcome kills the battery, but not so much when it might kill the user!

author: Stonent
author_rank: Super Contributor
timestamp: September 19, 2015, 04:28:07 am
content: Quote from: Delta on September 16, 2015, 10:26:31 pmSaid the Scotsman when his tailor asked what his inseam was when he went to purchase his first pair of trousers.(Mr. Humprhies, are you free?)