Lead Acid Battery Inrush current while charging - Page 1
Forum section: electronics / beginners

author: drdm
author_rank: Contributor
timestamp: October 22, 2024, 01:28:39 pm
content: Hello all, I have a Led Acid Battery question:Suppose the battery is deeply discharged. We are talking very discharged (3V between the terminals for a standard car battery). Now suppose that we want to charge this battery with another, fully charged battery with twice the capacity, so we have a practically limitless supply of current.What would be the inrush current into the dead battery? Do we have a theoretical way of calculating it?

author: wasedadoc
author_rank: Super Contributor
timestamp: October 22, 2024, 01:59:56 pm
content: 1. A lot of unknown variables. If heavily sulphated or severely lacking acid the internal resistance will be higher than normal. Resistance of the cables linking the batteries?2. Risk of very high current and acid being ejected from good battery.3. High probability that a 12v nominal battery that measures 3v will never be useful again.4. If you intend to proceed, an appropriate current limiting resistor is STRONGLY advised.

author: drdm
author_rank: Contributor
timestamp: October 22, 2024, 02:19:18 pm
content: Thank you for the quick answer.1. I know there is not much info on the topic. For now we assume that the current will be in the 10s of amps.2. I also suspect that in this case the charging battery could be damaged.3. This is also considered. The use case is that someone doesn't know the battery is dead and tries to charge it.

author: BillyO
author_rank: Super Contributor
timestamp: October 22, 2024, 03:10:41 pm
content: How long has the battery been like that? If a (nominally) 12V LA battery has been below 10V for more than a few days it can be considered post consumer waste. The sulfation rate will go through the roof resulting an extremely high self-discharge rate or even shorted cells. If the cells are not shorted and you manage to get the battery charged, it will not hold the charge. If it has shorted cells and you attempt to get it fully charged you will be grossly over charging the remaining cells and they will start gassing off hydrogen and oxygen (yeah, not good) and heating the plates to the point of them warping.My advice is to just don't do this.Good battery chargers will not attempt to charge a battery under 10V. Get a good battery charger and not worry about this situation.

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 22, 2024, 07:27:09 pm
content: Well you have 9V different in potential so the current will depend on the 9V and the internal resistance of both the good and bad battery. But it's something theoretical speaking I wouldn't try to charge it like that. First the 3V battery is considered no more good. If I try to charge it I would use a power supply with current limiting capability.

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: October 22, 2024, 07:47:29 pm
content: I've inadvertently killed a sealed lead acid battery by letting it sit at 11V. If your battery is at 3V it's pretty much guaranteed to be dead.

author: Gyro
author_rank: Super Contributor
timestamp: October 22, 2024, 08:28:10 pm
content: As an experiment to answer your question, if you have no other equipment other than the other car battery, putting a 12V 5W car bulb in series with the dead battery will limit the current to a reasonably safe level and give you some diagnostic information...- If the bulb does not light to some reasonable brightness then the battery is sulphated and completely dead.- If it does light, it should start to dim down fairly quickly. Give it a few minutes, and then disconnect.- Allow the battery to sit for ~30 mins then measure the battery voltage. If it isn't sitting at around 10V or more then again it is dead.- If the battery voltage is looking reasonable, as above, then continue to charge through the bulb, even if it isn't lit, it will still act as a safety current limiter.- After several hours, check again (30 mins wait again). If it is near the voltage of the good battery then it should be safe to put on a battery charger.The battery capacity is probably severely reduced regardless. As the others have said, if the battery has been down at 3V for any significant time, it is dead for sure.

author: Zero999
author_rank: Super Contributor
timestamp: October 22, 2024, 09:30:49 pm
content: If it's discharged to 3V, then it's dead, as everyone has said, but the inrush current will not be massive, because the internal resistance of a dead battery is typically very high.